Teledyne 3000TAEU Instruction Manual
Trace Oxygen Analyzer
OPERATING INSTRUCTIONS FOR
Model 3000TA-EU
Trace Oxygen Analyzer
D ANGER
HIGHLY TOXIC AND OR FLAMMABLE LIQUIDS OR GASES MAY BE PRESENT IN THIS MONITORING
SYSTEM.
PERSONAL PROTECTIVE EQUIPMENT MAY BE REQUIRED WHEN SERVICING THIS SYSTEM.
HAZARDOUS VOLTAGES EXIST ON CERTAIN COMPONENTS INTERNALLY WHICH MAY PERSIST
FOR A TIME EVEN AFTER THE POWER IS TURNED OFF AND DISCONNECTED.
ONLY AUTHORIZED PERSONNEL SHOULD CONDUCT MAINTENANCE AND/OR SERVICING. BEFORE
CONDUCTING ANY MAINTENANCE OR SERVICING CONSULT WITH AUTHORIZED SUPERVISOR/
MANAGER.
Teledyne Analytical Instruments
P/N M66316
11/24/04
ECO # 03-0126
i
Model 3000TA
Copyright © 1999 Teledyne Analytical Instruments
All Rights Reserved. No part of this manual may be reproduced, transmitted, transcribed, stored in a retrieval system, or translated into any other language or computer
language in whole or in part, in any form or by any means, whether it be electronic,
mechanical, magnetic, optical, manual, or otherwise, without the prior written consent of
Teledyne Analytical Instruments, 16830 Chestnut Street, City of Industry, CA 91749-1580.
Warranty
This equipment is sold subject to the mutual agreement that it is warranted by us free
from defects of material and of construction, and that our liability shall be limited to
replacing or repairing at our factory (without charge, except for transportation), or at
customer plant at our option, any material or construction in which defects become
apparent within one year from the date of shipment, except in cases where quotations or
acknowledgements provide for a shorter period. Components manufactured by others bear
the warranty of their manufacturer. This warranty does not cover defects caused by wear,
accident, misuse, neglect or repairs other than those performed by Teledyne or an authorized service center. We assume no liability for direct or indirect damages of any kind and
the purchaser by the acceptance of the equipment will assume all liability for any damage
which may result from its use or misuse.
We reserve the right to employ any suitable material in the manufacture of our
apparatus, and to make any alterations in the dimensions, shape or weight of any parts, in
so far as such alterations do not adversely affect our warranty.
Important Notice
This instrument provides measurement readings to its user, and serves as a tool by
which valuable data can be gathered. The information provided by the instrument may
assist the user in eliminating potential hazards caused by his process; however, it is
essential that all personnel involved in the use of the instrument or its interface, with the
process being measured, be properly trained in the process itself, as well as all instrumentation related to it.
The safety of personnel is ultimately the responsibility of those who control process
conditions. While this instrument may be able to provide early warning of imminent danger,
it has no control over process conditions, and it can be misused. In particular, any alarm or
control systems installed must be tested and understood, both as to how they operate and
as to how they can be defeated. Any safeguards required such as locks, labels, or redundancy, must be provided by the user or specifically requested of Teledyne at the time the
order is placed.
Therefore, the purchaser must be aware of the hazardous process conditions. The
purchaser is responsible for the training of personnel, for providing hazard warning
methods and instrumentation per the appropriate standards, and for ensuring that hazard
warning devices and instrumentation are maintained and operated properly.
Teledyne Analytical Instruments, the manufacturer of this instrument, cannot
accept responsibility for conditions beyond its knowledge and control. No statement
expressed or implied by this document or any information disseminated by the manufacturer or its agents, is to be construed as a warranty of adequate safety control under the
user’s process conditions.
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Trace Oxygen Analyzer
Specific Model Information
The instrument for which this manual was supplied may incorporate
one or more options not supplied in the standard instrument. Commonly
available options are listed below, with check boxes. Any that are incorporated in the instrument for which this manual is supplied are indicated by a
check mark in the box.
Instrument Serial Number: _______________________
Options Included in the Instrument with the Above Serial Number:
q 3000TA-C: In addition to all standard features, this model also
has separate ports for zero and span gases, and builtin control valves. The internal valves are entirely
under the control of the 3000TA electronics, to
automatically switch between gases in
synchronization with the analyzer’s operations
q 19" Rack Mnt: The 19" Relay Rack Mount units are available with
either one or two 3000 series analyzers installed in a
standard 19" panel and ready to mount in a standard
rack.
