Teledyne 9110TH - Nitrogen Oxides Analyzer - Manual (Page 239)

Troubleshooting & Repair Model 9110TH NOx Analyzer
Teledyne Analytical Instruments 219
re-calibrate the flow electronically using the procedure in Section 4.13.7.5, followed by
a regular review of these flows over time to see if the new setting is retained properly.
7.2.5. SAMPLE FLOW IS ZERO OR LOW BUT ANALYZER REPORTS
CORRECT FLOW
that the 9110TH/M 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 9110TH/M 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 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 the 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 9110TH/M features a orifice holder, which makes switching sample and ozone flow
orifices very easy, refer to Section 6.4.6 on how to change the sample orifices and
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. TAI 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.
7.3. CALIBRATION PROBLEMS
7.3.1. NEGATIVE CONCENTRATIONS
Negative concentration values can be caused by any of several reasons:
 A slight, negative signal is normal when the analyzer is operating under
zero gas and the signal is drifting around the zero calibration point. This is
caused by the analyzer’s zero noise and may cause reported concentrations
to be negative for a few seconds at a time down to -0.2 ppm, but should
randomly alternate with similarly high, positive values. The 9110TH/M has
a built-in Auto-zero function, which should take care of most of these
deviations from zero, but may yield a small, residual, negative value. If
larger, negative values persist continuously, check if the Auto-zero function
was accidentally turned off using the remote variables in Appendix A-2. In
this case, the sensitivity of the analyzer may be drifting negative.