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Model 9110T NOx Analyzer Troubleshooting & Service
Teledyne Analytical Instruments 282
12.6.3. AUTO ZERO WARNINGS
Auto Zero warnings occur if the signal measured during an Auto Zero cycle is higher than 200 mV.
Note 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 span gas reading (e.g., 800-900 mV at
400 ppb NO) 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 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 Auto Zero could be the ozone
air filter cartridge, if its contents have been exhausted and needs to be replaced.
This filter cartridge chemicals that can cause chemiluminescence and, if saturated, these chemicals
can break through to the reaction cell, causing an erroneously high Auto Zero value (background
noise).
A dirty reaction cell can cause high Auto Zero values.
Clean the reaction cell according to Section 11.3.9.
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 12.8.4 in order to minimize the HVPS.