Teledyne T200, Руководство по эксплуатации, Страница: 244 / 425

Troubleshooting & Service Teledyne API
– T200 NO/NO2/NOx Analyzer Operation Manual
242
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 T200 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 T200 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).

If the response offset test functions for NO (
NO OFFS
) or NO
X
(
NOX OFFS
) are
greater than 150 mV, a reaction cell contamination is indicated.

Clean the reaction cell as described in Section 11.3.9.
06858E DCN7057