Many non-vibration analysts assume that all vibration sensors are the same. Frequently the question is asked “I have an accelerometer I bought from CTC, why is it not providing a 4-20 mA output to my PLC?” The answer is usually the same as the customer is expecting a 4-20 mA loop powered output from a dynamic output IEPE (Integrated Electronic Piezo-Electric) accelerometer such as the AC102-1A.
Different sensors for different needs
4-20 mA sensors – Process control engineers do not necessarily need to know exactly what is causing a vibration, just that vibration has exceeded a certain level and it could be dangerous to continue operating the machine. 4-20 mA sensors can provide an output proportional to overall vibration in a selected frequency band for trending and alarm, but provide little information that can assist in diagnosing what is causing the vibration. In many locations, once an alarm level has been exceeded, an analyst is sent out to diagnose the problem.
Dynamic output IEPE accelerometers – Vibration analysts on the other hand, need to know exactly what is causing the vibration so they can either fix the problem immediately or schedule a repair during the next maintenance interval. Dynamic output accelerometers provide a time wave-form signal that a data analyzer can per-form a FFT (Fast Fourier Transformation) on to create a spectrum. The spectrum, in conjunction with the time waveform and historical data on the ma-chine, can provide sufficient information for trained ana-lysts to make a call about a particular problem in a piece of machinery.
Costs vs. Benefits
While 4-20 mA sensors are slightly higher in cost than dynamic IEPE accelerometers, they are able to provide continuous output to a PLC/DCS system for alarms or automatic shutdown before catastrophic failures and provide a benefit that far outweighs the cost. Dynamic accelerometers can provide diagnostic information that can pinpoint a vibration issue and reduce overall costs of maintenance and repairs, avoiding costly emergency shutdowns. Used together, 4-20 mA sensors and dynamic sensors utilized with the knowledge of a qualified analyst, can provide valuable cost reductions and improved uptime.