Ametek Dycor CG1100 Series User Manual page 13

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; the process temperature is continu-
ously measured and used in the software calculation. The oxygen content is
then determined from the Nernst equation.
E =
where R and F are constants, T is absolute temperature and o
oxygen partial pressures on either side of the cell.
For measuring oxygen in non-combustible gases, the calibration of an analyzer
is obtained from the formula:
E = A * T * Log
where A is constant, T is the cell temperature on an absolute scale (°c + 273)
and o unk% is the unknown oxygen concentration of the gas to be analyzed
2
(calculated by the analyzer).
The cell produces zero voltage when the same amount of oxygen is on both
sides. 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 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 cell will gener-
ate high millivolts and cause the display to indicate less oxygen than
NOTE
is actually in the gas (net oxygen content).
Hydrocarbons
When hydrocarbons are present in the gas sample, an oxidation process
occurs when this gas sample is exposed to the high temperature of the
zirconia cell. An indication that hydrocarbons may be present in the gas
sample is that the oxygen reading will be lower than expected. For ex-
ample, if a calibration gas cylinder has an oxygen value of 20 ppm and 5
ppm of hydrogen (balance nitrogen), the oxygen analyzer will read 17.5
ppm of oxygen. The reduction of oxygen is due to the combustion process
where 5 ppm of hydrogen will combine with 2.5 ppm of oxygen to form
water. Thus, the oxygen analyzer will read 17.5 ppm rather than the actual
20 ppm oxygen. Note that the amount of oxygen reduction is dependent
on the type of hydrocarbons present in the sample gas.
RT O
1
In
4F O
2
20.9%
AT = 48.0 at 695°C
O Unk%
2
and o are the
1 2
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