Appendix; Detailed Test Description; Diffusion Test; Plus Filter Tester) - Sartorius Sartocheck 4 plus Operating Instructions Manual

12. Appendix

12.1 Detailed Test Description

12.1.1 Diffusion Test (Only valid for the
®
Sartocheck

4 plus Filter tester)

72 Appendix
12. Appendix
When pressure is applied to the upstream side of a wet filter, a diffuse gas stream
starts to flow through the filter membrane after a short time.
This gas diffusion can be defined as follows:
p
N = D · L ––––––– · F
d
N = Molar flow per second
D = Diffusion coefficient
L = Solubility coefficient of the test gas in the wetting fluid
p = Differential pressure available at the membrane
d = Liquid layer thickness in the membrane
F = Effective filter area.
The equation above shows that the diffusion also depends on the solubility
coefficient "L" and the effective filter area "F," among other factors.
This means that different wetting fluids and test gases will yield different diffusion
values on account of the different solubility coefficients, and extremely low diffusion
values will be obtained for small filter surfaces (e.g., disc filters).
As a result, these low diffusion values cannot be determined with sufficient accuracy
by state-of-the-art measuring technology. This is why the Diffusion Test should
not be run on disc filter systems. On the other hand, this test method is ideal for
integrity-testing medium- and large-area filter cartridge systems with approximately
2
1000 cm and above.
The diffusion of a filter element can be calculated as follows from the pressure drop,
test time, upstream system volume and the reference pressure ("p0" = 1000 mbar),
according to the German industrial standard DIN 58356, Part 2:
p · V p
1 N 1
D = – – – – – – – – – – . I – – – – – – – – – [mI/min]
n
P · t p - p
0 1
D = Diffusion in ml/min or ml/test time
p = Test pressure in mbar at the beginning of a measurement
1
p = Pressure at the end of the measurement in mbar
2
p = p – p (pressure at the end of a measurement) in mbar
1 2
V = Upstream volume of a filter system in ml
N
t = Test time in min.
P = 1000 mbar (or actual measured barometric pressure).
0
The diffusion calculated is in reference to a constant test pressure and is
independent of the system's volume so that a direct correlation with the actual
retention capability of a filter is given.