Lakeshore 325 User Manual
User’s Manual
Model 325
Temperature Controller
Lake Shore Cryotronics, Inc.
575 McCorkle Blvd.
Westerville, Ohio 43082-8888 USA
E-mail addresses:
sales@lakeshore.com
service@lakeshore.com
Visit our website at:
www.lakeshore.com
Fax: (614) 891-1392
Telephone: (614) 891-2243
Methods and apparatus disclosed and described herein have been developed solely on company funds of Lake Shore Cryotronics, Inc. No government
or other contractual support or relationship whatsoever has existed which in any way affects or mitigates proprietary rights of Lake Shore Cryotronics,
Inc. in these developments. Methods and apparatus disclosed herein may be subject to U.S. Patents existing or applied for. Lake Shore Cryotronics,
Inc. reserves the right to add, improve, modify, or withdraw functions, design modifications, or products at any time without notice. Lake Shore shall
not be liable for errors contained herein or for incidental or consequential damages in connection with furnishing, performance, or use of this material.
Revision: 1.5 P/N 119-041 12 November 2013
Lake Shore Model 335 Temperature Controller User’s Manual
LIMITED WARRANTY STATEMENT
WARRANTY PERIOD: THREE (3) YEARS
1. Lake Shore warrants that products manufactured by Lake Shore (the "Product") will be free from defects in materials and workmanship for
three years from the date of Purchaser's physical receipt of the Product (the "Warranty Period"). If Lake Shore receives notice of any such
defects during the Warranty Period and the defective Product is shipped freight prepaid back to Lake Shore, Lake Shore will, at its option,
either repair or replace the Product (if it is so defective) without charge for parts, service labor or associated customary return shipping cost
to the Purchaser. Replacement for the Product may be by either new or equivalent in performance to new. Replacement or repaired parts, or
a replaced Product, will be warranted for only the unexpired portion of the original warranty or 90 days (whichever is greater).
2. Lake Shore warrants the Product only if the Product has been sold by an authorized Lake Shore employee, sales representative, dealer or an
authorized Lake Shore original equipment manufacturer (OEM).
3. The Product may contain remanufactured parts equivalent to new in performance or may have been subject to incidental use when it is
originally sold to the Purchaser.
4. The Warranty Period begins on the date of Purchaser's physical receipt of the Product or later on the date of operational training and
verification (OT&V) of the Product if the service is performed by Lake Shore, provided that if the Purchaser schedules or delays the Lake
Shore OT&V for more than 30 days after delivery then the Warranty Period begins on the 31st day after Purchaser's physical receipt of the
Product.
5. This limited warranty does not apply to defects in the Product resulting from (a) improper or inadequate installation (unless OT&V services
are performed by Lake Shore), maintenance, repair or calibration, (b) fuses, software, power surges, lightning and non-rechargeable
batteries, (c) software, interfacing, parts or other supplies not furnished by Lake Shore, (d) unauthorized modification or misuse, (e)
operation outside of the published specifications, (f) improper site preparation or site maintenance (g) natural disasters such as flood, fire,
wind, or earthquake, or (h) damage during shipment other than original shipment to you if shipped through a Lake Shore carrier.
6. This limited warranty does not cover: (a) regularly scheduled or ordinary and expected recalibrations of the Product; (b) accessories to the
Product (such as probe tips and cables, holders, wire, grease, varnish, feed throughs, etc.); (c) consumables used in conjunction with the
Product (such as probe tips and cables, probe holders, sample tails, rods and holders, ceramic putty for mounting samples, Hall sample
cards, Hall sample enclosures, etc.); or, (d) non-Lake Shore branded Products that are integrated with the Product.
7. To the extent allowed by applicable law,, this limited warranty is the only warranty applicable to the Product and replaces all other
warranties or conditions, express or implied, including, but not limited to, the implied warranties or conditions of merchantability and
fitness for a particular purpose. Specifically, except as provided herein.
Lake Shore undertakes no responsibility that the products will be fit for any particular purpose for which you may be buying the Products.
Any implied warranty is limited in duration to the warranty period. No oral or written information, or advice given by the Company, its
Agents or Employees, shall create a warranty or in any way increase the scope of this limited warranty. Some countries, states or provinces
do not allow limitations on an implied warranty, so the above limitation or exclusion might not apply to you. This warranty gives you
specific legal rights and you might also have other rights that vary from country to country, state to state or province to province.
8. Further, with regard to the United Nations Convention for International Sale of Goods (CISC,) if CISG is found to apply in relation to this
agreement, which is specifically disclaimed by Lake Shore, then this limited warranty excludes warranties that: (a) the Product is fit for the
purpose for which goods of the same description would ordinarily be used, (b) the Product is fit for any particular purpose expressly or
impliedly made known to Lake Shore at the time of the conclusion of the contract, (c) the Product is contained or packaged in a manner
usual for such goods or in a manner adequate to preserve and protect such goods where it is shipped by someone other than a carrier hired
by Lake Shore.
9. Lake Shore disclaims any warranties of technological value or of non-infringement with respect to the Product and Lake Shore shall have
no duty to defend, indemnify, or hold harmless you from and against any or all damages or costs incurred by you arising from the
infringement of patents or trademarks or violation or copyrights by the Product.
10. THIS WARRANTY IS NOT TRANSFERRABLE. This warranty is not transferrable.
11. Except to the extent prohibited by applicable law, neither Lake Shore nor any of its subsidiaries, affiliates or suppliers will be held liable for
direct, special, incidental, consequential or other damages (including lost profit, lost data, or downtime costs) arising out of the use, inability
to use or result of use of the product, whether based in warranty, contract, tort or other legal theory, regardless whether or not Lake Shore
has been advised of the possibility of such damages. Purchaser's use of the Product is entirely at Purchaser's risk. Some countries, states and
provinces do not allow the exclusion of liability for incidental or consequential damages, so the above limitation may not apply to you.
12. This limited warranty gives you specific legal rights, and you may also have other rights that vary within or between jurisdictions where the
product is purchased and/or used. Some jurisdictions do not allow limitation in certain warranties, and so the above limitations or
exclusions of some warranties stated above may not apply to you.
13. Except to the extent allowed by applicable law, the terms of this limited warranty statement do not exclude, restrict or modify the
mandatory statutory rights applicable to the sale of the product to you.
CERTIFICATION
Lake Shore certifies that this product has been inspected and tested in accordance with its published specifications and that this product met its
published specifications at the time of shipment. The accuracy and calibration of this product at the time of shipment are traceable to the United States
National Institute of Standards and Technology (NIST); formerly known as the National Bureau of Standards (NBS).
FIRMWARE LIMITATIONS
Lake Shore has worked to ensure that the Model 325 firmware is as free of errors as possible, and that the results you obtain from the instrument are
accurate and reliable. However, as with any computer-based software, the possibility of errors exists.
In any important research, as when using any laboratory equipment, results should be carefully examined and rechecked before final conclusions are
drawn. Neither Lake Shore nor anyone else involved in the creation or production of this firmware can pay for loss of time, inconvenience, loss of use
of the product, or property damage caused by this product or its failure to work, or any other incidental or consequential damages. Use of our product
implies that you understand the Lake Shore license agreement and statement of limited warranty.
FIRMWARE LICENSE AGREEMENT
The firmware in this instrument is protected by United States copyright law and international treaty provisions. To maintain the warranty, the code
contained in the firmware must not be modified. Any changes made to the code is at the user’s risk. Lake Shore will assume no responsibility for
damage or errors incurred as result of any changes made to the firmware.
Under the terms of this agreement you may only use the Model 325 firmware as physically installed in the instrument. Archival copies are strictly
forbidden. You may not decompile, disassemble, or reverse engineer the firmware. If you suspect there are problems with the firmware, return the
instrument to Lake Shore for repair under the terms of the Limited Warranty specified above. Any unauthorized duplication or use of the Model 325
firmware in whole or in part, in print, or in any other storage and retrieval system is forbidden.
TRADEMARK ACKNOWLEDGMENT
Many manufacturers and sellers claim designations used to distinguish their products as trademarks. Where those designations appear in this manual
and Lake Shore was aware of a trademark claim, they appear with initial capital letters and the ™ or
Alumel™ and Chromel™ are trademarks of Concept Alloys, LLC.
Apiezon® is a trademark of M&I Materials, Ltd.
CalCurve™, Cernox™, Duo-Twist™, Quad-Lead™, Quad-Twist™, Rox™, and SoftCal™ are trademarks of Lake Shore Cryotronics, Inc.
Cryogloves® is a trademark of Tempshield.
LabVIEW™ and NI-488.2™ are trademarks of National Instruments.
MS-DOS® and Windows® are trademarks of Microsoft Corp.
PC, XT, AT, and PS-2 are trademarks of IBM.
Stycast® is a trademark of Emerson & Cummings.
®
symbol.
Copyright © 2006–2013 by Lake Shore Cryotronics, Inc. All rights reserved. No portion of this manual may be
reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without the express written permission of Lake Shore.
Lake Shore Model 325 Temperature Controller User’s Manual
Lake Shore Model 325 Temperature Controller User’s Manual
Electromagnetic Compatibility (EMC) for the Model 325 Temperature Controller
Electromagnetic Compatibility (EMC) of electronic equipment is a growing concern worldwide. Emissions of
and immunity to electromagnetic interference is now part of the design and manufacture of most electronics. To
qualify for the CE Mark, the Model 325 meets or exceeds the requirements of the European EMC Directive
89/336/EEC as a CLASS A product. A Class A product is allowed to radiate more RF than a Class B product
and must include the following warning:
WARNING: This is a Class A product. In a domestic environment, this product may cause radio
interference in which case the user may be required to take adequate measures.
The instrument was tested under normal operating conditions with sensor and interface cables attached. If the
installation and operating instructions in the User’s Manual are followed, there should be no degradation in
EMC performance.
This instrument is not intended for use in close proximity to RF Transmitters such as two-way radios and cell
phones. Exposure to RF interference greater than that found in a typical laboratory environment may disturb the
sensitive measurement circuitry of the instrument.
