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Troubleshooting Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 236
10.3.2. Non-Repeatable Zero and Span
As stated earlier, leaks both in the GFC 7000TA and in the external system are a common source of
unstable and non-repeatable readings.
1. Check for leaks in the pneumatic systems as described in Section 9.3.3. Don’t forget to consider
pneumatic components in the gas delivery system outside the GFC 7000TA. Such as:
A change in zero air source such as ambient air leaking into zero air line, or;
A change in the span gas concentration due to zero air or ambient air leaking into the span
gas line.
2. Once the instrument passes a leak check, do a flow check (See Section 9.3.4) to make sure
adequate sample is being delivered to the sensor assembly.
3. A failing IR photo-detector may be at fault. Check the CO2 MEAS and CO2 REF test functions
via the front panel display to make sure the signal levels are in the normal range (See Appendix
A) and are quiet.
4. Confirm the sample pressure, wheel temperature, bench temperature, and sample flow readings
are correct and have steady readings.
5. Disconnect the exhaust line from the optical bench near the rear of the instrument and plug this
line into the SAMPLE inlet creating a pneumatic loop. The CO
2
concentration (either zero or
span) now must be constant. If readings become quiet, the problem is in the external pneumatics
supplies for sample gas, span gas or zero air.
6. If pressurized span gas is being used with a zero/span valve option, make sure that the venting is
adequate (See Section 3.1.2 and 5.4)
7. If it is the zero point that is non-repeatable, and if that non-repeatability seems to only occur at a
certain time of day, such as when worker occupancy is highest or lowest, make sure the flow of
purge gas to the GFC wheel housing has not stopped (See Sections 10.2.2 and 11.1.7 for more
information).
10.3.3. Inability to Span – Touchscreen SPAN Button Not Visible
1. Confirm that the carbon dioxide span gas source is accurate; this can be done by switching
between two span-gas tanks. If the CO
2
concentration is different, there is a problem with one of
the tanks.
2. Check for leaks in the pneumatic systems as described in Section 9.3.3.
3. Make sure that the expected span gas concentration entered into the instrument during
calibration is the correct span gas concentration and not too different from expected span value.
This can be viewed via the RNG Menu (See Section 6.7).
4. Check to make sure that there is no ambient air or zero air leaking into span gas line.