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Instruction Manual
IM-106-880, Rev 1.0
January 2007
1-3
OCX 8800
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 of the cell is given by the following Nernst equation:
EMF = KT log10 (P
1
/P
2
) + C
Where:
1. P
2
is the partial pressure of the oxygen in the measured gas on one side
of the cell.
2. P
1
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 OCX 8800 to provide
exceptional sensitivity at low oxygen concentrations.
The OCX 8800 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.
The OCX 8800 combustibles sensor is a catalytic sensor consisting of two
Resistance Devices (RTD). One RTD is the reference element covered with
an inert coating. The other RTD element is active, coated with a catalyst. As
the sample gases flow by the sensor, the combustible gases oxidize on the
surface of the active element. The oxidation that occurs produces heat and a
temperature rise in the active element. The temperature difference produces
a resistance relationship between the two elements that is directly
proportional to the concentration of combustibles in the sample gases.