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PMA45K1 | 1.00.01
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16.1.2
CONSIDERATION OF CROSS SENSITIVITIES
The selectivity of the above mentioned measuring principle is based on the high susceptibility of oxy-
gen to other gases (see table).
The following examples shall show how cross sensitivities can be considered for the zero calibration.
Example 1: Determination of the rest content of oxygen in a 100 % carbon dioxide
(CO
2
) protective atmosphere at 20 °C
In the table of cross sensitivities you can read the value for CO
2
at 20 °C of –0.27. This means that for
calibration with nitrogen the zero point must be set to +0.27 % in order to compensate the deviation of
the display.
In this example, the atmosphere contains exclusively CO
2
and O
2.
For this reason, the influence of
cross sensitivity can be eliminated without problem by using carbon dioxide (CO
2
) instead of nitrogen
(N
2
) for the zero calibration.
Example 2: Determination of the oxygen content of a gas mixture at 20°C
1 vol% C
2
H
6
(Ethan);
5 vol% O
2
;
40 vol% CO
2
;
54 vol% N
2
.
Zero point calibration with nitrogen (N
2
).
The cross sensitivity values of above table are based on 100 vol% of the respective gases. Therefore,
a conversion must be made to the effective volume concentration. In principle, the following is valid:
Table value x Volume concentration
Effective cross sensitivity =
[vol%]
100
For the components of the gas mixture, the following values are found:
C
2
H
6
: -0.0043 Vol.%;
CO
2
: -0.1080 Vol.%;
N
2
: 0.0000 Vol.%.
∑
= -0.1123 Vol.%
To determine the sum of cross sensitivity as exactly as possible, a correction factor has to be deter-
mined, because the sum of cross sensitivities relates not on 100 % but on 100 % minus the oxygen
concentration (here 95 %).
