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12.1.3
CONSIDERATION OF CROSS-SENSITIVITIES
The selectivity of the above-mentioned measurement method is based on the great susceptibility
of oxygen to other gases (see table).
The following examples are intended to show how cross-sensitivities can be taken into account in
zero point calibration.
Example 1:
Determining the residual oxygen content in 100% carbon dioxide
(CO
) inert gas atmosphere at 20 ° C.
2
From the table for CO
during calibration with nitrogen, the zero point must be set to 0.27% in order to compensate for the
display declination in good approximation.
Since in this example there is only an atmosphere consisting of CO
can be eliminated easily by using carbon dioxide (CO
(N2),
Example 2:
Determining the oxygen content of a gas mixture at 20 °C.
1 vol% C
H6 (ethane);
2
5 vol% O
;
2
40 vol% CO
;
2
54 vol% N
.
2
Zero point calibration with nitrogen (N
The cross-sensitivity values in the above table are based on 100% by volume of the corresponding
gas. Therefore a conversion to the actual volume concentration has to be made. In general:
Actual cross-sensitivity =
For the components of the gas mixture the following values result:
C
H6 :
-0.0043% by volume;
2
CO2 :
-0.1080% by volume;
N
:
0.0000 Vol. %
2
=
-0.1123% by volume
To determine as accurately as possible the actual total cross sensitivity, a correction factor must be
determined, since the sum of the cross sensitivities does not relate to 100% but to 100% minus the
oxygen concentration (here 95%).
9-3.20-MD
at 20° C, a value of -0.27 can be read for cross-sensitivity. This means that
2
).
2
Table value x volume concentration
Gas sampling and gas conditioning technology
) for zero point calibration instead of nitrogen
2
100
and O
, the interference effect
2
2
[Vol.%]
15
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