The NPL primary pressure balance standard

Describes the NPL primary pressure standard for the realization and maintenance of the SI pressure unit (the pascal). The accurate determination of the effective area of the piston-cylinder assembly of the pressure balance is central to its use and the various measurement techniques are discussed in detail. These include dimensional measurement of pistons and cylinders, comparison with a precision mercury column via a transfer standard and three different methods of providing information on the relative dimensions of the pistons and cylinders. The design and operation of the instrument are discussed and the method used for the comparison of other pressure balances with it is outlined. A detailed analysis of uncertainties is given and it is estimated that the overall uncertainty associated with a pressure generated within the optimum operating range of the balance (1-4 MPa) is +or-20 ppm at the 99.7% confidence level.