Condition monitoring is where the health of a rotating machine is monitored by using vibration levels. As the health of a machine (Example: becomes unbalanced, fan blades corrode, bearing surfaces degrade) deteriorates so the amplitude of the vibration the machine generates increases. By monitoring the vibration levels over a long period of time, this gradual deterioration of the health of the machine can be assessed until the vibration levels get to a point where the machine needs to be taken out of service and overhauled. Analysis of the frequency content of the machine vibration signal will indicate not only that the health of the machine has deteriorated, but also the root causes can be attributed to the problem.

Example: An eight-bladed pump running at 6,000 RPM (100 Hz) will produce a vibration signal with 100 Hz frequency if it becomes unbalanced, 200 Hz if it becomes misaligned, 800 Hz if the blades become corroded and 43-47 Hz if the bearings start to go into an oil whirl.