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6890 Back FID Flow will not go to zero.
Posted: Thu Oct 31, 2024 1:52 am
by rxnengr
Currently using a 6890 dual split/splitless inlet GC with a back FID, front TCD and 5975C MSD (with GSV). When using my 6890 in "GC-MS" mode with an ALS injection my machine performs perfectly (front inlet to MSD). However when I am using my back inlet to back FID (even with same column) I am getting terrible peaks and background noise. I initially believed that my GSV was to blame however, when doing more troubleshooting I see both inlets pass the pressure test with the septum vent capped they can hold an acceptable pressure and the GSV actuates correctly. However, when turning off the FID detector flows or simply turning off the flame, my hydrogen flow rate immediately drops to 0 ml/min but by air will drop from 450 ml/min to around 250 mL/mi, and even after 15 minutes the flow still be around 190 mL/min. Does this mean I have a dead EPC board? For what it is worth, there are no vaccum issues with the MSD.
Re: 6890 Back FID Flow will not go to zero.
Posted: Fri Nov 01, 2024 11:44 am
by Peter Apps
Your first step neds to be to check whether there really is that flow of air through the FID, using an independent flow meter.
If there is a flow your instrument flow gauge readout is working, but the control is faulty. If there is no flow then the controller is OK, but the readout is faulty - you may be able to fix that by recalibrating.
Re: 6890 Back FID Flow will not go to zero.
Posted: Fri Nov 01, 2024 2:43 pm
by Consumer Products Guy
Back in the old days, Agilent GCs came with a glass/bubble flow meter....maybe still do....
Re: 6890 Back FID Flow will not go to zero.
Posted: Fri Nov 01, 2024 9:28 pm
by Jake
I second what Peter is suggesting - you've got to measure the flow with an independent flow meter, then go from there.
Re: 6890 Back FID Flow will not go to zero.
Posted: Sat Nov 02, 2024 9:44 pm
by rxnengr
Your first step neds to be to check whether there really is that flow of air through the FID, using an independent flow meter.
If there is a flow your instrument flow gauge readout is working, but the control is faulty. If there is no flow then the controller is OK, but the readout is faulty - you may be able to fix that by recalibrating.
I just checked several setpoints with a supelco bubble meter, we are getting flow (~140 mL min matching readout, even at setpoint of 0.
Re: 6890 Back FID Flow will not go to zero.
Posted: Sun Nov 03, 2024 5:16 pm
by Peter Apps
Your first step neds to be to check whether there really is that flow of air through the FID, using an independent flow meter.
If there is a flow your instrument flow gauge readout is working, but the control is faulty. If there is no flow then the controller is OK, but the readout is faulty - you may be able to fix that by recalibrating.
I just checked several setpoints with a supelco bubble meter, we are getting flow (~140 mL min matching readout, even at setpoint of 0.
Then the flow controller is faulty and probably needs to be replaced.