GasCheck
Operating Manual V2.4
2
2.
PRINCIPLE OF OPERATION
Many gases, including CO
2
, absorb energy in the infrared band. This
absorption is selective and occurs at specific frequencies corresponding to the
resonant frequencies of bond vibrations within the molecule. Measuring a
characteristic absorption enables the gas to be detected and the strength of the
absorption gives a measure of the gas concentration.
The GasCheck sensor optics consists of an infrared source, a sampling cell, an
infrared filter and a detector. In addition, the sensor includes electronics to
drive the source and to process the signals from the detector.
source drive
electronics
sample cell
IR source
detector
signal processing
electronics
Figure 2.1
Schematic of the GasCheck sensor
The infrared source is the hot filament of a tungsten lamp emitting a broad
band of radiation, which is then guided through the gas in the sample cell. The
infrared then passes through a filter before reaching the detector. The filter
selects a very narrow band of frequencies corresponding to a characteristic
absorption band for CO
2
. The amount of energy reaching the detector falls as
the CO
2
concentration increases. Because the absorption of infrared by the gas
is non-linear, the output signal is also non-linear.
