Probe Path ERO
 
Probe Path ERO
API requires that vibration probes effectively measure the vibration of the shaft at a location that has the least amount of run-out. The Eddy Current probe measures the change in voltage generally caused by the shaft moving due to vibration. Mechanical run-out, caused by improper grinding can cause the voltage at the tip of the probe to change. This change in voltage is relayed to the vibration monitor as being "vibration". Any change in the "flux" lines around the tip of the probe is observed as "vibration" because it is a change in voltage.
Qualifying the probe path is essentially measuring the change in voltage while rotating the shaft at a very slow speed.
The voltage change due to mechanical run-out (poor grinding) is separated from the change in voltage due to change in flux lines (electrical).
Improving the mechanical run-out, may very well qualify the probe path to be in accordance with API. Hence, regrinding may be the answer. If however, the electrical is excessive, grinding the probe path will not achieve the desired results. BURNISHING will be required. Burnishing will not alter the physical roundness of the path, but will modify the electrical field on top of the ground surface. Once the shaft is placed into its bearings in the machine, it is too late to measure the probe path run-out because of the movement in the bearings.
The vibration probe is a dumb probe. It doesn't care if the change in voltage is caused by mechanical or electrical. It only measures the change in the voltage. Qualifying the probe path, traceable to NIST is the only way to insure that any change in the voltage at the probe path is caused by vibration. When the vibration monitor registers vibration, it is all vibration (minus the total ERO of the path).
 
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