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Teledyne API Model 100AH SO
2
Analyzer Instruction Manual, 02417, Rev. D
10.2.4 Negative Concentration Display
1. Mis-calibration. The 'zero' gas that was used to zero the M100AH contained some SO
2
gas -
that is it had more SO
2
gas than that of the sample air. May also be caused by doing a zero
calibration using ambient air.
2. Broken PMT temperature control circuit, causing high zero offset. Check PMT temperature
which should be 7 ± 1°C.
3. Aging UV filter causing high stray light. Check STR. LGT reading of TEST function. It
should be less than 100 PPM.
4. Check for light leak.
10.2.5 Excessive Noise
Common reasons for excessive noise are:
1. Leak in pneumatic system.
2. Light leak - check the sensor module with strong light.
3. HVPS noisy - see HVPS test procedure. See Section 10.3.9.
4. Defective electronic components on preamp board. - use optic test and electric test to check
electronics, optics and observe noise.
5. Contamination of reaction cell and optical system - This can be wet air or impurities. This
can be detected by high PMT readings with zero air as sample gas. Clean reaction cell as
described in Section 9.5.
6. Broken PMT temperature control circuit. Check PMT TEMP - TEST function.
7. Mis-calibration. Check SLOPES in TEST function.
8. Too low UV lamp output. Should be higher than 600 mV. Replace the lamp if it is lower than
this value.
9. High stray lights - UV filter is aging.
10. Reaction cell temperature is not stable.
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