Flow rate below set value

[kdallen]

I've been having problems with our Shimadzu UFLC pumps. The flow rate is below the set value ONLY when both pumps are running and ONLY when there is a column attached. If I set it at 100% A with or without a column, the flow is correct. If I set it at 100 % B with or without a column, the flow is correct. Without a column, the flow is fine no matter the A/B composition. I have sonicated the check valves and ran a ton of water, MeOH, and isopropanol through the system. I have tried it with multiple columns and all give the same result. The flow rate is ~80% of what it should be. I don't really understand how the column can decrease the flow rate...unless the increased back pressure is doing something...?

I would appreciate any suggestions!!!

[tom jupille]

Compressibility compensation not properly set? Bad pressure transducer? Leaking seals or inlet check valve?

If this were my problem I'd verify that the pressure tranducer is working (compare to a physical pressure gauge) then do a leak-down test on the pump.

[Hollow]

in additon to Tom's suggestions.

what about the compositional accuracy?
Did you run a gradient step test with one solvent having a UV absorbing compound in it e.g. Aceton 0.1%?
see http://www.lcresources.com/resources/TSWiz/hs450.htm

Test it with a restrictor capillary to get some backpressure.

[dblux_]

To eliminate volume contraction conduct your experiment with the same liquid in bottle A and B. Simply insert tubing from bottle B to A.

[uzman]

Simply , plug the outlet of the column and pump the mobile phase at 0.1 ml /min.

Stop the pump around 300 bar and wait for 2-3 minutes.

If the pressure drops more than 10% , it will give you an indication that there is a leak somewhere.