Filter
Top Products
Instruction Manual
760007-A
July 2003
3-8 Operation Rosemount Analytical Inc. A Division of Emerson Process Management
Model NGA2000 PMD
3-6 BACKGROUND GAS COMPENSATION
Any gas having a composition other than 100% oxygen contains background gas, that is, non-oxygen
components. Sometimes, the PMD Module response to background gas is significant, depending largely
on the span and range used.
If the operator uses zero and span gases that contain the same background gas as the sample, calibration
procedures automatically compensate. No adjustments are necessary.
If the background gas in the sample is different from that in the zero and/or span gases, the operator must
take into consideration background effects to ensure correct readout. During entry of zero and span gas
values in the Calibration Gas List, the instrument is not set to indicate the true oxygen content of the zero
and span standard gases. It is set to indicate a slightly different value, relative to background gas, calcu-
lated to provide correct readout during subsequent analysis of sample gas.
Oxygen Equivalent Values of Gases
For computation of background corrections, the analyzer's response to each component of the sample
must be known. Table 3-4 on page 3-10 lists the percentage oxygen equivalent values for many common
gases. For a more comprehensive list of oxygen equivalent values, refer to a resource text such as the
Handbook of Chemistry and Physics for tables of magnetic susceptibility of substances. The percentage
oxygen equivalent of a gas can be determined by the following equation, assuming both gases are supplied
at the same pressure:
For example, if the analyzer's response to oxygen is +100%, the response to xenon would be -1.34%.
The oxygen equivalent of a gas mixture is the sum of the contribution of the individual gas components.
Example: Zero Based Range
At lower range limit (i.e., 0% O
2), composition of sample is: 80% CO2, 20% N2.
From Table 3-4 on page 3-10, the percent oxygen equivalents are: CO
2 -0.623%, N2 -0.358%.
The percent oxygen equivalent of the mixture = 0.8(-0.623) + 0.2(-0.358) = (-0.4984) + (-0.0716) = 0.570% O
2.
A
nal
y
zer Res
p
onse to Gas
Analyzer Response to O2
%O2 Equivalent of Gas = X 100%