LI-COR li-6262 User manual

LI-6262
CO2/H2O Analyzer
®
INSTRUCTION MANUAL

LI-6262 CO2/H2O Analyzer
Operating and Service Manual
®
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Publication Number 9003-59
March, 1996
Software Version 2.02
LI-COR, inc.
P.O. Box 4425
4421 Superior Street
Lincoln, Nebraska 68504
Phone: [phone]
FAX: [phone]
Toll-free 1-800-447-3576 (U.S. & Canada)
U.S. and Foreign Patents Pending

ii
Federal Communications Commission
Radio Frequency Interference Statement
This instrument generates and uses radio frequency energy and if not installed and used
properly, that is, in strict accordance with these instructions, may cause interference to
radio and television reception. This equipment is required to comply with the limits for a
Class 'A' computing device pursuant to Subpart J of Part 15 of the FCC rules. A Class 'B'
device is designed to be used in a residential area. Class 'A' specifications are designed to
provide reasonable protection against such interference in a commercial environment.
This equipment has been type tested and found to comply with the limits for Class 'A'
computing devices, and is verified in accordance with FCC Part 15 Subpart J for the Class
'B' limits for a computing device. However, there is no guarantee that interference will not
occur in a particular installation. If this equipment does cause interference to radio or
television reception that can be determined by turning the instrument off and on, the user is
encouraged to try to correct the interference by one or more of the following measures:
1. Reorient the receiving antenna.
2. Relocate the instrument with respect to the receiver.
3. Plug the instrument into a different outlet (if so equipped) so that the instrument and
receiver are on different branch circuits.
If necessary, the user should consult LI-COR or an experienced radio/television technician
for additional suggestions.
The Federal Communications Commission has prepared a booklet entitled "How to
Identify and Resolve Radio-Television Interference Problems" which may be helpful to
you. This booklet (stock # 004-000-00345-4) is available from the Superintendent of
Documents, U.S. Government Printing Office, Washington, D.C. 20402.

iii
NOTICE
The information contained in this document is subject to change without notice.
LI-COR MAKES NO WARRANTY OF ANY KIND WITH REGARD TO THIS MATERIAL,
INCLUDING, BUT NOT LIMITED TO THE IMPLIED WARRANTIES OF MERCHANT-
ABILITY AND FITNESS FOR A PARTICULAR PURPOSE. LI-COR shall not be liable for
errors contained herein or for incidental or consequential damages in connection with the
furnishing, performance, or use of this material.
This document contains proprietary information which is protected by copyright. All rights are
reserved. No part of this document may be photocopied, reproduced, or translated to another
language without prior written consent of LI-COR, Inc.
Copyright 1990, LI-COR, Inc.
______________________________________________________________
IBM is a registered trademark of International Business Machines Corp.
PC-TALK is a trademark of Freeware.
ProComm is a trademark of Data Storm Technologies.
Printing History
New editions of this manual will incorporate all material since the previous editions. Update
packages may be used between editions which contain replacement and additional pages to be
merged into the manual by the user.
The manual printing date indicates its current edition. The printing date changes when a new
edition is printed. (Minor corrections and updates which are incorporated at reprint do not cause
the date to change).
1st Printing - April, 1990
2nd Printing - October, 1991
3rd Printing - March, 1996

iv
Table of Contents
Section I. Unpacking and Inspection
1.1 What's What ...................................................................................... 1-1
1.2 Checking the Batteries ...................................................................... 1-3
Section II. Setup and Operation
2.1 Setup and Operation ......................................................................... 2-1
2.2 Power On ........................................................................................... 2-3
2.3 Gelman Filter Installation.................................................................. 2-4
2.4 Absolute vs. Differential Mode Operation ........................................ 2-5
Section III. Theory Of Operation
3.1 General Description .......................................................................... 3-1
3.2 Calculating Gas Concentration - General .......................................... 3-2
3.3 Calculating CO2Concentration ......................................................... 3-6
3.4 Calculating H2O Concentration ........................................................ 3-6
3.5 Pressure Broadening Due to Water Vapor......................................... 3-8
3.6 Dilution Corrections .......................................................................... 3-11
3.7 Cris Unknown. Measuring CrAgainst Cs= 0 ................................. 3-12
Section IV. Calibration
4.1 Calibration - General Information ..................................................... 4-1
4.2 Differential Mode Calibration ........................................................... 4-2
4.3 Absolute Mode Calibration ............................................................... 4-5
4.4 H2O Absolute Mode Caution ............................................................ 4-5
4.5 Pressure Corrections .......................................................................... 4-9
4.6 Setting the Zero and Span in Software .............................................. 4-9
4.7 Using the LI-670 Flow Control Unit ................................................. 4-12

