Chromatography Forum

When I checked split flow of varian 3800 by flow meter it is not same as the amount calculated by software. Usually the measured amount is higher than calculated. I checked column dimensions and septum purge but all of them were correct.also it seems no any leak in EFC parts What could be reason of such difference? a

How did you do the measurement ?

What were the measured, and the calculated split flow ?

NB that the total flow shown on the front panel is column flow + septum purge + split.

Was the oven turned on ?

Peter

flow measured by electronic flowmeter and actualy it is gas comes from gc to out by EFC exit. calculated flow shown by GC software as total flow. for an example measured flow is 160 ml/min while calculated flow is 125 ml/min. spetum purge is about 3 and cloum flow 1.8

How do you know that the electronic flow meter is accurate ? When was it last calibrated, and is it calibrated for the gas you are trying to measure ? Do you have an old-fashioned bubble flow meter that you can use instead ?

Peter

Hi!,

where you measure the flow?

You should calibrate the EFC and septum purge.

The computed split rate on the GC assumes that you have given the correct column geometry and the column is installed in the inlet. And, it assumes that vacuum correction has been set to match the installation of the column. (I don't know the term Varian wojuld use for this correction. If the column outlet is atmospheric pressure, the vacuum compensation is off.) And the correct gas must be specified in the GC configuration.

If you have removed a bit of the column - that can affect the calculation. A few CM from the end of a column, you can usually ignore. But loss of a couple of meters needs to be reflected in the value in the instrument configuration.

Measurement of flow with a flow meter must be done correctly. There needs to be a good seal between the outlet of the GC you are measuring and the tube leading into the flow meter. If your trying to measure the flow coming out of a column by pinching a rubber tube around the end of the column - it will give you an accurate reading. There will be flow along the end of the column and out into the room no matter how hard you squeeze the tube. (I've tried it.) Typically the flow from a GC column is measured through the detector with an adapter that gives a tight seal into the detector outlet and with the tube to the flow meter.

And the system should be leak free.

Measurement of gas flow out to the split vent and septum purge also need good connections. Add empting to push the tubing from the flow meter over a threaded fitting does not work. And, I don't know the fittings on the Varian instrument.

And, as Peter notes - electronic flow meters are sensitive to the type of gas being measured. So be sure that it is set for the correct gas.