System Description - Emerson Rosemount Oxymitter 4000 Reference Manual

Rosemount Oxymitter 4000

System Description

12
The Hazardous Area Oxymitter 4000 is designed to measure the net concentration of
oxygen in an industrial process; i.e., the oxygen remaining after all fuels have been
oxidized. The probe is permanently positioned within an exhaust duct or stack and
performs its task without the use of a sampling system.
The equipment measures oxygen percentage by reading the voltage developed across a
heated electrochemical cell, which consists of a small yttria-stabilized, zirconia disc. Both
sides of the disc are coated with porous metal electrodes. When operated at the proper
temperature, the millivolt output voltage of the cell is given by the following Nernst
equation:
EMF = KT log10(P1/P2) + C
Where:
• P2 is the partial pressure of the oxygen in the measured gas on one side of the cell.
• P1 is the partial pressure of the oxygen in the reference air on the opposite side of
the cell.
• T is the absolute temperature.
• C is the cell constant.
• K is an arithmetic constant.
NOTE
For best results, use clean, dry, instrument air (20.95% oxygen) as the reference air.
When the cell is at operating temperature and there are unequal oxygen concentrations
across the cell, oxygen ions will travel from the high oxygen partial pressure side to the
low oxygen partial pressure side of the cell. The resulting logarithmic output voltage is
approximately 50 mV per decade. The output is proportional to the inverse logarithm of
the oxygen concentration. Therefore, the output signal increases as the oxygen
concentration of the sample gas decreases. This characteristic enables the Hazardous
Area Oxymitter 4000 to provide exceptional sensitivity at low oxygen concentrations.
The Hazardous Area Oxymitter 4000 measures net oxygen concentration in the presence
of all the products of combustion, including water vapor. Therefore, it may be considered
an analysis on a "wet" basis. In comparison with older methods, such as the portable
apparatus, which provides an analysis on a "dry" gas basis, the "wet" analysis will, in
general, indicate a lower percentage of oxygen. The difference will be proportional to the
water content of the sampled gas stream.
July 2018
Description and Components