v
Section V. System Software
5.1 Using the Keypad .............................................................................. 5-1
5.2 Software Overview ............................................................................ 5-2
5.3 Console Commands ........................................................................... 5-3
Section VI. Interfacing
6.1 General Information ......................................................................... 6-1
6.2 RS-232C Data Transfer ..................................................................... 6-2
6.3 Analog Output ................................................................................... 6-7
6.4 Auxiliary Channel Inputs................................................................... 6-13
6.5 Temperature Output ........................................................................... 6-14
6.6 Using the 6262-03 Pressure Transducer ............................................ 6-15
Section VII. Maintenance
7.1 Recharging the 6000B and 6200B Batteries ..................................... 7-1
7.2 Opening the LI-6262 ....................................................................... 7-1
7.3 Internal Soda Lime/Desiccant ........................................................... 7-2
7.4 External Soda Lime/Desiccant ......................................................... 7-4
7.5 Fan Filter ........................................................................................... 7-6
7.6 Fuses ................................................................................................. 7-6
Section VIII. Troubleshooting
APPENDIX A. Specifications
APPENDIX B. List of Suppliers
APPENDIX C. Software Command Index
APPENDIX D. Channel Code Headers
APPENDIX E. LI-6262 Console Commands
APPENDIX F. Saturation Vapor Pressure Table
APPENDIX G. Connector Descriptions
APPENDIX H. Sample Program
Warranty

Unpacking & Inspection 1-1
1
1Unpacking &
Inspection
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1.1 What's What
This procedure should be followed if you have just taken delivery on your
LI-6262. Check the packing list to verify that you have received everything
that was ordered and that you have also received the following items:
●Calibration Sheet - This data sheet contains a copy of the calibration
information entered into the analyzer at the factory. Keep it in a safe
place for future reference, or in case calibration data must be re-entered.
●Console Command Sheet - A quick reference card for entering console
commands via the LI-6262 keypad or from an external terminal.
●Spare Parts Kit - This kit contains replacement parts for your LI-6262.
As you become familiar with the analyzer you will learn which items to
keep close at hand and which items can be stored away.
●External Scrubber and Desiccant Tube - This tube is used during
normal operation. Several spare gaskets and adhesive disks have been
included for future use.
●1000-90 Data Communications and Utility Software - This software
can be used to communicate with an IBM PC/XT/AT/PS2 or
compatible. The software is provided on 3 1/2 inch diskettes.
●RS-232C Cable - (Part #392-02874) DTE to DCE, used to connect the
LI-6262 (DCE) to a DTE device such as an RS-232C printer.
●RS-232C Gender Changer - (Part #392-02977) Female-to-female
gender changer, used to connect the RS-232C cable to most computers.

Section 1
1-2 Unpacking & Inspection
There are several optional accessories available for use with the LI-6262,
including:
6262-03 Pressure Transducer - measures pressure in the sample
optical cell and its millivolt signal is used by the LI-6262 analyzer
software to calculate pressure values. CO2and H2O readings are then
automatically corrected by the software for any detected pressure
changes.
6262-04 Reference Pump - designed to purge the reference cell of H2O
during absolute mode operation. Air flow created by movement of the
chopping shutter disc is sufficient to purge the reference cell of CO2, but
it is not sufficient to remove all the water vapor in the reference cell.
For this reason, an auxiliary pump such as the 6262-04 or the LI-670
Flow Control Unit must be used when operating in absolute mode.
6000B Rechargeable Battery (provides 3.2 hours of battery life).
6200B Rechargeable Battery (provides 6.4 hours of battery life).
LI-6020 Battery Charger (92-138/184-276 VAC, 47 to 63 Hz).
1800-04 RS-232C Cable - Used to connect the LI-6262 (DCE) to a
standard IBM AT-type 9-pin RS-232C communications port).
In addition, LI-COR manufactures a portable dew point generator (LI-610),
and a flow control unit (LI-670), which are valuable accessories for
calibrating and operating the LI-6262.
LI-610 Portable Dew Point Generator
The LI-610 is an ideal field-portable system for calibrating the LI-6262, and
can also be used to provide a known water vapor source for use in environ-
mentally controlled measurement chambers.
The LI-610 is a precision instrument that provides a controlled water vapor
source of known dew point. The LI-610 has the ability to generate stable
dew points from 0 to 50 °C with high accuracy (±0.2 °C). The water vapor
source can be derived from any input air stream, including ambient air,
eliminating the need for external tanks or mixing of gases.
The dew point temperature of the air stream can be viewed on the LI-610’s
4 1/2 digit LCD display, or output to the LI-6262 directly, as an analog

