M&C GenTwo PMA1000V2.4, Instruction Manual

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Note
If you change the P-IN set point and don’t disconnect the gas
connections, both P-IN and P-OUT will accept the same value. In this
case the gas flow rate is set to zero and the flow measurement after
this change will not reflect the true flow value.
The sample gas flow can be adjusted, when a preset gas flow is present. The correction
factor for the flow rate can also be changed in the channel specific settings list (see page
46 'Fig. 48 Channel-specific settings list')
After leaving the section M5/S2, the set points will adopt the actual values shown on the
screen. If you open this section again, the actual values and the set points will have the
same values.
11.5 Cross-sensitivity of coexisting gases
11.5.1 Cross-sensitivity of oxygen sensor (PMA2)
Oxygen is a paramagnetic gas, which means that oxygen molecules are attracted into a
strong magnetic field. This paramagnetic susceptibility distinguishes oxygen from most
other gases.
The PMC (paramagnetic measuring cell) uses this paramagnetic characteristic to measure
the concentration of oxygen in a gas mixture.
Here are two examples of coexisting gases which have an effect on the accuracy of the
oxygen concentration measurement.
„
Example 1
To determine the residual oxygen content in a 100 % carbon dioxide (CO
2
) inert gas atmo-
sphere at 20 °C [+68 °F],the values of the cross-sensitivities can be taken from the table at
the end of this chapter. A value of -0.27 can be read there for the cross-sensitivities of CO
2
at 20 °C [+68 °F]. This means that when calibrating with nitrogen, the zero point must be
set to +0.27 % to compensate for the display misalignment.
This is an example for a gas composition with CO
2
and O
2
only. To eliminate the cross-sen-
sitivity effects, we can simply use CO
2
instead of N
2
for the zero point adjustment.
„
Example 2:
To determine the oxygen content of a gas mixture at +20 °C [+68 °F], please take a look at
the following values from the table.
C2H6
(Ethane)
1 vol%
O2
5 vol%
CO2
40 vol%
N2
54 vol%