TL;DR
Annually.
But if you only use the tool once or twice in a year, then every other year.
How Often is Often Enough?
Like any calibration, it's a matter of risk and confidence. A brand new tool (or one freshly calibrated) offers the highest confidence in the accuracy of its readings. Every day, that confidence level drops a little. Assuming regular use, one could expect the confidence in accuracy to drop to zero after 20 years—nearly any tool with no maintenance for 20 years would almost certainly have inaccurate readings. So between day-one and year-20, there is a point when the confidence drops below some threshold that is acceptable for your purposes.
Anecdotally, we recently had a Concentricity Alignment Tool (CAT) that came in for maintenance that had been in the field for 14 years, and by luck it was still within specifications. But this is certainly the exception. Roughly speaking, over 90% of CATs calibrated annually still meet specifications; that drops to about 75% for tools maintained every other year.
Preventing ERAG Failures
Put another way, there is a likelihood of failure that slowly increases over time, and may be accelerated by each use. Although an on-site dramatic failure would likely be detected (readings drifting as they are being read, blank display, etc.), there are other kinds of failures that we detect and repair during calibration may not be apparent during an alignment job but would be equally detrimental to a correctly executed turbine alignment (readings varying because of a loose cable clamp, nonlinear response of actual distance versus reported measurement, etc.)
ERAG Calibration for Accuracy, Consistency, and Confidence
At EDS Measurement Systems, Inc., we take precision seriously. Our calibration stands undergo an annual check against gage blocks. In turn, those gage blocks are sent to a service that validates them against NIST-traceable standards. NIST—the National Institute of Standards and Technology—is the gold standard for measurement accuracy. Thus, the results provided from a calibration are reliable and accurate.
Another benefit is to make different tools provide consistent measurements. If you used one ERAG on an alignment then a different one the next alignment, the results would be the same. Calibration increases the confidence that measurements are accurate.
ERAG Calibration for Quality Assurance and Traceability
Calibration to NIST standards is often a requirement for quality assurance programs, whether through internal corporate requirements, for industry regulations, or for certifications. By providing a traceable reference point, if there's ever a question about the accuracy of your measurements, you can trace them back to a recognized standard.
Conclusion
As you increase the time between regular maintenance, the potential for inaccuracy and failure increases. But because each customer uses their tool differently, there is no easy answer. The only way to find out with absolute certainty is to start increasing the time between maintenance and wait for an issue. This would result in additional turbine downtime, either to perform turbine repairs, or until the tool was repaired or a replacement acquired. This does not seem like a wise course of action.
Regular calibration helps identify and correct any drift in tool performance, reducing the risk of errors in measurements. Knowing that your tools are calibrated to a recognized standard gives you confidence in your measurements and results.
