Particle sizing apparatus and method of use thereof

Apparatus for determining the size of a particle or cell within a fluid, comprising:

• a sample chamber for the fluid;
• a flow restrictor, comprising:
• an orifice,
• a pair of electrodes disposed on opposite sides of the orifice,
• measuring means for measuring a signal representative of impedance variation between the electrodes to enable determination of the size of the particle within the fluid passing through the orifice, the particle passage through the orifice causing the signal to comprise a pulse which is measured by the measuring means,
• a memory means for storing a value of width of said measured pulse, and
• means for detecting blockage of the orifice, whether partial or complete, which comprises:
• means for monitoring the signal which detects occurrences of a predetermined variation of the signal indicative of blockage of the orifice,
• wherein the means for monitoring the signal measures the width of the pulse and compares the width of the pulse with a predetermined value thereby to detect partial or complete blockage of the orifice, and
• wherein the predetermined value is determined from an average of previously measured and stored pulse widths.