Leakage between fluid molecules that cause eddy currents creates a large number of ultrasonic waves. Therefore, ultrasonic detectors can be used to measure fluid leakage, regardless of the type of fluid, and both positive and negative pressure leaks can be detected. Of course, the leakage inside the valve body or the water heater or the blockage of the pipeline can also be detected from the outside. Even leaks without differential pressure can use the ultrasonic generator as a sound source to test the leakage of the cabin, the vessel wall, and the heat exchanger. The friction between solid objects also produces a large number of ultrasonic waves. Therefore, bearings or gearboxes that are under-lubricated or damaged can be detected with an ultrasonic detector. In addition, partial discharge of high and low voltage electrical equipment is also easy to detect.