Ametek CEM/Humox, User Manual

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Overview
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1-3
tion of the ratio of the two oxygen partial pressures. If the oxygen partial pres -
sure of one gas is known, the voltage produced by the cell indicates the oxygen
content of the other gas. A reference gas, usually air (20.9% O
2
), is used for one
of the gases.
Since the voltage of the cell is temperature dependent, the cell is maintained at a
constant temperature. Some newer high temperature insitu models use the heat from
the process to heat the sensor, and the process temperature is continuously measured
and used in the software calculation. The oxygen content is then determined from
the Nernst equation:
where R and F are constants, T is absolute temperature, and O
1
and O
2
are the
oxygen partial pressures on either side of the cell.
For measuring oxygen in non-combustibles gases, the calibration of an analyzer is
obtained from the formula:
Where A is a constant, T is the cell temperature on an absolute scale (°C + 273)
and O
2
Unk% is the unknown oxygen concentration of the gas to be analyzed, and
which is calculated by the analyzer.
The cell produces zero voltage when the same amount of oxygen is on both sides,
and the voltage increases as the oxygen concentration of the sample decreases. The
voltage created by the difference in the sample gas and the reference air is carried by
a cable to the microprocessor control unit, where it is linearized to an output signal.
Because of the high operating temperature of the cell, combustible gases
that are present may burn. When this occurs, the display to indicate
less oxygen than is actually in the gas (net oxygen content).
The moisture calculations are listed below:
% moisture in flue gas - C + [1-(Ow/Od)] * 100%
Ow - Wet O
2
in % by volume
Od - Dry O
2
in % by volume
C - % residual moisture by volume calculated using the cooler temperature
NOTE

E = In
RT
4F
O
1
O
2
AT = 48.0 at 695°C
AT = 45.8 at 650°C
E = A*T*Log
20.9%
O
2
Unk%