LI-COR LI7200RS, Instruction Manual

Page: 179 / 226

A Note About Pressure And Temperature
Since the instrument is calibrated for number density, accurate temperature is not
required for the calculation, and accurate pressure measurement is not required,
either (equations 8-20 and 8-26). For example, if you introduce a 1% error in the
pressure sensor on a perfectly calibrated instrument, the resulting CO
2
mole dens-
ity error would be about 0.25%, and the H
2
O mole density error about 0.5% in typ-
ical ambient conditions.
Optical cell temperature is measured by fine wire temperature thermocouples loc-
ated in the air inlet and outlet ports that measure the air temperature of incoming
and outgoing air. The cell temperature reported is a weighted average of
T
in
and
T
out
, where
8-30 T
cell
= 0.2T
in
+ 0.8T
out
at a flow rate of 12-17 lpm. In the event that one of the thermocouples (T
in
or T
out
)
should fail, the instrument will automatically ignore output from the broken ther-
mocouple and use only the functioning thermocouple to compute T
cell
.
Pressure is measured by sampling an absolute pressure sensor in the LI-7550 (Box
Pressure, kPa) and a fast differential pressure sensor in the sensor head (Head Pres-
sure, kPa). The two pressure measurements are summed together to get ambient
pressure in the optical cell (Total Pressure, kPa).
When calibrating (specifically when setting spans), temperature and pressure are
more important. Calibrating with a 1% pressure error will cause the resulting CO
2
mole density to have a 1% error, but no error in the resulting H
2
O mole density
(because the water span target is computed from dew point, not mole fraction). A
1% error in temperature (3 °C) will cause a 1% error in both CO
2
and H
2
O mole
density.
Diagnostics
CO
2
Signal Strength
The raw reference signals A
co
and A
wo
(for CO
2
and H
2
O, respectively) can be used
for diagnostic purposes. A
co
and A
wo
are insensitive to CO
2
and H
2
O con-
centrations, so if they are reduced, for example, it could be due to obstructions (dirt,
Section 8. Theory of Operation
8-7 A Note About Pressure And Temperature