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Model 9110T NOx Analyzer Troubleshooting & Service
Teledyne Analytical Instruments 280
Verify proper venting of the pump exhaust.
Span gas overflow is not properly vented and creates a back-pressure on the sample inlet port.
Also, if the span gas is not vented at all and does not supply enough sample gas, the
analyzer may be evacuating the sample line.
Ensure to create and properly vent excess span gas.
Diffusion of oxygen into Teflon-type tubing over long distances.
PTFE or related materials can act as permeation devices. In fact, the permeable membrane
of NO
2
permeation tubes is made of PTFE.
When using very long supply lines (> 1 m) between high concentrations span gases and the
dilution system, oxygen from ambient air can diffuse into the line and react with NO to form
NO
2
.
This reaction is dependent on NO concentration and accelerates with increasing NO concentration,
hence, affects linearity only at high NO levels.
Using stainless steel for long span gas supply lines avoids this problem.
12.5.7. DISCREPANCY BETWEEN ANALOG OUTPUT AND DISPLAY
If the concentration reported through the analog outputs does not agree with the value reported on the
front panel, you may need to recalibrate the analog outputs.
This becomes more likely when using a low concentration or low analog output range.
Analog outputs running at 0.1 V full scale should always be calibrated manually.
See Section 5.9.3.2 for a detailed description of this procedure.
12.5.8. DISCREPANCY BETWEEN NO AND NOX SLOPES
If the slopes for NO and NO
X
are significantly different after software calibration (more than 1%),
consider the following three problems:
NO
2
impurities in the NO calibration gas. NO gases often exhibit NO
2
on the order of 1-2% of the
NO value.
This will cause differences in the calibration slopes. If the NO2 impurity in NO is known, it
can easily be accounted for by setting the expected values for NO and NO2 accordingly to
different values, e.g., 448 ppb NO and 450 ppb NOX.
This problem is worse if NO gas is stored in a cylinder with balance air instead of balance
gas nitrogen or large amounts of nitrous oxide (N2O).
The oxygen in the air slowly reacts with NO to yield NO2, increasing over time.
The expected concentrations for NO and NO
X
in the calibration menu are set to different values.
If a gas with 100% pure NO is used, this would cause a bias.
See Section 9.2.3.1 on how to set expected concentration values.