The MM1 laser interferometer low-range mercury manometer of the IMGC

A compact version, designated MM1, of the HG5 primary standard manometer of the Istituto di Metrologia "G. Colonnetti" (IMGC) is described. The instrument was constructed in order to exploit thoroughly, in the range of pressures up to 5 kPa, the potential accuracy of the measurement principle and instrumentation already adopted in HG5 - the same optical components, instrument hardware and software can be used on either of the manometers. The improvement in measurement accuracy of the MM1 instrument over that of HG5 in the same range was largely obtained by reducing by an order of magnitude the lengths of both mercury and gas columns. This provision can reduce the uncertainties associated with the computation of aerostatic heads in the U-tube and with the correction of interferometer readings for gas refractivity, as well as reduce the detrimental effects of residual temperature gradients inside the manometer body. In addition, the systematic adoption of purpose-built cat's-eye optical devices allows the reflection of laser beams to be made directly on mercury menisci, whereas the massive, all-metal body construction of the manometer ensures an improved mechanical stability. The paper includes the uncertainty budget, with evaluated standard uncertainties ranging between 12 mPa at 1 Pa and 53 mPa at 5 kPa, in the relative or gauge measurement mode.