A Laser Interferometer Mercury Manometer used as primary standard for measurement of relative pressures up to 10 kPa

摘要

An improved version, designated MM2, of the MM1 5 kPa full-scale standard manometer of the IMGC-CNR is described. The instrument has been studied and constructed jointly by SPRING Singapore and IMGC-CNR in order to establish a reliable and accurate primary standard for relative gas pressures from 1 Pa to 10 kPa. The new manometer consists of a mercury-filled U-tube (60 mm bore) machined inside a massive stainless-steel body. The measurement of the vertical displacement between the two mercury menisci is made through a commercially heterodyne laser interferometer. Direct reflection of laser beams on mercury menisci is obtained by means of focusing optical devices of the so-called caf's-eye type, as developed at the IMGC (very lightweight floats carrying 1/4" corner cube reflectors are also available for fast but less accurate pressure measurements up to 10kPa). A software was developed by SPRING for instant calculation of pressure value and corrections; using suitable procedures, measurement results of high repeatability have been achieved. The paper presents the pressure uncertainty budget for the new version of manometer. The estimated standard uncertainty spans from 12 mPa at 1 Pa up to 46 mPa at 10 kPa, showing improvement over the prototype in most part of the range. For the purpose of validation, an experimental comparison between MM2 and MM1 has been carried out in 2001-2003. After a description of the adopted transfer standard and of the measurements that were carried out, the comparison results are presented and discussed. The comparison has demonstrated a good agreement and compatibility of measurements between the standard low-range manometers of Singapore and Italy.