Pay special attention to instrument cabling. Improperly installed cabling may defeat even the best EMC
protection. For the best performance from any precision instrument, follow the grounding and shielding
instructions in the User’s Manual. In addition, the installer of the Model 325 should consider the following:
• Shield measurement and computer interface cables.
• Leave no unused or unterminated cables attached to the instrument.
• Make cable runs as short and direct as possible. Higher radiated emissions is possible with long cables.
• Do not tightly bundle cables that carry different types of signals.
Lake Shore Model 325 Temperature Controller User’s Manual
TABLE OF CONTENTS
Chapter/Section Title Page
1 INTRODUCTION .................................................................................................................................................... 1-1
1.0 PRODUCT DESCRIPTION ............................................................................................................... 1-1
1.1 SENSOR SELECTION ...................................................................................................................... 1-4
1.2 SPECIFICATIONS ............................................................................................................................. 1-6
1.3 SAFETY SUMMARY ......................................................................................................................... 1-8
1.4 SAFETY SYMBOLS .......................................................................................................................... 1-9
2 COOLING SYSTEM DESIGN ................................................................................................................................. 2-1
2.0 GENERAL ......................................................................................................................................... 2-1
2.1 TEMPERATURE SENSOR SELECTION .......................................................................................... 2-1
2.1.1 Temperature Range ....................................................................................................................... 2-1
2.1.2 Sensor Sensitivity .......................................................................................................................... 2-1
2.1.3 Environmental Conditions .............................................................................................................. 2-1
2.1.4 Measurement Accuracy ................................................................................................................. 2-2
2.1.5 Sensor Package ............................................................................................................................. 2-2
2.2 CALIBRATED SENSORS ................................................................................................................. 2-2
2.2.1 Traditional Calibration .................................................................................................................... 2-2
2.2.2 SoftCal™........................................................................................................................................ 2-2
2.2.3 Standard Curves ............................................................................................................................ 2-3
2.2.4 CalCurve™ .................................................................................................................................... 2-4
2.3 SENSOR INSTALLATION ................................................................................................................. 2-4
2.3.1 Mounting Materials ......................................................................................................................... 2-4
2.3.2 Sensor Location ............................................................................................................................. 2-4
2.3.3 Thermal Conductivity ..................................................................................................................... 2-4
2.3.4 Contact Area .................................................................................................................................. 2-4
2.3.5 Contact Pressure ........................................................................................................................... 2-5
2.3.6 Lead Wire....................................................................................................................................... 2-5
2.3.7 Lead Soldering ............................................................................................................................... 2-6
2.3.8 Heat Sinking Leads ........................................................................................................................ 2-6
2.3.9 Thermal Radiation .......................................................................................................................... 2-6
2.4 HEATER SELECTION AND INSTALLATION .................................................................................... 2-6
2.4.1 Heater Resistance and Power ....................................................................................................... 2-6
2.4.2 Heater Location .............................................................................................................................. 2-7
2.4.3 Heater Types ................................................................................................................................. 2-7
2.4.4 Heater Wiring ................................................................................................................................. 2-7
2.5 CONSIDERATIONS FOR GOOD CONTROL ................................................................................... 2-7
2.5.1 Thermal Conductivity ..................................................................................................................... 2-7
2.5.2 Thermal Lag ................................................................................................................................... 2-7
2.5.3 Two-Sensor Approach ................................................................................................................... 2-7
2.5.4 Thermal Mass ................................................................................................................................ 2-8
2.5.5 System Nonlinearity ....................................................................................................................... 2-8
2.6 PID CONTROL .................................................................................................................................. 2-8
2.6.1 Proportional (P) .............................................................................................................................. 2-8
2.6.2 Integral (I)....................................................................................................................................... 2-9
2.6.3 Derivative (D) ................................................................................................................................. 2-9
2.6.4 Manual Heater Power (MHP) Output ............................................................................................. 2-9
2.7 MANUAL TUNING ........................................................................................................................... 2-11
2.7.1 Setting Heater Range .................................................................................................................. 2-11
2.7.2 Tuning Proportional ...................................................................................................................... 2-11
2.7.3 Tuning Integral ............................................................................................................................. 2-12
2.7.4 Tuning Derivative ......................................................................................................................... 2-12
2.8 AUTOTUNING ................................................................................................................................. 2-12
2.9 ZONE TUNING ................................................................................................................................ 2-13
Table of Contents i
Lake Shore Model 325 Temperature Controller User’s Manual
TABLE OF CONTENTS (Continued)
Chapter/Section Title Page
3 INSTALLATION ...................................................................................................................................................... 3-1
3.0 GENERAL ......................................................................................................................................... 3-1
3.1 INSPECTION AND UNPACKING ...................................................................................................... 3-1
3.2 REAR PANEL DEFINITION ............................................................................................................... 3-2
3.3 LINE INPUT ASSEMBLY ................................................................................................................... 3-3
3.3.1 Line Voltage ................................................................................................................................... 3-3
3.3.2 Line Fuse and Fuse Holder ............................................................................................................ 3-3
3.3.3 Power Cord .................................................................................................................................... 3-3
3.3.4 Power Switch ................................................................................................................................. 3-3
3.4 DIODE/RESISTOR SENSOR INPUTS .............................................................................................. 3-4
3.4.1 Sensor Input Connector and Pinout ............................................................................................... 3-4
3.4.2 Sensor Lead Cable ........................................................................................................................ 3-4
3.4.3 Grounding and Shielding Sensor Leads ......................................................................................... 3-5
3.4.4 Sensor Polarity ............................................................................................................................... 3-5
3.4.5 Four-Lead Sensor Measurement ................................................................................................... 3-5
3.4.6 Two-Lead Sensor Measurement .................................................................................................... 3-5
3.4.7 Lowering Measurement Noise........................................................................................................ 3-6
3.5 THERMOCOUPLE SENSOR INPUTS .............................................................................................. 3-6
3.5.1 Sensor Input Terminals .................................................................................................................. 3-6
3.5.2 Thermocouple Installation .............................................................................................................. 3-7
3.5.3 Grounding and Shielding ................................................................................................................ 3-7
3.6 HEATER OUTPUT SETUP ............................................................................................................... 3-7
3.6.1 Loop 1 Output ................................................................................................................................ 3-7
3.6.2 Loop 1 Heater Output Connector ................................................................................................... 3-7
3.6.3 Loop 1 Heater Output Wiring ......................................................................................................... 3-7
3.6.4 Loop 1 Heater Output Noise .......................................................................................................... 3-8
3.6.5 Loop 2 Output ................................................................................................................................ 3-8
3.6.6 Loop 2 Output Resistance .............................................................................................................. 3-8
3.6.7 Loop 2 Output Connector ............................................................................................................... 3-8
3.6.8 Loop 2 Heater Protection ............................................................................................................... 3-8
3.6.9 Boosting the Output Power ............................................................................................................ 3-8
3.7 INITIAL SETUP AND SYSTEM CHECKOUT PROCEDURE ............................................................ 3-9
4 OPERATION ........................................................................................................................................................... 4-1
4.0 GENERAL ......................................................................................................................................... 4-1
4.1 FRONT PANEL DESCRIPTION ........................................................................................................ 4-1
4.1.1 Keypad Definitions ......................................................................................................................... 4-1
4.1.2 Annunciators .................................................................................................................................. 4-2
4.1.3 General Keypad Operation ............................................................................................................ 4-2
4.1.4 Display Definition ........................................................................................................................... 4-3
4.2 TURNING POWER ON ..................................................................................................................... 4-4
4.3 DISPLAY FORMAT AND SOURCE (UNITS) SELECTION ............................................................... 4-4
4.4 INPUT SETUP ................................................................................................................................... 4-6
4.4.1 Diode Sensor Input Setup – 10 µA Excitation Current ................................................................... 4-6
4.4.2 Diode Sensor Input Setup – 1 mA Excitation Current .................................................................... 4-6
4.4.3 Resistor Sensor Input Setup .......................................................................................................... 4-7
4.4.3.1 Thermal EMF Compensation ...................................................................................................... 4-8
4.4.4 Thermocouple Sensor Input Setup ................................................................................................. 4-8
4.4.4.1 Room-Temperature Compensation ................................ ............................................................ 4-9
4.4.4.2 Room-Temperature Calibration Procedure ................................................................................. 4-9
4.4.5 Temperature Limit (For Firmware Version 1.5 and Later) ........................................................... 4-10
4.5 CURVE SELECTION ....................................................................................................................... 4-10
4.5.1 Diode Sensor Curve Selection ..................................................................................................... 4-11
4.5.2 Resistor Sensor Curve Selection ................................................................................................. 4-12
4.5.3 Thermocouple Sensor Curve Selection ........................................................................................ 4-12
4.5.4 Filter ............................................................................................................................................. 4-12
ii Table of Contents
Lake Shore Model 325 Temperature Controller User’s Manual
TABLE OF CONTENTS (Continued)
Chapter/Section Title Page
4.6 TEMPERATURE CONTROL ........................................................................................................... 4-13
