M&C PMA Series Instruction Manual page 29

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 calibration.
Example 1: Determination of the rest content of oxygen in a 100 % carbon dioxide (CO
atmosphere at 20 °C [68 °F]
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 good approximation.
In this example, the atmosphere contains exclusively CO
sitivity can be eliminated without problem by using carbon dioxide (CO
calibration.
Example 2: Determination of the oxygen content of a gas mixture at 20 °C [68 °F]
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 the table above, are based on 100 vol% of the respective gases. A conversion
to the actual volume concentration must therefore take place. In principle, the following is valid:
Actual cross sensitivity =  [vol%]
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 total cross sensitivity as exactly as possible, a correction factor has to be determined, be-
cause the total cross sensitivity relates not to 100 % but to 100 % minus the oxygen concentration (here 95
%).
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and O
2
).
2
Table value * Volume concentration
100
PMA 50 EX | 1.01.04
at 20 °C [68 °F] of –0.27. This means that for
2
For this reason, the influence of cross sen-
2.
) instead of nitrogen for the zero
2
) protective
2
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