How do temperature and pressure effect CO2 measurement?

CO2 gas is compressible; therefore the volume concentrations change with changing ambient atmospheric pressure and temperature. All non-dispersive infrared instruments fundamentally measure mole density (the number of molecules in the path of the beam). Most users prefer output in volume percent, so the CO2 instruments are adjusted to display this by correlating the number of molecules to a known CO2 volume concentration. The effect of changing temperature and pressure can be eliminated by applying correction.

Compensation is only needed if the measurement conditions deviate significantly from the calibration conditions, which are 1013 hPa and 25 °C.

The simplest form of correction can be applied using a formula according to the ideal gas law:

c_corrected(%/ppm) = c_measured(%/ppm)*(1013*(t(°C)+273)/(298*p(hPa)))

c_corrected     =  corrected volume concentration in% or ppm

c_measured   =  un-corrected reading

p             =  ambient pressure in hPa

t              =  ambient temperature in °C