Instruction Manual

245364-V May 2002

3-5 COMPENSATION FOR COMPOSITION OF BACKGROUND GAS

Model 755A

Any gas having a compensation other than 100% oxygen contains background gas. The background gas comprises all non- oxygen constituents.

Although instrument response to most gases other than oxygen is comparatively slight, it is not in all cases negligible. Contribution of these components to instrument response cab be computed for each individual case.

If the downscale and upscale standard gases contain the same background gas as the sample, the routine standardization procedure automatically compensates for the background components; therefore, they introduce no error. If the background gas in the sample is different from that in the downscale and/or upscale standard gas(es), however, background effects must be taken into consideration to ensure correct readout.

During adjustment of the ZERO and SPAN controls (on the analyzer door), the instrument is set to indicate, not the true oxygen content of the downscale and upscale standard gases, but slightly different values calculated to provide correct readout during subsequent analysis of the sample gas. The calculations are explained in Section 3-5c below.

a.Oxygen Equivalent Values of Gases

For computation of background corrections, the analyzer response to each component of the sample must be shown.

Table 3-3 (page 3-11) lists the percentage oxygen equivalent values for many common gases.

The percentage oxygen equivalent of a gas is the instrument response to the given gas compared to the response to oxygen, assuming that both gases are supplied at the same pressure.

In equation form:

%O2 Equivalent of Gas =

Analyzer Response to Gas

X 100

Analyzer Response to O2

 

To select a random example from Table 3-3 (page 3-11), if analyzer response to oxygen is +100%, the response to xenon would be -1.34%.

b.Oxygen Equivalents of Gas Mixtures

The oxygen equivalent of a gas mixture is the sum of the contribution of the individual gas components.

Example: At lower range limit, i.e., 0% oxygen, composition of sample is 80% CO2, 20% N2.

From Table 3-3(page 3-11), the % oxygen equivalents are CO2. -0.623 and N2, -0.358.

% oxygen equivalent of the mixture =

0.8x (-0.623) + 0.2 x (-0.358) = (- 0.4984) + (-0.0716) = -0.570

c.Computing Adjusted Settings for Zero and Span Controls

During instrument calibration, Adjusted Values may be required in setting the ZERO and SPAN control to correct for the magnetic susceptibility of the background gas. Terms used in the equation are defined as:

Standard Oxygen equivalent of background gas in the standard. Refer to Table 3-3 (page 3-11).

Special Oxygen equivalent of background gas in the sample. Refer to Table 3-3 (page 3-11) for values.

Calibration and measurement must be made at the same pressure in the detector cell unless the user makes the compensation referred to in Section 3-8, page 3-14.

Adjusted value = % O2 in Std +

% O2 in Std

100[100(Spl-Std)]-100(Spl-Std) 100

3-10

Operation

Rosemount Analytical Inc. A Division of Emerson Process Management