Test & measurement

Detect leaks with acoustic imaging

In this article, Fluke explains why it might be worth considering reaching beyond ultrasonic leak detection to find pesky compressed air leaks.

manufacturing plant. Through the years, the method of locating leakage has gone from using the human ear, to applying a soapy solution to pipes and hoses, to using sensitive electronic ultrasonic audio instruments to enable operators to find and record leaks. Recently, a method of detecting leaks has been developed called “acoustic imaging,” which uses audible and visual inputs and has the potential to lower industrial compressed air and process gas costs.


UltrasonIc leak detectIon Ultrasonic leak detection has been around for many years. From the start, equipment suppliers

or years, compressed air leaks have been among the first items to attack when looking for energy savings in a

realised that using only the human ear has limitations in finding leakage, especially in very noisy industrial settings. Loud background noise and legitimate compressed air uses in typical industrial plants very quickly mask any audible sound emitted by leaks, making the hunt an almost impossible task, even during quiet times during evenings and weekends. Long ago, it was discovered that a flow of gas

moving from one pressure to another emits sound in the ultrasonic frequency spectrum. Using electronic mixing circuits this ultrasonic signature can be brought down to a frequency range that humans can hear, but at the same time, general low frequency industrial noise can be filtered out. This gives ultrasonic detector operators “superhuman” hearing that allows gas leaks to be easily heard in noisy factory environments, even from hundreds of feet away. The detection method for ultrasonic guns is

typically a “point and shoot” method, where a directional audible pickup device is waved around until something is heard in a certain direction. The operator is then led on a search while following the sound, with the signal getting increasingly louder as the detector approaches the source. Sometimes, when near the leak, it is difficult to pinpoint the exact location due to various factors. At times, the leakage may be behind a protective


barrier or in an inaccessible location. Other times, the leak might be among a complex network of piping and hoses, requiring the operator to attempt to feel for the leak within the many fittings, sometimes creating a safety risk. Reflections and interference from other ultrasonic emitting devices also can drown out the leak signal, making it difficult to exactly locate.

Once the source of the sound is found, the leak is documented and recorded so the cost can be estimated, and a repair crew can find it again at a later time. This step usually involves measuring and recording a decibel reading at a fixed distance away from the leak, marking the location with a paper tag, then taking several photographs to describe the location for the future repair crew. Once a survey is done, a report is generated that estimates the leakage flow, calculates the theoretical cost of the leaks and displaying the location description and pictures. This process is a tedious exercise where the operator juggles the detector, a camera, a notebook and a bundle of tags each time a leak is detected, often repeating the task hundreds of times per day if working in a large plant.

enter acoUstIc ImagIng A more efficient way of finding and recording leaks has recently been developed by Fluke

April 2021 Instrumentation Monthly

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