4.6.1 Control Loops ............................................................................................................................... 4-13
4.6.2 Control Modes .............................................................................................................................. 4-14
4.6.3 Tuning Modes .............................................................................................................................. 4-14
4.7 CONTROL SETUP .......................................................................................................................... 4-15
4.8 MANUAL TUNING ........................................................................................................................... 4-16
4.8.1 Manually Setting Proportional (P) ................................................................................................ 4-16
4.8.2 Manually Setting Integral (I) ......................................................................................................... 4-17
4.8.3 Manually Setting Derivative (D) .................................................................................................... 4-17
4.8.4 Setting Manual Heater Power (MHP) Output ............................................................................... 4-18
4.9 AUTOTUNE (Closed-Loop PID Control) .......................................................................................... 4-18
4.10 ZONE SETTINGS (Closed-Loop Control Mode).............................................................................. 4-19
4.11 SETPOINT....................................................................................................................................... 4-22
4.12 RAMP .............................................................................................................................................. 4-23
4.13 HEATER RANGE AND HEATER OFF ............................................................................................ 4-24
4.14 HEATER RESISTANCE SETTING .................................................................................................. 4-24
4.15 LOCKING AND UNLOCKING THE KEYPAD .................................................................................. 4-25
4.16 REMOTE/LOCAL ............................................................................................................................ 4-25
4.17 INTERFACE .................................................................................................................................... 4-25
4.18 DEFAULT VALUES ......................................................................................................................... 4-26
5 ADVANCED OPERATION ..................................................................................................................................... 5-1
5.0 GENERAL ......................................................................................................................................... 5-1
5.1 CURVE NUMBERS AND STORAGE ................................................................................................ 5-1
5.1.1 Curve Header Parameters ............................................................................................................. 5-1
5.1.2 Curve Breakpoints ......................................................................................................................... 5-1
5.2 FRONT PANEL CURVE ENTRY OPERATIONS .............................................................................. 5-3
5.2.1 Edit Curve ...................................................................................................................................... 5-3
5.2.1.1 Thermocouple Curve Considerations ......................................................................................... 5-5
5.2.2 Erase Curve ................................................................................................................................... 5-5
5.2.3 Copy Curve .................................................................................................................................... 5-6
5.3 SOFTCAL™ ...................................................................................................................................... 5-6
5.3.1 SoftCal With Silicon Diode Sensors ............................................................................................... 5-7
5.3.2 SoftCal Accuracy With Silicon Diode Sensors ............................................................................... 5-7
5.3.3 SoftCal With Platinum Sensors ...................................................................................................... 5-8
5.3.4 SoftCal Accuracy With Platinum Sensors ...................................................................................... 5-8
5.3.5 SoftCal Calibration Curve Creation ................................................................................................ 5-9
6 COMPUTER INTERFACE OPERATION ................................................................................................................ 6-1
6.0 GENERAL ......................................................................................................................................... 6-1
6.1 IEEE-488 INTERFACE ................................................................................................ ...................... 6-1
6.1.1 IEEE-488 Interface Parameters ..................................................................................................... 6-1
6.1.2 Remote/Local Operation ................................................................................................................ 6-2
6.1.3 IEEE-488 Command Structure ....................................................................................................... 6-2
6.1.3.1 Bus Control Commands ............................................................................................................. 6-2
6.1.3.2 Common Commands ................................................................................................................. 6-3
6.1.3.3 Device Specific Commands ........................................................................................................ 6-3
6.1.3.4 Message Strings ......................................................................................................................... 6-3
6.1.4 Status System ................................................................................................................................ 6-3
6.1.4.1 Overview .................................................................................................................................... 6-3
6.1.4.2 Status Register Sets ................................................................................................................... 6-6
6.1.4.3 Status Byte and Service Request (SRQ) .................................................................................... 6-7
6.1.5 IEEE Interface Example Programs .............................................................................................. 6-10
6.1.5.1 IEEE-488 Interface Board Installation for Visual Basic Program .............................................. 6-10
6.1.5.2 Visual Basic IEEE-488 Interface Program Setup ...................................................................... 6-10
6.1.5.3 Program Operation ................................................................ ................................ ................... 6-14
6.1.6 Troubleshooting ................................................................ ........................................................... 6-14
Table of Contents iii
Lake Shore Model 325 Temperature Controller User’s Manual
TABLE OF CONTENTS (Continued)
Chapter/Section Title Page
6.2 SERIAL INTERFACE OVERVIEW .................................................................................................. 6-15
6.2.1 Physical Connection ..................................................................................................................... 6-15
6.2.2 Hardware Support ........................................................................................................................ 6-15
6.2.3 Character Format ......................................................................................................................... 6-16
6.2.4 Message Strings .......................................................................................................................... 6-16
6.2.5 Message Flow Control ................................................................................................................. 6-16
6.2.6 Changing Baud Rate .................................................................................................................... 6-17
6.2.7 Serial Interface Example Program ............................................................................................... 6-17
6.2.7.1 Visual Basic Serial Interface Program Setup ............................................................................ 6-17
6.2.7.2 Program Operation ................................................................................................................... 6-20
6.2.8 Troubleshooting ........................................................................................................................... 6-20
6.3 COMMAND SUMMARY .................................................................................................................. 6-21
6.3.1 Interface Commands (Alphabetical Listing) .................................................................................. 6-23
7 OPTIONS AND ACCESSORIES ............................................................................................................................ 7-1
7.0 GENERAL ......................................................................................................................................... 7-1
7.1 MODELS ........................................................................................................................................... 7-1
7.2 OPTIONS .......................................................................................................................................... 7-1
7.3 ACCESSORIES ................................................................................................................................. 7-2
8 SERVICE ................................................................................................................................................................ 8-1
8.0 GENERAL ......................................................................................................................................... 8-1
8.1 CONTACTING LAKE SHORE CRYOTRONICS ................................................................................ 8-1
8.2 RETURNING PRODUCTS TO LAKE SHORE .................................................................................. 8-1
8.3 FUSE DRAWER ................................................................................................................................ 8-2
8.4 LINE VOLTAGE SELECTION ........................................................................................................... 8-2
8.5 FUSE REPLACEMENT ..................................................................................................................... 8-3
8.6 ELECTROSTATIC DISCHARGE ....................................................................................................... 8-3
8.6.1 Identification of Electrostatic Discharge Sensitive Components ..................................................... 8-3
8.6.2 Handling Electrostatic Discharge Sensitive Components ............................................................... 8-3
8.7 REAR PANEL CONNECTOR DEFINITIONS .................................................................................... 8-4
8.7.1 Serial Interface Cable Wiring ......................................................................................................... 8-6
8.7.2 IEEE-488 Interface Connector ....................................................................................................... 8-7
8.8 TOP OF ENCLOSURE REMOVE AND REPLACE PROCEDURE.................................................... 8-8
8.9 FIRMWARE AND NOVRAM REPLACEMENT .................................................................................. 8-8
8.10 JUMPERS ......................................................................................................................................... 8-9
8.11 ERROR MESSAGES ......................................................................................................................... 8-9
8.12 CALIBRATION PROCEDURE ......................................................................................................... 8-11
8.12.1 Equipment Required for Calibration ............................................................................................. 8-11
8.12.2 Diode/Resistor Sensor Input Calibration ...................................................................................... 8-12
8.12.2.1 Sensor Input Calibration Setup and Serial Communication Verification ................................... 8-12
8.12.2.2 10 µA Current Source Calibration and 1 mA Current Source Verification................................. 8-12
8.12.2.3 Diode Input Ranges Calibration ................................................................................................ 8-13
8.12.2.4 Resistive Input Ranges Calibration ........................................................................................... 8-14
8.12.3 Diode Sensor Input Calibration – 1 mA Excitation Current ........................................................... 8-15
8.12.4 Thermocouple Sensor Input Calibration ....................................................................................... 8-15
8.12.4.1 Sensor Input Calibration Setup ................................................................................................. 8-15
8.12.4.2 Thermocouple Input Ranges Calibration .................................................................................. 8-15
8.12.5 Loop 2 Heater Calibration ............................................................................................................ 8-16
8.12.5.1 Loop 2 Voltage Output Calibration ............................................................................................ 8-16
8.12.6 Calibration Specific Interface Commands .................................................................................... 8-17
APPENDIX A – GLOSSARY OF TERMINOLOGY ....................................................................................................... A-1
APPENDIX B – TEMPERATURE SCALES .................................................................................................................. B-1
APPENDIX C – HANDLING OF LIQUID HELIUM AND NITROGEN ........................................................................... C-1
APPENDIX D – CURVE TABLES................................................................................................................................. D-1
iv Table of Contents
Lake Shore Model 325 Temperature Controller User’s Manual
LIST OF ILLUSTRATIONS
Figure No. Title Page
1-1 Model 325 Front View .................................................................................................................................. 1-1
1-2 Model 325 Rear Panel Connections ............................................................................................................. 1-2
2-1 Silicon Diode Sensor Calibrations and CalCurve ......................................................................................... 2-3
2-2 Typical Sensor Installation In A Mechanical Refrigerator ............................................................................. 2-5
2-3 Examples of PID Control ............................................................................................................................ 2-10
3-1 Model 325 Rear Panel .................................................................................................................................. 3-2
3-2 Line Input Assembly ..................................................................................................................................... 3-3
3-3 Diode/Resistor Input Connector ................................................................................................................... 3-4
3-4 Thermocouple Input Definition and Common Connector Polarities .............................................................. 3-6
4-1 Model 325 Front Panel ................................................................................................................................. 4-1
4-2 Display Definition.......................................................................................................................................... 4-3
4-3 Display Format Definition ................................................................ ............................................................. 4-4
4-4 Record of Zone Settings ............................................................................................................................. 4-20
5-1 SoftCal Temperature Ranges for Silicon Diode Sensors .............................................................................. 5-7
5-2 SoftCal Temperature Ranges for Platinum Sensors..................................................................................... 5-8
6-1 Model 325 Status System ............................................................................................................................ 6-4
6-2 Standard Event Status Register ................................................................................................................... 6-6
6-3 Operation Event Register ............................................................................................................................. 6-7
6-4 Status Byte Register and Service Request Enable Register ........................................................................ 6-8
6-5 GPIB Setting Configuration ........................................................................................................................ 6-11
6-6 DEV 12 Device Template Configuration ..................................................................................................... 6-11
7-1 Model 325 Sensor and Heater Cable Assembly ........................................................................................... 7-4
7-2 Model RM-1/2 Rack-Mount Kit ..................................................................................................................... 7-5
7-3 Model RM-2 Dual Rack-Mount Kit ................................................................................................................ 7-6
8-1 Fuse Drawer ................................................................................................................................................. 8-2
8-2 Power Fuse Access...................................................................................................................................... 8-2
8-3 Sensor INPUT A and B Connector Details ................................................................................................... 8-4
8-4 HEATER OUTPUT Connector Details .......................................................................................................... 8-4
8-5 RELAYS and ANALOG OUTPUT Terminal Block ........................................................................................ 8-5
8-6 RS-232 Connector Details ............................................................................................................................ 8-5
8-7 IEEE-488 Rear Panel Connector Details ...................................................................................................... 8-7
8-8 Location of Internal Components ............................................................................................................... 8-10
B-1 Temperature Scale Comparison .................................................................................................................. B-1
C-1 Typical Cryogenic Storage Dewar ................................................................................................................ C-1
Table of Contents v
Lake Shore Model 325 Temperature Controller User’s Manual
LIST OF TABLES
Table No. Title Page
1-1 Sensor Temperature Range ......................................................................................................................... 1-4
1-2 Typical Sensor Performance ........................................................................................................................ 1-5
4-1 Sensor Input Types ...................................................................................................................................... 4-6
4-2 Sensor Curves ............................................................................................................................................ 4-10
4-3 Comparison of Control Loops 1 and 2 ........................................................................................................ 4-13
4-4 Default Values ............................................................................................................................................ 4-26
5-1 Curve Header Parameters ............................................................................................................................ 5-2
5-2 Recommended Curve Parameters ............................................................................................................... 5-2
6-1 Binary Weighting of an 8-Bit Register ........................................................................................................... 6-5
6-2 Register Clear Methods ................................................................................................................................ 6-5
6-3 Programming Example to Generate an SRQ ............................................................................................... 6-9
6-4 IEEE-488 Interface Program Control Properties ......................................................................................... 6-12
6-5 Visual Basic IEEE-488 Interface Program .................................................................................................. 6-13
6-6 Serial Interface Specifications ................................................................ .................................................... 6-15
6-7 Serial Interface Program Control Properties ............................................................................................... 6-18
6-8 Visual Basic Serial Interface Program ........................................................................................................ 6-19
6-9 Command Summary .................................................................................................................................. 6-22
8-1 Calibration Table for Diode Ranges ........................................................................................................... 8-13
8-2 Calibration Table for Resistive Ranges ...................................................................................................... 8-15
8-3 Calibration Table for Thermocouple Ranges .............................................................................................. 8-16
B-1 Temperature Conversion Table .................................................................................................................... B-2
C-1 Comparison of Liquid Helium and Liquid Nitrogen ...................................................................................... C-1
D-1 DT-470 Silicon Diode Curve (Curve 10) ...................................................................................................... D-1
D-2 DT-670 Silicon Diode Curve ........................................................................................................................ D-2
D-3 DT-500 Series Silicon Diode Curves ........................................................................................................... D-2
D-4 PT-100/-1000 Platinum RTD Curves ........................................................................................................... D-3
D-5 RX-102A Rox™ Curve ................................................................................................................................ D-4
D-6 RX-202A Rox™ Curve ................................................................................................................................ D-5
D-7 Type K Thermocouple Curve ....................................................................................................................... D-6
D-8 Type E Thermocouple Curve ....................................................................................................................... D-7
D-9 Type T Thermocouple Curve ....................................................................................................................... D-8
D-10 Chromel-AuFe 0.03% Thermocouple Curve ................................................................................................ D-9
D-11 Chromel-AuFe 0.07% Thermocouple Curve .............................................................................................. D-10
vi Table of Contents
Lake Shore Model 325 Temperature Controller User’s Manual
CHAPTER 1
INTRODUCTION
1.0 PRODUCT DESCRIPTION
The Model 325 dual-channel temperature controller is capable of supporting nearly any diode, RTD, or thermocouple
temperature sensor. Two independent PID control loops with heater outputs of 25 W and 2 W are configured to drive
either a 50 Ω or 25 Ω load for optimal cryocooler control flexibility. Designed with ease of use, functionality, and value
in mind, the Model 325 is ideal for general-purpose laboratory and industrial temperature measurement and control
applications.
Sensor Inputs
The Model 325 temperature controller features two inputs with a high-resolution 24-bit analog-to-digital converter and
separate current sources for each input. Constant current excitation allows temperature to be measured and controlled
down to 2.0 K using appropriate Cernox™ RTDs or down to 1.4 K using silicon diodes. Thermocouples allow for
temperature measurement and control above 1,500 K. Sensors are optically isolated from other instrument functions for
quiet and repeatable sensor measurements. The Model 325 also uses current reversal to eliminate thermal EMF errors in
resistance sensors. Sensor data from each input is updated up to ten times per second, with display outputs twice each
second. Standard temperature response curves for silicon diodes, platinum RTDs, ruthenium oxide RTDs, and many
thermocouples are included. Up to fifteen 200-point CalCurves® (for Lake Shore calibrated temperature sensors) or user
curves can be stored into non-volatile memory. A built-in SoftCal® algorithm can be used to generate curves for silicon
diodes and platinum RTDs for storage as user curves. The Lake Shore curve handler software program allows sensor
curves to be easily loaded and manipulated.
Sensor inputs for the Model 325 are factory configured and compatible with either diodes/RTDs or thermocouple
sensors. Your choice of two diode/ RTD inputs, one diode/RTD input and one thermocouple input, or two thermocouple
inputs must be specified at time of order and cannot be reconfigured in the field. Software selects appropriate excitation
current and signal gain levels when the sensor type is entered via the instrument front panel.
325_Front.bmp
Figure 1-1. Model 325 Front View
Introduction 1-1
Lake Shore Model 325 Temperature Controller User’s Manual
Product Description (Continued)
Temperature Control
The Model 325 temperature controller offers two independent proportional-integral-derivative (PID) control loops.
A PID algorithm calculates control output based on temperature setpoint and feedback from the control sensor.
Wide tuning parameters accommodate most cryogenic cooling systems and many small high-temperature ovens.
A high-resolution digital-to-analog converter generates a smooth control output. The user can set the PID values
or the AutoTuning feature of the Model 325 can automate the tuning process.
Control loop 1 heater output for the Model 325 is a well-regulated variable DC current source. The output can provide up
to 25 W of continuous power to a 50 or 25 heater load, and includes a lower range for systems with less cooling
power. Control loop 2 heater output is a single-range, variable DC voltage source. The output can source up to 0.2 A,
providing 2 W of heater power at the 50 setting or 1 W at the 25 setting. When not being used for temperature
control, the loop 2 heater output can be used as a manually controlled voltage source. The output voltage can vary from
0 to 10 V on the 50 setting, or 0 to 5 V on the 25 setting. Both heater outputs are referenced to chassis ground. The
setpoint ramp feature allows smooth continuous setpoint changes and can also make the approach to setpoint more
predictable. The zone feature can automatically change control parameter values for operation over a large temperature
range. Ten different temperature zones can be loaded into the instrument, which will select the next appropriate value on
setpoint change.
Interface
The Model 325 includes both parallel (IEEE-488) and serial (RS-232C) computer interfaces. In addition to data
gathering, nearly every function of the instrument can be controlled via computer interface. Sensor curves can also be
entered and manipulated through either interface using the Lake Shore curve handler software program.
Figure 1-2. Model 325 Rear Panel Connections
1-2 Introduction
Loop 1 Heater output
Serial (RS-232C) I/O (DTE)
Line input assembly
Loop 2 Heater output
Sensor input connectors
IEEE-488 interface
Lake Shore Model 325 Temperature Controller User’s Manual
Configurable Display
The Model 325 offers a bright, easy to read OLED display that simultaneously displays up to four readings. Display data
includes input and source annunciators for each reading. All four display locations can be configured by the user. Data
from either input can be assigned to any of the four locations, and the user’s choice of temperature or sensor units can be
displayed. Heater range and control output as current or power can be continuously displayed for immediate feedback on
control operation. The channel A or B indicator is underlined to indicate which channel is being controlled by the
displayed control loop.
Normal (Default) Display Configuration
The display provides four reading locations. Readings from each input and the control setpoint can be expressed in any
combination of temperature or sensor units, with heater output expressed as a percent of full scale current or power.
Flexible Configuration
Reading locations can be configured by the user to meet application needs. The character preceding the reading indicates
input A or B or setpoint S. The character following the reading indicates measurement units.
Curve Entry
The Model 325 display offers the flexibility to support curve, SoftCal™, and zone entry. Curve entry may be performed
accurately and to full resolution via the display and keypad as well as computer interface.
Introduction 1-3
Model
Useful Range
Magnetic Field Use
Diodes
Silicon Diode
DT-670-SD
1.4 K to 500 K
T 60 K & B 3 T
Silicon Diode
DT-670E-BR
30 K to 500 K
T 60 K & B 3 T
Silicon Diode
DT-414
1.4 K to 375 K
T 60 K & B 3 T
Silicon Diode
DT-421
1.4 K to 325 K
T 60 K & B 3 T
Silicon Diode
DT-470-SD
1.4 K to 500 K
T 60 K & B 3 T
Silicon Diode
DT-471-SD
10 K to 500 K
T 60 K & B 3 T
GaAlAs Diode
TG-120-P
1.4 K to 325 K
T 4.2 K & B 5 T
GaAlAs Diode
TG-120-PL
1.4 K to 325 K
T 4.2 K & B 5 T
GaAlAs Diode
TG-120-SD
1.4 K to 500 K
T 4.2 K & B 5 T
Positive Temperature
Coefficient (PTC) RTDs
100 Platinum
PT-102/3
14 K to 873 K
T > 40 K & B 2.5 T
100 Platinum
PT-111
14 K to 673 K
T > 40 K & B 2.5 T
Rhodium-Iron
RF-800-4
1.4 K to 500 K
T > 77 K & B 8 T
Rhodium-Iron
RF-100T/U
1.4 K to 325 K
T > 77 K & B 8 T
Negative Temperature
Coefficient (NTC) RTDs 1
Cernox™
CX-1010
2 K to 325 K 4
T > 2 & B 19 T
Cernox
CX-1030-HT
3.5 K to 420 K
2,5
T > 2 & B 19 T
Cernox
CX-1050-HT
4 K to 420 K
2,5
T > 2 & B 19 T
Cernox
CX-1070-HT
15 K to 420 K 2
T > 2 & B 19 T
Cernox
CX-1080-HT
50 K to 420 K 2
T > 2 & B 19 T
Germanium
GR-200A/B-1000
2.2 K to 100 K 3
Not Recommended
Germanium
GR-200A/B-1500
2.6 K to 100 K 3
Not Recommended
Germanium
GR-200A/B-2500
3.1 K to 100 K 3
Not Recommended
Carbon-Glass
CGR-1-500
4 K to 325 K 4
T > 2 K to 19 T
Carbon-Glass
CGR-1-1000
5 K to 325 K 4
T > 2 K to 19 T
Carbon-Glass
CGR-1-2000
6 K to 325 K 4
T > 2 K to 19 T
Rox™
RX-102A
1.4 K to 40 K 4
T > 2 K to 10 T
Thermocouples
Type K
9006-006
3.2 K to 1505 K
Not Recommended
Type E
9006-004
3.2 K to 934 K
Not Recommended
Chromel-AuFe 0.07%
9006-002
1.2 K to 610 K
Not Recommended
1.1 SENSOR SELECTION
Lake Shore Model 325 Temperature Controller User’s Manual
Table 1-1. Sensor Temperature Range
1
Single excitation current may limit the low temperature range of NTC resistors.
