Flow rate problem in low pressure system

[ScottHorn]

I'm doing low pressure LC with proteins, SEC with superdex, and affinity with sepharose. Our lab has always used Lab Alliance pumps (series 1 and series 3) because they seem to be rugged an reliable. For the most part we've used them for low pressure analytical SEC. Why we buy a pump capable of operating at 5000psi when we seldom exceed 100 is beyond me. If the flow rate was off by a bit we never noticed it because it was at least constant, therefore our retention times were constant as well. Recently, since we've started using them for affinity, flow rate accuracy has become more important. If we set the software controlling the pump to tell it to load a 500 ml sample and it tries to load 550 ml, that extra 50 ml of air causes a problem. We recently noticed that one of our pumps was doing just that. We tested it by telling it to pump 5 ml/min for 8 min (40 ml). When we measured (difference in volume of buffer at the inlet reservoir), it had pumped 42.5, around a 6% excess. We then tested our other pumps (same manufacturer, series 1 and series 3) and found the same problem, all had higher flow rates than they were set for.

My question is, does the pump, which is capable of handling 5000 psi of back pressure, require a minimum amount of back pressure to give an accurate flow rate? I noticed on the qa/qc form that came with the pump, that the flow rate test was conducted at 1000 psi (flow rate was 99.5% accurate). I currently have a 20 psi back pressure regulator on the system to reduce bubbles, but removing it doesn't seem to change the flow rate problems any. Does anyone with experience with these pumps know the minimum pressure I need to use, or can you think of another reason that a few of these pumps are delivering a higher flow rate than they should?

[tom jupille]

I don't know about those specific pumps, but --

liquids are compressible (about 1% per 1,000 psi is a number that comes to mind, although I wouldn't swear to its accuracy). Pumps designed for high-pressure use typically incorporate some form of compressibility compensation. In a basic pump this may simply be a "fudge factor" on the flow rate -- in which case the flow will only be accurate at one back pressure. In a more sophisticated system, it would be dynamic, based on the measured pressure.

In either case, 6% high sounds waay out of spec. It's easy enough to confirm the manufacturer's test. Just re-run the flow rate test with a piece of capillary tubing connected to the outlet. At 5 mL/min, a meter or so of 0.010" id tubing should put you in the ball park for pressure (you can adjust the flow and/or the length of tubing as necessary).

[alwelch]

Perhaps you have an elevated supply bottle? If you have little back pressure and you elevate the supply it will feed into the system. These pumps do have a pulse dampener in them that requires some back pressure in order to work. That issue would not explain the flow rate problem.

If you still have a problem you can contact SSI and ask for service department. I am sure they can advise you how to correct the problem.

[JGK]

Have you considered fitting a flow through back pressure regulator into the system to give the pumps a back pressure to work against.

I've had normal phase methods where this has been necessary to maintain a stable flow for the system and they have worked well (fitted between the pump and Injector).

http://www.postnova.com/PDFDocuments/pr ... 009/10.pdf