Section 4
Calibration
4-3
7.
There will likely be a small zero offset. Adjust the zero potentiometer to
read zero.
8.
To check the span, flow a different gas through the sample cell. FCT 22
should show that value. If you don’t have a second gas, you could scrub
the sample cell, and see if FCT 22 reads zero. Adjust the span as
needed.
9.
Lock the potentiometers.
H
2
O Zero and Span Calibration
It is likely that zero and reference gases containing known amounts of water
vapor will be produced by a dewpoint generator, such as the LI-610, or
measured with a dewpoint hygrometer. However, the LI-6262 requires that
the reference cell water vapor mole fraction be entered as a parameter before
differential measurements can be performed. Therefore, it will often be
necessary to convert a dewpoint temperature into a vapor pressure, and then
into mole fraction.
Vapor pressure can be obtained from dewpoint temperature by making
reference to Appendix F, or by calculation. Note that Appendix F gives
vapor pressure in millibars; these values must be divided by 10 to convert to
kilopascals.
Vapor pressure e (kPa) can be calculated as
e
T
T
d
d
=
×
+
0 61083 10
7 6448
242 62
.
.
.
where T
d
is the dewpoint temperature. The reference water vapor mole
fraction (mmol/mol) is
W
e
p
ref
=
1000
where P is the local barometric pressure in kilopascals. See Section 3.4 for
further information.
