Thermal Conductivity Analyzer
OPERATING INSTRUCTIONS
FOR
Model 235 Series
THERMAL CONDUCTIVITY ANALYZER
235
236
237
238
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.
DANGER
Teledyne Analytical Instruments
P/N M32845
08/06/99
ECO # 99-0323
i
Model 235
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 (TAI), 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.
Teledyne Analytical Instrumentsii
Thermal Conductivity Analyzer
Table of Contents
1 Introduction (Models 235, 236, 237, 238)
1.1 Electronic Circuitry ...................................................... 1-2
1.2 Plug-in Circuit Boards.................................................. 1-3
1.2.1 T.C. Cell Power Supply/Amplifier Board ........... 1-3
1.2.2 Differential Power Supply Board.......................1-3
1.2.3 Alarm Comparator Board (optional).................. 1-3
1.2.4 E to I Converter Board (optional)......................1-3
1.2.5 Linearizer Board (optional) ............................... 1-4
1.2.6 220 to 240 Volt Operation (optional) ...................1-4
2 Installation
2.1 Location....................................................................... 2-1
2.2 Electrical Connections.................................................2-1
2.3 Gas Connections......................................................... 2-2
2.3.1 Reference and Zero Gas .................................. 2-2
2.3.2 Vent Lines........................................................ 2-2
2.4 Pressure Regulation.................................................... 2-3
2.5 Accessory Sample System Components .................... 2-4
2.6 Recommended Flowmeter Readings .......................... 2-5
3 Operation
3. 1 Preliminary ..................................................................3-1
3.2 Gas Flowrate............................................................... 3-1
3.3 Zero Standardization................................................... 3-1
3.4 Span standardization .................................................. 3-2
3.5 Onstream Operation.................................................... 3-2
3.6 Normal Operation........................................................ 3-3
3.7 Maintenance................................................................ 3-3
4 Linearizer
4.1 Theory of Operation ....................................................4-1
4.2 Output Signal Reversal ............................................... 4-3
Appendix
Spare Parts List ................................................................... A-1
Calibration Data ................................................................... A-2
Drawing Package................................................................. A-3
Teledyne Analytical Instruments
iii
Model 235
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.
Teledyne Analytical Instrumentsiv
Thermal Conductivity Analyzer Introduction 1
Introduction (Models 235, 236, 237, 238)
The 235 Series Thermal Conductivity Analyzers measure the concentration of one component in a binary stream of gas, or the purity of a sample
stream containing a composite mixture of impurities, by comparing the
difference in thermal conductivity of the sample stream with that of a
reference gas of fixed composition.
Control of the sample and supporting gases is not provided for in the
basic design TAI offers a variety of supporting gas control panels as companion accessories to the analyzer to fill this need. In any case, means must
be provided for controlling the flowrates through the sample and reference
paths of the analyzer, and a control manifold will be required for the introduction of zero and span gas, as well as sample gas, into the sample path.
Appropriate pressure reducing regulators will have to be installed at all gas
supply sources; for those customers wishing to incorporate their own
sample controls, a recommended system piping schematic is included among
the drawings at the rear of the manual.
Thermal conductivity measurements are non-specific by nature. This
fact imposes certain limitations and requirements. If the user intends to
employ the analyzer to detect a specific component in a sample stream, the
sample must be composed of the component of interest and one other gas in
order to be accurate.
If, on the other hand, the user is primarily interested in the purity of a
process stream, and does not require specific identification of the impurity,
the analyzer can be used on more complex mixtures. The impurities, then,
can be a composition in themselves.
Teledyne Analytical Instruments
1-1
1 Introduction Model 235
Because analysis by thermal conductivity is not an absolute measurement, standardization gases of known composition will be required to fix
the upper and lower parameters of the range (or ranges) of analysis. These
gases will be used to periodically check the accuracy of the analyzer.
The difference in thermal conductivity between the fixed reference gas
and the sample is sensed by hot wire elements. The elements are mounted in
a cell assembly so that one set is in the reference and the other in the sample
stream. Each set of elements is a component in an electrical bridge circuit.
During calibration, the bridge circuit is balanced in zero and reference
gas at one end of the measurement range, and sensitized in reference and
span gas at the other end, so that intervening points along the range (or
ranges) of interest will produce a DC electrical signal representative of the
analysis. The resulting electrical signal is fed to an amplifier and span pot,
which produce a standard 0-1V output signal. An E to I converter PC board
is also installed and produces an isolated 4–20 mA DC current output in
addition to the voltage output.
