An apparatus for counting and/or measuring small particles suspended in a fluid medium, is provided with an improved flow cell in which the particle carrying fluid medium is fed into a flow channel through which also a particle free liquid is passed for narrowing and extending the flow of the suspension fluid within the channel which by its configuration including a section of continuously decreasing cross sectional area, allows a more precise measurement and avoids clogging of the suspension fluid inlet the diameter of which is not any longer a critical dimension.

A sheath flow type flow-cell device for plow-cytometer which comprises a first inlet for sheath fluid, a flow passage communicated with the first inlet and contracted toward downstream, the flow passage having a substantially rectangular cross section, a straight capillary flow passage connected to the flow passage downstream thereof, the capillary flow passage having a substantially rectangular cross section, a second inlet for sample fluid, a nozzle communicating with the second inlet and opened within the flow passage in the same direction as the flow direction of the straight capillary flow passage, a discharge port provided at a terminal end of the straight capillary flow passage, and flow regulating means for regulating the flow of the sheath fluid in the straight capillary flow passage to be a laminar flow having a gradient of flow velocity.

The sample air inlet nozzle into a particle counter test cell is formed with an elongated, flattened tip to provide a thin, flat laminar flow jet which intersects a laser beam. A secondary tube surrounding the nozzle provides a sheath of air which helps maintain the jet shape.

A microelectromechanical system mixes a fluid using predominantly planar laminar flow. The microelectromechanical system includes a mixing chamber and a set of valves to establish the planar laminar flow in the mixing chamber. In one embodiment, bubble-controlled pumps are operated with bubble-controlled valves to establish the predominantly planar laminar flow in the mixing chamber. The bubble-controlled pumps and valves may be used to establish a pulsed double-dipole flow field in the mixing chamber.

A compact, rugged sensor for use in the measurement of particle sizes in a fluid stream is disclosed. The sensor permits the use of a light emitting diode as a light source by mounting a high grade optical system with its focal point at the point light source provided by the light emitting diode to collimate the generated light. This collimated light passes through an aperture formed in one dimension by the width of the fluid flow stream and formed in a second perpendicular dimension by a narrow transparent slit in an otherwise opaque shield adjacent the fluid flow stream. The collimated light source, together with the use of the flow stream as a light path boundary, substantially increases the accuracy of the sensor. All of this is accomplished in conjunction with a solid state light emitting diode source which permits the sensor to be used in shock and vibration environments which would have prohibited particle measurement using prior art sensors.