Section 1
Unpacking & Inspection 1-3
signal. Analog output is linear, and the 0-50 °C dew point range is scaled
over 0-5 volts (100mV/°C).
LI-670 Flow Control Unit
The LI-670 is a valuable accessory for use with the LI-6262, which can be
used for a variety of calibration and gas exchange measurement functions.
In its simplest form, the LI-670 is used as a constant source of air flow to the
LI-6262. Independent pumps and rotameters allow variable flow rates
through the LI-6262 sample and reference cells, if desired.
For calibration of the LI-6262, soda lime and desiccant tubes provide a zero
gas source; a span gas can be swapped between cells to adjust for any zero
offset present during calibration.
The LI-670 also can be used as a power source for the LI-6262; a built-in
low battery detection circuit monitors battery life of the system. Mounting
brackets are included for attaching the LI-670 and LI-6262, making the
system completely portable.
1.2 Checking the Batteries (If purchased)
Batteries are tested and fully charged when they leave the factory, but they
may discharge during shipping. It is a good idea to test each battery to see if
it is charged. If the batteries are below 12V, they should be charged before
use. Refer to Section 7.1 for charging instructions.
NOTE: Never store batteries in a discharged state. Charge stored bat-
teries every 3 months.

Setup & Operation 2-1
2
2Setup &
Operation
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This section will describe the steps necessary for you to set up the hardware
and software in your LI-6262 to begin making measurements. Follow these
steps to configure your analyzer; where noted, the steps are described in
more detail in this and other sections.
2.1 Setup and Operation
1. Install Gelman filters.
The Gelman filters are used externally, and must be inserted in the
Sample and Reference airstreams upstream from the analyzer. See
Section 2.3, Gelman Filter Installation below.
2. Prepare a soda lime/desiccant tube, as described in Section 7.4, and
attach to the SCRUBBER holder on the back panel.
3. Absolute or Differential mode operation?
The LI-6262 can be operated in absolute or differential mode. In
absolute mode, the reference cell contains a zero concentration of CO2
and H2O. The sample cell is connected to the unknown gas to be
measured. The signal from the sample cell is compared to the zero gas
reference signal to provide absolute measurements of CO2 and H2O. In
differential mode, the reference cell contains a non-zero gas of known
concentration. The difference between absorption in the sample cell
and in the reference cell is measured. Configuring the LI-6262 for
differential or absolute mode operation requires slightly different hose
connections; see Absolute vs Differential Mode Operation below.
4. Turn the power ON.
The LI-6262 can be operated with AC or DC power. See Section 2.2,
Power On below.
5. Calibrate the analyzer as described in Section 4.

Section 2
2-2 Setup & Operation
6. Connect sample airstream to be measured.
7. Configure analyzer for data output.
Data can be obtained from the LI-6262 in several different ways; from
the display, via RS-232C output, or from one of the analog output
channels. See Section 6, Interfacing, for more information.
8. Configure the software.
All software commands are executed by pressing FUNCTION and then
entering a two-digit function code. Some codes execute a function
directly, while others access scrollable lists of options, or other
functions. Items in lists can be selected by scrolling the list with the
arrow keys and pressing ENTER.
●Enter the station barometric pressure. Press FCT, followed by 77,
and enter the barometric pressure, in kPa.
Pressure may also be input continuously using the 6262-03 Pressure
Transducer from LI-COR, or through auxiliary channel 15 with an
external pressure sensor, as described in section 6.4.
●Set the parameters for RS-232C output, if necessary. Press FCT,
followed by 17, and enter the data output parameters.
●Set the Print List (FCT 13).
Data from up to ten different channels may be sent to the RS-232C
port at a specified interval. See Section 5, FCT 13.
●Set Auto Print (FCT 14).
Enter specified interval in seconds (0.1-3600) at which to transmit
data (defined at FCT 13) to the RS-232C port. See Section 5, FCT
14.
●Set Auto Header (FCT 15).
The column header interval is the number of data lines which will
be printed between column headers. See Section 5, FCT 15.

