Establishing SI traceability for measurements of mercury vapour

The majority of measurements of mercury vapour, for example those to determine mass concentration in air, are currently ultimately traceable to the vapour pressure of mercury, usually via a bell-jar calibration apparatus. This allows a saturated concentration of mercury vapour in air to develop in a confined space in equilibrium with ambient conditions, from which a known mass of mercury can be removed for calibration purposes. Several empirical equations are available to describe the vapour pressure of mercury at a given temperature, but the agreement between them is not good, with data from different equations sometimes differing by 5% or more. In order to remove the dependence of mercury vapour measurement on these empirical equations, and to provide stability, comparability and coherence for mercury vapour measurements, this paper describes work undertaken to link directly mercury vapour measurements to standards of mass, and therefore to establish traceability for these measurements to the SI system of units. This has been achieved by measuring the mass output rate of a dynamic mercury vapour generator gravimetrically, and linking this to the expected mass concentration in the bell-jar apparatus. The SI traceable mercury vapour measurements have been shown to agree with the predicted output from the bell-jar, as defined by the most commonly used empirical mercury vapour pressure equation, within the uncertainty of the measurement.