Abstract: A plugging factor (PF) monitor unit is provided for measuring the quality of the prefiltered feed water supplied to the membrane of a membrane-type desalting plant in accordance with a standard PF test. Samples from sample point selector booster pump unit are selectively diverted to the monitor after being pumped to the required pressure level. A filter membrane holder assembly receives test membranes from a continuous supply and clamps a test membrane in place so that the sample water flows therethrough to a filtrate tank used in measuring out a predetermined standard volume. A control unit controls supplying of the sample water to the monitor unit as well as discharging of the filtrate from the tank such that the time taken to measure out the standard volume is determined at two spaced time periods in accordance with the standard test. Provision is made for preventing breakage of the membrane during testing.