Net vs. Gross/Wet vs. Dry Oxygen Measurements0 pages

Application Note

POWER

Net vs. Gross/Wet vs. Dry Oxygen Measurements
BACKGROUND
When a portable oxygen analyzer is utilized to check the
calibration of an in situ oxygen analyzer, the portable analyzer
will often produce a measurement which is higher than the
measurement produced by the in situ analyzer.
Although the two readings differ, they may in fact indicate
that the in situ oxygen analyzer is properly calibrated. The
difference in readings is explained by the difference in “net”
oxygen measurement versus “gross” oxygen measurement;
and the difference between “wet” oxygen measurement
versus “dry” oxygen measurement.

Figure 1 - Net vs. Gross O2 Measurement

NET/GROSS

WET/DRY

The definition of “net” oxygen measurement is based on the
assumption that free oxygen will combine or “burn” with
combustibles in the flue gas stream. The remaining free
oxygen is considered the “net” oxygen. This “net” effect
applies to zirconium oxide sensors, such as the Model 6888 O2
Analyzer, due to the required elevated temperature necessary
for the sensor’s operation.

The definition of oxygen measurement on a “wet” or “total”
basis is based on the assumption that the moisture content
in a flue stream is not removed and is an element included
when the oxygen content is measured. The Model 6888 O2
Analyzer, along with other in situ oxygen probes, provides a
“wet” oxygen measurement.

The definition of “gross” oxygen measurement is based on
the assumption that free oxygen will NOT combine with
combustibles in the flue stream. Thus, the free oxygen is the
“gross” oxygen. The “gross” effect applies to those sensors
NOT requiring an elevated temperature for sensor operation,
including those utilized with portable oxygen analyzers.
The two methods of measuring oxygen will produce different
results. The assumption is that the difference is due solely
to the presence of combustibles. As combustibles appear in
the flue stream, “gross” oxygen measures higher than “net”
oxygen.

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43-106-300A.AO1©2011 Rosemount Analytical, Inc. All rights reserved

The definition of oxygen measurement on a “dry” basis
is based on the assumption that the moisture content
in a flue stream is removed before the oxygen content is
measured. Portable oxygen analyzers provide a “dry” oxygen
measurement.
There will be a difference in the two methods of measuring
oxygen. The assumption is that this difference is due solely to
moisture (H2O). The following relationship holds:
O2 dry = O2 wet (1/1-H2O)
As H2O appears in the flue stream, O2 dry measures higher
than O2 wet.