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Teledyne API Model 200A NO
X
Analyzer Instruction Manual, 02246, Rev. G, DCN 5247
9.2.4 Negative Output
1. Mis-calibration. The 'zero' gas that was used to zero the M200A contained some NO gas -
that is, it had more NO gas than that of the sample air. May also be caused by doing a zero
calibration using ambient air. If NO/NOx OFFSET - TEST functions are greater than
150 mV, reaction cell contamination is indicated.
2.
Corruption of the AutoZero filter. If a significant signal was detected during the AutoZero
cycle, that higher reading can enter the AutoZero filter. The value of the AutoZero filter is
subtracted from the current reading, thus producing a negative reading. High AutoZero
readings can be caused by:
A.
Leaking AutoZero valve.
B.
Electronic fault in the preamp causing it to have a voltage on the PMT output pin during
the AutoZero cycle.
C.
Reaction cell contamination causing high background ( >40 mV) light readings.
D.
Broken PMT temperature control circuit, allowing high zero offset.
After fixing the cause of the high AutoZero filter readings, the M200A will take 15
minutes for the filter to clear itself.
3.
Check for leaks.
4.
Check for used up zero air canister, if the instrument has the IZS option and the canister is
being used to zero the instrument.
9.2.5 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 9.3.8.5.
4.
Defective electronic components on preamp board. - use optic test and electric test to
check electronics, optics and observe noise.
5.
Contamination of ozone generator and/or reaction cell - This can be wet air or impurities.
This can be detected by high PMT readings with zero air as sample gas. Verify this condition
by turning off the ozone generator using the DIAG mode command and observing a drop in
PMT reading of more than 50 mV or 25 ppb. If the ozone generator or reaction cell is
contaminated, disassemble and clean.
6.
Broken PMT temperature control circuit. Check PMT TEMP - TEST function.
7.
Mis-calibration. Check NO/NOx SLOPES in TEST function.
8.
Reaction cell pressure too high. Check RCEL - TEST function.
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