Teledyne 9110T - Nitrogen Oxides Analyzer - Manual (Page 282)

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.