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Model 9110T NOx Analyzer Troubleshooting & Service
Teledyne Analytical Instruments 276
12.5. CALIBRATION PROBLEMS
This section describes possible causes of calibration problems.
12.5.1. 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 high, positive values.
The 9110T 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.
A corruption of the Auto Zero filter may also cause negative concentrations.
If a short, high noise value was detected during the Auto Zero cycle, that higher reading will
alter the Auto Zero filter value.
As the value of the Auto Zero filter is subtracted from the current PMT response, it will
produce a negative concentration reading.
High Auto Zero readings can be caused by
a leaking or stuck Auto Zero valve (replace the valve),
by an electronic fault in the preamplifier causing it to have a voltage on the
PMT output pin during the Auto Zero cycle (replace the preamplifier),
by a reaction cell contamination causing high background (>40 mV) PMT
readings (clean the reaction cell),
by a broken PMT temperature control circuit, allowing high zero offset
(repair the faulty PMT cooler). After fixing the cause of a high Auto Zero
filter reading, the 9110T will take 15 minutes for the filter to clear itself, or
by an exhausted chemical in the ozone cleanser (see Section 11.3.3).
Calibration error is the most likely explanation for negative concentration values.
If the zero air contained some NO or NO
2
gas (contaminated zero air or a worn-out zero air
scrubber) and the analyzer was calibrated to that concentration as “zero”, the analyzer may
report negative values when measuring air that contains little or no NO
x
.
The same problem occurs, if the analyzer was zero-calibrated using zero gas that is
contaminated with ambient air or span gas (cross-port leaks or leaks in supply tubing or
user not waiting long enough to flush pneumatic systems).