Aperture module for use in particle testing apparatus

An aperture module for use in particle testing apparatus including a container of particulate liquid suspension to be tested, said module comprising, a housing having a first chamber and a second chamber, an aperture holder formed of optically clear material disposed in the second chamber and having an aperture provided therein, the aperture holder having a closed end and the closed end forming a wall in the second chamber, the aperture holder extending into the container with the aperture in communication on one side thereof with the liquid suspension, a passageway in the module connecting the chambers and passing through the aperture holder, the aperture being in communication on the side opposite said one side with the passageway, a first electrode in the container and a second electrode in the first chamber to establish an electrical field in the aperture between the container and the aperture module, there being a zone in the passageway proximate to the aperture in which spurious signals may normally be produced, means for connecting the passageway at an entrance thereof to a source of particle free liquid and means for connecting the pssageway at an exit thereof to fluid moving means to move the particle free liquid through the passageway and simultaneously move the suspension from the container through the aperture into the spurious signal producing zone, means including electrical leads connected to said electrodes and adapted to extend connections to a detector to respond to electrical measuring signals produced across said electrodes with passage of particles through said aperture, the aperture holder having a projection formed on the closed end of the aperture holder and extending into the passageway to form a restriction therein proximate the aperture to cause the flow velocity of the particle free liquid to increase in the spurious signal producing zone and continuously wash the zone simultaneously with passage of the suspension through the aperture such that particles which have passed through the aperture immediately are swept out of the spurious signal producing zone by the particle free liquid and moved to the exit of the passageway, and an objective lens assembly positioned in the second chamber adjacent said well formed by the closed end of the aperture holder on the side thereof opposite that having said projection for visually examining the aperture during movement of the suspension therethrough.