Chromatography Forum

Hi all -

I have a question regarding setting flow rates on a 5890 GC/FID. I am attempting to use the following flow rates:

He column flow: 7 mL/min
He make-up: 20 mL/min
Hydrogen: 30 mL/min
Air: 400 mL/min

I am using an Agilent electronic flow meter (Veriflow 500), which can be set to the gas that is flowing at the time. I've noticed that if I set the flow meter to meter Helium flow it will read around 460 mL/min when all three gases are flowing. When I set the flow meter to Air, it reads around 70 mL/min with all gases flowing.

It seems surprising that the meter would show such different readings when the meter is toggled between Air and He settings. Does anyone know if this sounds normal?

Does anyone have a good procedure for setting flows using a meter like this one?

Thanks!

I am not 100% sure of this, but I think that these flowmeters measure the rate heat is transfered between the process fluid and a heating element. The setting Air/He essentially "tells" the instrument what the thermal conductivity of the fluid is in order to calculate the flow rate. Since He has a much higher thermal conductivity than air (see here: http://www.engineeringtoolbox.com/therm ... d_429.html), this is to be expected.

As to setting the gas flows, I would recommend setting up the four gases separately. Turn off the makeup from the knob in the front of the GC and measure column flow. Then, turn on the air, and adjust the total flow to 407 ml/min (400 air + 7 He from the column). Turn off the air and adjust the H2 gas in the same way. Lastly, set the makeup He flow. You can have all this straight from the horse's mouth (Agilent's mouth, that is) right here: http://www.chem.agilent.com/Library/Sup ... A16017.pdf.

Good luck!

Kostas

Thanks, Kostas! Those links are very helpful.

Yeah do them all separately and when changing gases let it flow through the meter for awhile to make sure you are measuring pure gas and not a mix of say He and H2. For Air its not a big deal because of how high the flow rate is, but you will be off 5-10 mL/min if you switch from He to H2 and try to measure immediately.