Abstract: A metering device for providing fine control of the movement of a capillary tube and hence the rate of aspiration of a fluid flowing through the tube is disclosed. A reduction gear is threadedly mounted in a housing and has an internal threaded bore. An elongated member which holds the capillary tube with the aid of a vice or collet is threaded into the internal bore of the reduction gear. The number of threads per unit axial length on the outside threaded surface of the reduction gear, which surface is threaded into the housing, is different from the number of threads per unit axial length on the internal bore. This provides the reduction gear effect to thereby provide longitudinal displacement of the elongated member as well as the capillary tube in linear proportion to the rotational displacement of the reduction gear. The housing has a pin which is adapted to be inserted in a groove in a nuthead of the elongated member to maintain the elongated member rotationally stationary with respect to the housing.