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Teledyne API Model 200AU NO
X
Analyzer Instruction Manual, 02293, Rev. F
9-28
9.2.4 Negative Output
1. Mis-calibration. The 'zero' gas that was used to zero the M200AU 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/NO
x
OFFSET - TEST functions are greater than 150
mV, reaction cell contamination is indicated. Verify quality of zero air used it should have
less than 1 ppt NO and correspondingly low equivalent values of interferent gasses .
2. Corruption of the Pre-reactor filter. If a significant signal was detected during the Pre-reactor
cycle, that higher reading can enter the Pre-reactor filter. The value of the Pre-reactor filter is
subtracted from the current reading, thus producing a negative reading. High Pre-reactor
readings can be caused by:
A. Leaking Pre-reactor valve.
B. Electronic fault in the preamp causing it to have a voltage on the PMT output pin during
the Pre-reactor cycle. Area around the high impedance section of the preamp contains
conductive dirt.
C. Reaction cell contamination causing high background ( >40 mV) light readings.
D. Broken PMT temperature control circuit, or broken PMT cooler power supply - allowing
high zero offset.
After fixing the cause of the high Pre-reactor filter readings, the M200AU will take 15
minutes for the filter to clear itself.
3. Check for leaks.
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.