M&C TechGroup PMA 20 Operator's manual - Manual (Page 18)

18
Gas sampling and gas conditioning technology 9-3.5.1-ME
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 calibr a-
tion.
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 for the zero calibration.
Example 2:
Determination of the oxygen content of a gas mixture at 20°C
1 Vol.% C
2
H
6
(Ethane);
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 ga ses.
Therefore, a conversion must be maid to the effective volume concentration. In principle, the fol-
lowing is valid:
Table value * 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
determined, because the sum of cross sensitivities relates not on 100% but on 100% minus the
oxygen concentration (here 95%).