GasCheck
Operating Manual V2.4
11
4.2
Temperature and Pressure correction
Because gases are compressible, there are various ways of expressing the gas
concentration: volume percent, partial pressure, density, mole density. Infrared
instruments operate by measuring the amount of infrared energy absorbed by a
sample of the gas and therefore fundamentally measure mole density (the
number of molecules in the path of the beam). Most users calibrate their
equipment against gases supplied in cylinders and therefore consider the
output to be an indication of volume concentration.
The mole density of the gas released from the cylinder depends not only on the
gas mixture in the cylinder but also on the ambient pressure and temperature.
This means that the output volts from a sensor connected to the same cylinder
will vary from day to day depending on the ambient conditions.
The sensor has built in compensation for temperature variation. Pressure
correction can be calculated from the ideal gas law, but there is also an
increase in absorption per molecule with pressure due to line broadening
effects. The sum of theses two effects is that the True %V/V is related to the
displayed value from the GasCheck by the following relationship.
(mBar)
25
.
1013
P
(mBar)
pressure
ambient
P
where
5
.
0
5
.
1
/
%
0
0
0
P
P
P
Value
Indicated
V
V
True
For example: if a GasCheck 3000ppm sensor output is 0.978 V, then the
linearisation data (Appendix1) states that the reading is 2852ppm. If the
ambient pressure is 980 mbar then the true reading can be derived:
corrected %V/V = 2852ppm x
)
25
.
1013
5
.
0
980
5
.
1
(
25
.
1013
= 3000ppm
NOTE
All units are factory set to the corrected gas
concentration on the day of the manufacture. For
maximum accuracy all on-site calibrations should also
use pressure and temperature correction.