The temperature of the measuring cell is regulated to within 0.1 degree
C by a sophisticated control circuit. A thermistor is used to measure the
temperature, and a zero-crossing switch regulates the power in a cartridgetype heater. Temperature control is precise enough to eliminate diurnal
effects in the output over the operating ranges of the analyzer.
The overall design of the instrument is intended to facilitate servicing
and troubleshooting, should that ever be necessary. The controls are all
mounted on the front panel, which can swing down, allowing access to the
cell compartment. The cell is enclosed in an insulated compartment that is
readily removable from the chassis; the electronics are mounted on a series
of circuit boards at the rear of the enclosure, accessible by removing the
back panel.
Explosion-proof models of the series use sealed explosion-proof
enclosures for the analysis section (Model 237) or both the analysis section
and control unit (Model 238). Model 235 is general purpose with remote
control unit, and Model 235 is general purpose with integral control unit.
1.1 Electronic Circuitry
The electronic components are mounted on a number of circuit boards
that plug into sockets on a larger board, dubbed the “Mother Board”. This
allows for rapid troubleshooting and repair of any defective parts, and also
for rapid field installation of optional features not ordered with the unit.
1-2
Teledyne Analytical Instruments
Thermal Conductivity Analyzer Introduction 1
All electrical interconnections are made to the terminal strips on the
mother board; this board also contains an unusual feature — a series of
regularly-spaced holes in a rectangular pattern, known as a “kludge” space,
is set aside for the installation of circuitry for special customer requirements.
1.2 Plug-in Circuit Boards
Several options are available as convenient plug-in circuit boards;
although these may not all be present in the specific instrument under
consideration, a brief description of some of the more common ones is
offered below, and noted as (optional); PC boards which are not noted as
(optional) are standard features.
1.2.1 T.C. Cell Power Supply/Amplifier Board
This circuit contains an IC regulator that holds the voltage through the
cell to 4.5 V. It also contains a 2-stage IC amplifier, with range resistors.
1.2.2 Differential Power Supply Board
15 Volts, regulated (for electronic amplifiers, etc.), and +24 volts, nonregulated (for alarm and relay circuitry and certain other functional uses) are
supplied by this circuit.
1.2.3 Alarm Comparator Board (optional)
The comparator alarm circuit is available in single or dual configurations, which can be supplied as high or low alarms, energized above or
below setpoint; adjustment of each alarm setpoint is made using a potentiometer provided on the instrument’s front panel. Power failure or “fail-safe”
alarming can also be provided. Refer to the specifications covering one
individual analyzer for details regarding specific alarm or other optional
provisions.
Teledyne Analytical Instruments
1-3
1 Introduction Model 235
1.2.4 E to I Converter Board (standard)
The standard current output in the form of an isolated 4–20 mA dc
current is supplied by the E to I converter circuit. The output of this board
is proportional to the percentage of range, for example, 4 mA for 0% and
20 mA for 100% of range. This current output is in addition to the 0–1 V dc
voltage output.
1.2.5 Linearizer Board (optional)
An excellent alternative to the use of correction curves is available as
an option with the Series 235 Analyzer. A digital linearizer circuit is available as a plug-in PC board. This is a very flexible circuit that produces a
linear correction to a wide variety of non-linear curves. The result is an
output signal which is linear over the specified analysis range or ranges.
When employed, the digital linearizer is transparent to the user and requires
no adjustment.
1.2.6 220 to 240 Volt Operation (optional)
The Series 235 analyzer is available for either 110-120 (standard) or
220-240 (optional), 50 or 60 Hz operation.
1-4
Teledyne Analytical Instruments
Thermal Conductivity Analyzer Installation 2
Installation
2.1 Location
The analyzer should be installed where it will not be subject to the
following conditions:
1. Direct sunlight
2. Drafts of air
3. Shock and vibration
4. Temperatures below 30° F or above 110° F
The analyzer should be placed as close as possible, subject to the above
conditions, to the sample point to minimize the effects of sample line lag
time on the analysis.
An outline diagram, showing the location and identification of the gas
line and electrical conduit connections, as well as the physical dimensions of
the analyzer case, is included in the drawings at the rear of the manual.
