Analysis Non-Destructive Testing Reliability & Maintenance Center Ultrasound

Set Baselines, Alarms on Mechanical Assets

Jane Alexander | December 17, 2018

Taking a historical approach to setting baselines and associated alarm levels for a plant’s mechanical assets is an ultrasound best practice.

With advancements in instrumentation and software, structure-borne ultrasound has become a major player in condition monitoring of rotating equipment.

Adrian Messer of UE Systems, Elmsford, NY (, noted that increasing numbers of reliability professionals are adding structure-borne ultrasound to their condition-based maintenance programs. Its value-added capabilities and benefits are substantial (see sidebar below), as is the information that an ultrasound instrument collects, i.e., temperature, decibel (dB), and FFT and TWF data from the recorded ultrasound sound file. 

Messer stated that one of the most common questions asked by first-time users of structure-borne ultrasound technology concerns the setting of baseline decibel levels and associated alarm levels on mechanical assets. His answer: Take the historical approach (a best practice).


The historical approach to setting baselines begins with establishing data-collection routes in the ultrasound-database software on the assets that are to be monitored. Once a route has been established, it is uploaded into the ultrasound device and the user collects ultrasound data on those points. The data should include decibel level and sound-file recording. 

When collecting the initial round of information for building a “history,” the ultrasound data may need to be acquired more frequently than normal. As an example, it may need to be collected weekly for a month to build a history or trend. Once a baseline has been established, the data can then be acquired less frequently, typically monthly. 

When the history has been established and an initial trend started, the user can select any historical reading as the baseline. The ultrasound software may even set the first reading downloaded into the established route as the baseline. Keep in mind, however, that the baseline can be changed at any time. 

Additionally, once the baseline dB has been established, going forward, only the dB reading should be taken. The only time to re-record the sound file would be when the point reaches an alarm level. The sound file can then be re-recorded and compared to or overlayed to the baseline sound file.

According to ISO29821-1, a standard for ultrasound use and associated guidelines, the ultrasound equipment’s alarm indicators should be set at 8 dB above baseline, which represents a lack of lubrication, and 16 dB above baseline, which indicates that a bearing or equipment is in a failure mode beyond a lack of lubrication, or the point where there is physical damage to the equipment. EP

Capabilities and Benefits of Ultrasound

According to the P-F curve, ultrasound has been shown to find early-stage premature bearing defects, even before they can be detected by traditional vibration analysis. Ultrasound is also easily used to precisely lubricate bearings to ensure that neither too much nor too little grease is added. Structure-borne ultrasound is also very effective for monitoring slow-speed bearings, even as slow as 1 rpm. 

Adrian Messer is U.S. manager of operations for UE Systems, Elmsford, NY. To learn more about ultrasound applications and solutions, visit



Jane Alexander

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