when the membranes are flat 50 ml of suspension of calibrated polystyrene particles of diameter 0 27 0 4 or 0 64 microns marked rhone poulenc under the trademark estapor diluted to 0 1 with distilled water with addition of 1 surfactant alkylarylsulphonate trademark sinozon has 60 of the sinnova company are loaded into a cell of type amicon model 52 the amicon cell is fitted with a sample of the membrane supported on a mesh an air pressure corresponding to 20 cm of water is established the first six milliliters of filtrate are recovered for determination of the concentration cf of the calibrated particles membranes having the above characteristics are generally of synthetic material for example cellulose esters cellulose nitrate and the like regenerated cellulose polycarbonate and the like these membranes can also be based on polyether urethanes containing heparinized ammonium groups or be of acrylonitrile copolymer these membranes are advantageously reinforced by a mesh when they are in the form of flat membranes and advantageously have a thickness of between 50 and 200 microns fig4 represents an embodiment equivalent to that of fig1 but in which are shown the electrical connections of the different components to a logic unit 22 for control and monitoring the electrical leads being represented by dashed lines the unit 22 is connected to a current source not shown naturally all of the elements of the subject apparatus can be consolidated on a console or desk having castors for example to facilitate shifting the description of.