Teledyne 9110TH - Nitrogen Oxides Analyzer - Manual (Page 240)

Model 9110TH NOx Analyzer Troubleshooting & Repair
Teledyne Analytical Instruments 220
 A corruption of the Auto-zero filter may also cause negative concentrations.
If a short, high noise value was detected during the AutoZero cycle, that
higher reading will alter the Auto-zero filter value. As the value of the
Auto-zero filter is subtracted from the current PMT response, it will
produce a negative concentration reading. High AutoZero readings can be
caused by:
 a leaking or stuck AutoZero valve (replace the valve),
 by an electronic fault in the preamplifier causing it to have a voltage on the PMT
output pin during the AutoZero cycle (replace the preamplifier),
 by a reaction cell contamination causing high background (>40 mV) PMT
readings (clean the reaction cell),
 by a broken PMT temperature control circuit, allowing high zero offset (repair the
faulty PMT cooler). After fixing the cause of a high Auto-zero filter reading, the
9110TH/M will take 15 minutes for the filter to clear itself, or
 by an exhausted chemical in the ozone scrubber cartridge (Section 6.4.4).
 Miscalibration is the most likely explanation for negative concentration
values. If the zero air contained some NO or NO
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 NO
x
. The same problem occurs, if
the analyzer was zero-calibrated using zero gas that is contaminated with
ambient air or span gas (cross-port leaks or leaks in supply tubing or user
not waiting long enough to flush pneumatic systems).
 If the response offset test functions for NO (NO OFFS) or NO
X
(NOX
OFFS) are greater than 150 mV, a reaction cell contamination is indicated.
Clean the reaction cell according to Section 6.4.5.
7.3.2. 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.
 Check if the ozone generator is turned on. Usually, the analyzer issues a
warning whenever the ozone generator is turned off. Go to SETUP-
MORE-DIAG-ENTR, then scroll to the OZONE GEN OVERRIDE and
see if it shows ON. If it shows OFF, turn it ON and EXIT the DIAG
menu. If this is done and the ozone flow is correct, the analyzer should be
properly supplied with ozone unless the generator itself is broken. A more
detailed description of the ozone generator subsystem checks are in Section
11.5.17.
 Confirm the lack of response by supplying NO or NO
2
span gas of about
80% of the range value to the analyzer.
 Check the sample flow and ozone flow rates for proper values.