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Model 3000TA
.
Model 3000TA-EU complies with all of the requirements of the
Commonwealth of Europe (CE) for Radio Frequency Interference,
Electromagnetic Interference (RFI/EMI), and Low Voltage Directive
(LVD).
The following International Symbols are used throughout the Instruction Manual for your visual and immediate warnings and when
you have to attend CAUTION while operating the instrument:
STAND-BY, Instrument is on Stand-by,
but circuit is active
GROUND
Protective Earth
CAUTION, The operator needs to refer to the manual
for further information. Failure to do so may
compromise the safe operation of the equipment.
CAUTION, Risk of Electric Shock
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Trace Oxygen Analyzer
Table of Contents
1 Introduction
1.1 Overview........................................................................ 1-1
1.2 Typical Applications ....................................................... 1-1
1.3 Main Features of the Analyzer ....................................... 1-1
1.4 Model Designations....................................................... 1-2
1.5 F ront Panel (Oper ator Interface) ..................................... 1-3
1.6 Recognizing Difference Between LCD & VFD............... 1-5
1.7 Rear Panel (Equipment Interface) .................................. 1-5
2 Operational Theory
2.1 Introduction .................................................................... 2-1
2.2 Micro-Fuel Cell Sensor .................................................. 2-1
2.2.1 Principles of Operation ............................................ 2-1
2.2.2 Anatomy of a Micro-Fuel Cell .................................. 2-2
2.2.3 Electrochemical Reactions...................................... 2-3
2.2.4 The Effect of Pressure.............................................. 2-4
2.2.5 Calibration Characteristics ...................................... 2-4
2.3 Sample System.............................................................. 2-5
2.4 Electronics and Signal Processing ................................ 2-7
3 Installation
3.1 Unpacking the Analyzer................................................. 3-1
3.2 Mounting the Analyzer ................................................... 3-1
3.3 Rear Panel Connections................................................ 3-2
3.3.1 Gas Connections ................................................... 3-3
3.3.2 Electrical Connections........................................... 3-4
3.3.2.1 Primary Input Power....................................... 3-4
3.3.2.2 50-Pin Equipment Interface Connector .......... 3-5
3.3.2.3 RS-232 Port................................................... 3-10
3.4 Installing the Micro-Fuel Cell ......................................... 3-11
3.5 Testing the System......................................................... 3-11
4 Operation
4.1 Introduction .................................................................... 4-1
4.2 Using the Data Entry and Function Buttons ................... 4-2
4.3 The System Function..................................................... 4-3
4.3.1 Tracking the O2 Readings during Calibration ........ 4-4
4.3.2 Setting up an Auto-Cal........................................... 4-5
4.3.3 Passw ord Protection.............................................. 4-6
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Model 3000TA
4.3.3.1 Entering the Password....................... ............ 4-6
4.3.3.2 Installing or Changing the Password ............. 4-7
4.3.4 Logout.................................................................... 4-9
4.3.5 System Self-Diagnostic Test .................................. 4-9
4.3.6 Version Screen ...................................................... 4-10
4.3.7 Showing Negative Oxygen Readings.................... 4-10
4.4 The Zero and Span Functions ....................................... 4-111
4.4.1 Zero Cal ................................................................. 4-12
4.4.1.1 Auto Mode Zeroing ........................................ 4-12
4.4.1.2 Manual Mode Zeroing.................................... 4-13
4.4.1.3 Cell Failure .................................................... 4-13
4.4.2 Span Cal................................................................ 4-14
4.4.2.1 Auto Mode Spanning ..................................... 4-14
4.4.2.2 Manual Mode Spanning................................. 4-15
4.4.3 Span Failure .......................................................... 4-16
4.5 The Alarms Function...................................................... 4-16
4.6 The Range Function ...................................................... 4-18
4.6.1 Setting the Analog Output Ranges......................... 4-19
4.6.2 Fixed Range Analysis ............................................ 4-19
4.7 The Analyze Function.................................................... 4-20
4.8 Signal Output ................................................................. 4-20
Maintenance
5.1 Routine Maintenance..................................................... 5-1
5.2 Cell Replacement .......................................................... 5-1
5.2.1 Storing and Handling Replacement Cells ............... 5-1
5.2.2 When to Replace a Cell........................................... 5-2
5.2.3 Removing the Micro-Fuel Cell ................................. 5-2
5.2.4 Installing a New Micro-Fuel Cell.............................. 5-4
5.2.5 Cell W arranty ........................................................... 5-4
5.3 Fuse Replacement ......................................................... 5-5
5.4 System Self Diagnostic Test........................................... 5-6
5.5 Major Internal Components............................................ 5-6
5.6 Cleaning ........................................................................ 5-7
5.7 Troubleshooting ............................................................. 5-8
Appendix
A-1 Model 3000TA Specifications ........................................ A-1
A-2 Recommended 2-Year Spare Parts List ......................... A-2
A-3 Drawing List................................................................... A-3
A-4 19-Inch Relay Rack Panel Mount................................... A-4
A-5 Application Notes on Restrictors, Pressures and Flow .. A-5
A-6 Material Safety Data Sheet ............................................ A-6
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Trace Oxygen Analyzer
DANGER
COMBUSTIBLE GAS USAGE WARNING
This is a general purpose instrument designed for usage in a
nonhazardous area. It is the customer's responsibility to ensure safety especially when combustible gases are being analyzed since the potential of gas leaks always exist.
