Teledyne 3020T User Manual

Trace Oxygen Analyzer

OPERATING INSTRUCTIONS FOR

Model 3020T

Trace Oxygen Analyzer

DANGER

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 PER-

SIST 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 M65908

11/22/99

ECO # 99-0459

i

Model 3020T

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 was supplied are indicated by a check mark in
the box.

Instrument Serial Number: __________________________

The instrument with the above serial number has the following
Options:

❏ 3020T-C Three gas inputs, for sample, zero and span gases, with

three solenoid-actuated gas-flow control valves built in.
Valves are automatically synchronized to the analyzer's
electronic control sequences.

❏ 3020T–F Built-in flame arresters for Groups C and D service.
❏ 3020T–G Built-in flame arresters for Groups C and D service, plus

gas-control valves as in –C option, above.

❏ 3020T–H Built-in flame arresters for Group B (hydrogen) service.
❏ 3020T–I Built-in flame arresters for Group B (hydrogen) service,

plus gas-control valves as in –C option, above.

❏ 19" Rack Mount

The 19" Relay Rack Mount units are available with
either one or two series 3000 analyzer Control Units
installed in a standard 19" panel and ready to mount in a
standard rack. See Appendix for details.

❏ Cell Class* ____________________ (L-2C standard).

Enter Class Designation

* See Part II, Chapter 2 and/or any addendum that may be

attached to this manual for cell specifications.

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iii

Model 3020T

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 Operator Interface .......................................................... 1-3

1.5.1 UP/DOWN Switch .................................................. 1-4

1.5.2 ESCAPE/ENTER Switch....................................... 1-4

1.5.3 Displays ................................................................. 1-5

1.6 Recognizing Difference Between LCD & VFD............... 1-5

1.7 Rear Panel Equipment Interface .................................... 1-5

1.7.1 Electrical Connector Panel .................................... 1-5

1.7.2 Gas Connector Panel............................................. 1-7

Table of Contents

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 Eff ect of Pressure.............................................. 2-4

