A6.2
APPENDIX 6. EFFECT OF BACKGROUND GASES
Oxygen is a paramagnetic gas, i.e. it is attracted into a magnetic field. Virtually all other gases are
diamagnetic, ie they are repelled by a magnetic field. Servomex oxygen analysers are calibrated
on a scale which is normalised for nitrogen at 0 and oxygen at 100. For high accuracy
measurements it may be necessary to introduce a zero offset into the calibration to compensate
for the background gas. The Table below presents, for many common gases, the data required
to calculate the zero offset.
For example, an analyser calibrated with nitrogen as the zero gas will, when 100% carbon dioxide
is passed through it, give a reading of ! 0.30% oxygen. If it is required to measure oxygen in
carbon dioxide then this will give an error. There are two ways to compensate for this:
1. CO
2
is used as the zero gas.
2. N
2
is used as the zero gas and the zero is offset to a value equal but opposite to the
reading produced by the background gas.
In the example above this is ! 0.30% and the value +30 is entered as the gas zero instead
of 0.00.
If the background gas is a mixture then the proportional sums of the zero offsetsare used.
E.g. For a background gas with a composition of 12% CO
2
, 5% CO, 5% n-Octane,
78% N
2
, the zero offset will be:
12% CO
2
= 12% of ! 0.30 = -0.04
5% CO = 4% of +0.07 = +0.00
5% n-Octane = 5% of ! 2.78 = 0.14
78% N
2
= 78% of 0.00 = +0.00
Total: = ! 0.18
(Where ! 0.30, +0.07 and ! 2.78 are the zero offsets of 100% carbon dioxide, carbon monoxide
and n-octane respectively relative to pure nitrogen. See following table)
In this case gas zero should be set to +0.18.
Note 1. Nitrogen dioxide exists in equilibrium with dinitrogen tetroxide. The relative proportions
vary greatly with temperature. As nitrogen dioxide is paramagnetic and dinitrogen
tetroxide is diamagnetic, the relative molar susceptibility of the equilibrium gas also varies.
The data given in the Table are for cell temperatures of either 60/C or 11/C. Neither of
these temperatures may actually be the temperature of the process.
Note 2. Servomex Application Note AP01 lists the zero offsets for a range of technically important
gases at cell temperatures of 60/C and 110/C.
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