October 15, 2013

Net vs. Gross/Wet vs. Dry Oxygen Measurements

By Doug Simmers, Rosemount Analytical

Today’s AnalyticExpert blog post gives you an easy way to compare the difference in measurements from an in situ zirconium oxide O2 probe, and an extractive portable oxygen analyzer.  The portable analyzer will often produce a measurement which is higher than the measurement produced by the in situ analyzer.

The difference in readings is often explained by the difference in “net” oxygen measurement versus “gross” oxygen measurement; and the difference between “wet” oxygen measurement versus “dry” oxygen measurement.

The ZrO2 sensing cell is heated to 736oC, and utilizes platinum electrode materials.  Any combustible components in the flue gas stream will burn on the cell, consuming some oxygen in the process. 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.

Figure1The 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 typically used in portable analyzers that are NOT requiring an elevated temperature for sensor operation.

As combustibles appear in the flue stream, “gross” oxygen measures higher than “net” oxygen.

The in situ ZrO2 probe is inserted directly into the process stream, so the water vapor typically found in flue gases is part of the volume of gases being measured. The portable analyzer removes the water vapor as it is extracted. The portable analyzer will typically measure a higher O2 reading because of this moisture removal. 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.

In order to verify that the in situ analyzer is properly calibrated, a known oxygen balance nitrogen test gas can be injected into both the in situ analyzer calibration port and the portable analyzer’s input port. In this case, because no combustibles or moisture are present in the known test gas, the readings should be identical. This verification can be completed using a Rosemount Analytical Portable O2 Test Gas Kit.

In situ oxygen analyzers measure oxygen content on a “net” and “wet” basis. Portable oxygen analyzers provide oxygen measurements on a “gross” and “dry” basis. Consequently, the two may provide different readings. A known test gas can be used to verify the accuracy of both the portable and in situ analyzers.