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Section 4
4-2
Calibration
4.2 Differential Mode Calibration
It is important to remember that in differential mode the pressures on the two
sides of the analyzer must be kept the same. For this reason, never connect
the exit ports (
SAMPLE OUT
and
REFERENCE OUT
) to anything - just vent
them to the atmosphere, or to tubes of equal length. If you wish to flow the
same gas through both sides of the analyzer,
try to avoid
doing it by putting
the sample and reference cells in series, as you will likely have different
pressures. Instead, split the flow upstream of the analyzer, and run separate
hoses to the reference and sample cells. Also, be sure the flow rates through
both sides are the same.
The concentrations used to set the zero and span should bracket the range of
concentrations expected in subsequent measurements. For example, using
340 ppm for the zero and 300 ppm for the span might be the proper
selections for flow-thru photosynthesis applications where drawdowns from
ambient are expected.
There is generally a small zero shift when the reference concentration
changes. Therefore, if operating in a mode in which there are significant
changes in reference gas concentration, you should re-zero the analyzer
following each change in concentration.
CO
2
Zero and Span Calibration
1.
Set the CO
2
reference to zero (FCT 59). If you are not using the
6262-03 Pressure Transducer, enter the barometric pressure in FCT 77.
2.
Display absolute CO
2
µ
mol/mol (FCT 22).
3.
Flow a dry CO
2
-free gas through both sides of the analyzer.
4.
Unlock the CO
2
zero potentiometer and adjust until the absolute CO
2
concentration on the display reads zero.
5.
With the CO
2
-free gas on the reference side, flow a span gas through the
sample cell. Set the span potentiometer to read the span gas mole
fraction (
µ
mol/mol).
6.
Flow the span gas through both cells of the analyzer at the same flow
rate. The flow rate should match that used in the reference cell. Set the
CO
2
reference (FCT 59) to the span gas mole fraction (
µ
mol/mol).
Display differential CO
2
(FCT 23).