Instruction Manual

760007-A July 2003

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, calculated 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:

%O2 Equivalent of Gas =

Analyzer Response to Gas

X 100%

Analyzer Response to O2

 

 

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% O2), composition of sample is: 80% CO2, 20% N2.

From Table 3-4 on page 3-10, the percent oxygen equivalents are: CO2 -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% O2.

3-8 Operation

Rosemount Analytical Inc. A Division of Emerson Process Management