A Low Differential-Pressure Primary Standard for the Range 1 Pa to 13 kPa
A P. Miiller, Charles R. Tilford, Jay H. Hendricks
The National Institute of Standards and Technology (NIST) has completed the development of a low differential-pressure primary standard covering a range from 1 Pa to 13 kPa for operation with line pressures up to 200 kPa. The standard is based on a UIM (Ultrasonic Interferometer Manometer) primary pressure standard and includes a test-instrument manifold with pressure control systems. The standard was found to be equivalent to low differential-pressure primary standards at three other national metrology institutes (NMIs) in a recent international key comparison. Initial performance of the standard was limited in large part by pressure instabilities. This problem has been addressed with the development of two types of active pressure control, one for calibrating pressure-measuring instruments such as capacitance diaphragm gauges (CDGs), resonant silicon gauges (RSGs), and Bell-type Micromanometers, and the other for characterizing pressure-generating instruments such as conical-piston, ball, and force-balanced piston gauges. The UIM is characterized by a standard (k = 1) uncertainty due to systematic effects of [(3 x 10-3 Pa)2 + (3.2 x 10-6 P)2]1/2 where P is the pressure in Pa. Random uncertainties are dominated by pressure instabilities which can be controlled to a level that varies from standard deviations of 3 mPa at lowest differential pressures to about 60 mPa at full range. The NIST primary standard is described, along with results of comparisons with primary standards developed at other NMIs.