2.2 Electrical Connections
A single-phase, 110 to 120 Volt, or 220 to 240 Volt, 50 or 60 Hz line,
capable of delivering 2-1/2 amperes of current continuously, is required to
operate the analyzer. Primary power connections are made on the terminal
strip mounted on the mother board, behind the rear access cover. A solid
water-pipe ground should be provided for personnel protection. When
connecting the power source, polarize the connections as indicated on the
interconnection diagram at the rear of the manual.
Teledyne Analytical Instruments
2-1
2 Installation Model 235 Series
Use 2-conductor shielded cable (nominally No. 22 wire size) to interconnect the analyzer output signal with the recording equipment. The shield
should be terminated on the appropriate terminal (see interconnection
diagram) at the analyzer—and be left disconnected at the recorder.
2.3 Gas Connections
Customer gas connection points are located on the underside of the
analyzer case. (Standard, basic instrument)
(See Outline Diagram for identification of each point.)
2.3.1 Reference and Zero Gas
A constant supply of gas, of a fixed composition, is needed as the
reference to which the sample gas will be compared. The reference gas is
normally selected to represent the main background of the analysis. For
certain applications, an optional sealed air reference is available where the
reference side of the detector cell is filled with air and sealed. This eliminates the need to have reference gas constantly passing through the cell. For
instruments equipped with the optional sealed air reference, there will not be
reference inlet or vent ports.
A supply of gas, containing little or none of the components of interest,
is required to zero-standardize the analyzer.
In order to satisfy the requirements, both of these gases must be
supplied from purchased cylinder sources — as no other economical means
is readily available that will guarantee the user that impurities are maintained
at a low, fixed level.
Because most cylinder gases are supplied 99.95 to 99.98% pure, TBE
recommends that one cylinder of gas be used to fill both needs for most
applications (i.e., zero and reference.)
Specific recommendations as to the number and type of supporting
gases required will be found listed in the calibration section of the manual.
It is essential to the accuracy of the analyzer that the purity of the zero
gas be known. The zero control would be adjusted during zero standardization, so that the recorder indicates the impurity content of the zero gas,
rather than zero.
2-2
Teledyne Analytical Instruments
Thermal Conductivity Analyzer Installation 2
2.3.2 Vent Lines
The selected gas introduced into the sample path of the cell (zero,
span, and sample) is vented from one connection at the bottom of the
analyzer, and the reference gas is vented from another.
If it is desirable to carry these gases to an area remote from the analyzer to vent them, the following precautions will have to be observed in
vent line installation:
1. The vent lines should be constructed of 1/4 inch tubing, so that
no appreciable back pressure resulting from restricted flow is
experienced by the analyzer.
2. Both the sample and reference lines must be vented into an area
where the ambient pressure is the same.
3. The ambient pressure in the vent area must undergo no more
than normal barometric pressure changes.
4. The vent lines must be installed so that water and dirt cannot
accumulate in them.
2.4 Pressure Regulation
All incoming gas lines should be equipped with pressure regulators.
The sample line pressure regulator should be installed as close to the
sample point as possible to minimize sample line lag time.
Sample pressure should be set somewhere between 5 and 50 psig—10
psig is nominal.
To minimize flowrate adjustments, the pressure regulators on the
supporting gas supply cylinders should be adjusted to provide the same
output pressure as the sample line regulator.
When installing pressure regulators on supply cylinders, crack the
cylinder valves so that gas is flowing during installation. Using this procedure will eliminate the most common cause of standardization gas contamination. Air trapped during assembly can, and will, diffuse back into the
cylinder. This is particularly important in applications where impurities of 1
and 2% are the range of interest.
Teledyne Analytical Instruments
2-3
2 Installation Model 235 Series
2.5 Accessory Sample System Components
An integral gas selector panel is available as an option. This panel
mounts the gas controls on a panel where they can be operated conveniently
In applications where TBE furnishes an accessory gas control panel, or
a completely interconnected panel or cubicle system, installation can be
simply accomplished by using the supporting drawings included at the rear
of the manual. However, if the customer is selecting and interconnecting his
own gas system components, the following conditions should be adhered to:
1. Do not deviate from the system outlined in the piping schematic
when constructing your system.
2. Select a flowmeter capable of resolving 0.08 SCFH (40 to
50 cc/min) for the reference path of the analyzer.
