Model 100E Instruction Manual TROUBLESHOOTING & REPAIR
045150102 Rev XB1 205
11.3. Calibration Problems
11.3.1. Negative Concentrations
Negative concentration values can be caused for several things:
• 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 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. Clean the sample chamber according to Section 9.3.4.
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,
• 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, see Section 6.7.5. 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, see Section 6.7.6. 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.
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