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Section 4
Calibration
4-9
4.5 Pressure Corrections
NOTE
: This section may be disregarded if the 6262-03 Pressure Transducer
is installed.
The LI-6262 is sensitive to pressure differences between the sample and
reference cells. The span of the analyzer should be set with a span gas flow
rate equal to the flow rate at which it will be used. With the flow of the
analyzer vented to the atmosphere the effect is minimal, but if the flow from
the analyzer must pass through tubing, fittings, etc., the pressure increase
must be taken into account.
If it is not convenient or possible to have the span gas flow rate equal to the
flow rate used during measurement, the pressure effect can be characterized,
and the span set at zero flow, after complete purging of the sample cell. The
procedure for this is as follows:
1.
Connect a stable source of air to the input of the analyzer, and adjust the
flow rate to be equal to the flow rate at which measurements will be
taken.
2.
Monitor CO
2
or H
2
O on the display, and record the concentration after
the reading has stabilized.
3.
Shut the flow off, leaving this same concentration in the sample cell.
4.
Record the concentration from the display.
Correction factor
= Conc. (with flow)/ Conc. (without flow)
When setting the span, multiply the reading obtained without flow by the
correction factor, and set the displayed concentration equal to the corrected
value.
This flow/concentration relationship is linear over the entire measurement
range of the LI-6262. If another flow rate is used, a new correction will have
to be calculated.
4.6 Setting the Zero and Span in Software
FCT 08 (Test Menu) contains CO
2
and H
2
O Zero/Span functions that
provide a software method of setting the zero and span without manually
adjusting the potentiometers. There are two advantages of allowing software
zero and span adjustments; the zero and span can be set using a remote