Development of a horizontal interferometric dilatometer for gauge blocks

Abstract

Based on a heterodyne interferometric measurement, an original system to measure the linear thermal expansion coefficient (LTEC) of parallel gauge blocks (GB) has been developed. The automatic instrument measures the displacement of the parallel reference faces of the GB, which is mounted in horizontal way. The dilatometer has to measure the change of temperature and displacement of the GB simultaneously. The measurement displacement is carried out by a laser interferometer using the reference surface of a GB as part of the flat mirror optical interferometer. Termistor sensors in the body of the block measure the temperature. The GB changes its temperature by thermal conduction from electrical heaters. The optical system consists of two plane mirror high stability interferometers and an optical fiber to receive the measurement signal from interferometers. Two laser beams reflect the surface of the GB face in order to provide an optical resolution of λ/4. The total resolution of the commercial interferometric system is λ/128. For the measurement of GB of steel and tungsten carbide, a reproducibility of 0,03 x 10 -6 K -1 has been achieved. The estimated uncertainty in the measurement of LTEC is less than 3 x 10 -7 K -1 for GB of 100 mm. The results for short GBs, the uncertainty analysis and uncertainty budget, the measurement time optimization and the capability to measure objects of different shape of GB are discussed.