3. Select a flowmeter capable of resolving 0.3 SCFH (150 cc/min)
for the sample path of the analyzer. (See Addendum A for
recommended flowmeter readings for gases heavier or lighter
than air.)
2.6 Recommended Flowmeter Readings for
Gases Heavier or Lighter Than Air
Due to the wide range of applications and gases that are measured with
the Thermal Conductivity Analyzer, the density of different sample gases
may vary considerably; for example, air is more dense than hydrogen. When
setting the sample and reference flowrate, note that gases lighter than air
will have an actual flowrate higher than indicated on the flowmeter, while
gases heavier than air will have a lower actual flowrate. The following chart
(with hypothetical figures) illustrates this fact:
GAS FLOWMETER ACTUAL
READING FLOWRATE
Lighter than air 0.3 SCFH 1.2 SCFH
Heavier than air 0.3 SCFH 0.2 SCFH
Air 0.3 SCFH 0.3 SCFH
2-4
Teledyne Analytical Instruments
Thermal Conductivity Analyzer Installation 2
The analyzer is not flow sensitive during measurement; i.e., the OUTPUT does not vary with the flow, but for maximum accuracy and repeatability, measurements should be made at the same flowrate used when
calibrating the analyzer.
TBE recommends, for lighter-than-air gas backgrounds, setting the
flowmeter to a lower reading for reference and measurement; this will
conserve gas. For example, for hydrogen or helium, set the flowmeter
reading to 0.1 SCFH. A higher reading is recommended for heavier-than-
air gas backgrounds, e.g., for carbon dioxide or argon, set the flowmeter
to 0.4 SCFH.
Teledyne Analytical Instruments
2-5
2 Installation Model 235 Series
2-6
Teledyne Analytical Instruments
Thermal Conductivity Analyzer Operation 3
Operation
3. 1 Preliminary
• Check to see that all gases have been connected to the proper
ports of the analyzer and that all gas connection lines are leakfree.
• Check to see that the power and signal wiring has been properly
installed.
• Check to see that the fuses in the analyzer are intact.
• Check to see that all PC Boards are intact and securely plugged
in.
• Turn the recorder and power switches to the “ON” position.
3.2 Gas Flowrate
Start the REFERENCE gas flow, and adjust the flowrate to approxi-
mately 0.08 SCFH (40 cc/min.)
Start ZERO gas through the sample path of the analyzer, and adjust
the flowrate to approximately 0.3 SCFH (150 cc/min).
See Section 2.6 for additional flowrate information for gases lighter or
heavier than air.
Allow the analyzer to run with zero and reference gas flowing for
several hours before attempting calibration. This will permit the cell to come
to thermal equilibrium.
3.3 Zero Standardization
After the necessary temperature stabilization period, the analyzer can
be zero-standardized as follows:
NOTE: Before zero-standardization of the analyzer is possible, and while
the power is off, the mechanical zero of the meter must be
checked. If the pointer does not rest at zero with the power off,
then adjust the slotted screw found at a low center position of the
meter face to correct. DO NOT allow this screw to be readjusted
after the zero-standardization has been performed. No mechanical
zero is used with digital meters.
Teledyne Analytical Instruments
3-1
3 Operation Model 235 Series
1. Check to see that the span control is set at about 50% of its
travel. Some readjustment of this control may be necessary
during standardization, but our concern at this point is to see
that a reasonable level of output signal is available to the
recorder for zero standardization.
2. With multi-range analyzers, make sure that the range selection
switch is on the “Range 1” position. As in Step No. 1, this
insures a proper signal level for deriving a correct zero setting.
3. Check the sample path flowmeter to see that the zero gas
flowrate is 0.3 SCFH.
4. Adjust the Zero control on the analyzer control panel until the
meter indicates the impurity (if any) contained in the zero gas.
3.4 Span Standardization
After the zero setting has been accomplished, the span (or sensitivity)
of the analyzer can be checked as follows:
1. Arrange the sample path so that span gas is flowing through the
analyzer.
2. Check the sample path flowmeter to see if the span gas is
flowing at a 0.3 SCFH rate.
3 With mufti-range instruments, set the range selector switch on
the position that provides the highest resolution of the span gas
concentration.
4. Adjust the span control until the meter reads the correct value of
impurity in the span gas.
3.5 Onstream Operation
After standardization has been successfully concluded, arrange the
sample path so that sample gas is flowing through the analyzer at approximately. 0.3 SCFH.