The customer should ensure that the principles of operating of
this equipment is well understood by the user. Misuse of this
product in any manner, tampering with its components, or
unauthorized substitution of any component may adversely
affect the safety of this instrument.
Since the use of this instrument is beyond the control of
Teledyne, no responsibility by Teledyne, its affiliates, and
agents for damage or injury from misuse or neglect of this
equipment is implied or assumed.
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Model 3000TA
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Trace Oxygen Analyzer Introduction 1
Introduction
1.1 Overview
The Teledyne Analytical Instruments Model 3000TA Trace Oxygen
Analyzer is a versatile microprocessor-based instrument for detecting
oxygen at the parts-per-million (ppm) level in a variety of gases. This
manual covers the Model 3000TA General Purpose flush-panel and/or
rack-mount units only. These units are for indoor use in a nonhazardous
environment.
1.2 Typical Applications
A few typical applications of the Model 3000TA are:
• Monitoring inert gas blanketing
• Air separation and liquefaction
• Chemical reaction monitoring
• Semiconductor manufacturing
• Petrochemical process control
• Quality assurance
• Gas analysis certification.
1.3 Main Features of the Analyzer
The Model 3000TA Trace Oxygen Analyzer is sophisticated yet
simple to use. The main features of the analyzer include:
• A 2-line alphanumeric display screen, driven by microprocessor
electronics, that continuously prompts and informs the operator.
• High resolution, accurate readings of oxygen content from low
ppm levels through 25%. Large, bright, meter readout.
• Stainless steel cell block.
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1 Introduction Model 3000TA
• Advanced Micro-Fuel Cell, designed for trace analysis, has a
one year warranty and an expected lifetime of two years.
• Versatile analysis over a wide range of applications.
• Microprocessor based electronics: 8-bit CMOS microprocessor
with 32 kB RAM and 128 kB ROM.
• Three user definable output ranges (from 0-10 ppm through 0250,000 ppm) allow best match to users process and equipment.
• Air-calibration range for convenient spanning at 20.9 %.
• Auto Ranging allows analyzer to automatically select the proper
preset range for a given measurement. Manual override allows
the user to lock onto a specific range of interest.
• Two adjustable concentration alarms and a system failure alarm.
• Extensive self-diagnostic testing, at startup and on demand,
with continuous power-supply monitoring.
• CE Compliance.
• RS-232 serial digital port for use with a computer or other
digital communication device.
• Four analog outputs: two for measurement (0–1 V dc and
Isolated 4–20 mA dc) and two for range identification.
• Convenient and versatile, steel, flush-panel or rack-mountable
case with slide-out electronics drawer.
1.4 Model Designations
3000TA: Standard model.
3000TA-C: In addition to all standard features, this model also has
separate ports for zero and span gases, and built-in control
valves. The internal valves are entirely under the control of
the 3000TA electronics, to automatically switch between
gases in synchronization with the analyzer’s operations.
1-2
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Trace Oxygen Analyzer Introduction 1
1.5 Front Panel (Operator Interface)
The standard 3000TA is housed in a rugged metal case with all controls and displays accessible from the front panel. See Figure 1-1. The
front panel has thirteen buttons for operating the analyzer, a digital meter,
an alphanumeric display, and a window for viewing the sample flowmeter.
Function Keys: Six touch-sensitive membrane switches are used to
change the specific function performed by the analyzer:
• Analyze Perform analysis for oxygen content of a sample gas.
