pressure fluctuation on lc-8A

[jiang295]

what kind of pressure fluctuation is normal? does it depends on instrument? if so, could anybody has lc-8A system tell me what kind of pressure fluctuation you have . thanks a lot. i have 10bar fluctuation.......

[JGK]

A 10 bar (145 psi) pressure fluctuation is bad on any HPLC system. the only time I've seen any thing that bad was in a (normal phase) method with a very low column head pressure. (solved by placing a 500 psi flow restricor in line)

I'd check the mobile phases are degassed, the pump heads don't contain static bubbles and then sequentially check the solvent path up to the column head.

[jiang295]

thanks. my total pressure is 200bar at 6ml/min. is total pressure related to the fluctuation you can tolerate? the more total pressure you have, more fluctuation you will have?


and I think mobile phase, air bubble problem is already eliminated.
mobile phase degas at vacuum with ultrosonic for 30 minutes. and used 30ml/min flow rate to purge for 5minutes.

A 10 bar (145 psi) pressure fluctuation is bad on any HPLC system. the only time I've seen any thing that bad was in a (normal phase) method with a very low column head pressure. (solved by placing a 500 psi flow restricor in line)

I'd check the mobile phases are degassed, the pump heads don't contain static bubbles and then sequentially check the solvent path up to the column head.

[danko]

Hi Jiang295,

5% pressure fluctuation is not that bad. I know people that would live with 10 %.
And yes, the relative fluctuation is the relevant one in this situation. Many people work with pressurs in the range of 10 - 20 bars, so in their case it would've been a catastrophy to experiance a 10 bar fluctuation.

Best Regards

[JGK]

Is total pressure related to the fluctuation you can tolerate? the more total pressure you have, more fluctuation you will have?

In my experience, large pressure fluctuations are encountered when the operating pressure within the HPLC is low. The "back pressure" provided by the column is insufficient to "bed in" the pump seals or to correctly seat check vaves resulting in leakage around seals and through the check valves.

In standard RP-HPLC, if the operating pressure is ~2000 psi I would expect a variation of < ±1% on a well functioning system.

[danko]

Lucky you!
What brands of HPLC equipment do you have? Less than 1% fluctuation is a remarkable achievement – I’ll have to admit.

Best Regards

[jiang295]

thanks a lot for all your reply.


Today, I found that pressure fluctuation probably coming form insufficient solvent feeding from reservoirs.

flow rate is 25ml/min if i disconnect pump inlet. and my desire flow rate is around 18ml/min. Is this flow rate not enough since pump spend more time in delivering solvent than refilling solvent?

Hi Jiang295,

5% pressure fluctuation is not that bad. I know people that would live with 10 %.
And yes, the relative fluctuation is the relevant one in this situation. Many people work with pressurs in the range of 10 - 20 bars, so in their case it would've been a catastrophy to experiance a 10 bar fluctuation.

Best Regards