Quantitative dispenser for a liquid

Claim 1

A quantitative dispenser for a liquid comprising a pipette (3) having a downwardly directed nozzle (4) adapted to pick up and deliver a predetermined quantity of the liquid, means for moving the pipette (3) downwards to immerse the lower end of the nozzle (4) in the liquid, a liquid level sensor (10) for determining the position of the liquid surface and a control system (9) for determining the stopping point for the downward movement of the pipette (3), characterised in that the liquid level sensor (10) comprises a lens system (34,36), a point light source (31) arranged to direct a beam of light through the lens system (34) onto the surface of the liquid, and a point light sensor (32) for receiving light reflected from the surface of the liquid through the lens system (36), the control system (9) being operatively connected to the point light sensor (32) and to the means for moving the pipette (3) for stopping the downward movement of the pipette (3) at a position corresponding to that where a maximum intensity of the reflected light is detected by the point light sensor (32), the lens system comprising first and second lens placed side by side, the first lens being supplied with light from the point light source and the second lens detecting light with the point light sensor, the optical axes of the two lens being inclined with respect to each other and meeting at a crossing point, the said point being coincident with the focal point of the first lens and with the focal point of the second lens whereby light from the point light source reflected from the surface of the liquid will be detected by the point light sensor, when the surface of the liquid is at the said focal point.

Claim 4

A method for quantitatively dispensing a liquid in which a pipette (3) having a nozzle (4) is moved downwards to immerse the lower end of the nozzle (4) in the liquid, the downward movement is stopped by a liquid level sensor (10) and a control system (9), the liquid sample is taken up, the pipette (3) is withdrawn, and the sample is dispensed, characterised in that the downward movement is stopped by directing a beam of light on to the liquid surface from a point light source (31) via a first lens (34), detecting the reflected light from the surface via a second lens (36) which focuses the light onto a point light sensor (32), the first and second lens being disposed side by side, the optical axes of the two lenses being inclined with respect to each other and meeting at a crossing point, which is coincident with the focal point of the first lens and the focal point of the second lens, and stopping the downward movement of the pipette (3) at a position corresponding to that where the crossing point is positioned on the surface of the liquid and a maximum intensity of the reflected light is detected by the point-light sensor(32).