Section 2
Setup & Operation 2-3
●Set the displays (FCT 91-99).
The LCD display has two lines, and each line can be configured to
show any channel code 21-29 (CO2), 31-39 (H2O), or 41-47
(MISC). As many as nine two-line display configurations can be
defined by the Set Displays routine. Each display is identified by a
number 1 through 9, and can be viewed by pressing the appropriate
number without pressing FCT. See Section 5, FCT 04.
9. Begin making measurements.
2.2 Power On
1. If a LI-COR rechargeable battery is being used, connect it to the
10.5-16VDC battery connector on the rear panel. The 6000B
Rechargeable Battery will provide power for approximately 3.2 hours at
25 °C, or 2.4 hours at 40 °C. The 6200B Rechargeable Battery has
twice the capacity of the 6000B. Instructions for recharging these
batteries are given in Section 7.1.
One set of battery leads with a 3-pin plug is included in your spare parts
kit for connection to a user-supplied battery (10.5 - 16VDC, 1.5 amp
maximum current required) or other DC power supply.
If AC line voltage is being used, make sure the AC VOLTAGE selector
on the back panel is set correctly (choose the 115 setting for 100-
130VAC, or the 220 setting for 200-260VAC), and plug the line cord
into the receptacle on the back panel.
2. Turn the power switch on the front panel ON. The fan will run, and
after a few seconds, the chopper motor will come up to speed. The
READY light on the front panel will come on after 1-3 minutes. (The
higher the ambient temperature, the longer it takes). Any one of the
following conditions will prevent the READY light from illuminating:
●Ambient temperature greater than about 55 °C.
●Level of CO2is too high in the reference cell (greater than about
3000 ppm).
●CO2or water vapor in the detector, caused by the internal soda
lime/desiccant bottle being exhausted.
●Span potentiometer on the front panel is defective (open).

Section 2
2-4 Setup & Operation
Low Battery Voltage
IMPORTANT: The LI-6262 has no built-in circuitry for monitoring
battery voltage. When battery voltage drops below approximately 10.5
volts, the analyzer will shut down, and data transmission will stop. It is
therefore suggested that battery voltage be monitored regularly to avoid
system shutdown.
You may wish to monitor battery voltage with an external voltmeter, or use a
timer with an alarm to alert the user to possible system shutdown. If you
intend to operate the LI-6262 with battery power for more than 3 hours with
the 6000B (6 hours with the 6200B), it is recommended that a user-supplied
battery with a larger storage capacity be connected using the battery leads
included with the LI-6262, or that several batteries be used in parallel for
longer operation.
2.3 Gelman Filter Installation
IMPORTANT: Always install the external air filters before operating
the LI-6262. Insert filters into both the SAMPLE and REFERENCE
airstreams before they enter the LI-6262. Failure to do this will lead to
contamination of the sample and reference optical paths, which can
cause large zero offsets that may require expensive repairs.
The Gelman polypropylene filters included use PTFE filter elements with a
1 µm pore size. They have excellent particle retention, very low water
sorption, little flow restriction and small internal volumes. These filters will
not fit inside the LI-6262 case, and must be used externally.
In absolute mode, insert a filter between the scrubber tube outlet (top) and
the REFERENCE IN port (Figure 2-3). Use a jumper tube to connect the
REFERENCE OUT port to the TO CHOPPER port. The scrubber tube inlet
(bottom) is connected to the FROM CHOPPER port. Air supplied to the
sample cell must also pass through a filter before it enters the LI-6262.
In differential mode, place a filter in the sample airstream before it enters the
SAMPLE IN port on the LI-6262. Place a second filter in the reference
airstream before it enters the REFERENCE IN port.

