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Troubleshooting & Service
7.3. CALIBRATION PROBLEMS
7.3.1. NEGATIVE CONCENTRATIONS
7.3.2. NO RESPONSE
232
flows regularly will reveal such problems, because the pressures would slowly or
suddenly change from their nominal, mean values. Teledyne API 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.
Negative concentration values can occur for any of 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 -0.2 ppm, but should randomly alternate with
similarly high, positive values. The T200H/M 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 AutoZero 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 AutoZero readings can be caused by:
•
a leaking or stuck AutoZero valve (replace the valve),
•
by an electronic fault in the preamplifier causing it to have a voltage on the PMT
output pin during the AutoZero 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
T200H/M will take 15 minutes for the filter to clear itself, or
•
by an exhausted chemical in the ozone scrubber cartridge (Section 6.3.4).
•
Miscalibration is the most likely explanation for negative concentration values. If the
zero air contained some NO or NO
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
. The same problem occurs, if the analyzer was zero-calibrated using zero gas
x
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
greater than 150 mV, a reaction cell contamination is indicated. Clean the reaction
cell according to Section 6.3.5.
If the instrument shows no response (display value is near zero) even though sample gas
is supplied properly and the instrument seems to perform correctly.
Teledyne API - Model T200H/T200M Operation Manual
gas (contaminated zero air or a worn-out zero
2
(NOX OFFS) are
X
07270D DCN7141
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