He purification system, with an acoustic purity monitor*

A system for Helium extraction and purification (SHeEP) is constructed around a chromatographic cylinder filled with activated charcoal to remove ⁴He contamination from ³He gas [1]. In order to monitor the composition of the mixture during the process, an acoustic cavity is designed. The acoustic cavity measures the average atomic mass of the mixture by measuring the speed of sound [2][3]. The sensitivity of the acoustic cavity is estimated to be 0.04%. It provides a cheap alternative to a mass spectrometer.

[1] Dmitriev, V. V. (2004). "Purification of ³He from a ⁴He impurity using adsorption". Instruments and Experimental Techniques, 47, 567-569.
[2] Kagiwada, R. S., and Rudnick, I. (1970). "Note on a simple method for determining the isotropic concentration of a 3He-4He gas". J. Low Temp. Phys. 3, 113.
[3] Lin, H., et al, (2013). "Improved determination of the Boltzmann constant using a single fixed-length cylindrical cavity". Metrologia, 50, 417-432.