Teledyne 9110T instruction manual Calibration Problems, Negative Concentrations

Model 9110T NOx Analyzer Troubleshooting & Service

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 NO2 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 NOx.

∙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).

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