Ultrasound for Predictive Maintenance Applications

When companies begin exploring incorporating ultrasound technology and devices into their preventive maintenance (PM) and predictive maintenance (PdM) programs it can be very confusing for first time buyers. In the ultrasound marketplace today there is a lot of hype, a wide range in prices, and more often than not, mixed and confusing messages as to the true capabilities of these devices and the results one might come to expect.

Let’s begin by stating that ultrasound is not a replacement for other technologies such as infrared thermography, vibration analysis or fluid analysis but rather a complimentary and valuable technology. Ultrasound provides companies more information about the health of their physical plant and often provides this information earlier than other technologies. By being able to “listen” to your plant’s equipment and machinery, one can detect nuisances and abnormalities well in advance of the development of other characteristics such as heat buildup (detectable through thermography and fluid analysis) or audible detectable physical changes (vibration analysis). Here are some general properties and considerations to effective ultrasound testing:

• Ultrasound is sound that is above the human audible range (greater than 20kHz). Due to its three primary properties, attenuation (loss of energy), directionality, and ease of shielding, it is ideal for diagnosing critical components and detecting leaks. Ultrasound is created by friction, impact, turbulence, electrical interference, or artificially such as with a transmitter. Ultrasonic technology provides several advantages over other commonly used maintenance methods: earliest warning signs, instantaneous results, accuracy in pinpointing, more versatile, non-destructive, more cost effective and easier to learn and use.

• Ability to listen to ultrasound at its “sweet spot” - this is the frequency that allows you to block out lower frequencies in the audible range while still allowing you to detect from a distance. Higher frequencies require more proximity to the source to be detectable while lower frequencies allow competing sources of sound to merge and blend with the target sound source. Naturally produced sound travels across a frequency spectrum, not just at one single frequency. Therefore, effective ultrasound devices should be set at a fixed frequency or “sweet spot” combined with a narrow bandwidth to enhance ultrasound detection and directional capabilities. Most ultrasound microphones on the market today for PdM diagnostic applications are centered at 40kHz. And while some devices can change frequencies, usually from 30 kHz to 100 kHz, it is uncommon that the ultrasound would not be detectable at 40 kHz.

• Sensitivity of the ultrasound sensors should be very high in order to detect sources of ultrasound that are fairly quiet (low pressure vacuum leaks, slow turning bearings, etc.) and also to be able to discern sources from each other such as listening to different bearings within the same housing.

• To be able to be effective in noisy environments, the headset should be of such quality as to not only block or dampen audible plant noise but also to enhance the detectable ultrasound so as to be clear and crisp. This quality of headset is often found in the aviation field where the quality of communications is critical. Remember that your technicians will be wearing these headsets for several hours at a time and it would be very advisable to have comfortable fitting headsets that are meant for extended wear use.

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