Feasibility study of application of ultrasonic method for precise measurement of the long distances in air

Abstract

The ultrasonic devices for distance measurement in air are simple in exploitation and their price in most cases is one of the smallest. However, high-precision measurements are affected by deviation of parameters of the surrounding medium, like temperature, relative humidity, CO₂ concentration and other parameters. Changes in air temperature by ±1°C cause deviation of ultrasound velocity by ±0.59 m/s. Such deviation gives the source of the distance measurement uncertainty of 0.17%, in the case of measurement of relatively long distance (4 m). The change of relative humidity by 5% gives the source of distance measurement uncertainty of 0.02%. In the case of carbon dioxide CO₂ concentration increases by 1%, ultrasound velocity decreases and it gives source of distance measurement uncertainty of 0.3%. In order to eliminate such factors, the additional calibration channel should be used. Also, the appropriate signal processing technique should be used in order to increase the resolution of time of flight (TOF) measurement. The uncertainty of TOF measurement was ±0.5 ns, which corresponds to the uncertainty of distance measurement ±0.18 mm (with a confidence probability 68%) and ± 0.36 mm (with a confidence probability 95%).