Two methods are presented for using conventional pressure balances in the pressure range 10 Pa to 10 kPa. Both these methods overcome the inability of a pressure balance to generate directly pressures below a limit determined by the mass of the floating element. The first method uses twin pressure balances, with the desired low pressure established above one pressure balance in terms of a small added mass on the other pressure balance. This method is shown to be capable of realizing low absolute pressures with a standard uncertainty below 2 mPa + (9 10 -6 ) , when operating the pressure balances at 100 kPa. The second method uses one pressure balance in conjunction with a high-resolution pressure gauge. A calculable low pressure is established above the pressure balance either by reference to an added mass on the pressure balance or by a change in the pressure gauge reading. A 100 kPa pressure balance is used in combination with a commercial high-precision barometric pressure gauge, modified to improve the temperature control of its sensor. The standard uncertainty associated with this implementation is shown to be about 14 mPa + (9 10 -6 ) .
The design and analysis of a pressure balance using a finite element method is described. The design precisely defines the pressure gradient in the gap and provides isolation from end-mounting effects. It also simplifies the finite element analysis. The change of area with pressure is calculated for four different operating modes, along with the gap profiles and pressure gradients. The sensitivity of the change in area with pressure to the piston-cylinder geometry, materials and fluid is analysed. The results indicate that the simple mode of operation (no external jacket pressure) is preferred as it produces a small uncertainty in the change of area with pressure and is easy to implement.
This paper describes a pressure comparison, covering the range 25 MPa to 260 MPa, carried out between the CSIRO National Measurement Laboratory (NML, Australia) and the Measurement Standards Laboratory of New Zealand (MSL) in 1997 and 1999. A feature of each part of the comparison was the high-resolution Digiquartz pressure transducer used as the transfer standard. This small and relatively inexpensive transducer is easier to transport between countries than a bulky and heavy pressure balance. The measured performance of the transducer demonstrates that it can compare the pressure scales of each laboratory to a standard uncertainty of 2.3 kPa, which is much smaller than the uncertainties specified for each pressure scale. The results of both comparisons show that the two pressure scales agree to within their respective uncertainties, although the 1999 comparison produced a larger difference than the 1997 comparison. This may indicate a change in the pressure balance parameters or that the transducer performance is not yet fully characterized. An analysis is presented to show the relationship between the measured pressure difference and the parameters of each pressure balance.
The possibility of a watt balance based on gasoperated pressure balances is discussed in the context of both a measurement of the Planck constant and a non-artefact realisation of the kilogram. Gasoperated pressure balances meet several key requirements of a watt balance. For example, two pressure balances in combination act as a highprecision mass comparator for the weighing mode while a single pressure balance can be used to move the coil through the magnetic field in a strictly vertical manner for the dynamic mode.
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