Lodged debris detector for a particle analyzer

Abstract

A debris detector for a particle counter, such as a blood cell counter, of the COULTER (R) type. When a particle, or transient debris, passes through the sensing orifice of the particle detector, a voltage pulse occurs; whereas, when debris lodges in or against the orifice of the particle detector, a d.c. voltage shift occurs. The voltage at the orifice is capacitively coupled to an amplifier and the output of the amplifier is integrated. The capacitor coupling the orifice voltage to the amplifier causes an undershoot voltage at the trailing edge of each particle or transient debris pulse, such that a zero average voltage is applied to and, hence, from the amplifier for each voltage pulse. The integrated voltage is applied to a voltage comparator to provide an output signal therefrom whenever the integrated voltage exceeds a reference voltage. The comparator output signal is provided to a delay circuit having a delay related to the time required for a particle to pass through the COULTER detector orifice. If the comparator signal remains after the delay time, a debris alarm signal is provided. This occurs only if a d.c. voltage shift had occurred, since there is no trailing edge, and hence no undershoot to return the integrated voltage to zero prior to the delay time.