With multirange instruments, select the range of analysis that gives the
best recorder resolution of the process stream. The analyzer is now
“onstream” and ready for use.
3-2
Teledyne Analytical Instruments
Thermal Conductivity Analyzer Operation 3
3.6 Normal Operation
For routine operation of the analyzer, you should perform the follow-
ing checks:
• Sample flow: Check the sample flowrate daily to insure proper
operation.
• Reference gas flow: Check the reference gas flowrate daily—
and the reference supply cylinder periodically—to insure against
accidental depletion. Whenever it is necessary to replace the
reference gas supply, the analyzer standardization procedures
must be repeated.
• Standardization: The analyzer should be restandardized on a
monthly schedule as a check of its performance.
3.7 Maintenance
Since there are no moving parts in the analyzer, no routine maintenance is required other than normal care of the instrument. The checklist
above should be adequate to keep the analyzer functioning properly for
many years.
Teledyne Analytical Instruments
3-3
3 Operation Model 235 Series
3-4
Teledyne Analytical Instruments
Thermal Conductivity Analyzer Linearizer 4
Linearizer
4.1 Theory of Operation
The need for an electronics linearizer circuit arises in those applications
where the output of an instrument is not linearly related to the parameter the
instrument tries to measure. Often, this is the concentration of a chemical of
interest, color values, absorbance, or transmittance. When the calibration
curve, which is a plot of concentration versus instrument signal output, is
not a straight line, the linearizer can correct the curve and make it approach
a straight line. The linearizer does this by dividing the curve into eight
sections. Each section is amplified and added to the previously corrected
section.
Each section has a “breakpoint”, which connects it to the next section
away from zero; zero is the starting point of the curve. (Refer to Figures
4-1, 4-2, and 4-3.) The error left after linearization is due to the curvature of
each individual section. This error can be made quite small by correct selection of the breakpoints. The output of the linearizer is 0-1 Volt. See Figure
4-4 to visualize the linearization process.
Figure 4-5 shows how the linearizer works in actuality. It is exaggerated for clarity. For segment 1, the output will be some number (or fraction)
times the input voltage:
where V
Here, for this example,, the gain of the circuit is 0.8 for an input
voltage between 0 and 0.125 Volts.
When the input voltage exceeds 0.125 Volts, the second amplifier, as
well as the first, is working; it is adding or subtracting its output in proportion according to the setting of trimpot P2. In this case, its output is added
to the output of the first amp. The total gain (the slope of the line segment)
for the combined segment is now about 1.9.
= 0.8 x Vin.
V
out
is the output voltage and Vin is the input voltage.
out
TELEDYNE BROWN ENGINEERING
Analytical Instruments
4-1
4 Linearizer Model 235
Figure 4-1a: THE PROBLEM
The analyzer output is not directly proportional to the parameter it
is supposed to measure.
Figure 4-1b: THE SOLUTION
The Linearizer output is proportional to its input in a complimentary
fashion to the analyzer curve. As a result, the output is directly
proportional to the parameter being measured.
4-2
TELEDYNE BROWN ENGINEERING
Analytical Instruments
Thermal Conductivity Analyzer Linearizer 4
Figure 2a: THE IMPLEMENTATION - STEP 1
Adjust the gain of the first amplifier to bring the first segment (from 0 to 1) of the
analyzer curve into line with the ideal curve. This produces a small error in the
first section, but a larger error at higher inputs.
TELEDYNE BROWN ENGINEERING
Analytical Instruments
4-3
4 Linearizer Model 235
Figure 4-2b: THE IMPLEMENTATION - STEP 2
At point 1, the second amplifier begins to work in addition to the first. Its gain is
adjusted so that point 2 lies on the ideal curve. The error is small until point 2 is
reached. Here, the large error is due to the curvature of the original analyzer
curve.
4-4
TELEDYNE BROWN ENGINEERING
Analytical Instruments
Thermal Conductivity Analyzer Linearizer 4
FIGURE 4-2c: THE IMPLEMENTATION - STEP 3
At point 2, the third amplifier begins to work in addition to the first two. Its gain is
adjusted so that point 3 lies on the ideal curve.
TELEDYNE BROWN ENGINEERING
Analytical Instruments
4-5
4 Linearizer Model 235
Figure 4-3: THE RESULT
By using all the line segment amplifiers, the output of the analyzer is made to be
almost directly proportional to the parameter being measured.
