Development of an interferometric dilatometer for gauge blocks

Abstract

In the quest for a better measurement quality, a reduction of the measurement uncertainty must be foreseen. Gauge Block calibration by interferometry is an unavoidable link in the traceability chain of most dimensional measurements and it has an impact in the uncertainty budget of all measurements that derive from it. Length-dependent effects of this calibration contribute largely to the combined uncertainty and therefore, its reduction must be expected to improve the combined uncertainty of gauge block calibration by interferometry. The thermal expansivity expressed as a linear thermal expansion coefficient (LTEC) is one of the main sources of uncertainty in the uncertainty budget. The gauge block manufacturer commonly quotes this number with an uncertainty of about one part in 10 - 6 °C - 1 . A reduction of the LTEC uncertainty to one third of the quoted value would provide a decrease of at least 25 % in the combined uncertainty of gauge blocks longer than 50 mm calibrated by interferometry. The system has been developed with the goal of measuring the LTEC with an uncertainty of one third or better of the manufacturers quoted uncertainty. It uses a commercial heterodyne laser interferometer head and flat-mirror differential interferometer and it can perform the measurements in a few hours time. The reflecting surfaces used for the arms of the interferometer are the gauge block face and the wringing platen themselves. In order to provide rapid heating and cooling, thermoelectric Peltier elements in a batch set-up are used and energy is dissipated to a massive aluminum plate. The supporting structure is thermally isolated from the heat exchanging system and it is built with low thermal expansivity materials to minimize its length variation. Gauge Block temperature is measured with up to four contact thermistors. Air and optics temperatures are also monitored. The laser interferometer, the heat exchanging system and the temperature measurements are all monitored and controlled by a computer and all data acquisition is performed automatically. The optical arrangement, the mechanical design of the instrument, the heat exchanging system, the control and data acquisition system as well as the uncertainty budget of the measurement and the experimental results obtained are presented.