Using ultrasound to locate compressed gas leaks is relatively easy, but occasionally it can present some challenges. The reason ultrasound is so successful is that it is a high frequency, a short-wavelength signal that does not like to penetrate 2nd mediums. While performing compressed gas leak inspections, keep in mind that strong ultrasonic signals can bounce off most materials leading to false indications.
To overcome this challenge, turn your ultrasonic detector 180° and see if the signal is stronger coming from that direction.
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Maintenance Tips, Ultrasound by Paul Klimuc CRL - SDT Ultrasound Solutions