N2 Makeup Gas Not Reaching Setpoint

[AZBiodiesel]

Hi what could cause N2 makeup gas on an agilent dual FID 6890 GC not reach setpoint? Setpoint is 20mL/min for rear detector but its showing 0.2 regardless of pressure and set point increase. Front detector makeup gas hits setpoint no problem. GC is getting proper pressure (70psi) confirmed good flow to the rear inlet ports.

I just removed FID assembly,removed jet and inspected, replaced the large FID graphite seal, re cut and inserted column, removed 1mm from Jet tip and tightened column nut ferrule. Still no flow. Is there somewhere it could be plugged?

Detector lights no problem and I am getting good chromatography, just curious. This detector is used with an on column injector if that makes a difference?

Thanks,

[James_Ball]

Hi what could cause N2 makeup gas on an agilent dual FID 6890 GC not reach setpoint? Setpoint is 20mL/min for rear detector but its showing 0.2 regardless of pressure and set point increase. Front detector makeup gas hits setpoint no problem. GC is getting proper pressure (70psi) confirmed good flow to the rear inlet ports.

I just removed FID assembly,removed jet and inspected, replaced the large FID graphite seal, re cut and inserted column, removed 1mm from Jet tip and tightened column nut ferrule. Still no flow. Is there somewhere it could be plugged?

Detector lights no problem and I am getting good chromatography, just curious. This detector is used with an on column injector if that makes a difference?

Thanks,

Can you measure any flow at the outlet with a flow meter?

Since it is reading only 0.2 on the instrument panel, then it could be that the EPC is bad. We have had to have those replaced in the past. There are a few companies that will rebuild them for a lot less than Agilent wants for new ones.

If the instrument panel was reading flow but nothing at the detector then I would suspect a broken line, usually at the makeup gas fitting. Since they measure the flow at the flow controller then a low reading is either bad flow sensor or no flow getting through the control solenoids.

[peter_tran]

Hi,

Just to correct that H2 and Makeup gas share the same tubing from EPC outlet to the FID assembly thus if your H2 reading is good one should not suspect choke or leak of that line.

I agree with James that measuring the real makeup flow at the FID exhaust is the first thing to do.
If it does have some flow rate initially then the Pressure Sensor inside the EPC would be bad (by the way the EPC uses pressure sensor, no flow sensor).
If there is no flow at all I would suspect the proportional valve inside the EPC is bad. You would probably be able to verify it by measuring the coil resistance, if you get O.L. that indicates an open coil. For 7890 model Agilent does sell the valve but I'm not sure if they do for 6890 model.

Nevertheless I would go back to basis that if you are getting good enough chromatogram then it's not worth it spending too much effort and money to have makeup gas back up. The FID is just fine to operate without any makeup gas.

Best,

Peter.