AIR DENSITY MEASUREMENT FOR MASS CALIBRATION

摘要

The measurement of air density is crucial for any high accuracy mass calibration and is of particular importance when comparing weights of dissimilar materials such as stainless steel and platinum iridium. The current limit of accuracy to which air density can be determined is the major source of uncertainty in the dissemination of the mass scale for the majority of National Measurement Institutes. Until fairly recently the main method for the determination of air density was by calculation from measurements of temperature, pressure and humidity using the equation recommended by the CIPM (Comite International des Poids et Mesures). Recently a number of laboratories, including the National Physical Laboratory in the UK, have developed artefact sets for the direct measurement of air density gravimetrically. This paper describes the development of the NPL artefacts and compares their results with those from the CIPM equation. NPL have also been investigating alternative methods for the determination of air density including the use of an air refractometer, a vibrating U-tube densitometer and an Aerostat "floating" element device. The development, use and relative merits of these three methods will be described as will their current levels of performance and potential future accuracies. Discrepancies between the calculated value for air density from the CIPM equation and direct measurement by artefacts and the other methods described will also be discussed.