4-6
TELEDYNE BROWN ENGINEERING
Analytical Instruments
Thermal Conductivity Analyzer Linearizer 4
Figure 4-4: The Effect of Each Amplifier on the Final Result
The points labeled 1, 2, 3, etc. are breakpoints.
TELEDYNE BROWN ENGINEERING
Analytical Instruments
4-7
4 Linearizer Model 235
Figure 4-5: Exaggerated Illustration of How the Linearizer Works
4-8
TELEDYNE BROWN ENGINEERING
Analytical Instruments
Thermal Conductivity Analyzer Linearizer 4
When the voltage exceeds 0.25 Volts, the third amplifier works, along
with the first two, adding or subtracting its output in proportion, according
to the setting of its trimpot, P3. The gain is now the sum of all three gains.
In this case, the gain of the third amp is negative, so the total gain is about
0.3.
As the input voltage exceeds each breakpoint, another amplifier joins
in. The slope of each line segment is equal to the sum of the gain of all the
amplifiers in operation at that particular time. The gain of each amplifier is
set by its trimpot. The first amplifier has a gain range of 0 to +4, and all the
others about -3 to +3.
The maximum slope obtainable is limited. Setting the gain too high will
result in the amplifier saturating. However, with the dynamic range inherent
in these amplifiers, this is not likely to happen.
The breakpoints are factory-set by the values of resistors R6, R8, R10,
R-12, R14, R16, and R18. See Figure 4-10 for location of these resistors.
The most efficient way to check the operation of the linearizer circuit
is to drive it using a 1 kHz. triangular wave of 2 Volts peak-to-peak amplitude as shown in Figure 4-6. The effect of the breakpoints and trimpots can
then readily be seen; if you alternate the gain of the stages, a jagged step
effect can be produced. This will show the breakpoints clearly. Alternatively, a DVM may be attached to the junction of D2 and R20; this junction
point accesses the output of a line segment amplifier. As the input voltage is
gradually increased, the DVM at some point will indicate a negative voltage. At this point, a breakpoint has been passed. Repeat this test for each
line segment amplifier to determine its breakpoint.
Refer to Figure 4-10 for the component position in the following
discussion. AlA is a non-inverting buffer and amplifier with a gain of 2.5,
zeroed with P9. Its output is checked for zero at test point 1 (TP1). R1
provides a bias path in case the input is not DC-loaded.
The amplified output is brought to the inverting inputs of line segment
amplifiers AlB, A1C, A1D, A2A, A2B, A2C, A2D, and A3A, through
resistors R5, R7, R9, R11, R13, R15, R17, and R19.
A1D is configured differently from the other line segment amplifiers. It
is simply an inverting amplifier with P1 as its feedback resistor to set its
gain. The gain for this amplifier may be set between 0 and 4.
The other line segment amplifiers work in a similar fashion. Let us
examine A1C, for example. Refer to Figure 4-7 for the following analysis of
a typical line segment amplifier.
4.2 Linearizer Circuit Theory
TELEDYNE BROWN ENGINEERING
Analytical Instruments
4-9
4 Linearizer Model 235
Figure 4-6: Output of Linearizer on Oscilloscope Screen Using Triangle Wave
Notice the mirror image effect as the voltage comes down from its maximum. If
the vertical scale and the timebase are arranged so that the slope of the
unmodified triangle wave is 45 degrees and the amplitude of the wave is 2 Volts
peak to peak, then the value of the breakpoints may be read off the screen by
measuring the distance. For example, say the distance measured is X. Then the
value of the breakpoint is X times the Volts per centimeter appropriate to the
range employed on the Y axis.
4-10
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Thermal Conductivity Analyzer Linearizer 4
Figure 4-7: A Typical Line Segment Amplifier
Diode D1 effectively shorts the output to the inverting input for any
positive-going signals at the output of AlC, while D2 would not allow any
positive output at AlC to reach the circuit’s output. Negative-going signals,
however, do not get through D1, but can go through D2, and R20 then acts
as a feedback resistor to set the gain of the circuit (to 4 in the example for
A1C). Since D2 is inside the feedback loop, its voltage drop does not
appear at the output. R6 sums a fraction of the -15 V supply to the input
voltage; the output of the circuit"is thus:
Vo = -R20 (Vin/R7-15/R6) for V0 < O
Vo = O if -R20 (Vin/R7-15/R6) > O
From this it can be seen that the circuit amplifies input voltages above
a cutoff voltage, and otherwise has zero output.
