Enhancing the handling of standard substitution weights on a hydrostatic weighing apparatus

The hydrostatic weighing apparatus is a first-level method used to measure the density of solids and liquids. At the Portuguese Institute for Quality the hydrostatic tests for the determination of the liquids’ density implies the multiple manual placement of a set of standard substitution weights on apparatus’ balance pan, leading to undesirable perturbations in the weighing environment. Also, by this the eccentricity effects on the balance, contribute for an unwanted increase of the measurement uncertainty. To overcome these phenomena, an automated mechanism was designed and implemented, which allows the placing and recollection of the set of standard weights on the balance. To validate the new mechanism, tests to ultrapure water at 20 °C were executed before and after its implementation and the obtained results were compared. Despite a 3-fold increase in density measurement uncertainty, mainly due to the exposure of the standard substitution weights to air convection currents, the execution of hydrostatic tests was improved, not only for the fact that the measurements are now carried out quicker, but also for the fact that the operator only needs to intervene once during the whole test. Moreover, the smaller absolute deviation from the reference density value for ultrapure water at 20 °C results also in a normalised error E_n lower than 1 (0.3), thus validating the designed automated mechanism for the handling of standard substitution weights.