LumaSense Technologies INNOVA 1314i - Manual (Page 137)

Chapter 12
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BE6030-13 1314i Photoacoustic Gas Monitor LumaSense Technologies A/S
Page 137 of 199
Presence of Other Interferents
In many measurement situations water vapour is not likely to be the
only interferent present. Suppose that you have installed an optical
filter “A” to measure a gas (A), but another gas (B) is normally found
in the ambient air you are monitoring. If gas B absorbs some light
from filter “A”, it will “interfere” with the signal produced by gas A’s
absorption of this light. This is illustrated by the lower absorption
spectrum in Fig.12.3 (“Interference from Gas B when filter A is
used”). This interference is termed cross interference.
How is this interference measured? An optical filter “B” is chosen,
which transmits light with a wavelength which is most strongly
absorbed by gas B, and less strongly by gas A, this is illustrated by
the lower absorption spectrum shown in Fig.12.3 . During cross
interference calibration two sets of measurements are performed:
1. The signal in the cell is measured using filter “A” and the filter “B”
when a known concentration of gas A is in the analysis cell. The
ratio of these two signals is directly related to gas A’s relative
absorption of light at these two wavelengths (upper spectrum in
Fig.12.3 ). This ratio is a constant and is a measure of Gas A’s
Interference on filter “B”.
2. The signal in the cell is measured using filter “A” and filter “B”
when a known concentration of gas B is in the cell. The ratio of
these two signals is directly related to gas B’s relative absorption
of light at these two wavelengths (lower spectrum in Fig.12.3 ).
This ratio is a constant and is a measure of Gas B’s Interference
on filter “A”.
By measuring the ratios described above, the Monitor is able to
compensate any measurement made with filter “A” for any
interference signal produced by the presence of gas B in the cell, and
vice versa. This procedure is known as cross compensation.
12.3 Tasks Before Starting Any Calibration
Before starting any kind of calibration task there are several
operations, which need to be performed:
12.3.1 Obtaining a Print-out of Calibration Data in the
Monitor
Whether the Monitor is to be used online or as a stand-alone
instrument, the Calibration Software BZ7002 can be used to check
the Monitor’s existing calibration. Please refer to the User Manual
BE6034.