-R20 (Vin/R7-15/R6) = O or
Vin = 15 x R7/R6
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Analytical Instruments
4-11
4 Linearizer Model 235
The negative supply voltage is -15 Volts. A similar expression for the
output is V
Notice that V
= -4(Vin - V
out
is actually 2.5 times the voltage at the linearizer input,
in
due to the gain of AlA. This gain acts to minimize the effect of offset errors.
The cutoff voltage (V
referenced to the input is approximately V
).
cutoff
) is set by the choice of R6; the “breakpoint”
cutoff
/2.5. Thus:
cutoff
R6 = 15xR7x 12.5 x V
Note: Each amplifier amplifies everything above its cutoff voltage, and not
just a segment between two cutoff voltages.
bkpt
or (299.4/ V
) K ohms
bkpt
The output of each amplifier other than A1D is brought to the slider of
a trimpot. One end of the pot goes through a resistor (e.g., R28) to the
summing node of the output amp, A3B. The other end of the pot goes
through another resistor (e.g. R29) to the summing node of the inverter
A3C.
The output from the inverter is then also brought into the summing
node of the output amp A3B. Clearly the position of the slider on pot P2
will determine how much, signal goes directly into the summing input of
A3B, and now much goes through the inverter. If the slider is up at the top,
almost all of the output of AlC will add to the output of A1D. If the slider is
down at the bottom, then the output of A1C will be subtracted from that of
AlD. If the slider is in the middle, the output will be added and subtracted in
the same amount and thus will have no effect.
So, we see that the gain of the first section of the curve from 0 to the
first breakpoint is set at some value, (A), with P1. The gain of the second
section of the curve is the value (A) plus a value (B), which is set by P2.
The gain of the third breakpoint then, would be the sum, (A+B), plus a third
value, (C), set by P3. In other words, each pot affects the gain of all the
sections above where it starts working.
The gain of A3C is kept low by the small value of R43 (2K). This is to
stop it from saturating if it gets too much input from all the amplifiers. The
resistor that sums its output into A3B, (R44), is selected at 2K to compensate for this.
A3B has a gain of about 0.3 to compensate for the gain of AlA and to
reduce zero errors. It also sums all the positive contributions via R44 and
A3C. Finally, it provides a low impedance output for the circuit.
4-12
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Thermal Conductivity Analyzer Linearizer 4
4.3 Selection of Breakpoint Resistors
It may de desirable to concentrate the breakpoints in some areas of the
voltage range. For example: between 0 and 1in Figure 4-8, the curve is
fairly straight. Between 2 and 3, the curve is modestly straight but with less
slope. Between 1 and 2, however, the it appears substantially curved. Since
the linearizer is to approximate the curve with a series of straight lines, we
would like to have most of the segments on the curved segment; i.e.,
between 1 and 2. This means that most of the breakpoints must be between
1 and 2 rather than evenly spaced out. Similarly for the “S” curve shown in
Figure 4-9, the breakpoints would be concentrated between the points 1–2
and 3–4.
The resistors for the breakpoints are set according to the formula:
where V
R = 299.4/V
is the voltage at the particular breakpoint.
bkpt
bkpt
.
The resistors affected are R6, R8, R10, R12, R14, R16 and R18 where
R6 is the first breakpoint.
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4-13
4 Linearizer Model 235
Figure 4-8: A Simple Curve
4-14
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Analytical Instruments
Thermal Conductivity Analyzer Linearizer 4
Figure 4-9: An “S” Shaped Curve
TELEDYNE BROWN ENGINEERING
Analytical Instruments
4-15
4 Linearizer Model 235
Figure 4-10: Typical Component Placement on Linearizer
4-16
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Analytical Instruments
Thermal Conductivity Analyzer Linearizer 4
4.4 How to Use the Linearizer
Refering to Figure 4-10 for positioning of the Linearizer components,
check to see that the proper breakpoint resistors are installed. Also, refer to
the Linearizer drawings that have been included in the Appendix.
4.4.1 Nulling Amplifiers A1A
1. Short the input (pin 3) to the common of the diff. power
supply.
