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
oxygen to other gases (see table).
The following examples shall show how cross sensitivities can be considered for the zero calibra-
tion.
Example 1:
In the table of cross sensitivities you can read the value for CO
for calibration with nitrogen the zero point must be set to +0,27% in order to compensate the devi-
ation of the display.
In this example, the atmosphere contains exclusively CO
cross sensitivity can be eliminated without problem by using carbon dioxide (CO
gen for the zero calibration.
Example 2:
1 Vol.% C H (Ethane);
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. There-
fore, a conversion must be maid 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
determined, because the sum of cross sensitivities relates not on 100% but on 100% minus the
oxygen concentration (here 95%).
18
Determination of the rest content of oxygen in a 100% carbon dioxide
(CO ) protective atmosphere at 20°C
2
Determination of the oxygen content of a gas mixture at 20°C
).
2
Table value * Volume concentration

=
PMA20 | 1.00.01
at 20°C of –0,27. This means that
2
and O For this reason, the influence of
2 2.
100
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) instead of nitro-
2
[Vol.%]