I am using an HP5890 GC and set up my system with a carrier flow of 1 ml/min measured with a bubble meter. I ran my method and saw retention times that were longer than expected. I then measured the flow rate using a methane injection and calculated my flow rate to be ~0.2 ml/min. My column is 30m x 0.25mm and methane RT was 7.35 min. Am I calculating the flow rate correctly? If so, what could be causing the difference in measured flow rates? Thanks for your help.
By Anonymous on Tuesday, May 4, 2004 - 09:21 am:
Injecting methane and measuring the retention time gives you carrier velocity. In your case I calculate 6.8 cm/sec. Running this figure through my flow rate calculator program I got a flow of 0.28 ml/min, which is only an estimate because I don't know your temperature parameters or the nature of your carrier gas. I guess I can't explain the discrepancy in your flow measurements, other than to point out that there is some dispute on the accuracy of bubble flow meters. At any rate, your velocity seems too low for most applications I am familiar with.
By Anonymous on Tuesday, May 4, 2004 - 11:13 am:
Buy an electronic flow unit, few hundred dollars. Your linear velocity is way too low, should be at least 4 times faster, increase your head pressure to at least 12 psi.
By Anonymous on Tuesday, May 4, 2004 - 01:49 pm:
methane should have a retention time of about 150 seconds to achieve 20cm/sec. About 12 psi is correct for helium.
By dlorenzo on Wednesday, May 5, 2004 - 04:31 am:
It seems that you are calculating your flow in the correct way with methane.
How do you use your bubble meter? Are you sure air and make up are closed when you are measuring flow? I suppose one of your Air, Make up or Hydrogen valves are not closing well and then you are measuring your 0.2 mL/min carrier gas and approx 0.8 mL/min of other thing.
Bubble meters are not so accurate but this is a serious difference between data obtanined from Methane and flow meter.