Servomex SERVOPRO 4100 User Manual

A.1
APPENDIX A EFFECTS OF VARIATIONS IN SAMPLE
COMPOSITION
Oxygen is a paramagnetic gas, i.e. it is attracted into a magnetic field. Virtually all other gases
are diamagnetic, i.e. 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 +0.30 is entered as the gas zero instead
of 0.00.
If the background gas is a mixture then the proportional sums of the zero offsets are 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 = 5% 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 110°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.

Servomex SERVOPRO 4100 Specifications

General

Brand	Servomex
Model	SERVOPRO 4100
Category	Measuring Instruments
Language	English