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Troubleshooting & Repair Teledyne API - Model 200EH/EM Operation Manual
250
Dirty or plugged critical flow orifices. Check flows, pressures and, if necessary, change orifices (Section
9.3.8).
Wrong materials in contact with samp
le - use glass, stainless steel or Teflon materials only. Porous
materials, in particular, will cause memory effects and slow changes in response.
Dirty reaction cell. Clean the reaction cell.
Insufficient time allowed for purging of lines upstream of the analyzer. Wait until stability is low.
Insufficient time allowed for NO or NO
2
calibration gas source to become stable. Wait until stability is
low.
NO
2
converter temperature is too low. Check for proper temperature.
11.4.3. AUTO-ZERO WARNINGS
Auto-zero warnings occur if the signal measured during an auto-zero cycle is lower than –20 mV or higher than
200 mV. The Auto-Zero warning displays the value of the auto-zero reading when the warning occurs.
If this value is higher than 150 mV, check that the auto-zero valve is operating properly. To do so, use
the
SIGNAL I/O functions in the DIAG menu to toggle the valve on and off. Listen if the valve is
switching, see if the respective LED on the relay board is indicating functionality. Scroll the
TST
functions until PMT is displayed and observe the PMT value change between the two valve states.
If the valve is operating properly, you should be able to hear it switch (once a minute under normal
operation or when manually activated from the
SIGNAL I/O menu), the PMT value should drop from its
nominal reading for span gas level measurements to less than 150 mV and the LED on the relay board
should light up when the valve is activated. If the PMT value drops significantly but not to less than 150
mV, the valve is probably leaking across its ports. In this case, replace the valve. If the PMT value does
not change at all, the valve is probably not switching at all. Check the power supply to the valve (12 V to
the valve should turn on and off when measured with a voltmeter).
Note that it takes only a small leak across the ports of the valve to show excessive auto-zero values
when supplying high concentrations of span gas.
Another reason for high (although not necessarily out-of-range) values for AutoZero could be the ozone
air filter cartridge, if its contents has been exhausted and needs to be replaced. This filter cartridge
(Figure 3-1) filters chemicals that can cause chemilu
minescence and, if saturated, these chem
icals can
break through to the reaction cell, causing an erroneously high AutoZero value (background noise).
A dirty reaction cell can cause high AutoZero values. Clean the reaction cell according to Section 9.3.7.
Finally, a high
HVPS voltage value may cause excess background noise and a high AZERO value. The
HVPS value changes from analyzer to analyzer and could show nominal values between 450 and 800
V. Check the low-level hardware calibration of the preamplifier board and, if necessary, recalibrate
exactly as described in Section 11.6.5 in order to minimize the
HVPS.
04521C (DCN5731)