An apparatus for characterizing particles suspended in a liquid sample containing electrolyte including a sample chamber to hold the liquid sample and a collection chamber to hold an electrolyte solution. A wall separates the chambers and includes an aperture to allow passage of particles between the chambers; and a pair of electrodes are disposed on opposite sides of the aperture to induce a current through the aperture. A pressure source forces the sample to pass from the sample chamber into the collection chamber; and a processor measures a signal representative of electric resistance variation between the electrodes to enable determination of the size of the particle within the liquid passing through the aperture, wherein the processor is adapted to monitor two or more parameters characterizing the flow of particles through the aperture, and to detect a potential blockage of the aperture by detecting a change in any one of the parameters.