Ametek Thermox WDG-V, User Manual

Page: 26 / 164

2-4 | Thermox WDG-V / VC / VCM with Blow Back
For measuring oxygen in non-combustibles gases, the calibration of an
analyzer is obtained from the formula:
AT = 48.0 at 680°C E = A*T*Log
20.9%
O
2
Unk%
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 cell will gener-
ate high millivolts and cause the display to indicate less oxygen than
is actually in the gas (net oxygen content).
Hydrocarbons
When hydrocarbons are present in the gas sample, a combustion 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 exam-
ple, 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
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.
The amount of oxygen reduction is dependent on the type of hydro-
carbons present in the sample gas.
NOTE
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NOTE
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