User`s guide

Appendix D-5
Reference Informations Appendix D
Flow Rate Accuracy Principle
Flow Rate Accuracy Principle
To generate the selected flow rate with high accuracy,
maintained under high pressure and for a variety of
liquids, the 305 software adds to complementary
corrections to the basic “piston flow rate”. Defined
from the piston stroke volume only, the piston flow
rate is theoretically accurate at athmospheric pres-
sure only.
The objective flow rate, F, is considered as the sum
of three components:
F = F
0
+ F
1
+ F
2
F
0
, the piston flow rate decreases when pressure
increases;
F
1
, the compensation flow rate for the liquid com-
pressibility, increases with pressure;
F
2
, the compensation flow rate for all others factors,
also increases with pressure.
The piston flow rate, F
0
, is defined by:
F
0
= N
0
V
S
with N
0
, number of piston cycles per unit time; and
V
S
, piston stroke volume.
The compensation flow rate for the liquid compress-
ibility, F
1
, is calculated as a function of five variables:
F
1
= f
1
(F
0
, V
S
, V
D
, P, X)
where V
D
is the volume of the dead space inside the
compression chamber; P, the operating pressure;
and X, the compressibility of the liquid under the
pressure P.
In the 305 software, X is calculated using the simpli-
fied Tait equation:
X = c
P + d
Coeeficient c varies only slightly with the nature of
the liquid. It is a constant included in the software.
Coefficient d is calculated from the Liquid Com-
pressibility at athmospheric pressure, X
0
(for P = 0),
entered by the user as a “set-up” parameter. Values
of X
0
for some common solvents are tabulated in
the previous page.
Operating pressure, P, is continuously transmitted
to the pump by the Manometric Module (pressure
feedback).