Teledyne 9110T - Nitrogen Oxides Analyzer - Manual (Page 276)

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).