Map the pressure taps first
Identify module inlet, concentrate outlet and permeate outlet. Record each tap elevation and whether its sensor reads gauge or absolute pressure. Never combine the two scales directly. Readings must describe the same point in the operating cycle; an inlet reading during filtration cannot be paired with a permeate reading during backwash.
In pressurised dead-end filtration, feed pressure minus permeate pressure is useful where axial loss is small. Follow the module manufacturer when this assumption does not hold. Pressure losses through a remote valve or upstream filter should not accidentally become part of the membrane pressure calculation.
Crossflow example with permeate backpressure
Assume inlet pressure is 2.0 bar, concentrate pressure 1.4 bar and permeate pressure 0.3 bar, all measured at the same elevation. Approximate mean TMP is (2.0 + 1.4)/2 − 0.3 = 1.4 bar. Axial pressure loss is separately 0.6 bar and provides different diagnostic information.
These figures demonstrate arithmetic rather than an operating recommendation. Ignoring permeate backpressure would produce 1.7 bar and distort calculated permeability. At constant production, investigate whether a pressure change comes from fouling, the permeate valve or a change in pressure at the receiving tank.
Allow for liquid head in suction UF
For a submerged module, pressure outside the membrane depends on tank level. It is not necessarily zero gauge pressure. One metre of water head is approximately 0.098 bar. With a membrane 1.5 m below the surface and permeate at −0.20 bar, an idealised calculation gives approximately 0.347 bar TMP.
A real installation needs the elevation reference specified for its module and allowances for suction-line losses. Record level, temperature and density if it differs materially from water. Start-up transients, trapped air and reverse flow require operating-mode flags rather than being treated as stable filtration readings.
Check the signal used by the controller
Compare sensors under a known condition and check their zero, range and units. Trend TMP together with specific flux, temperature and cycle state. Calculate permeability using the matching TMP and then apply the approved temperature correction; correcting unrelated measurements will not recover meaningful performance data.
A blocked pressure tap can create a plausible but incorrect trend. Maintain the taps and cross-check them against a verified instrument. Before changing control settings, document the controller formula and confirm that its sign during filtration and cleaning agrees with the hydraulic drawing and commissioning records.
Common questions
Is TMP the same as differential pressure?
Only when the same two sides are specified. Inlet-to-concentrate pressure drop measures axial loss; TMP acts across the membrane towards the permeate.
Can TMP rise without fouling?
Yes. Tank level, backpressure, temperature and sensor errors can change the reading. Check these variables before assigning a fouling diagnosis.