When equipment begins to fail due to an air or gas leak or vibration, or electrical discharge, the leakage point emits an ultrasonic sound wave that is above the natural range of human hearing. The ULD-420 Ultrasonic Leak Detector converts this ultrasonic sound into a signal that can be used to pinpoint the exact location of the equipment failure. When working in unpressurized systems or the pressure is not sufficient enough to detect or verify a leak with the Receiver alone use the Transmitter.