Teledyne 200E, Instruction Manual, Page: 214 / 302

Model 200E Instruction Manual Troubleshooting & Repair
• 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 randomly alternate with similarly
high, positive values. The M200E has a built-in Auto-zero function, which should take
care of most of these deviations from zero, but may yield a small, residual, negative
value. If larger, negative values persist continuously, check if the Auto-zero function
was accidentally turned off using the remote variables in Appendix A-2. In this case,
the sensitivity of the analyzer may be drifting negative.
• 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
M200E will take 15 minutes for the filter to clear itself, or
• by an exhausted chemical in the ozone scrubber cartridge (Section 9.3.3).
• Mis-calibration 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 9.3.9.
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.
• 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 11.5.167.
• 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.
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