The HG5 laser interferometer mercury manometer of the IMGC

Mercury manometers have been designed and constructed at the Istituto di Me trologia "G. Colonnetti" (IMGC) since 1986, their main characteristic being the adoption of cube-corner reflectors, carried by lightweight floats, to reflect the beams of a laser interferometer that measures the vertical sepa ration between the two mercury menisci. The paper describes the most recent instrument, designated HG5, designed to measure up to 120 kPa in either ab solute or relative mode. The major improvements introduced in HG5 over its prototype (HG-3) are: the adoption of larger-bore columns; the installation of a single-beam laser interferometer that allows the use of smaller and l ess intrusive reflecting floats; the improvement of temperature control and measurement; and the development of cat's-eye floats of a novel design all owing the direct reflection of laser beams on mercury menisci. The uncertai nty budget and the equations giving the HG5 standard uncertainties (43 mPa to 314 mPa) under all conditions are included.