Schambeck SDS 9414I, Operation Manual, Page: 27 / 45

SDS 9414i HPLC Pump
Version: 2.2 (July 2007) Page: 27 / 45
8. COMPRESSIBILITY
A general problem for constant flow delivery with piston pumps is the compressibility of the
solvents under pressure.
Any liquid shows a specific volume elasticity. If the liquid is influenced by pressure changes
∆
p, the volume will change by
∆
V. The volume V will become the same as before, if the
pressure will be reduced to the original value again. The dimension of the change in volume is
the compressibility.
This can be shown as:
relative change of the volume
Compressibility =
the necessary pressure change
therefore:
-1
∆
V
K =
V
x
∆
p
The constant volume delivery can be seen as a stepwise displacing of liquid out of the pump
head. The dimension for the volume delivery is the amount of piston volume during a period of
time. But this is only correct as long as the delivery is done without any added pressure. As
soon as the liquid delivery has to be done against a back pressure, a specific amount of the
piston volume has to be used for compressing the liquid first, before the actual delivery can
start.
This can be described by the following diagram:
V
1
suction volume p
1
suction pressure
V
2
delivery volume
p
2
delivery pressure