Equation 6
Calculation for Transmembrane Pressure
It is recommended that water permeability be measured at three different
TMP values.
• Up to 1 barg (15 psig) for UF membranes ≤ 100 kD and
• Up to 0.65 barg (10 psig) for UF membranes over 100 kD
For example, choose the TMP values 0.3, 0.65, 1.0 barg (5, 10, 15 psig) for a
10 kD Omega membrane cassette.
Choose the highest TMP, so that the permeate flow rate does not exceed
150 LMH.
3. Quickly open and then close the retentate valve at least three times to expel any
trapped air.
4. Measure the permeate flow rate and calculate the permeate flux rate in units of
LMH (L/m
2
/hr).
5. Measure the temperature of the permeate stream.
6. Adjust the feed flow rate to the next transmembrane pressure and repeat steps
2 to 5.
7. Repeat steps 2 to 5 at the highest TMP.
8. Plot the water permeate flux rate vs. TMP for the 3 points measured.
9. Draw a straight line from the origin that best fits the data points plotted.
Note: The plot should be linear for UF membranes. If the points do not fall close to
the line, check the pressure gauges for accuracy and re-measure.
The accuracy and reproducibility of measurements are very dependent on the
accuracy and readability of the pressure gauges or transducers used. Be sure to use
calibrated equipment that is accurate within the measurement range.
10. From the graph, determine the water permeate flux rate at the transmembrane
pressure in Table 13.
Table 13
Measuring Water Permeability
Membrane NMWC TMP
Omega ≤ 100 kD 0.7 barg (10 psig)
> 100 kD 0.3 barg (5 psig)
Use pressure gauges and transmitters with a maximum range of 0 to 2 barg
(0 to 30 psig) for accurate, repeatable results.
11. Calculate the water permeability at the recorded TMP from Equation 7: Water
Permeability Calculation.
www.pall.com/biopharm 43
TMP =
P
feed + Pretentate
2
– P
permeate
( )
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