9-3.20-MD
Gas sampling and gas conditioning technology
15
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
2
) inert gas atmosphere at 20 ° C.
From the table for CO
2
at 20° C, a value of -0.27 can be read for cross-sensitivity. This means that
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
2
and O
2
, the interference effect
can be eliminated easily by using carbon dioxide (CO
2
) for zero point calibration instead of nitrogen
(N2),
Example 2:
Determining the oxygen content of a gas mixture at 20 °C.
1 vol% C
2
H6 (ethane);
5 vol% O
2
;
40 vol% CO
2
;
54 vol% N
2
.
Zero point calibration with nitrogen (N
2
).
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:
Table value x volume concentration
Actual cross-sensitivity =
[Vol.%]
100
For the components of the gas mixture the following values result:
C
2
H6 :
-0.0043% by volume;
CO2 :
-0.1080% by volume;
N
2
:
0.0000 Vol. %
= -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%).
