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Troubleshooting & Repair Teledyne API M100E Analyzer Operation Manual
264
12.4. CALIBRATION PROBLEMS
This section provides information regarding possible causes of various calibration problems.
12.4.1. NEGATIVE CONCENTRATIONS
Negative concentration values may be caused due to the following:
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 -5 ppb, but should alternate
with similarly high, positive values.
Mis-calibration is the most likely explanation for negative concentration values. If the zero air contained
some SO
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 SO
2
. The same problem occurs, if the analyzer was zero-calibrated using ambient air
or span gas.
If the response offset test function for SO
2
(OFFSET) are greater than 150 mV, a failed PMT or high
voltage supply, or sample chamber contamination, could be the cause.
12.4.2. 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,
Confirm response by supplying SO
2
span gas of about 80% of the range value to the analyzer.
Check the sample flow rate for proper value.
Check for disconnected cables to the sensor module.
Carry out an electrical test with the ELECTRICAL TEST procedure in the diagnostics menu, refer to
Section 6.9.6. If this test produces a concentration reading, the analyzer’
s electronic signal path is
working.
Carry out an optical test using the OPTIC TEST procedure in the diagnostics menu, refer to Section
6.9.5. If this test results in a concentration signal, then
the PMT sensor and the electronic signal path are
operating properly. If the M100E passes both ETEST and OTEST, the instrument is capable of detecting
light and processing the signal to produce a reading. Therefore, the problem must be in the pneumatics,
optics or the UV lamp/lamp driver.
04515F DCN6048