Teledyne 9110T - Nitrogen Oxides Analyzer - Manual (Page 323)

Model 9110T NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 323
During this time, only O
3
is present in the reaction cell, effectively turning off the chemiluminescence
reaction. Once the chamber is completely dark, the 9110T records the output of the PMT and keeps a
running average of these AZERO values. This average offset value is subtracted from the raw PMT
readings while the instrument is measuring NO and NO
X
to arrive at an Auto Zero corrected reading.
Figure 13-4: Pneumatic Flow During the Auto Zero Cycle
13.1.5. MEASUREMENT INTERFERENCES
It should be noted that the chemiluminescence method is subject to interferences from a number of
sources. The 9110T has been successfully tested for its ability to reject interference from most of these
sources. Table 13-1 list the most common types of interferents that could affect the performance of your
9110T.
13.1.5.1. Direct Interference
Some gases can directly alter the amount of light detected by the PMT due to chemiluminescence in the
reaction cell. This can either be a gas that undergoes chemiluminescence by reacting with O
3
in the
reaction cell or a gas that reacts with other compounds and produces excess NO upstream of the reaction
cell.