2
Non-HT version maximum temperature: 325 K.
3
Low temperature limited by input resistance range.
4
Low temperature specified with self-heating error: 5 mK.
5
Low temperature specified with self-heating error: 12 mK.
Silicon diodes are the best choice for general cryogenic use from 1.4 K to above room temperature. Diodes are
economical to use because they follow a standard curve and are interchangeable in many applications. They are
not suitable for use in ionizing radiation or magnetic fields.
Cernox™ thin-film RTDs offer high sensitivity and low magnetic field-induced errors over the 2 K to 420 K
temperature range. Cernox sensors require calibration.
Platinum RTDs offer high uniform sensitivity from 30 K to over 800 K. With excellent reproducibility, they are
useful as thermometry standards. They follow a standard curve above 70 K and are interchangeable in many
applications.
1-4 Introduction
Example Lake
Shore Sensor
Temp
Nominal
Resistance/
Voltage
Typical Sensor
Sensitivity 1
Measurement
Resolution:
Temperature
Equivalents
Electronic
Accuracy:
Temperature
Equivalents
Temperature
Accuracy
including
Electronic
Accuracy,
CalCurve™, and
Calibrated
Sensor
Electronic
Control
Stability 2:
Temperature
Equivalents
Silicon Diode
DT-670-SD-13
with 1.4H
calibration
1.4 K
1.644 V
-12.49 mV/K
0.8 mK
±13 mK
±25 mK
±1.6 mK
77 K
1.028 V
-1.73 mV/K
5.8 mK
±76 mK
±98 mK
±11.6 mK
300 K
0.5597 V
-2.3 mV/K
4.4 mK
±47 mK
±79 mK
±8.8 mK
500 K
0.0907 V
-2.12 mV/K
4.8 mK
±40 mK
±90 mK
±9.6 mK
Silicon Diode
DT-470-SD-13
with 1.4H
calibration
1.4 K
1.6981 V
-13.1 mV/K
0.8 mK
±13 mK
±25 mK
±1.6 mK
77 K
1.0203 V
-1.92 mV/K
5.2 mK
±69 mK
±91 mK
±10.4 mK
300 K
0.5189 V
-2.4 mV/K
4.2 mK
±45 mK
±77 mK
±8.4 mK
475 K
0.0906 V
-2.22 mV/K
4.6 mK
±39 mK
±89 mK
±9.2 mK
GaAlAs
Diode
TG-120-SD
with 1.4H
calibration
1.4 K
5.391 V
-97.5 mV/K
0.2 mK
±7 mK
±19 mK
±0.4 mK
77 K
1.422 V
-1.24 mV/K
16.2 mK
±180 mK
±202 mK
±32.4 mK
300 K
0.8978 V
-2.85 mV/K
7 mK
±60 mK
±92 mK
±14 mK
475 K
0.3778 V
-3.15 mV/K
6.4 mK
±38 mK
±88 mK
±12.8 mK
100
Platinum
RTD 500
Full Scale
PT-103 with
1.4J calibration
30 K
3.660
0.191 /K
10.5 mK
±23 mK
±33 mK
±21 mK
77 K
20.38
0.423 /K
4.8 mK
±15 mK
±27 mK
±9.6 mK
300 K
110.35
0.387 /K
5.2 mK
±39 mK
±62 mK
±10.4 mK
500 K
185.668
0.378 /K
5.3 mK
±60 mK
±106 mK
±10.6 mK
Cernox™
CX-1050-SD-
HT 3 with 4M
calibration
4.2 K
3507.2
-1120.8 /K
36 µK
±1.4 mK
±6.4 mK
±72 µK
77 K
205.67
-2.4116 /K
16.6 mK
±76 mK
±92 mK
±33.2 mK
300 K
59.467
-0.1727 /K
232 mK
±717 mK
±757 mK
±464 mK
420 K
45.030
-0.0829 /K
483 mK
±1.42 K
±1.49 K
±966 mK
Germanium
GR-200A-1000
with 1.4D
calibration
2 K
6674
-9930 /K
4 µK
±0.3 mK
±4.3 mK
±8 µK
4.2 K
1054
-526 /K
76 µK
±1 mK
±5 mK
±152 µK
10 K
170.9
-38.4 /K
1 mK
±4.4 mK
±9.4 mK
±2 mK
100 K
2.257
-0.018 /K
2.22 K
±5.61 K
±5.626 K
±4.44 K
CarbonGlass
CGR-1-2000
with 4L
calibration
4.2 K
2260
-2060 /K
20 µK
±0.5 mK
±4.5 mK
±40 µK
77 K
21.65
-0.157 /K
255 mK
±692 mK
±717 mK
±510 mK
300 K
11.99
-0.015 /K
2.667 K
±7 K
±7.1 K
±5.344 K
Thermocouple
50mV
Type K
75 K
-5862.9 µV
15.6 µV/K
26 mK
±0.25 K 4
Calibration not
available from
Lake Shore
±52 mK
300 K
1075.3 µV
40.6 µV/K
10 mK
±0.038 K 4
±20 mK
600 K
13325 µV
41.7 µV/K
10 mK
±0.184 K 4
±20 mK
1505 K
49998.3 µV
36.006 µV/K
12 mK
±0.73 K 4
±24 mK
Lake Shore Model 325 Temperature Controller User’s Manual
Table 1-2. Typical Sensor Performance
1
Typical sensor sensitivities were taken from representative calibrations for the sensor listed.
2
Control stability of the electronics only, in an ideal thermal system.
3
Non-HT version maximum temperature: 325 K.
4
Accuracy specification does not include errors from room temperature compensation.
Introduction 1-5
Sensor
Temperature
Coefficient
Input
Range
Excitation
Current
Display
Resolution
Measurement
Resolution
Electronic
Accuracy
(at 25 °C)
Measurement
Temperature
Coefficient
Electronic
Control
Stability1
Diode
Negative
0 V to 2.5 V
10 µA ±0.05%
2,3
100 µV
0.4 µV
±80 µV
±0.005% of rdg
(10 µV +
0.0005% of rdg)/°C
±20 µV
Negative
0 V to 7.5 V
10 µA ±0.05%
2,3
100 µV
10 µV
±320 µV
±0.01% of rdg
(20 µV +
0.0005% of rdg)/°C
±40 µV
PTC RTD
Positive
0 to 500
1 mA4
10 m
2 m
±0.004
±0.01% of rdg
(0.2 m +
0.0005% of rdg)/°C
±4 m
Positive
0 to 5000
1 mA4
100 m
20 m
±0.04
±0.02% of rdg
(0.2 m +
0.0005% of rdg)/°C
±40 m
NTC RTD
Negative
0 to 7500
10 µA ±0.05%
100 m
40 m
±0.1
±0.04% of rdg
(2 m +
0.0005% of rdg)/°C
±80 m
Thermocouple
Positive
±25 mV
NA
1 µV
0.4 µV
±1 µV
±0.05% of rdg
(0.2 µV +
0.0015% of rdg)/°C
±0.8 µV
Positive
±50 mV
NA
1 µV
20 µV
±1 µV
±0.05% of rdg
(0.2 µV +
0.0015% of rdg)/°C
±0.8 µV
Diode/RTD
Thermocouple
Measurement type
4-lead differential
2-lead, room temperature compensated
Excitation
Constant current with current reversal for
RTDs
NA
Supported sensors
Diodes: Silicon, GaAlAs RTDs: 100
Platinum, 1000 Platinum, Germanium,
Carbon-Glass, Cernox, and Rox
Most thermocouple types
Standard curves
DT-470, DT-500D. DT-670, PT-100, PT-1000,
RX-102A, RX-202A
Type E, Type K, Type T, AuFe 0.07% vs. Cr,
AuFe 0.03% vs Cr
Input connector
6-pin DIN
Ceramic isothermal block
Lake Shore Model 325 Temperature Controller User’s Manual
1.2 SPECIFICATIONS
Input Specifications
1
Control stability of the electronics only, in ideal thermal system
2
Current source error has negligible effect on measurement accuracy
3
Diode input excitation can be set to 1 mA
4
Current source error is removed during calibration
5
Accuracy specification does not include errors from room temperature compensation
Thermometry
Number of inputs 2
Input configuration Each input is factory configured for either diode / RTD or thermocouple
Isolation Sensor inputs optically isolated from other circuits but not each other
A/D resolution 24-bit
Input accuracy Sensor dependent, refer to Input Specifications table
Measurement resolution Sensor dependent, refer to Input Specifications table
Max update rate: 10 rdg/s on each input, (except 5 rdg/s on input A when configured as thermocouple)
User curves Room for 15, 200 point CalCurves™ or user curves
SoftCal™ Improves accuracy of DT-470 diode to ±0.25 K from 30 K to 375 K. Improves accuracy
of platinum RTDs to ±0.25 K from 70 K to 325 K. Stored as user curves.
