The mercury U-tube manometer is a traditional pressure-measuring instrument and has played an important role as the primary standard in the atmospheric pressure range. It has also been used as the basis of standards for other pressure ranges. About thirty-five years ago, the National Research Laboratory of Metrology developed the precise mercury manometer using a white-light Michelson interferometer to detect the mercury menisci. We now introduce a new mercury U-tube manometer, developed as a national primary standard, to which some improvements and refinements have been added. Though the central principle is the same, the optical path of the reference arm of the interferometer has been redesigned to reduce the index-of-refraction corrections and some improvements have been incorporated to achieve more precise measurements on the U-tube conditions. The pressure range of the new manometer is 116 kPa and the uncertainty at 100 kPa is estimated to be about 0,4 Pa, in both absolute and gauge modes.

A dead-weight piston manometer which serves as a standard for accurate measurements of pressures up to 2 GPa is described. The piston-cylinder assembly is of the controlled-clearance type and has a nominal area of 0.05 cm2. The interrelations between equilibrium pressure, external pressure and fall rate of piston, together with other inherent characteristics of the piston manometer are studied for pressures up to 1.6 GPa using two kinds of pressure-transmitting liquid. The overall accuracy of pressure measurement is confirmed to be within ±0.37 MPa or ±0.023%.