• System Perform system-related tasks (described in detail in
chapter 4, Operation.).
Figure 1-1: Model 3000TA Front Panel
• Span Span calibrate the analyzer.
• Zero Zero calibrate the analyzer.
• Alarms Set the alarm setpoints and attributes.
• Range Set up the 3 user definable ranges for the instrument.
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1 Introduction Model 3000TA
Data Entry Keys: Six touch-sensitive membrane switches are used to
input data to the instrument via the alphanumeric VFD display:
• Left & Right Arrows Select between functions currently
displayed on the VFD screen.
• Up & Down Arrows Increment or decrement values of
functions currently displayed.
• Enter Moves VFD display on to the next screen in a series.
If none remains, returns to the Analyze screen.
• Escape Moves VFD display back to the previous screen in a
series. If none remains, returns to the Analyze screen.
Digital Meter Display: The meter display is a LED device that
produces large, bright, 7-segment numbers that are legible in any lighting.
It produces a continuous readout from 0-10,000 ppm and then switches to a
continuous percent readout from 1-25%. It is accurate across all analysis
ranges without the discontinuity inherent in analog range switching.
Alphanumeric Interface Screen: The VFD screen is an easy-to-use
interface from operator to analyzer. It displays values, options, and
messages that give the operator immediate feedback.
Flowmeter: Monitors the flow of gas past the sensor. Readout is 0.2
to 2.4 standard liters per minute (SLPM).
Standby Button: The Standby turns off the display and outputs,
but circuitry is still operating.
CAUTION: The power cable must be unplugged to fully
disconnect power from the instrument. When
chassis is exposed or when access door is open
and power cable is connected, use extra care to
avoid contact with live electrical circuits .
Access Door: For access to the Micro-Fuel Cell, the front panel
swings open when the latch in the upper right corner of the panel is pressed
all the way in with a narrow gauge tool. Accessing the main circuit board
requires unfastening rear panel screws and sliding the unit out of the case.
1.6 Recognizing Difference Between LCD & VFD
LCD has GREEN background with BLACK characters. VFD has
DARK background with GREEN characters. In the case of VFD - NO
CONTRAST ADJUSTMENT IS NEEDED.
1-4
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Trace Oxygen Analyzer Introduction 1
1.7 Rear Panel (Equipment Interface)
The rear panel, shown in Figure 1-2, contains the gas and electrical
connectors for external inlets and outlets. Those that are optional are
shown shaded in the figure. The connectors are described briefly here and
in detail in the Installation chapter of this manual.
Figure 1-2: Model 3000TA Rear Panel
• Power Connection Universal AC power source.
• Gas Inlet and Outlet One inlet (must be externally valved)
and one exhaust out. Three inlet when
“C” option ordered.
• RS-232 Port Serial digital concentration signal
output and control input.
• Remote Valves Used in the 3000TA for controlling
external solenoid valves only.
• 50-Pin Equipment Interface Port:
• Analog Outputs 0–1 V dc concentration plus 0-1 V d c
range ID, and isolated 4–20 mA dc plus
4-20 mA dc range ID.
• Alarm Connections 2 concentration alarms and 1 system
alarm.
• Remote Span/Zero Digital inputs allow external control of
analyzer calibration.
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1 Introduction Model 3000TA
• Calibration Contact To notify external equipment that
instrument is being calibrated and
readings are not monitoring sample.
• Range ID Contacts Four separate, dedicated, range relay
contacts. Low, Medium, High, Cal.
• Network I/O Serial digital communications for local
network access. For future expansion.
Not implemented at this printing.
Optional:
• Calibration Gas Ports Separate fittings for zero, span and
sample gas input, and internal valves
for automatically switching the gases.
Note: If you require highly accurate Auto-Cal timing, use external
Auto-Cal control where possible. The internal clock in the
Model 3000TA is accurate to 2-3 %. Accordingly, internally
scheduled calibrations can vary 2-3 % per day.
1-6
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Operational Theory
2.1 Introduction
The analyzer is composed of three subsystems:
1. Micro-Fuel Cell Sensor
2. Sample System
3. Electronic Signal Processing, Display and Control
The sample system is designed to accept the sample gas and transport
it through the analyzer without contaminating or altering the sample prior
to analysis. The Micro-Fuel Cell is an electrochemical galvanic device that
translates the amount of oxygen present in the sample into an electrical
current. The electronic signal processing, display and control subsystem
simplifies operation of the analyzer and accurately processes the sampled
data. The microprocessor controls all signal processing, input/output and
display functions for the analyzer.