2.2.5 Calibration Characteristics ...................................... 2-4

2.3 Sample System.............................................................. 2-5

2.4 Electronics and Signal Processing ................................ 2-6

2.5 Temperature Control ...................................................... 2-8

3 Installation

3.1 Unpacking the Analyzer................................................. 3-1

3.2 Mounting the Analyzer ................................................... 3-1

3.3 Electrical Connections ................................................... 3-3

3.3.1 Primary Input P ow er............................................... 3-4

3.3.2 Fuse Installation..................................................... 3-4

3.3.3 Analog Outputs ...................................................... 3-4

3.3.4 Alarm Relays ......................................................... 3-6

3.3.5 Digital Remote Cal Inputs ...................................... 3-7

3.3.6 Range ID Relays ................................................... 3-9

3.3.7 Network I/O ............................................................ 3-9

3.3.8 RS-232 Port ........................................................... 3-9

3.3.9 Remote Sensor and Solenoid Valves ....................3-10

3.4 Installing the Micro-Fuel Cell ........................................ 3-11

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Trace Oxygen Analyzer

3.5 Gas Connections .......................................................... 3-12

3.6 Testing the System........................................................ 3-14

4 Operation

4.1 Introduction .................................................................... 4-1

4.2 Using the Controls ......................................................... 4-1

4.2.1 Mode/Function Selection ....................................... 4-2

4.2.1.1 Analysis Mode ............................................... 4-2

4.2.1.2 Setup Mode ................................................... 4-2

4.2.2 Data Entry.............................................................. 4-4

4.2.2.1 ENTER .......................................................... 4-4

4.2.2.2 ESCAPE........................................................ 4-4

4.3 The AUTO-CAL Function............................................... 4-5

4.4 The PWD Function ........................................................ 4-5

4.4.1 Entering the Password........................................... 4-6

4.4.2 Installing or Changing the Password ..................... 4-7

4.5 The LOGOUT Function.................................................. 4-8

4.6 The VERSION Screen ................................................... 4-8

4.7 The SELF TEST Function.............................................. 4-9

4.8 The ZERO and SPAN Functions ................................... 4-9

4.8.1 Zero Cal................................................................. 4-10

4.8.1.1 Auto Mode Zeroing ........................................ 4-10

4.8.1.2 Manual Mode Zeroing.................................... 4-11

4.8.1.3 Cell Failure .................................................... 4-11

4.8.2 Span Cal................................................................ 4-12

4.8.2.1 Auto Mode Spanning ..................................... 4-12

4.8.2.2 Manual Mode Spanning................................. 4-13

4.9 The ALARMS Function.................................................. 4-14

4.10 The RANGE Function.................................................... 4-16

4.10.1 Setting the Analog Output Ranges......................... 4-16

4.10.2 Fixed Range Analysis ............................................ 4-17

4.11 The CONTRAST Function............................................. 4-18

4.12 The STANDBY Function................................................ 4-18

4.13 The

Analysis Mode ........................................................

4-19

Maintenance

5.1 Routine Maintenance..................................................... 5-1

5.2 Major Internal Components............................................ 5-1

5.3 Cell Replacement .......................................................... 5-2

5.3.1 Storing and Handling Replacement Cells ............... 5-3

5.3.2 When to Replace a Cell........................................... 5-3

5.3.3 Removing the Micro-Fuel Cell ................................. 5-4

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Model 3020T

5.3.4 Installing a New Micro-Fuel Cell.............................. 5-6

5.2.5 Cell W arr anty ........................................................... 5-6

5.4 Fuse Replacement ......................................................... 5-7

5.5 System Self Diagnostic Test........................................... 5-7

Appendix

A-1 Specifications ................................................................ A-1

A-2 Recommended 2-Year Spare Parts List ......................... A-3

A-3 Drawing List................................................................... A-4

A-4 Application Notes on Restrictors, Pressures & Flow...... A-4

A-5 Material Safety Data Sheet ............................................ A-5

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Trace Oxygen Analyzer Introduction 1

Introduction

1.1 Overview

The Teledyne Analytical Instruments Model 3020T 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 3020T, trace oxygen, explosion-proof, bulkhead-mount units
only.

1.2 Typical Applications

A few typical applications of the Model 3020T 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 3020T 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 3020T

• Advanced Micro-Fuel Cell, redesigned for trace analysis, has an
expected life of one year.

• 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.

• 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.

1.4 Model Designations

3020T: Standard model.
3020T-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 3020T electronics, to automatically switch between gases
in synchronization with the analyzer’s operations

3020T-F: Flame arrestors for Groups C and D installed.
3020T-G: Flame arrestors for Groups C and D, & -C option, installed.
3020T-H: Flame arrestors for Group B (hydrogen) installed.
3020T-I: Flame arrestors for Group B, & -C option, installed.

1-2

All of the above options are available in combination.

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Trace Oxygen Analyzer Introduction 1

1.5 Operator Interface

All controls and displays on the standard 3020T are accessible from
outside the housing. The instrument has two simple operator controls. A
digital meter, an alphanumeric display, and a sample flowmeter give the
operator constant feedback from the instrument. See Figure 1-1. The controls
are described briefly here and in greater detail in chapter 4.

Figure 1-1: Model 3020T Controls, Indicators, and Connectors

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1 Introduction Model 3020T

1.5.1 UP/DOWN Switch

Functions: The UP/DOWN switch is used to select the function to be

performed. Choose UP or DOWN to scroll through the following list of
eleven functions:

• Auto-Cal Set up an automatic calibration sequence.

• PWD Install a password to protect your analyzer setup.

• Logout Locks Setup Mode.

• Version Displays model and version of analyzer.

• Self-Test Runs internal diagnostic program, displays results.

• 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.

• Contrast Allows adjustment of LCD contrast.

Contrast Function is

• Standby Leaves analyzer powered, but no outputs or displays.

(Refer to Section 1.6)

DISABLED

WARNING: THE POWER CABLE MUST BE DISCONNECTED TO

FULLY REMOVE POWER FROM THE INSTRUMENT.

Subfunctions: Once a Function is entered, the UP/DOWN switch is

used to select between any subfunctions displayed on the VFD screen.

Parameter values: When modifiable values are displayed on the

VFD, the UP/DOWN switch can be used to increment or decrement the
values.

