By Erin Miller
Updated Mar 24, 2022
Transmembrane pressure (TMP) is the differential pressure across a membrane, expressed in psi or pascals. It indicates the force needed to push the feed—water or any liquid to be filtered—through the membrane. A low TMP suggests a clean, efficient membrane, while a high TMP signals fouling and reduced performance. The optimal TMP range is specified by the membrane manufacturer.
Insert the pressure transducer’s sensing tip into the feed stream outside the membrane. Record the displayed pressure; this is the feed pressure.
Place the transducer in the retentate reservoir, between the membrane and the feed tank. Note the reading; this is the retentate pressure.
Gently insert the transducer into the permeate side, inside the membrane. Read and record the pressure; this is the permeate pressure.
Calculate TMP using the formula: TMP = – (Feed pressure + Retentate pressure)/2 + Permeate pressure or equivalently: TMP = (Feed + Retentate)/2 – Permeate.
Place the membrane assembly on a stable, level surface. Measure feed, retentate, and permeate pressures, then calculate TMP using the formula above. Express the result in psi.
Never let the transducer’s sensing tip touch the membrane directly. Direct contact can distort readings and may damage thin membranes.
