Consideration Of Cross Sensitivities - M&C PMA Series Instruction Manual

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 ) protective atmosphere at 20 °C
2
In the table of cross sensitivities you can read the value for CO
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
cross sensitivity can be eliminated without problem by using carbon dioxide (CO
(N ) for the zero calibration.
2
Example 2: Determination of the oxygen content of a gas mixture at 20°C
1 vol% C H (Ethan);
2 6
5 vol% O ;
2
40 vol% CO ;
2
54 vol% N
.
2
Zero point calibration with nitrogen (N
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:
Effective cross sensitivity
For the components of the gas mixture, the following values are found:
C H : -0.0043 Vol.%;
2 6
CO : -0.1080 Vol.%;
2
N : 0.0000 Vol.%.
2
= -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 %).
20
).
2
Table value x Volume concentration
= 
PMA45K1 | 1.00.01
at 20 °C of –0.27. This means that for
2
and O For this reason, the influence of
2 2.
100
www.mc-techgroup.com
) instead of nitrogen
2
[vol%]