Gas flow in GC

[alcorn]

Hi all

How is the total gas flow calculated in GC software? When I set column flow to 1 ml/ml and split ratio to 10, a total gas flow is 22 ml/min (septum purge is off). What other flow contributes in total flow?
I test these values in two Varian CP-3800 instruments.

Another question is about gas saver mode function:
I set total gas flow at 5 ml/min in gas saver mode (which is activated after 2 min when active method is not run). Is it possible knowing how much column flow rate is, when gas saver mode is activated and column flow is set at 1 ml/min and split ratio of 10 in active method (septum purge flow =0). How gas flow rates are set in gas saver mode? I want to my column flow rate must be at 1 ml/min in all times.

Thanks.

[Peter Apps]

Column flow depends on the pressure in the inlet and the temperature of the column oven. When the gas save is operating it maintains the inlet pressure to provide whatever flow you set, and reduces the flow of gas into the inlet to the flow that you set. Beware that at 5 ml/min quite small leaks can lead to a loss of inlet pressure because there is too little gas going into the inlet.

The total flow is controlled by a mass flow controller upstream of the inlet, which monitors flow and restricts or opens a valve appropriately. If you have set a 10:1 split and you are getting a 22 ml/min total flow with a 1 ml/min column flow the most likely causes are; your column dimensions are set wrong, you have set the wrong carrier gas, or you have selected atmospheric for the column outlet pressure when it should be at vacuum.

You need to check your settings, and use an independent flow meter to measure the flow out of the split vent. Inject an unretained substance such as lighter gas and measure its elution time to calculate the volume flow through the column.

Peter

[alcorn]

Thanks Peter

Assuming my total gas flow is correct (11 ml/min, with a column flow of 1 ml/min and split ration of 10). I want to know when the gas saver is activated (at 5 ml/min total flow) and total flow reduced from 22 to 5 ml/min, my column flow is reduced by a factor of 5/22 or split vent flow reduced or something else is occurred?

[Peter Apps]

Thanks Peter

Assuming my total gas flow is correct (11 ml/min, with a column flow of 1 ml/min and split ration of 10). I want to know when the gas saver is activated (at 5 ml/min total flow) and total flow reduced from 22 to 5 ml/min, my column flow is reduced by a factor of 5/22 or split vent flow reduced or something else is occurred?

"When the gas save is operating it maintains the inlet pressure to provide whatever flow you set, and reduces the flow of gas into the inlet to the flow that you set."

In other words split vent flow is reduced.

Peter

[MSCHemist]

There are sensors throughout the inlet side. The column flow is calculated based on the inlet pressures and the entered in column dimmensions and column outlet pressure (1 atm, vacuum, or positive pressure).