Filter Averages 2 to 64 input readings
Sensor Input Configuration
1-6 Introduction
Type
Variable DC current source
D/A resolution
16-bit
25 Setting
50 Setting
Max power
25 W
25 W
Max current
1 A
0.71 A
Voltage compliance (min)
25 V
35.4 V
Heater load range
20 to 25
40 to 50
Heater load for max power
25
50
Ranges
2 (2.5 W/25 W)
Heater noise (<1 kHz)
1 µA + 0.01% of output
Grounding
Output referenced to chassis ground
Heater connector
Dual banana
Safety limits
Curve temperature, power up heater off, short circuit protection
Type
Variable DC voltage source
D/A resolution
16-bit
25 Setting
50 Setting
Max power
1 W
2 W
Max voltage
5 V
10 V
Current compliance (min)
0.2 A
0.2 A
Heater load range
25
50
Heater load for max power
25
50
Ranges
1
Heater noise (<1 kHz)
50 µV + 0.01% of output
Grounding
Output referenced to chassis ground
Heater connector
Detachable terminal block
Safety limits
Curve temperature, power up heater off, short circuit protection
Lake Shore Model 325 Temperature Controller User’s Manual
Specifications (Continued)
Control
Control loops 2
Control type Closed loop digital PID with manual heater output or open loop
Tuning Autotune (one loop at a time), PID, PID zones
Control stability Sensor dependent, refer to Input Specifications table
PID control settings:
Proportional (Gain) 0 to 1000 with 0.1 setting resolution
Integral (Reset) 1 to 1000 (1000/s) with 0.1 setting resolution
Derivative (Rate) 1 to 200% with 1% resolution
Manual output 0 to 100% with 0.01% setting resolution
Zone control 10 temperature zones with P, I, D, manual heater out, and heater range
Setpoint ramping 0.1 K/min to 100 K/min
Loop 1 Heater Output
Loop 2 Heater Output
Front Panel
Display Before Q4, 2013: 2-line 20-character liquid crystal display with 5.5 mm high characters;
After Q4, 2013: 2-line × 20-character, OLED display with 5.5 mm high characters
Number of reading displays 1 to 4
Display units K, °C, V, mV,
Reading source Temperature, sensor units
Display update rate 2 rdg/s
Temperature display resolution 0.001° from 0° to 99.999°, 0.01° from 100° to 999.99°, 0.1° above 1000°
Sensor units display resolution Sensor dependent, to 5 digits
Other displays Setpoint, heater range and heater output (user selected)
Setpoint setting resolution Same as display resolution (actual resolution is sensor dependent)
Heater output display Numeric display in percent of full scale for power or current
Heater output resolution 1%
Display annunciators Control Input, Remote, Autotune
Keypad 20-key membrane, numeric and specific functions
Front panel features Front panel curve entry, keypad lock-out
Introduction 1-7
Lake Shore Model 325 Temperature Controller User’s Manual
Specifications (Continued)
Interface
IEEE-488.2 interface:
Features SH1, AH1, T5, L4, SR1, RL1, PP0, DC1, DT0, C0, E1
Reading rate To 10 rdg/s on each input
Software support LabVIEW™ driver (contact Lake Shore for availability)
Serial interface
Electrical format RS-232C
Baud rates 9600, 19200, 38400, 57600
Connector 9-pin D-style, DTE configuration
Reading rate To 10 rdg/s on each input
General
Ambient temperature 15 °C to 35 °C at rated accuracy. 5 °C to 40 °C at reduced accuracy
Power requirement 100, 120, 220, 240 VAC, +6% –10%, 50 or 60 Hz, 85 VA
Size 216 mm W × 89 mm H × 368 mm D (8.5 in × 3.5 in × 14.5 in), half rack
Weight 4.0 kg (8.8 lb)
Approval CE mark (contact Lake Shore for availability)
Ordering Information
Standard Temperature Controllers, all features included:
Part Number Description (Input configuration cannot be changed in the field)
325 Two diode / RTD inputs
325-T1 One diode / RTD, one thermocouple input
325-T2 Two thermocouple inputs
Refer to Chapter 7 of this manual for a complete description of Model 325 options and accessories.
Specifications subject to change without notice.
1.3 SAFETY SUMMARY
Observe these general safety precautions during all phases of instrument operation, service, and repair. Failure to comply
with these precautions or with specific warnings elsewhere in this manual violates safety standards of design,
manufacture, and intended instrument use. Lake Shore Cryotronics, Inc. assumes no liability for Customer failure to
comply with these requirements.
The Model 325 protects the operator and surrounding area from electric shock or burn, mechanical hazards, excessive
temperature, and spread of fire from the instrument. Environmental conditions outside of the conditions below may pose
a hazard to the operator and surrounding area.
• Indoor use.
• Altitude to 2000 m.
• Temperature for safe operation: 5 °C to 40 °C.
• Maximum relative humidity: 80% for temperature up to 31 °C decreasing linearly to 50% at 40 °C.
• Power supply voltage fluctuations not to exceed ±10% of the nominal voltage.
• Overvoltage category II.
• Pollution degree 2.
1-8 Introduction
Safety Summary (Continued)
Lake Shore Model 325 Temperature Controller User’s Manual
Ground the Instrument
To minimize shock hazard, the instrument is equipped with a three-conductor AC power cable. Plug the power cable
into an approved three-contact electrical outlet or use a three-contact adapter with the grounding wire (green) firmly
connected to an electrical ground (safety ground) at the power outlet. The power jack and mating plug of the power
cable meet Underwriters Laboratories (UL) and International Electrotechnical Commission (IEC) safety standards.
Ventilation
The instrument has ventilation holes in its side covers. Do not block these holes when the instrument is operating.
Do Not Operate in an Explosive Atmosphere
Do not operate the instrument in the presence of flammable gases or fumes. Operation of any electrical instrument in
such an environment constitutes a definite safety hazard.
Keep Away from Live Circuits
Operating personnel must not remove instrument covers. Refer component replacement and internal adjustments to
qualified maintenance personnel. Do not replace components with power cable connected. To avoid injuries, always
disconnect power and discharge circuits before touching them.
Do Not Substitute Parts or Modify Instrument
Do not install substitute parts or perform any unauthorized modification to the instrument. Return the instrument to an
authorized Lake Shore Cryotronics, Inc. representative for service and repair to ensure that safety features are
maintained.
Cleaning
Do not submerge instrument. Clean only with a damp cloth and mild detergent. Exterior only.
1.4 SAFETY SYMBOLS
Introduction 1-9
Lake Shore Model 325 Temperature Controller User’s Manual
This Page Intentionally Left Blank
1-10 Introduction
Lake Shore Model 325 Temperature Controller User’s Manual
CHAPTER 2
COOLING SYSTEM DESIGN
2.0 GENERAL
Selecting the proper cryostat or cooling source is probably the most important decision in designing a temperature
control system. The cooling source defines minimum temperature, cool-down time, and cooling power. (Information on
choosing a cooling source is beyond the scope of this manual.) This chapter provides information on how to get the best
temperature measurement and control from cooling sources with proper setup including sensor and heater installation.
2.1 TEMPERATURE SENSOR SELECTION
This section attempts to answer some of the basic questions concerning temperature sensor selection. Additional useful
information on temperature sensor selection is available in the Lake Shore Temperature Measurement and Control
Catalog. The catalog has a large reference section that includes sensor characteristics and sensor selection criteria.
2.1.1 Temperature Range
Several important sensor parameters must be considered when choosing a sensor. The first is temperature range. The
experimental temperature range must be known when choosing a sensor. Some sensors can be damaged by temperatures
that are either too high or too low. Manufacturer recommendations should always be followed.
Sensor sensitivity is also dependent on temperature and can limit the useful range of a sensor. It is important not to
specify a range larger than necessary. If an experiment is being done at liquid helium temperature, a very high sensitivity
is needed for good measurement resolution at that temperature. That same resolution may not be required to monitor
warm up to room temperature. Two different sensors may be required to tightly cover the range from helium to room
temperature, but lowering the resolution requirement on warm up may allow a less expensive, one sensor solution.
Another thing to consider when choosing a temperature sensor is that instruments like the Model 325 are not able to read
some sensors over their entire temperature range. Lake Shore sells calibrated sensors that operate down to 50 millikelvin
(mK), but the Model 325 is limited to above 1 kelvin (K) in its standard configuration.
2.1.2 Sensor Sensitivity
Temperature sensor sensitivity is a measure of how much a sensor signal changes when the temperature changes. It is an
important sensor characteristic because so many measurement parameters are related to it. Resolution, accuracy, noise
floor, and even control stability depend on sensitivity. Many sensors have different sensitivities at different temperatures.
For example, a platinum sensor has good sensitivity at higher temperatures but has limited use below 30 K because its
sensitivity drops sharply. It is difficult to determine if a sensor has adequate sensitivity over the experimental
temperature range. This manual has specifications (Section 1.2) that include sensor sensitivity translated into temperature
resolution and accuracy at different points. This is typical sensor response and can be used as a guide when choosing a
sensor to be used with the Model 325.
2.1.3 Environmental Conditions
The experimental environment is also important when choosing a sensor. Environmental factors such as high vacuum,
magnetic field, corrosive chemicals, or even radiation can limit the use of some types of sensors. Lake Shore has devoted
much time to developing sensor packages that withstand the temperatures, vacuum levels, and bonding materials found
in typical cryogenic cooling systems.
Experiments done in magnetic fields are becoming very common. Field dependence of temperature sensors is an
important selection criteria for sensors used in these experiments. This manual briefly qualifies the field dependence of
most common sensors in the specifications (Section 1.2). Detailed field dependence tables are included in the Lake Shore
Temperature Measurement and Control Catalog. When available, specific data on other environmental factors is also
included in the catalog.
