Teledyne 9110T Auto Zero Warnings, Dirty reaction cell can cause high Auto Zero values

Model 9110T NOx Analyzer Troubleshooting & Service

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.

Teledyne Analytical Instruments 282