2. Connect a DVM to TP1 (testpoint 1 is located on the PC
board)
3. Adjust trimpot P9 until the DVM reads 0 mV ±10 mV.
4.4.2 Nulling the Entire Linearizer Input to Output
1. Maintain the shorted input of A1A. (See step 1 above.)
2. Connect the DVM to the output of the linearizer (pin 7
of A3B).
3. Adjust trimpot P10 until the DVM reads 0 mV ±10 mV.
4.4.3 Linearizing the Calibration Curve
The calibration curve must be known at this point and found to be non-
linear. This calibration can be done using known samples.
The curve has been studied, and breakpoint positions determined on
the most curved portions of the curve. Appropriate breakpoint resistors
have been installed.
1. Remove the shorting jumper previously installed for
nulling steps.
2. Connect a DC voltage source to the input of AlA (pin 3)
and ground. (The DVM is still connected to pin 7 of
A3B.)
3. Apply 0 volts to the input, (V
). The output, (V
in
) must
out
read 0 volts as well.
4. Make V
voltage. NOTE: V
until V
changes V
out
= V
in
= V
(1) per chart. This is the first breakpoint
test
must always be positive. Adjust P1
in
(1) per chart. NOTE: make sure P1
lin
.
out
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4-17
4 Linearizer Model 235
5. Make Vin = V
breakpoint voltage. Adjust P2 until V
(2) per chart. This is the second
test
= V
out
lin
(2).
6. Continue up each line segment, repeating the procedures
of the sections just covered, using pots P3, P4, P5, P6,
P7, and P8, to linearize line segments 3, 4, 5, 6, 7, and 8.
7. Repeat the calibration against known samples, and verify
that the values obtained at various concentrations are
linearly displayed.
8. If the linearity is not quite satisfactory, determine which
line segment requires touch-up. If more than one
segment is not properly adjusted, readjust the segment
closest to zero first. All other following segments must
be toucned up, since they are affected by the former one.
If results are still not satisfactory, re-evaluate the
breakpoints. Change their positions on the curve as
required by installing different values for breakpoint
resistors. Repeat the line segment trimpot adjustment
procedures as outlined in the above sections.
4-18
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Thermal Conductivity Analyzer Appendix
Appendix
Spare Parts List
QTY. P/N DESCRIPTION
1 C-14449 PC BOARD—TEMP CONTROLLER FOR TG OPTION
(220V USE C-69410)
1 B-30868 PC BOARD—TEMP CONTROL (220 V USE B-36026)
1 B-34856 PC BOARD—AMPLIFIER
1 A-9306 PC BOARD—POWER SUPPLY
1* C-58991 PC BOARD—LINEARIZER
1* A-10045 PC BOARD—SINGLE ALARM (-1 OPTION)
1* A-9309 PC BOARD—DUAL ALARM (-2 OPTION)
1 B-29600 PC BOARD—E TO I CONVERTER, ISOLATED 4–20 mA dc
5 F-10 FUSE, 2A (220 V USE F-9)
5 F-75 FUSE, 1/2 A (110 V, 220 V)
1 H-158 HEATER (110 V, 220 V)
1 A-31157 CELL ASSEMBLY
1 A-33748 THERMISTOR ASSEMBLY
* These items are options to the standard instrument and unless ordered,
will not be present.
IMPORTANT: Orders for replacement parts should include the part
number, the model, and serial numbers of the analyzer
in which they are to be used.
Orders should be sent to:
TELEDYNE Analytical Instruments
16830 Chestnut Street
City of Industry, CA 91749-1580
Phone (626) 934-1500, Fax (626) 961-2538
TWX (910) 584-1887 TDYANYL COID
Web: www.teledyne-ai.com
or your local representative.
Teledyne Analytical Instruments
A-1
Appendix Model 235
Calibration Data
The following data, along with any Addenda that may be included in
the front part of this manual, pertain to your specific Thermal Conductivity
Analyzer.
Calibration data for Model: ______________________
Serial Number: ______________________
Range: ______________________
Non-measured components: ______________________
Output Signal: ______________________
Reference and Zero Gas: ______________________
Note: If the zero gas contains a known (or equivalent) impurity,
the zero control should be set so that the analyzer indicates the impurity during the standardization procedure.
Span Gas: ______________________
Selected Resistor Values: ______________________
______________________
______________________
______________________
Alarm Strapping: ______________________
Zero Setting: ______________________
Span Setting: ______________________
A-2
Teledyne Analytical Instruments
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