Panametrics CGA 351, User Manual, Page: 82 / 119

Chapter 5.
76 CGA 351 User’s Manual
5.1.3.2 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 6 on page 14).
Note: For information regarding component locations and/or wiring connections in an optional enclosure, see
Appendix C.
5.1.3.3 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
CGA 351 on page 62, for the correct procedure.
5.1.3.4 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 77.
WARNING!
The sensor enclosure is hot. Use heat-resistant gloves when handling the enclosure.
IMPORTANT:
This symbol indicates Caution - dangerously hot surfaces: