Conventional pressure balances as reference standards for pressures in therange 10 Pa to 10 kPa

Two methods are presented for using conventional pressure balances in the p ressure 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 b alances, with the desired low pressure established above one pressure balan ce in terms of a small added mass on the other pressure balance. This metho d is shown to be capable of realizing low absolute pressures with a standar d uncertainty below 2 mPa + (9 x 10(-6))p, when operating the pressure bala nces at 100 kPa. The second method uses one pressure balance in conjunction with a high-resolution pressure gauge. A calculable low pressure is establ ished above the pressure balance either by reference to an added mass on th e 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 ba rometric pressure gauge, modified to improve the temperature control of its sensor. The standard uncertainty associated with this implementation is sh own to be about 14 mPa+(9 x 10(-6))p.