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Model 9110T NOx Analyzer Troubleshooting & Service
Teledyne Analytical Instruments 275
When supplying sample, zero or span gas at ambient pressure, a high flow would indicate that one or
more of the critical flow orifices are physically broken (very unlikely case), allowing more than nominal
flow, or were replaced with an orifice of wrong specifications.
If the flows are within 15% higher than normal, we recommend measuring and recalibrating the flow
electronically using the procedure in Section 10, followed by a regular review of these flows over
time to see if the new setting is retained properly.
Also, check the flow assembly o-rings and replace as needed.
12.4.1.4. Sample Flow is Zero or Low but Analyzer Reports Correct Flow
Note that the 9110T analyzer can report a correct flow rate even if there is no or a low actual sample
flow through the reaction cell.
The sample flow on the 9110T is only calculated from the sample pressure and critical flow condition
is verified from the difference between sample pressure and vacuum pressure.
If the critical flow orifice assembly is partially or completely clogged, both the sample and vacuum
pressures are still within their nominal ranges (the pump keeps pumping, the sample port is open to
the atmosphere), but there is no flow possible through the reaction cell.
Although measuring the actual flow is the best method, in most cases, this fault can also be diagnosed by
evaluating the two pressure values.
Since there is no longer any flow, the sample pressure should be equal to ambient pressure, which
is about 1 in-Hg-A higher than the sample pressure under normal operation.
The reaction cell pressure, on the other hand, is significantly lower than under normal operation,
because the pump no longer has to remove 500 cm³/min of sample gas and evacuates the reaction
cell much better.
Those two indicators, taken together with a zero or low actual flow, indicate a clogged sample
orifice.
The 9110T features a new orifice holder, which makes switching sample and ozone flow orifices very
easy; refer to Section 11.3.10 on how to change the sample orifices and to Appendix B for part numbers
of these assemblies.
Again, monitoring the pressures and flows regularly will reveal such problems, because the pressures
would slowly or suddenly change from their nominal, mean values. Teledyne recommends to review all
test data once per week and to do an exhaustive data analysis for test and concentration values once per
month, paying particular attention to sudden or gradual changes in all parameters that are supposed to
remain constant, such as the flow rates.