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Constant flow v/s Constant pressure
Posted: Sat Sep 25, 2010 5:22 pm
by Rhodium
What are the basic guidelines for using constant flow v/s constant pressure on an Agilent 6890?
Posted: Sun Sep 26, 2010 1:38 am
by Don_Hilton
My rule of thumb is easy. Unless I need to follow a method that specifies constant pressure, I use constant flow. I try to run columns close to the optimum linear velocity - or just a bit above that. If the low rate drops - as happens with temperture programs and contant head pressure - the linear velocity may drop below optimum. And the van Deemter curve is much less forgiving at low flow than high flow.
Posted: Mon Sep 27, 2010 2:54 pm
by willnatalie
I agree, constant flow is better.
Posted: Tue Sep 28, 2010 4:16 am
by aldehyde
yeah
Posted: Tue Sep 28, 2010 7:26 pm
by larkl
I worked in one lab that ran a LOT of methods in constant pressure. Problem was, they used a lot of 100 meter capillary columns and temperature program methods. With constant flow, the head pressure has to increase as the temperture increases. You run into a limit if you have a very restrictive column, like the long ones.
Either one will work. There have been many, many samples run in constant pressure for many years before electronic flow controllers were developed.