The molecular drag gauge as a calibration standard

The National Institute of Standards and Technology has used molecular drag gauges for six years as transfer standards in the high-vacuum range, 1×10−4 to 1×10−1 Pa. We report on the experience gained with these gauges and, in particular, on their long- and short-term calibration stability, on factors affecting accuracy, on the predictability of the effective accommodation coefficient, and on factors affecting the stability of the offset correction.