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Non-contact measurement & inspection


Six things to look for in an acoustic imaging camera


C


ompressed air leaks, vacuum system leaks, electrical partial discharge—these are all expensive system issues that consume power, causing companies to deal with unforeseen costs and


potential production/uptime issues. Ultrasound imaging with an acoustic camera is an effective way to detect these equipment issues as part of a complete asset management plan. This easy-to-use technology typically allows professionals to complete their inspections 10 times faster than with traditional methods. So, what should you look for when selecting an acoustic imaging camera?


Here are six must-haves that could help you make your purchasing decision. Effective Frequency Range


One of the first features to consider is the camera’s frequency range. You might think that you need the widest range possible in order to pick up the widest range of sounds. However, in reality, the most effective frequency range for detecting a compressed air leak is between 20 and 30 kHz. This is because using the 20 to 30 kHz range aids in distinguishing compressed air leaks from the background noise in a factory. The amplitude of machinery noise usually peaks below 10 kHz and trends down to zero at 60 kHz, whereas air leaks peak between 20 and 30 kHz. Since there is a greater difference between the air leak noise


and the background noise between 20-30 khz, compared with higher frequencies, it is easier to detect the air leak in this frequency range. Both the compressed air and machinery noise follow the same


downward amplitude trend in the 30 to 60 kHz frequency range, making it difficult to distinguish between them. Therefore, working within the 20 to 30 kHz range is more effective. For users looking for partial discharge from a safe distance, the 10 to


30 kHz range is optimal. This is because higher frequency ranges travel shorter distances. In order to detect partial discharge from high voltage equipment in an outdoor setting, your camera needs to be attuned to lower frequency, farther-traveling sounds.


Optimal Number Of Microphones


In the pursuit of quieter noises, more is better. Acoustic imaging cameras typically employ dozens of microelectric-mechanical system (MEMS) microphones to collect and characterise sound. While MEMS are small, use little power, and are very stable, they also generate their own noise that interferes with an individual microphone’s ability to pick up very quiet sounds. The solution is to increase the number of microphones in use; simply doubling the number of microphones improves the signal-to- noise ratio enough to remove three decibels of unwanted sounds.


For example, one microphone might create enough self-noise to make


it impossible for the system to pick up a compressed air leak generating a 16.5 kHz signal.


48


March 2021 Instrumentation Monthly


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