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what is "back pressure"
Posted: Fri Jan 25, 2008 4:24 pm
by firework1
what is "back pressure"? I understand it is a measurement of the pressure in an HPLC system. I suppose it measures the fluid pressure right after pump but before column, is it right? Why is it called BACK pressure? Is there a "front pressure", or is there any other pressures that matters? What do we call the difference in pressure before column and after column?
Posted: Fri Jan 25, 2008 5:43 pm
by Bruce Hamilton
It's the resistance to flow - the pushing back...
It's called backpressure because it's established by the column/lines and depends on the fluid viscosity, flow rate, temperature etc., not directly contralled by the pump setting, as the pressure is a consequence of the actual pump setting for the system resistance to flow.
If the pump went to 1000 psi and changed the flow to always maintain that same pressure downstream the system would be forward pressure controlled, but the chromatography would be rather variable...
You can put "backpressure" regulators ( even simple relief valves ) after the detector which maintain a constant pressure in the upstream detector to prevent bubbles, regardless of pump flowrate.
I hope that's not too confusing...
Please keep having fun,
Bruce Hamilton
Posted: Fri Jan 25, 2008 6:50 pm
by firework1
That's very clear, thanks
It's the resistance to flow - the pushing back...
It's called backpressure because it's established by the column/lines and depends on the fluid viscosity, flow rate, temperature etc., not directly contralled by the pump setting, as the pressure is a consequence of the actual pump setting for the system resistance to flow.
If the pump went to 1000 psi and changed the flow to always maintain that same pressure downstream the system would be forward pressure controlled, but the chromatography would be rather variable...
You can put "backpressure" regulators ( even simple relief valves ) after the detector which maintain a constant pressure in the upstream detector to prevent bubbles, regardless of pump flowrate.
I hope that's not too confusing...
Please keep having fun,
Bruce Hamilton
Posted: Mon Jan 28, 2008 7:20 am
by Mattias
Same topic but difference question:
Do different systems have different measuring points for the backpressure?
I run the same method on Agielnt 1100 and Dionex Summit system, and can see that the backpressure is always lower on the Dionex system.
Typical pressure Dionex: 45 bar
Typical pressure Agilent: 65 bar
Pressure on both systems without column is 1-2 bar
I have checked the flow rate, and it is correct. Also the retention times and the general separation "appearance" is identical. So, it really doesn't matter - just curious of the difference!