Teledyne 9110EH - Nitrogen Oxides Analyzer - Manual (Page 216)

Troubleshooting & Repair Model 9110EH Instruction Manual
11.2.2. High Flow
Flows that are significantly higher than the allowed operating range (typically ±10-11% of
the nominal flow) should not occur in the M9110EH unless a pressurized sample, zero or span
gas is supplied to the inlet ports. Ensure to vent excess pressure and flow just before the
analyzer inlet ports.
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 to re-calibrate the flow
electronically using the procedure in Section 6.7.8, followed by a regular review of these
flows over time to see if the new setting is retained properly.
11.2.3. Sample Flow is Zero or Low But Analyzer Reports
Correct Flow
Note that the M9110EH 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 M9110EH is only
calculated from the sample pressure and critical flow condition is verified from the differ-
ence 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 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 M9110EH features a new orifice holder, which makes switching sample and ozone flow
orifices very easy, refer to Section 9.3.10 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.
11.3. Calibration Problems
11.3.1. Negative Concentrations
Negative concentration values can be caused for 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 -20 ppb, but should randomly alternate with similarly
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