P-F Interval
To appropriately determine monitoring intervals a couple of things must be known. First,  the point in time (P) that the potential failure becomes detectible must be known (detected with vibration monitoring,  etc.). Second, the time (F) at which the potential failure would degrade to a functional failure must be known. This difference in time (P-F Interval) is the window to take corrective action and avoid the negative consequences of the failure. This difference in time will determine how often conditional tasks such as vibration monitoring must be done to detect potential failures such as bearing issues, etc. Typically, the monitoring interval would be set to half of the P-F interval. This allows enough time for the technology to detect the problem and for corrective action to be taken. However, in certain circumstances, it may be necessary to collect data at shorter intervals than half of the P-F interval.

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by Trent Phillips