System Description - Emerson Rosemount Oxymitter 4000 Instruction Manual

Instruction Manual
IM-106-340, Rev. 4.3
July 2017

System Description

The Oxymitter 4000 is designed to measure the net concentration of oxygen
in an industrial combustion processes 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:
1. P2 is the partial pressure of the oxygen in the measured gas on one
side of the cell.
2. P1 is the partial pressure of the oxygen in the reference air on the
opposite side of the cell.
3. T is the absolute temperature.
4. C is the cell constant.
5. 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 Oxymitter 4000 to
provide exceptional sensitivity at low oxygen concentrations.
The 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.
Oxymitter 4000
1-3