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Instruction Manual
245364-V
May 2002
3-10 Operation Rosemount Analytical Inc. A Division of Emerson Process Management
Model 755A
3-5 COMPENSATION FOR COMPOSITION OF
BACKGROUND GAS
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:
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%
CO
2
, 20% N
2
.
From Table 3-3(page 3-11), the %
oxygen equivalents are CO
2
. -0.623 and
N
2
, -0.358.
% oxygen equivalent of the mixture =
0.8 x (-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 Equivalent of Gas =
Analyzer Response to Gas
Analyzer Response to O
2
X 100
[100(Spl-Std)]-100(Spl-Std)
100
% O2 in Std
100