Research On Air Buoyancy Correction For High Precision Measurement Of Mass Standards

To meet needs for assuring reliability of mass metrology, the traceability system of mass standards linked finally to the international prototype of the kilogram has been established. As comparison measurements of weights in the system are accomplished in atmosphere, the air buoyancy correction evaluated from the volume difference of weights and the air density is necessary. So, research on measurements of air density and weights volume are made, in order to improve reliability of air buoyancy correction for high precision measurement of mass standards.Conventionally, two methods are used to determine air density:(a) application of the CIPM formula; (b) direct determination using air buoyancy artefacts having the same surface area but a large volume difference. Compare with the latter, the former can obtain the same accuracy by high precision measurement system, and it is easier to be realized. An air density measurement system is established base on CIPM formula for its advantages. In this system, the latest formula, CIPM-2007 equation, is used. Four environment parameters:temperature, humidity, pressure and CO2, are integrated for measuring air density. For assuring reliability of measurement result, all measurement signals are separated and processing automatically by well design of software and hardware. Application software is developed to extend the capabilities of system. It provides well support for mass standards measurement work.Although hydrostatic weighing is the most accurate method currently used in determining volume, but its measurement procedure is complicated relatively. And the weights need to be wetted in a reference fluid, the contamination and mass change must be considered. For the demands to improve the efficiency of determining volume and avoid influence of fluid, another method, acoustic volumeter, that satisfies uncertainty requirements, is used in here. An acoustic volumeter system is developed and a practical procedure of measuring the volume of weights ranging from 10 g to 100 g using an acoustic volumeter is proposed. The volumes obtained using an acoustic volumeter were compared with results obtained by the hydrostatic, and the relative error ranging from 1×10-5 to 8.3×10-4 is acquired.A great deal of experiment have been done base on these research. All the results prove that the air density measurement system and the acoustic volumeter system are efficient and reliable and can provide good support to air buoyancy correction for high precision measurement of mass standards.