LumaSense Technologies INNOVA 1412i - Manual (Page 138)

Chapter 12
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BE6025-15 1412i Photoacoustic Gas Monitor LumaSense Technologies A/S
Page 138 of 202
Monitor. The signal in the cell is measured with the water vapour
filter and with all the other installed optical filters.
The water vapour in the cell absorbs light from the water-vapour
filter and from the other installed filters, producing signals which are
related to water-vapour’s relative absorption of the light at the
wavelengths transmitted by the different optical filters (see
Fig.12.2 ). We know, however, that the ratio of the signal measured
with any one filter (e.g. “A”) and the signal measured with the water-
vapour filter is a constant. This ratio is related to the Humidity Gain
Factor calculated during the humidity-interference calibration of
filter “A”. For example, if the signal produced by water vapour when
using the water-vapour filter, is 160µV and using Filter “A” is 16µV,
then, if the signal measured with the water-vapour filter is found to
be 80µV, we know that this concentration of water vapour will
produce a signal of 8µV when Filter “A” is used. Measurements made
during humidity-interference calibration of the installed filters
therefore enable the Monitor to compensate any measured signal for
water-vapour’s interference.
Fig.12.3 Schematic diagram showing the absorption spectrum of gas
A and gas B. The dashed lines represent the half-power
bandwidths of the filters used to measure each gas