﻿ Varian 3900 inlet pressure issues - Chromatography Forum

## Varian 3900 inlet pressure issues

Discussions about GC and other "gas phase" separation techniques.

### Varian 3900 inlet pressure issues

Having problems with injector Pressure and Flow.

The pressure will not stay at the Desired Pressure or Desired Flow. Usually one or the other will have the ready light on for a second or two and then go off as the pressure raises or lowers.

I have changed the septum and insert. I have checked for leaks and cannot find any. Any ideas?

### Re: Varian 3900 inlet pressure issues

- Carrier gas
- Desired flow
- Pressure
- Column dimensions
- Oven temperature
- Split ratio (if split)

### Re: Varian 3900 inlet pressure issues

Carrier gas: He
Desired Flow: 114.9 mL/min
Pressure: 55 psi from cylinder
Col Dem: 30m x 25mm x .25um
Oven temp: 190
Split Ratio: 50

### Re: Varian 3900 inlet pressure issues

dkelly wrote:
Carrier gas: He
Desired Flow: 114.9 mL/min
That will make a hurricane in the column
Pressure: 55 psi from cylinder
Irrelevant parameter, provided it is sufficient, usually 5 bar is "safe"
Col Dem: 30m x 25mm x .25um
I doubt it may even have enough room in the oven
Oven temp: 190
Split Ratio: 50

### Re: Varian 3900 inlet pressure issues

dkelly wrote:
Carrier gas: He
Desired Flow: 114.9 mL/min
Pressure: 55 psi from cylinder
Col Dem: 30m x 25mm x .25um
Oven temp: 190
Split Ratio: 50

I hope that is 0.25mm diameter. If so according to my calculator which maxes out at 100ml/min flow it would take 287psi to achieve the flow, so you would need over 300psi to reach 114.9ml/min helium.

If the total flow is 114.9 with a split ratio of 50:1 then you have a column flow of 2.298 which is more achievable. At 114.9ml/min column flow and 50:1 split you would have 5745 ml/min at the split vent which would empty your helium cylinder in a day.

With a column diameter of 0.25mm and flow of 2.298ml/min you require a head pressure of 31.1psi to maintain that flow rate so it could be you need more than just 55psi from the cylinder to keep the head pressure up and flow that much to the split vent.
The past is there to guide us into the future, not to dwell in.

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