An accelerometer is often used with a magnet to couple the sensor to the machine. The coupling between the magnet and machine is critical to ensure quality vibration data is acquired.
Placing the magnet onto the machine can create an impact that can shock the sensor for several seconds. Collecting data during this time can skew the results. The magnet should be rolled onto the machine to minimize the impact. Once the magnet is “Rolled” onto the machine the analyst should feel the magnet to see if it has a good contact or is loose. If the magnet feels loose try rotating the magnet either clockwise or counterclockwise to obtain a more secure fit. Additionally, move the top of the sensor to check for a secure fit as well.
These steps will help you check for sensor placement issues that could impact the quality of data collected. By making these checks you have also allowed any impact signals from attaching the magnet to the machine to decay out of the signal.
Maintenance Tips, Vibration Analysis by Gary James CRL