Development and Performance Characteristics of a Capillary Dosage Unit with in Situ Weight Sensor for the Preparation of Known Amounts of Gaseous Voc's in Air

This paper describes the design, performance and operation of a home-made capillary dosage unit with in situ weight sensor. This continuous injection technique is developed to produce dynamic gas phase calibration mixtures of VOC's in a diluent gas at occupational hygiene levels. A new advance in the technique allows the calculation of the amounts of pollutants emitted into the carrier gas, directly from the gravimetric measurements. This is achieved by a built-in weight sensor which is monitored without interfering with the injection process. The survey of the capillary dosage unit(s) is managed by a microcomputer which automatically lists the amounts of pollutants introduced into the diluent gas. The capillary dosage technique thus becomes related directly to a primary standard. The homogeneity and stability of the test gas generated is shown to be better than 1% relative standard deviation over periods of up to 24 hours. The VOC concentrations in the manifold are known with a total uncertainty of between 2 and 3%SS.