FID fault

Hi all,

The 7890B GC shows Back detector fault. The FID is reading 5.6 E06 pA.

The GC is installed within a glove box and access is limited and maintenance cannot be performed.

Any ideas? I had recently also a failure on a trace heating controller (trace heating installed on a custom made FID chimney) and i think it might be electrical issue.

Thank you,


  • Hi

    Either FID is contaminated or electrometer / fid signal board is faulty.

    Meanwhile, you can check gases going to fid are not contaminated..

  • Hi,

    All the GC gases are pure although we had few samples...

    Can the electrometer be damaged if cold gas from liquid nitrogen boil off reaches the FID?

    Thank you,

  • One common cause of this is the electrometer is grounding somewhere.  Often it is due to the spring at the end of the interconnect assembly is not resting correctly inside the groove on the collector itself.  If you can remove the entire collector assembly and the signal drops to zero, there is a problem with the collector assembly.  Either dirty/contaminated as ISH suggests or not assembled correctly.  If the signal stays high, that points to an electronics problem, either interconnect, electrometer, or signal board.  I know you said access to the GC is limited but I think this is the easiest way to diagnose the problem.

    More details on FID maintenance, see the 7890 Maintenance manual starting on page 195:

  • Is the FID signal moving? If it's grounded somewhere e.g. the collector touching the top of the jet or the spring not seated correctly, it'll be a static high signal (usually around 9 million pA from my experience). If it's moving a bit though this is less likely and you might want to try switching the GC off and disconnecting the FID electrometer from the main board, waiting about 10 seconds and plugging it back in again. As daft as it sounds this has solved at least two issues exactly like this that I can think of.

  • Thank you all for the replies,

    The FID signal goes up and down a bit.

    Shut down and restart was my initial thought but due to the GC operation requirement I cannot shut down and cannot open the glove box until June next year. 

    Is there any way I could safely reset only the FID?

    Thank you,

  • tbh, if your signal is consistently as bad as above I'd say you won't be able to use the GC at present so the requirement for operation is mute, but that's just me. If you can get to the right side panel where the main board is located via the glove box then you can disconnect the FID electrometer from the main board whilst the GC is on but I wouldn't recommend it!

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