GE CGA 351, User Manual

Page: 87 / 133

Service and Maintenance 5-9
December 2004
Oxygen Reading Above
100%
An oxygen display reading greater than 100% is almost always
caused by reversed wiring of the oxygen sensor. For example, 4%
oxygen at 700 °C would generate an oxygen sensor output of +34.69
mV. However, reversed oxygen sensor wiring would cause this
reading to be –34.69 mV. The Nernst equation calculation would then
convert this voltage to an oxygen display of 109.49%!
•
To correct this problem, check the oxygen sensor wiring at
terminal block
TB2
for the correct polarity (refer to Figure 2-2 on
page 2-10).
Note:
For information regarding component locations and/or wiring
connections in an optional enclosure, see Appendix C.
Oxygen Reading Too Low
This condition is usually caused by combustibles in the sample gas,
but it may also be the result of the oxygen sensor being out of
calibration. To correct the condition, complete the following steps:
1.
The sample gas may be contaminated with combustibles from
grease or oil in the plumbing. The oxygen sensor will cause
combustibles such as CO, H
2
, or hydrocarbons in the sample to
react with oxygen, thus depleting the oxygen in the sample gas.
This will give a reading that may be less than the expected amount
of oxygen. To fix the problem, remove the source of the
contamination.
2.
If the sample gas is not contaminated with hydrocarbons,
recalibrate the oxygen sensor. See
Calibrating the Model CGA
351 on page 4-31, for the correct procedure.
Oxygen Reading Too High
This condition is most likely caused either by poor quality reference
air, a leak in the plumbing or an oxygen sensor that is too cool. A
brief description of possible solutions to this problem is presented on
page 5-10.
!WARNING!
The sensor enclosure is hot. Use heat-resistant gloves
when handling the enclosure.
IMPORTANT:
This symbol indicates Caution - dangerously hot
surfaces:
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