LumaSense Technologies INNOVA 1314i - Manual (Page 134)

Chapter 12
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BE6030-13 1314i Photoacoustic Gas Monitor LumaSense Technologies A/S
Page 134 of 199
12.2 What is Calibration and Why is it Necessary?
In order to understand calibration, it is necessary to look at what
happens inside the analysis cell during a gas concentration
measurement. A sample of air is drawn into the cell. The cell is then
sealed off and light is sent from the infra-red source via a chopper
(to pulsate it) through an optical filter. The optical filter only
transmits light in a defined wavelength range and this light enters
the cell.
If there is a gas in the cell, which absorbs light of this wavelength, a
pressure wave is created and this is measured by the microphones
mounted within the cell. The greater the concentration of the
absorbing gas in the cell, the greater the pressure (sound) wave it
creates.
How do we measure the relationship between the measured sound
signal and the concentration of the absorbing gas in the cell? We
calibrate the optical filter.
Our description of what happens in the cell is, however, a little too
simplified. In reality, we have to take three other factors into
consideration: namely: (1) cell noise; (2) the presence of water
vapour in the sample (humidity interference); and (3) the possible
presence of other interferent gases.
Fig.12.1 High resolution absorption spectrum of water vapour
Cell Noise
When there is zero-gas in the cell (that is, a gas which does not
absorb any infra-red light) a signal is measured in the cell. This
signal is due to what is termed cell noise. Cell noise is created by
the imperfect reflection of infra-red light from the cell walls. It is
dependent upon the properties of the cell itself (for example, its
dimensions and the reflectivity of its walls) as well as the wavelength
(and thus energy) of the infra-red light which is incident on its walls.