1.5.2 ESCAPE/ENTER Switch

Data Entry: The ESCAPE/ENTER switch is used to input data, from

the alphanumeric VFD screen into the instrument:

• Escape Moves VFD display back to the previous screen in a

series. If none remains, returns to the

Analyze

screen.

• Enter With a Subfunction Selected: Moves VFD display on to

the next screen in a series. If none remains, returns to
the

Analyze

screen.

With a Value Selected: Enters the value into the
analyzer as data. AdvancesVFD to next operation.

1-4

(See Chapter 4 for details.)

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Trace Oxygen Analyzer Introduction 1

1.5.3 Displays

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).

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.

1.7 Equipment Interface

1.7.1 Electrical Connector Panel

The electrical connector panel, shown in Figure 1-2, contains the
electrical connections for external inlets and outlets. The connectors are
described briefly here and in detail in the Installation chapter of this manual.

CAUTION: The power cable must be disconnected to fully

remove power from the instrument.

Access: To access the electrical connector panel, or the sensor block,
the control panel doubles as a door that can be unbolted and swung open.

Electrical Connections: The electrical connections on the electrical
connector panel are described briefly here, and in more detail in chapter 3
Installation.

• Power Connection 115 or 230 V dc, 50 or 60 Hz.

• Analog Outputs 0-1 V dc concentration plus 0-1 V dc

range ID, and isolated 4-20 mA dc plus
4-20 mA dc range ID.

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1 Introduction Model 3020T

• Alarm Connections 2 concentration alarms and 1 system

alarm.

• RS-232 Port Serial digital concentration signal output

and control input.

• Remote Valves Used for controlling external solenoid

valves, if desired.

1-6

Figure 1-2: Electrical Connector Panel

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Trace Oxygen Analyzer Introduction 1

• Remote Sensor Used for external sensor and

thermocouple, if desired.

• Remote Span/Zero Digital inputs allow external control of

analyzer calibration.

• 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.

1.7.2 Gas Connector Panel

The gas connector panel, shown in Figure 1-3, contains the gas con­nections 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 1-3: Model 3020T Gas Connector Panel

• Gas Inlet and Outlet One inlet (must be externally valved)

and one exhaust out.

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1 Introduction Model 3020T

Optional:

• Calibration Gas Ports Separate fittings for zero, span and

sample gas input, plus internal valves for
automatically switching the gases in
sync with the 3020 electronics.

Note: If you require highly accurate Auto-Cal timing, use external

Auto-Cal control where possible. The internal clock in the
Model 3020T is accurate to 2-3 %. Accordingly, internally
scheduled calibrations can vary 2-3 % per day.

1-8

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Trace Oxygen Analyzer Operational Theory 2

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 3020T 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 (oxygen)
comes from outside the device as a constituent of the sample gas being

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2 Operational Theory Model 3020T

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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Trace Oxygen Analyzer Operational Theory 2

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 3020T

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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Trace 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 gas panel inlets. Depending on the mode of operation either
sample or calibration gas is delivered.

The Model 3020T 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 flowing through the system is
monitored by a flowmeter downstream from the cell.

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2 Operational Theory Model 3020T

s

a

o

Figure 2-4 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 and/or F
options are ordered. The valves, when supplied, are installed inside the
3020T enclosure and are regulated by the instruments internal electronics.
The flame arrestors, when supplied, are installed in the Gas Connector Panel.

Span In

Zero In

Sample In

Solenoid

In vacuum service the
restrictor should b
placed here.

Exhaust O u

Restrictor

In normal service the
restrictor should b
placed here.

Valves

Components in the
the -C option (intern
only and are not sh
diagram abov e

Cell

Figure 2-4: Flow Diagram

2.4 Electronics and Signal Processing

The Model 3020T 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
most international power sources. See chapter 5 Maintenance for the location
of the power supply and the main electronic PC boards.

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 access panel. Figure 2-5 is a block diagram of the Analyzer
electronics.

2-6

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Trace Oxygen Analyzer Operational Theory 2

Figure 2-5: Block Diagram of the Model 3020T Electronics

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2 Operational Theory Model 3020T

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.

2.5 Temperature Control

For accurate analysis this instrument is temperature controlled not to fall
beneath a certain temperature. This temperature is 22oF. This is to prevent
the sensor from freezing in cold environments.

2-8

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