Cooling System Design 2-1
Lake Shore Model 325 Temperature Controller User’s Manual
2.1.4 Measurement Accuracy
Temperature measurements have several sources of error that reduce accuracy. Be sure to account for errors induced by
both the sensor and the instrumentation when computing accuracy. The instrument has measurement error in reading the
sensor signal and error in calculating a temperature using a temperature response curve. Error results from the sensor
being compared to a calibration standard and the temperature response of a sensor will shift with time and with repeated
thermal cycling (from very cold temperatures to room temperature). Instrument and sensor makers specify these errors
but there are things a user can do to maintain good accuracy. For example, choose a sensor that has good sensitivity in
the most critical temperature range, as sensitivity can minimize the effect of most error sources. Install the sensor
properly following guidelines in Section 2.3. Have the sensor and instrument periodically recalibrated, or in some other
way null the time dependent errors. Use a sensor calibration that is appropriate for the accuracy requirement.
2.1.5 Sensor Package
Many types of sensors can be purchased in different packages. Some types of sensors can even be purchased as bare
chips without any package. A sensor package generally determines its size, thermal and electrical contact to the outside,
and sometimes limits temperature range. When different packages are available for a sensor, the user should consider the
mounting surface for the sensor and how leads will be heat sinked when choosing.
2.2 CALIBRATED SENSORS
There can sometimes be confusion in the difficult task of choosing the right sensor, getting it calibrated, translating the
calibration data into a temperature response curve that the Model 325 can understand, then getting the curve loaded into
the instrument. Lake Shore provides a variety of calibration and curve loading services to fit different accuracy
requirements and budgets.
2.2.1 Traditional Calibration
Calibration is done by comparing a sensor with an unknown temperature response to an accepted standard. Lake Shore
temperature standards are traceable to the U.S. National Institute of Standards and Testing (NIST) or the National
Physical Laboratory in Great Britain. These standards allow Lake Shore to calibrate sensors from 50 mK to above room
temperature. Calibrated sensors are more expensive than uncalibrated sensors of the same type because of the labor and
capitol equipment used in the process.
This type of calibration provides the most accurate temperature sensors available from Lake Shore. Errors from sensor
calibration are almost always smaller than the error contributed by the Model 325. The Lake Shore Temperature
Measurement and Control Catalog has complete accuracy specifications for calibrated sensors.
Calibrated sensors include the measured test data printed and plotted, the coefficients of a Chebychev polynomial that
has been fitted to the data, and two tables of data points to be used as interpolation tables. Both interpolation tables are
optimized to allow accurate temperature conversion. The smaller table, called a breakpoint interpolation table, is sized to
fit into instruments like the Model 325 where it is called a temperature response curve. Getting a curve into a Model 325
may require a CalCurve™ described below or hand entering through the instrument front panel.
It is important to look at instrument specifications before ordering calibrated sensors. A calibrated sensor is required
when a sensor does not follow a standard curve if the user wishes to display in temperature. Otherwise the Model 325
will operate in sensor units like ohms or volts. The Model 325 may not work over the full temperature range of some
sensors. The standard inputs in are limited to operation above 1 K even with sensors that can be calibrated to 50 mK.
2.2.2 SoftCal™
SoftCal is a good solution for applications that do not require the accuracy of a traditional calibration. The SoftCal
algorithm uses the well-behaved nature of sensors that follow a standard curve to improve the accuracy of individual
sensors. A few known temperature points are required to perform SoftCal.
Lake Shore sells SoftCal calibrated sensors that include both the large interpolation table and the smaller breakpoint
interpolation table. A CalCurve may be required to get the breakpoint table into a Model 325 where it is called a
temperature response curve. Refer to Section 2.2.4.
The Model 325 can also perform a SoftCal calibration. The user must provide one, two, or three known temperature
reference points. The range and accuracy of the calibration is based on these points. Refer to Section 5.3.
2-2 Cooling System Design
Lake Shore Model 325 Temperature Controller User’s Manual
2.2.3 Standard Curves
Some types of sensors behave in a very predictable manner and a standard temperature response curve can be created for
them. Standard curves are a convenient and inexpensive way to get reasonable temperature accuracy. Sensors that have a
standard curve are often used when interchangeability is important. Some individual sensors are selected for their ability
to match a published standard curve and sold at a premium, but in general these sensors do not provide the accuracy of a
calibrated sensor. For convenience, the Model 325 has several standard curves included in firmware.
C-325-2-1.bmp
Figure 2-1. Silicon Diode Sensor Calibrations and CalCurve
Cooling System Design 2-3
Lake Shore Model 325 Temperature Controller User’s Manual
2.2.4 CalCurve™
The CalCurve service provides the user with a convenient way get the temperature response curve from Lake Shore
calibrated sensors into instruments like the Model 325. It can be performed at the factory when calibrated sensors and
instruments are ordered together. The factory installed CalCurve option is Model 8001-325 and should be ordered with
the calibrated sensor. A CalCurve can be done in the field when additional or replacement sensors are installed.
Customers that have a PC-compatible computer with an RS-232C or IEEE-488 interface can load the curve into the
instrument using one of the computer interfaces. The Model 8000 CalCurve is offered on CD or via e-mail free of charge
at time of order to any customer who orders a calibrated sensor. See Section 7.2 for details.
2.3 SENSOR INSTALLATION
This section highlights some of the important elements of proper sensor installation. For more detailed information, Lake
Shore sensors are shipped with installation instructions that cover that specific sensor type and package. The Lake Shore
Temperature Measurement and Control Catalog includes an installation section as well. To further help users properly
install sensors, Lake Shore offers a line of cryogenic accessories. Many of the materials discussed are available through
Lake Shore and can be ordered with sensors or instruments.
2.3.1 Mounting Materials
Choosing appropriate mounting materials is very important in a cryogenic environment. The high vacuum used to
insulate cryostats is one source of problems. Materials used in these applications should have a low vapor pressure so
they do not evaporate or out-gas and spoil the vacuum insulation. Metals and ceramics do not have this problem but
greases and varnishes must be checked. Another source of problems is the wide extremes in temperature most sensors
are exposed to. The linear expansion coefficient of materials becomes important when temperature changes are so large.
Never try to permanently bond materials with linear expansion coefficients that differ by more than three. A flexible
mounting scheme should be used or the parts will break apart, potentially damaging them. The thermal expansion or
contraction of rigid clamps or holders could crush fragile samples or sensors that do not have the same coefficient.
Thermal conductivity is a property of materials that can change with temperature. Do not assume that a heat sink grease
that works well at room temperature and above will do the same job at low temperatures.
2.3.2 Sensor Location
Finding a good place to mount a sensor in an already crowded cryostat is never easy. There are fewer problems if the
entire load and sample holder are at the same temperature. Unfortunately, this not the case in many systems.
Temperature gradients (differences in temperature) exist because there is seldom perfect balance between the cooling
source and heat sources. Even in a well-controlled system, unwanted heat sources like thermal radiation and heat
conducting through mounting structures can cause gradients. For best accuracy, sensors should be positioned near the
sample, so that little or no heat flows between the sample and sensor. This may not, however, be the best location for
temperature control as discussed below.
2.3.3 Thermal Conductivity
The ability of heat to flow through a material is called thermal conductivity. Good thermal conductivity is important in
any part of a cryogenic system that is intended to be the same temperature. Copper and aluminum are examples of metals
that have good thermal conductivity, while stainless steel does not. Non-metallic, electrically-insulating materials like
alumina oxide and similar ceramics have good thermal conductivity, while G-10 epoxy-impregnated fiberglass does not.
Sensor packages, cooling loads, and sample holders should have good thermal conductivity to reduce temperature
gradients. Surprisingly, the connections between thermally conductive mounting surfaces often have very poor thermal
conductivity.
2.3.4 Contact Area
Thermal contact area greatly affects thermal conduction because a larger area has more opportunity to transfer heat. Even
when the size of a sensor package is fixed, thermal contact area can be improved with the use of a gasket material. A soft
gasket material forms into the rough mating surface to increase the area of the two surfaces that are in contact. Good
gasket materials are soft, thin, and have good thermal conductivity. They must also withstand the environmental
extremes. Indium foil and cryogenic grease are good examples.
2-4 Cooling System Design
Drawing
Not To Scale
To Room
Temperature
Refrigerator
Expander
Refrigerator
Second Stage
Dental Floss
Tie-Down
Thermal Anchor
(Bobbin)
Thermal Anchor
(Bobbin)
Radiation Shield
Sensor
Cold Stage and
Sample Holder
Optical Window
(If Required)
Cryogenic Tape
Cryogenic Wire
(small diameter,
large AWG)
-or-
Vacuum Shroud
Vacuum Space
Heater
(wiring not shown
for clarity)
Lake Shore Model 325 Temperature Controller User’s Manual
2.3.5 Contact Pressure
When sensors are permanently mounted, the solder or epoxy used to hold the sensor act as both gasket and adhesive.
Permanent mounting is not a good solution for everyone because it limits flexibility and can potentially damage sensors.
Much care should be taken not to over heat or mechanically stress sensor packages. Less permanent mountings require
some pressure to hold the sensor to its mounting surface. Pressure greatly improves the action of gasket material to
increase thermal conductivity and reduce thermal gradients. A spring clamp is recommended so that different rates of
thermal expansion do not increase or decrease pressure with temperature change.
2.3.6 Lead Wire
Different types of sensors come with different types and lengths of electrical leads. In general a significant length of lead
wire must be added to the sensor for proper heat sinking and connecting to a bulk head connector at the vacuum
boundary. The lead wire must be a good electrical conductor, but should not be a good thermal conductor, or heat will
transfer down the leads and change the temperature reading of the sensor. Small 30 to 40 AWG wire made of an alloy
like phosphor bronze is much better than copper wire. Thin wire insulation is preferred and twisted wire should be used
to reduce the effect of RF noise if it is present. The wire used on the room temperature side of the vacuum boundary is
not critical so copper cable is normally used.
P-325-2-2.bmp
Figure 2-2. Typical Sensor Installation In A Mechanical Refrigerator
Cooling System Design 2-5
Max Power (W) at 25 Setting
Max Power (W) at 50 Setting
Voltage Limit:
(25 V)2
Resistance ()
(35.4 V)2
Resistance ()
Current Limit:
(1 A)2 × Resistance ()
(0.71 A)2 × Resistance ()
Lake Shore Model 325 Temperature Controller User’s Manual
2.3.7 Lead Soldering
When additional wire is soldered to short sensor leads, care must be taken not to overheat the sensor. A heat sink such as
a metal wire clamp or alligator clip will heat sink the leads and protect the sensor. Leads should be tinned before bonding
to reduce the time that heat is applied to the sensor lead. Solder flux should be cleaned after soldering to prevent
corrosion.
