Ultrasonic techniques for detecting helium leaks

We have examined two ultrasonic techniques for their applications in helium leak detectors. In one technique, a surface acoustic wave (SAW) device is used to detect helium gas through changes of thermal conductivity in helium/air mixtures. The second technique measures variations of sound speed (SS) in such mixtures. Sensitivities and detection limits of both techniques were predicted with simple models and verified by measurements. The SAW sensor and the SS techniques can detect helium leaks as small as 10−4 and 10−5 cm3/s, respectively.