We often hear about the need to acquire vibration measurements at precisely the same location each time we measure a point. Why is this important? In order for vibration measurements to be trendable they must be closely repeatable, and we need to eliminate measurement error. With high-frequency measurements, the vibration attenuates rapidly as it travels away from its source. The author has seen readings vary by as much as 50% when the collection transducer was moved by as little as ¼ inch. If the transducer is not placed in the same location, the trended data will show an error that may be mistaken for a change in machine condition. When the collection point is different, the transmission path is either longer or shorter. This affects the amount of energy perceived by the transducer. Standing waves also exist in vibrating machinery. The transducer may sometimes be located at a nodal point of one of these waves; and if care isn’t taken in transducer placement, the next measurement may be at an anti-node. This is more apparent in larger machines because of the amount of surface area available for standing wave formation. There are several ways to precisely mark data points for measurement with a magnet-mounted transducer. Paint, glue-on-pads, stud-mounted pads, machined surfaces, and dimples made by a small drill bit are all used with success. Best of all are permanently mounted coded attachment studs (such as VIBCODE®) that guarantee precise re-placement of the transducer every time, at the same location, angle, and pressure. Regardless of the method employed, it is important to always precisely identify data collection points.
Maintenance Tips, Vibration Analysis by Bill Hillman CMRP