A robot for weighing syringes used in reference gas mixture preparation

Abstract

In the preparation of some types of reference gas mixtures, one of the starting materials is a liquid. The liquid is introduced in the cylinder by means of a syringe. The weighing of these syringes before and after injection is critical for the calculation of the gas composition. A robot has been developed to facilitate this weighing of syringes used in the gravimetric preparation of these gas mixtures. From the validation work, it is clear that the robot performs the measurements more reliably and with better repeatability than the manual method. The percentage of non-conforming gas mixtures dropped from approximately 5% to less than 1%. Subsequent experiments to revisit assumptions made in the preparation process have gained new insight into effects such as convection and evaporation of liquid. Additional preventive measures are taken, to further improve the weighing accuracy, and by implication, the accuracy of the gas composition data calculated from preparation. The gain in uncertainty is different for various types of gas mixtures and depends largely on the thermophysical properties of the liquid and the gas mixture being prepared.

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