2.2 Micro-Fuel Cell Sensor
2.2.1 Principles of Operation
The oxygen sensor used in the Model 3000T series is a Micro-Fuel
Cell designed and manufactured by Analytical Instruments It is a sealed
plastic disposable electrochemical transducer.
The active components of the Micro-Fuel Cell are a cathode, an anode,
and the 15% aqueous KOH electrolyte in which they are immersed. The cell
converts the energy from a chemical reaction into an electrical current in
an external electrical circuit. Its action is similar to that of a battery.
There is, however, an important difference in the operation of a
battery as compared to the Micro-Fuel Cell: In the battery, all reactants are
stored within the cell, whereas in the Micro-Fuel Cell, one of the reactants
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2 Operational Theory Model 3000TA
(oxygen) comes from outside the device as a constituent of the sample gas
being analyzed. The Micro-Fuel Cell is therefore a hybrid between a
battery and a true fuel cell. (All of the reactants are stored externally in a
true fuel cell.)
2.2.2 Anatomy of a Micro-Fuel Cell
The Micro-Fuel Cell is a cylinder only 1¼ inches in diameter and 1¼
inches thick. It is made of an extremely inert plastic, which can be placed
confidently in practically any environment or sample stream. It is effectively sealed, although one end is permeable to oxygen in the sample gas.
The other end of the cell is a contact plate consisting of two concentric foil
rings. The rings mate with spring-loaded contacts in the sensor block
assembly and provide the electrical connection to the rest of the analyzer.
Figure 2-1 illustrates the external features.
Figure 2-1: Micro-Fuel Cell
Refer to Figure 2-2, Cross Section of a Micro-Fuel Cell, which illustrates the following internal description.
2-2
Figure 2-2. Cross Section of a Micro-Fuel Cell (not to scale)
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At the top end of the cell is a diffusion membrane of Teflon, whose
thickness is very accurately controlled. Beneath the diffusion membrane
lies the oxygen sensing element—the cathode—with a surface area almost
4 cm2. The cathode has many perforations to ensure sufficient wetting of
the upper surface with electrolyte, and it is plated with an inert metal.
The anode structure is below the cathode. It is made of lead and has a
proprietary design which is meant to maximize the amount of metal available for chemical reaction.
At the rear of the cell, just below the anode structure, is a flexible
membrane designed to accommodate the internal volume changes that
occur throughout the life of the cell. This flexibility assures that the sensing membrane remains in its proper position, keeping the electrical output
constant.
The entire space between the diffusion membrane, above the cathode,
and the flexible rear membrane, beneath the anode, is filled with electrolyte. Cathode and anode are submerged in this common pool. They each
have a conductor connecting them to one of the external contact rings on
the contact plate, which is on the bottom of the cell.
2.2.3 Electrochemical Reactions
The sample gas diffuses through the Teflon membrane. Any oxygen
in the sample gas is reduced on the surface of the cathode by the following
HALF REACTION:
O2 + 2H2O + 4e– → 4OH
–
(cathode)
(Four electrons combine with one oxygen molecule—in the presence
of water from the electrolyte—to produce four hydroxyl ions.)
When the oxygen is reduced at the cathode, lead is simultaneously
oxidized at the anode by the following HALF REACTION:
Pb + 2OH– → Pb+2 + H2O + 2e
–
(anode)
(Two electrons are transferred for each atom of lead that is oxidized.
Therefore it takes two of the above anode reactions to balance one cathode
reaction and transfer four electrons.)
The electrons released at the surface of the anode flow to the cathode
surface when an external electrical path is provided. The current is proportional to the amount of oxygen reaching the cathode. It is measured and
used to determine the oxygen concentration in the gas mixture.
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2 Operational Theory Model 3000TA
The overall reaction for the fuel cell is the SUM of the half reactions
above, or:
2Pb + O2 → 2PbO
(These reactions will hold as long as no gaseous components capable
of oxidizing lead—such as iodine, bromine, chlorine and fluorine—are
present in the sample.)
The output of the fuel cell is limited by (1) the amount of oxygen in
the cell at the time and (2) the amount of stored anode material.
In the absence of oxygen, no current is generated.
2.2.4 The Effect of Pressure
In order to state the amount of oxygen present in the sample in partsper-million or a percentage of the gas mixture, it is necessary that the
sample diffuse into the cell under constant pressure.
