LumaSense Technologies 3434i SF6 Leak Detector - Manual (Page 137)

Chapter 12
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BE6028-14 3434i SF6 Leak Detector LumaSense Technologies A/S
Page 137 of 191
As it is the optical filter, which determines the wavelength of light,
cell noise will depend upon the optical filter being used.
The cell-noise signal is measured during the Zero-point Calibration
of each filter. A supply of dry, zero gas is attached to the air-inlet of
the Monitor and the signal in the cell is measured with the installed
optical filter (“A”). This signal is called the concentration offset
factor for the filter. Whenever the filter is used, this signal is
subtracted from the total measured signal as it is not related to the
concentration of any gas.
Presence of Water Vapour
Water vapour is nearly always present in ambient air and it absorbs
infra-red light, to a greater or lesser extent, at nearly all wavelengths
(see Fig.12.1 ). This means that no matter which optical filter is
transmitting light into the cell the water vapour in the cell will absorb
some of this light and create a signal. This signal is said to “interfere”
with the signal produced by the gas we wish to measure with this
filter.
Fig.12.2 Schematic diagram showing the relative absorption of water
vapour by different optical filters
How is this interference measured? A water-vapour optical filter,
which transmits light with a wavelength, which is absorbed by very
few gases, except water vapour, is always installed in position “W” of
the filter carousel. Using this filter, water-vapour’s interference is
measured during the Humidity Interference Calibration on the
installed filter. A supply of zero gas containing a constant
concentration of water vapour is attached to the air-inlet of the