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Troubleshooting & Repair Teledyne API - Model 200EH/EM Operation Manual
246
11.3. CALIBRATION PROBLEMS
11.3.1. 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 -0.2 ppm, but should
randomly alternate with similarly high, positive values. The M200EH/EM 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 M200EH/EM will take 15 minutes for the
filter to clear itself, or
by an exhausted chemical in the ozone scrubber cartridge (Section
Mis-cali
bration 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 according to Section 9.3.7.
11.3.2. NO RESPONSE
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.
Check if the ozone generator is turned on. Usually, the analyzer issues a warning whenever the ozone
generator is turned off. Go to
SETUP-MORE-DIAG-ENTR, then scroll to the OZONE GEN OVERRIDE
and see if it shows ON. If it shows OFF, turn it ON and EXIT the DIAG menu. If this is done and the
ozone flow is correct, the analyzer should be properly supplied with ozone unless the generator itself is
broken. A more detailed description of the ozone generator subsystem checks are in Section 11.5.17.
Confirm the lack of response by supplying NO or NO
2
span gas of about 80% of the range value to the
analyzer.
Check the sample flow and ozone flow rates for proper values.
04521C (DCN5731)