NDIR Gas Analyzer
Calibration 5
Teledyne Analytical Instruments., - Rev. 3
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Set whether the display and output should TRACK the data
while calibrating or HOLD the last value before the calibration.
This is done in the MODE/CAL menu. See Section 4.3.
Choosing HOLD institutes a CAL DELAY before and after the
ZERO and SPAN calibrations. This is to allow for adequate
purging of the sample cell. At the end of the SPAN calibration,
the instrument will send a signal to the span gas valve to close.
If you select TRACK, make sure that you allow a period of
time after calibration for the concentration to readjust as
sample gas replaces the calibration gas in the cell. You can
determine what is an “adequate” time by watching the display
as it changes to reflect the dynamic conditions within the cell.
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The instrument is now set to automatically calibrate at the
frequency entered into the CAL TIME submenu.
NOTE:
The priority in the timing chip is low for the CAL TIME feature. Thus
the actual time between calibrations maybe different than the
specified value in CAL TIME.
5.5 Calibration
Issues
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Pressure or flow variatio
n
The detection process inherent in the patented dual chamber
balance detector essentially involves a “counting” of the
molecules of the gas of interest within the sample cell. Any
pressure or flow variation changes the instantaneous number of
gas molecules in the fixed volume cell and hence causes a span
error proportional to the pressure variation.
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Obtaining a valid difference between ZERO and SPAN
The absolute count from a given concentration of the gas of
interest will vary with time but the difference (delta) between
the zero level and same level will remain essentially constant.
Thus the need to get a valid difference between zero and span
is crucial.
In principle, the detector will zero on almost any gas however,
in practice a true zero gas such as dry nitrogen with minimum
absorption in the IR spectrum is used. Using any other gas as a
ZERO gas may cause calibration errors.
At the factory, during initial setup and calibration, a valid delta
is established using dry nitrogen and an appropriate span gas
for the application at hand. When the instrument is first