How Often Should Measurements Be Taken?
Regardless of the condition monitoring technology, you must decide how often measurements will be taken. At one extreme, it could be a permanent monitoring system that takes measurements every split second of every day. At the other extreme, it may be infrared analysis that is performed once a year. But how do you make that decision?
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The most common answer we receive is that it is based on the criticality of the equipment. More frequent measurements are taken on the more critical equipment. The next most common answer is that it is based on reliability. If you have been monitoring a machine with vibration analysis every 30 days and have not detected a fault for a year you may decide to test it every 60 days, or 90 days. Now, it is true that you have to decide how best to use your precious time. But the one factor often forgotten is the “PF interval”.
The PF interval, also known as the “lead time to failure”, is the time between when you can detect the fault condition and when the equipment will have “functionally failed” – i.e. it can no longer be used. If we use the right technologies with the correct settings and we take frequent measurements, then we will get the earliest warning, and therefore we have the greatest lead time to act. However, if the PF interval is short, then it is possible that if you have extended the measurement period to 90 days, the equipment may develop a fault and fail before you take the next measurement.
There is a lot more that could be said on this topic but suffice to say that it is essential that you understand the PF interval and continue to monitor equipment so that you take at least two measurements between the time the fault is detectable and when the asset will have functionally failed.
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