Ultrasonic sensors detect ultrasound transmitted by turbulent flow produced by gas molecules escaping through a crack, hole, or seam from a high-pressure system to a low-pressure system. A piezoelectric microphone is used to receive sound pressure waves at a specific ultrasonic frequency. Most airborne piezoelectric microphones have a characteristic center of frequency at 40 kHz. The human ear can only detect sound pressure at frequencies up to 20 kHz. Once ultrasound is detected by the receiver’s microphone, the receiver will convert the incoming signal into a frequency range that can be heard by the human ear. The new signal is output to headphones.
When a leak occurs, the turbulent flow produces sound pressure waves all along the spectrum from 0 Hz to100 kHz and beyond. Lower frequency sounds travel greater distances and interfere with ambient noise such as running machinery. Also, these sounds have greater energy and can easily reflect off surfaces, minimizing the ability of a low frequency microphone to accurately locate the leak. High frequency waves (those far above 40 kHz) do not have sufficient energy to be detectable from reasonable distances. An ultrasonic sensor that is used for the detection and location of leaks should:
Have a narrow bandwidth with center of frequency 40 kHz
Have a narrow directional pattern of reception
Have controls for adjusting the sensitivity of the receiver in order to pinpoint location
Have an analog meter, that rapidly displays small changes in the input signal
Have a good signal-to-noise ratio as noise will minimize the sensor’s ability to detect a leak
Have a long battery life
A good ultrasonic leak detector is important for the detection and location of leaks throughout a manufacturing facility, but no ultrasonic leak detector will be able to determine how much money you are saving or how much air you are losing. Ultrasonic sensors are incapable of determining leakage flow rate or CFM loss. There are many factors that change the intensity of the turbulent flow through an orifice such as thickness, size, and shape of the opening; roughness of edges; directional pattern; distance from receiver; humidity; temperature; atmospheric pressure; etc.
Consider the fact that under the same pressure differential inside and outside a system, the same leakage flow rate through a crack with rough sharp edges will produce ultrasound that is dozens and even hundreds of times as intense as the same flow rate through an opening with round, smooth edges. Therefore, in order to accurately determine cost savings, it is necessary to monitor electrical consumption and production output through other means. It is possible, using complex and expensive equipment to make such an accurate measurement, but it is not necessary. Routine air and steam audits will save money, though, even if you are unsure of how much. If you are not using ultrasonic technology, you are missing out on the fastest and easiest means of leak detection.