Acoustic system for average temperature measurement in an industrial environment

International audienceThe accuracy of length measurements performed in harsh industrial environments by optical systems, such as laser trackers and absolute distance meters, for the dimensional inspection of large mechanical parts, strongly depends on the air refractive index. The latter is usually calculated as a function of ambient conditions i.e. temperature, pressure, humidity and CO2 content using the Edlen formula. Its value should be carefully controlled during length measurements. In industrial environments, monitoring all these parameters, especially temperature, with sufficient accuracy can be most difficult. Indeed, the industrial workspaces can be subjected to temperature gradients of few degrees, and an uncertainty of 1°C in the temperature estimation leads to an uncertainty of 1 μm/m in distances measured optically. The aim of this work is to develop and optimize an acoustic thermometer able to perform real-time measurements of the average air temperature along an optical pathway. This apparatus is destined to be integrated into a laser absolute distance meter to improve its performances. The acoustic thermometer presented in this paper is based on the measurement of the time of flight of acoustic pulses between two measuring heads. The requirements for such a thermometer, for measurements in industrial environments, make the technical choices and practical realization challenging. Indeed, the working frequency range should be in the ultrasonic region, which implies a strong attenuation of the acoustic waves in air and correspondingly a low signal-to-noise ratio. Moreover, the whole measurement system should remain compact, with wireless communication between the measuring heads