Vaisala GMP343 - Manual (Page 47)

Chapter 4 ________________________________________________________________ Operation
VAISALA_______________________________________________________________________ 45
Linearization
Due to the principle of the CO
2
absorption, the sensor of GMP343
produces a signal which is not linear in relation to the CO
2
concentration.
However, the output signal is linearized with the internal linearization
function. The user can disable the internal linearization function to achieve
a signal proportional to the absorption.
Setting the Linearization ON/OFF
LINEAR x
SAVE
x=ON or OFF (default=ON)
Example:
>linear
LINEAR : ON ?
>
>save
EEPROM saved successfully.
>
Temperature, Pressure, Relative Humidity, and
Oxygen Compensations
The measurement result of an NDIR CO
2
sensor such as the GMP343 is
proportional to the absolute number of CO
2
molecules in the active volume
of the sensor. Hence, according to the ideal gas law, the 'ppm CO
2
' output
is pressure and temperature dependent. Additionally, background gases
such as humidity and oxygen have an effect on the absorption strength of
CO
2
.
The factory calibration of GMP343 is carried out using dry N
2
and CO
2
mixtures. In other words, the relative humidity and the oxygen
concentration of the calibration gases are 0 %.
To achieve the most accurate 'ppm CO
2
' measurement, the measurement
should be compensated for T, P, % RH, and O
2
concentration in the
environment. In GMP343 these compensations are a built-in option. The
compensated output corresponds to 'ppm CO
2
' in the actual environment