Practical cylindrical acoustic gas thermometer of constant performance for space exploration

It is difficult to recalibrate thermometers currently used in harsh environment such as thermistors and thermocouples for in-situ observation of meteorological temperature on Mars or the moon once they are installed in site. However, environmental factors like space irradiation change the resistance–temperature relation, which results in large temperature measurement uncertainties. Acoustic gas thermometry (AGT) works on a physically invariant fundamental relation between gas particles’ acoustic energy and thermodynamic temperature with long-term reliability, which makes calibration unnecessary. Nevertheless, the existing acoustic resonators of AGT are bulk with typical diameters more than 60 mm, which makes the thermometers impractical. The principal difficulty of reducing resonator dimension lies in that the quality factor of measurement tends to be lower with a smaller size. This limit becomes more serious with increasing temperature and introducing acoustic waveguides. In this work, a miniaturised cylindrical resonator (30 mm in inner-diameter, 40 mm in inner-length) with acoustic waveguides for AGT is proposed. The results indicate that the resonant frequencies in argon at temperature from 213 K to 400 K were measured with the relative standard deviation less than $1.0\times {10}^{-4}$. The thermodynamic temperature with the standard uncertainty from 0.05 K to 0.43 K was obtained. Calibration uncertainty varied from 0.05 K to 1 K considering thermal inhomogeneity. The portable AGT has the potential for in-situ calibration of thermometers in harsh environment like space with an uncertainty of several times lower than present estimation.