If the total pressure increases, the rate that oxygen reaches the cathode
through the diffusing membrane will also increase. The electron transfer,
and therefore the external current, will increase, even though the oxygen
concentration of the sample has not changed. It is therefore important that
the sample pressure at the fuel cell (usually vent pressure) remain relatively
constant between calibrations.
2.2.5 Calibration Characteristics
Given that the total pressure of the sample gas on the surface of the
Micro-Fuel Cell input is constant, a convenient characteristic of the cell is
that the current produced in an external circuit is directly proportional to
the rate at which oxygen molecules reach the cathode, and this rate is
directly proportional to the concentration of oxygen in the gaseous mixture. In other words it has a linear characteristic curve, as shown in Figure
2-3. Measuring circuits do not have to compensate for nonlinearities.
In addition, since there is zero output in the absence oxygen, the
characteristic curve has close to an absolute zero (within ± 1 ppm oxygen).
In practical application, zeroing may still used to compensate for the
combined zero offsets of the cell and the electronics. (The electronics is
zeroed automatically when the instrument power is turned on.)
2-4
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T race Oxygen Analyzer Operational Theory 2
Figure 2-3. Characteristic Input/Output Curve for a Micro-Fuel Cell
2.3 Sample System
The sample system delivers gases to the Micro-Fuel Cell sensor from
the analyzer rear panel inlet. Depending on the mode of operation either
sample or calibration gas is delivered.
The Model 3000TA sample system is designed and fabricated to
ensure that the oxygen concentration of the gas is not altered as it travels
through the sample system. The sample encounters almost no dead space.
This minimizes residual gas pockets that can interfere with trace analysis.
The sample system for the standard instrument incorporates ¼ inch
tube fittings for sample inlet and outlet connections at the rear panel. For
metric system installations, 6 mm adapters are supplied with each instrument to be used if needed. The sample or calibration gas flows through the
system is monitored by a flowmeter downstream from the cell. Figure 2-4
shows the piping layout and flow diagram for the standard model.
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2 Operational Theory Model 3000TA
Figure 2-4: Piping Layout and Flow Diagram for Standard Model
Figure 2-5 is the flow diagram for the sampling system. In the standard instrument, calibration gases (zero and span) can be connected directly to the Sample In port by teeing to the port with appropriate valves.
The shaded portion of the diagram shows the components added when the
–C option is ordered. The valving is installed inside the 3000TA-C enclosure and is regulated by the instruments internal electronics.
2-6
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T race Oxygen Analyzer Operational Theory 2
Figure 2-5: Flow Diagram
2.4 Electronics and Signal Processing
The Model 3000TA Trace Oxygen Analyzer uses an 8031 microcontroller with 32 kB of RAM and 128 kB of ROM to control all signal processing, input/output, and display functions for the analyzer. System power
is supplied from a universal power supply module designed to be compatible with any international power source. Figure 2-6 shows the location of
the power supply and the main electronic PC boards.
Figure 2-6: Electronic Component Location Inside the Model 3000TA
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2 Operational Theory Model 3000TA
The signal processing electronics including the microprocessor,
analog to digital, and digital to analog converters are located on the
motherboard at the bottom of the case. The preamplifier board is mounted
on top of the motherboard as shown in the figure. These boards are accessible after removing the back panel. Figure 2-7 is a block diagram of the
Analyzer electronics.
2-8
Figure 2-7: Block Diagram of the Model 3000TA Electronics
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T race Oxygen Analyzer Operational Theory 2
In the presence of oxygen the cell generates a current. A current to
voltage amplifier converts this current to a voltage, which is amplified in
the second stage amplifier.
The second stage amplifier also supplies temperature compensation
for the oxygen sensor output. This amplifier circuit incorporates a thermistor, which is physically located in the cell block. The thermistor is a
temperature dependent resistance that changes the gain of the amplifier in
proportion to the temperature changes in the block. This change is inversely proportional to the change in the cell output due to the same temperature changes. The result is a signal that is temperature independent.
The output from the second stage amplifier is sent to an 18 bit analog to
digital converter controlled by the microprocessor.
The digital concentration signal along with input from the control
panel is processed by the microprocessor, and appropriate control signals
are directed to the display, alarms and communications port. The same
digital information is also sent to a 12 bit digital to analog converter that
produces the 4-20 mA dc and the 0-1 V dc analog concentration signal
outputs, and the analog range ID outputs.
Signals from the power supply are also monitored, and through the
microprocessor, the system failure alarm is activated if a malfunction is
detected.
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