Section 2
Setup & Operation 2-5
Replace the filters when flow rates drop due to particle retention, or when
the apparent LI-6262 response to changes in humidity becomes slow due to
filter retention of hygroscopic material.
When using the LI-6262 in high flow rate applications (e.g. eddy correlation)
or where sample air is particularly dirty, you may consider stacking two
filters in series. For maintenance, replace the filter furthest upstream from
the analyzer with the filter closest to the analyzer and replace the filter
closest to the analyzer with a new filter.
µ
P
a
r
t
#
9
9
6
7
-
0
0
8
I
N
1
µ
m
P
T
F
E
I
N
Air In
Air Out
Install the new filter(s) with the blue lettering facing away from the
SAMPLE IN or REFERENCE IN ports.
Instruments with serial numbers of IRG3-239 and below were shipped with
internal Balston filters. Balston filters have good particle retention with low
flow resistance, but they equilibrate slowly with changing humidities, thus
degrading the apparent response time and performance of the LI-6262.
Replace the Balston filters with external Gelman filters as described above.
We recommend that the SAMPLE IN and REFERENCE IN ports be covered
with the dust caps provided or attached to an airline when the instrument is
not in use. This will prevent dust from entering the instrument downstream
from the filters where it can enter the optical paths.
Replacement Gelman Acro 50 filters can be obtained from LI-COR (part #
9967-008).
2.4 Absolute vs. Differential Mode Operation
The LI-6262 may be operated in either absolute or differential modes to
simultaneously measure CO2and H2O in any units. The following
discussion will help you to set up the analyzer to measure these outputs.

Section 2
2-6 Setup & Operation
Prepare a soda lime/desiccant tube, as described in Section 7.4, and attach to
the SCRUBBER holder on the back panel.
Differential Mode Operation
●Orient the scrubber tube so that the soda lime is on the bottom and the
magnesium perchlorate is on top.
●Attach the bottom hose (nearest the soda lime) to the FROM CHOPPER
fitting.
●Attach the top hose (nearest the perchlorate) to the TO CHOPPER fitting
(Figure 2-1).
Figure 2-1. Hose connections for operation in differential mode.
IMPORTANT:Attach the hoses as shown in Figure 2-2. The hose
nearest the soda lime is attached to the FROM CHOPPER fitting, and
the hose near the magnesium perchlorate is attached to the TO
CHOPPER fitting. This ensures that CO2is purged first, and then water
vapor. If this sequence is reversed, the air will pick up water vapor
from the soda lime before being returned to the chopper housing.

Section 2
Setup & Operation 2-7
To chopper
From chopper
Soda Lime
Fiberglass Wool
Magnesium
Perchlorate
Figure 2-2. Hose attachments to external soda lime/desiccant tube assembly.
Absolute Mode Operation
IMPORTANT: Air flow created by movement of the chopping shutter
disc is sufficient to purge the reference cell of CO2, but is not sufficient
to remove all of the water vapor in the reference cell. For this reason an
auxiliary pump such as the 6262-04 must be used when operating in
absolute mode. See Section 4.4 for more information.
●Orient the scrubber tube so that the soda lime is on the bottom and the
magnesium perchlorate is on the top.
●Attach the bottom hose (nearest the soda lime) to the FROM CHOPPER
fitting. An auxiliary pump should be placed between the FROM
CHOPPER fitting and the bottom hose of the scrubber tube. Make sure
the pump is oriented to pump in the direction indicated in Figure 2-3.
●Attach the top hose (nearest the perchlorate) to the REFERENCE IN
fitting.
●Attach a jumper hose between the REFERENCE OUT and TO CHOPPER
fittings.

Section 2
2-8 Setup & Operation
SAMPLE
IN
REFERENCE
IN
SAMPLE
OUT
REFERENCE
OUT
TO
CHOPPER
FROM
CHOPPER
Mg(ClO4)2
SODA LIME
PUMP
Filter
Figure 2-3. Hose connections for operation in absolute mode.

Theory of Operation 3-1
3
3Theory of
Operation
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3.1 General Description
The LI-6262 is a differential, non-dispersive, infrared (NDIR) gas analyzer.
The CO2and H2O measurements are based on the difference in absorption
of infrared (IR) radiation passing through two gas sampling cells. The
reference cell is used for a gas of known CO2or H2O concentration, and the
sample cell is used for a gas of unknown concentration. Infrared radiation is
transmitted through both cell paths, and the output of the analyzer is
proportional to the difference in absorption between the two.
Sample
Reference
Motor
Chopping Shutter
Source Feedback Photodiode
Lens
Lens
Dichroic Beam
Splitter
Thermoelectric
Cooler
Thermoelectric
Cooler
H
2
O Detector
H
2
O Filter
CO
2
Detector
CO
2
Filter
The infrared source is vacuum sealed for long life (> 10,000 hours) and high
stability. A separate optical feedback circuit with a photodiode maintains the
source at a constant color temperature (1250 °K). A gold reflector
surrounding the IR source maximizes the radiation output from the source
and decreases the power required.
The chopping shutter disc is spun by a motor, whose shaft turns on high
precision bearings for long life and low noise. Rotation is precisely
controlled at 500 hertz on a phase locked loop circuit. The optical path
between the source and optical bench is sealed and continuously purged of
CO2and H2O by an attached tube containing soda lime and magnesium
perchlorate. This eliminates interference due to ambient CO2or H2O vapor.