2.3.8 Heat Sinking Leads
Sensor leads can be a significant source of error if they are not properly heat sinked. Heat will transfer down even small
leads and alter the sensor reading. The goal of heat sinking is to cool the leads to a temperature as close to the sensor as
possible. This can be accomplished by putting a significant length of lead wire in thermal contact with every cooled
surface between room temperature and the sensor. Lead wires can be adhered to cold surfaces with varnish over a thin
electrical insulator like cigarette paper. They can also be wound onto a bobbin that is firmly attached to the cold surface.
Some sensor packages include a heat sink bobbin and wrapped lead wires to simplify heat sinking.
2.3.9 Thermal Radiation
Thermal (black body) radiation is one of the ways heat is transferred. Warm surfaces radiate heat to cold surfaces even
through a vacuum. The difference in temperature between the surfaces is one thing that determines how much heat is
transferred. Thermal radiation causes thermal gradients and reduces measurement accuracy. Many cooling systems
include a radiation shield. The purpose of the shield is to surround the load, sample, and sensor with a surface that is at or
near their temperature to minimize radiation. The shield is exposed to the room temperature surface of the vacuum
shroud on its outer surface, so some cooling power must be directed to the shield to keep it near the load temperature.
If the cooling system does not include an integrated radiation shield (or one cannot be easily made), one alternative is to
wrap several layers of super-insulation (aluminized mylar) loosely between the vacuum shroud and load. This reduces
radiation transfer to the sample space.
2.4 HEATER SELECTION AND INSTALLATION
There is a variety of resistive heaters that can be used as the controlled heating source for temperature control. The
mostly metal alloys like nichrome are usually wire or foil. Shapes and sizes vary to permit installation into different
systems.
2.4.1 Heater Resistance and Power
Cryogenic cooling systems have a wide range of cooling power. The resistive heater must be able to provide sufficient
heating power to warm the system. The Model 325 can supply up to 25 W of power to a heater (if the heater resistance is
appropriate). The Model 325 heater output current source has a maximum output of 1 A at the 25 setting, or 0.71 A at
the 50 setting. Even though the Model 325 main heater output is a current source, it has a voltage limit (called the
compliance voltage) which is set to either 25 V or 35.4 V when the heater resistance is set to 25 or 50 , respectively.
This compliance voltage also limits maximum power.
Both limits are in place at the same time, so the smaller of the two computations gives the maximum power available to
the heater. A heater of 50 at the 50 setting allows the instrument to provide its maximum power of 25 W. A smaller
resistance of 40 at the 50 setting allows about 20 W of power, while a larger resistance of 60 is limited by
compliance voltage to about 21 W. The Model 325 is designed to limit the internal power dissipation as a measure of
self-protection. This internal power limit will not allow the output current to rise once the power limit is reached.
The resistor chosen as a heater must be able to withstand the power being dissipated in it. Pre-packaged resistors have a
power specification that is usually given for the resistor in free air. This power may need to be derated if used in a
vacuum where convection cooling cannot take place and it is not adequately heat sinked to a cooled surface.
2-6 Cooling System Design
Lake Shore Model 325 Temperature Controller User’s Manual
2.4.2 Heater Location
For best temperature measurement accuracy the heater should be located so that heat flow between the cooling power
and heater is minimized. For best control the heater should be in close thermal contact with the cooling power. Geometry
of the load can make one or both of these difficult to achieve. That is why there are several heater shapes and sizes.
2.4.3 Heater Types
Resistive wire like nichrome is the most flexible type of heater available. The wire can be purchased with electrical
insulation and has a predictable resistance per given length. This type of heater wire can be wrapped around a cooling
load to give balanced, even heating of the area. Similar to sensor lead wire, the entire length of the heater wire should be
in good thermal contact with the load to allow for thermal transfer. Heat sinking also protects the wire from over heating
and burning out.
Resistive heater wire is also wound into cartridge heaters. Cartridge heaters are more convenient but are bulky and more
difficult to place on small loads. A typical cartridge is 0.25 inch in diameter and 1 inch long. The cartridge should be
snugly held in a hole in the load or clamped to a flat surface. Heat sinking for good thermal contact is again important.
Foil heaters are thin layers of resistive material adhered to, or screened on to, electrically insulating sheets. There are a
variety of shapes and sizes. The proper size heater can evenly heat a flat surface or around a round load. The entire active
area should be in good thermal contact with the load, not only for maximum heating effect, but to keep spots in the
heater from over heating and burning out.
2.4.4 Heater Wiring
When wiring inside a vacuum shroud, we recommend using 30 AWG copper wire for heater leads. Too much heat can
leak in when larger wire is used. Heat sinking, similar to that used for the sensor leads, should be included so that any
heat leaking in does not warm the load when the heater is not running. The lead wires should be twisted to minimize
noise coupling between the heater and other leads in the system. When wiring outside the vacuum shroud, larger gage
copper cable can be used, and twisting is still recommended.
2.5 CONSIDERATION FOR GOOD CONTROL
Most of the techniques discussed above to improve cryogenic temperature accuracy apply to control as well. There is an
obvious exception in sensor location. A compromise is suggested below in Section 2.5.3 – Two Sensor Approach.
2.5.1 Thermal Conductivity
Good thermal conductivity is important in any part of a cryogenic system that is intended to be at the same temperature.
Most systems begin with materials that have good conductivity themselves, but as sensors, heaters, sample holders, etc.,
are added to an ever more crowded space, the junctions between parts are often overlooked. In order for control to work
well, junctions between the elements of the control loop must be in close thermal contact and have good thermal
conductivity. Gasket materials should always be used along with reasonable pressure.
2.5.2 Thermal Lag
Poor thermal conductivity causes thermal gradients that reduce accuracy and also cause thermal lag that make it difficult
for controllers to do their job. Thermal lag is the time it takes for a change in heating or cooling power to propagate
through the load and get to the feedback sensor. Because the feedback sensor is the only thing that lets the controller
know what is happening in the system, slow information to the sensor slows the response time. For example, if the
temperature at the load drops slightly below the setpoint, the controller gradually increases heating power. If the
feedback information is slow, the controller puts too much heat into the system before it is told to reduce heat. The
excess heat causes a temperature overshoot, which degrades control stability. The best way to improve thermal lag is to
pay close attention to thermal conductivity both in the parts used and their junctions.
2.5.3 Two-Sensor Approach
There is a conflict between the best sensor location for measurement accuracy and the best sensor location for control.
For measurement accuracy the sensor should be very near the sample being measured, which is away from the heating
and cooling sources to reduce heat flow across the sample and thermal gradients. The best control stability is achieved
when the feedback sensor is near both the heater and cooling source to reduce thermal lag. If both control stability and
measurement accuracy are critical it may be necessary to use two sensors, one for each function. Many temperature
controllers including the Model 325 have two sensor inputs for this reason.
Cooling System Design 2-7
Lake Shore Model 325 Temperature Controller User’s Manual
2.5.4 Thermal Mass
Cryogenic designers understandably want to keep the thermal mass of the load as small as possible so the system can
cool quickly and improve cycle time. Small mass can also have the advantage of reduced thermal gradients. Controlling
a very small mass is difficult because there is no buffer to adsorb small changes in the system. Without buffering, small
disturbances can very quickly create large temperature changes. In some systems it is necessary to add a small amount of
thermal mass such as a copper block in order to improve control stability.
2.5.5 System Nonlinearity
Because of nonlinearities in the control system, a system controlling well at one temperature may not control well at
another temperature. While nonlinearities exist in all temperature control systems, they are most evident at cryogenic
temperatures. When the operating temperature changes the behavior of the control loop, the controller must be retuned.
As an example, a thermal mass acts differently at different temperatures. The specific heat of the load material is a major
factor in thermal mass and the specific heat of materials like copper change as much as three orders of magnitude when
cooled from 100 K to 10 K. Changes in cooling power and sensor sensitivity are also sources of nonlinearity.
The cooling power of most cooling sources also changes with load temperature. This is very important when operating at
temperatures near the highest or lowest temperature that a system can reach. Nonlinearities within a few degrees of these
high and low temperatures make it very difficult to configure them for stable control. If difficulty is encountered, it is
recommended to gain experience with the system at temperatures several degrees away from the limit and gradually
approach it in small steps.
Keep an eye on temperature sensitivity. Sensitivity not only affects control stability but it also contributes to the overall
control system gain. The large changes in sensitivity that make some sensors so useful may make it necessary to retune
the control loop more often.
2.6 PID CONTROL
For closed-loop operation, the Model 325 temperature controller uses a algorithm called PID control. The control
equation for the PID algorithm has three variable terms: proportional (P), integral (I), and derivative (D). See Figure 2-3.
Changing these variables for best control of a system is called tuning. The PID equation in the Model 325 is:
where the error (e) is defined as: e = Setpoint – Feedback Reading.
Proportional is discussed in Section 2.6.1. Integral is discussed in Section 2.6.2. Derivative is discussed in Section 2.6.3.
Finally, the manual heater output is discussed in Section 2.6.4.
2.6.1 Proportional (P)
The Proportional term, also called gain, must have a value greater than zero for the control loop to operate. The value of
the proportional term is multiplied by the error (e) which is defined as the difference between the setpoint and feedback
temperatures, to generate the proportional contribution to the output: Output (P) = Pe. If proportional is acting alone,
with no integral, there must always be an error or the output will go to zero. A great deal must be known about the load,
sensor, and controller to compute a proportional setting (P). Most often, the proportional setting is determined by trial
and error. The proportional setting is part of the overall control loop gain, and so are the heater range and cooling power.
The proportional setting will need to change if either of these change.
2-8 Cooling System Design
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