Section 3
3-2 Theory of Operation
The sample cells are gold-plated to enhance IR reflection and resist
tarnishing over time. One set of sample cells is used for both H2O and CO2
measurements by using a dichroic beam splitter to provide radiation to two
separate detectors.
A 150 nm bandpass optical filter is used to tune the CO2detector to the 4.26
micron absorption band for CO2, and a 50 nm bandpass optical filter tunes
the H2O detector to the 2.59 micron absorption band. Both filters provide
excellent rejection of IR radiation outside the desired band, allowing the
analyzer to reject the response of other IR absorbing gases. The filters are
mounted directly on the detectors for thermal stability.
Each detector is a lead selenide solid state device that is insensitive to
vibration. The detectors are cooled and regulated to -5 °C by thermo-
electric coolers, and electronic circuits continuously monitor and maintain a
constant detector sensitivity. The result of this detector circuitry is a detector
system that is very stable.
Infrared radiation from the source is focused through the gas cells and onto
the detector by a lens at each end of the optical bench. Focusing the
radiation maximizes the amount of radiation that reaches the detector in
order to provide maximum signal sensitivity.
All of these features provide a CO2noise level that is typically 0.3 ppm
peak-to-peak (at 350 ppm) when using 1 second signal averaging, and 1 ppm
peak-to-peak with 0.1 second signal averaging; the H2O noise level is
approximately 0.002 kPa at 2.0 kPa when using 1 second signal averaging,
and 0.006 kPa when using 0.1 second signal averaging. Signal averaging
times are selectable (in software) between 1 and 30 seconds and are used to
achieve even lower noise levels. For example, with 4 seconds of signal
averaging, the noise levels typically decrease 50%.
3.2 Calculating Gas Concentration - General
LI-COR gas analyzers use a lead selenide detector that operates
approximately as a linear quantum counter; that is, over much of its range
the detector signal output νis proportional to the number of photons
reaching the detector. The output voltage V that is used to compute CO2
mole fraction is proportional to the difference between the signals generated
by the detector when it sees the sample cell (νs) and when it sees the
reference cell (νr).

Section 3
Theory of Operation 3-3
V = k(νr- νs)3-1
The analyzer operates in such a way as to keep νrconstant, so we can factor
νrout of the quantity in parentheses, define K = kνr, and obtain,
VK s
r
=−





1υ
υ3-2a
In absolute mode, the instrument is operated with no CO2in the reference
cell so νris proportional to maximum photon throughput. The sample cell
normally contains CO2, which reduces the photon flux reaching the detector
through the sample cell and reduces νs. Therefore, the ratio νs/νrgives the
ratio of photon flux in the presence of CO2and in its absence, which is just
the transmittance τ. Since absorptance A equals 1-τ, we can also write
equation 3-2 as
V = K(1 - τ)3-2b
and
V = KA 3-2c
The constant K is given on the calibration sheet.
Equation 3-2c indicates that the analyzer output voltage is proportional to
absorptance; however, absorptance is a non-linear function of CO2mole
fraction. Absorptance and analyzer output voltage both increase with
increasing CO2mole fraction in the sample cell. Figure 3-1 illustrates a
typical relationship between gas concentration, transmittance (νs/νr) and
analyzer output voltage.
1.2
1.0
0.8
0.6
0
(A)
Gas Conc.
νs
νr
(B)
Gas Conc.
V (volts)
V= K(1- τ)
F(V)
0
Figure 3-1. (A) The ratio of detector output while viewing the sample cell (νs) and
the reference cell (νr) decreases with increasing gas concentration. (B) The output
voltage V is related to the ratio of νsto νr by Equation (3-2a). Gas concentration in
terms of V is the calibration function F(V).