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Calibrating the Calibration System

Control, confidence and low uncertainties are the hallmark characteristics of a quality metrology laboratory.  The basis for this operation is quality calibration equipment, sensor specific operator knowledge, as well as solid business and calibration processes.  One vital key in this type of operation is maintaining proper control and calibration of the calibration system.  In general, there are three reference calibration paths a laboratory can choose which provide various tradeoffs in downtime, cost and risk…

1. Outsource
In this most common case, the user typically blocks out 2 to 4 weeks (or as long as 16 weeks for some vendors) of calibration downtime. The user disassembles the calibration system into the critical components (reference accelerometer, signal conditioning and data acquisition), sends them to the accredited vendor or an accredited regional metrology service provider for calibration and then waits for the test/return of the system. When conducted with an accredited provider, this option is often the easiest method while still being economical. A quality calibration system vendor can provide reasonable turn around, has deep expertise in both electrical calibration of the components and the specialized knowledge/skills for mechanical calibration of the reference accelerometer.  Additionally, a quality vendor has the experience and confidence found only through handling hundreds of reference accelerometer calibrations and can provide the necessary skill and service for the lowest reference uncertainty through laser primary calibration in accordance with ISO16063-11.

Once the user receives return shipment of the recalibrated calibration system components, it is critical to reassemble and revalidate for proper system operation. Newer automated accelerometer calibration systems will automatically import and update the reference file as well as assist with proper system validation. It is critical to have an accelerometer (or set of accelerometers if validating different frequency ranges and transduction types like PE, PR, VC, etc.) to calibrate and check system output versus the validation sensor(s)' known performance and history trend.  (Note: This validation transducer can also serve as a daily validation to ensure quality operation of both system and the operators’ skill/techniques.)  Subsequent to validation, calibration certificates are reviewed and filed, calibration trends are updated, and next calibration due date stickers are checked or affixed. Confidence in this overall method is extremely high due to the use of an expert service provider with large volumes of similar equipment to ensure that both skills and insight are at their highest levels.

2. Service Contract
In this case, the high volume or extremely critical user values minimal to virtually no downtime and pays for an accredited service provider to bring all the requisite skills and equipment to perform immediate on-site component calibration and/or provide controlled, calibrated loaned components. A quality service provider handles all recalibration and revalidation of the system including checks against the service provider validation set of transducers and any customer requests. While this is generally the higher priced option of the three discussed here, service contracts supplied by the calibration system vendor can include significant benefits like software updates, maintenance of the precision air-bearing calibration exciter and PC controller, as needed on-site training and throughout the year no charge repairs with emergency loaner equipment. These benefits are generally valued by the highest volume critical users. A service contract is also valued by infrequent users who would like to safely rely on the vendor for all aspects of excellence and operation during the recalibration process.

3. Do It Yourself
In some cases, users have broad metrology capabilities and maintain the equipment and expertise to handle virtually all of the recalibration steps by themselves. Commonly, metrology labs opt to handle their own electrical recalibration of both the signal condition and the time/frequency and amplitude references for the data acquisition system. However, regarding the mechanical excitation and recalibration of the reference accelerometer, it is advisable to either return the reference to an accredited provider of a laser primary calibration for lowest uncertainty or shuttle a transfer standard for laser primary calibration which is then compared against the back to back working reference accelerometer to provide a traceable, yet slightly higher uncertainty due to adding an extra link in the mechanical calibration traceability chain. Some modern precision air bearing calibration exciters have been engineered to allow for easy removal of the mounting platform housing the integral working reference accelerometer and thereby simplifying the removal/reinstall process to encourage/facilitate the low cost, accurate recalibration at the vendor's primary calibration technology center(s). Critical for the do-it-yourself customers is the system upload of the reference accelerometer recalibration file and the system revalidation. As noted earlier, a set of revalidation transducers is a critical asset in ensuring the reassembled capability and confidence in the operation. Overall, the do-it-yourself option lowers the downtime by allowing for quick and flexible in-house scheduling of most of the component recalibration, as well as reducing the total costs.

Conclusion
The single biggest component of uncertainty in the accelerometer calibration system is the mechanical calibration of the reference accelerometer. Convenient recalibration can be facilitated through the use of an in-house transfer standard; however, this method provides an additional link in the traceability chain and raises the system uncertainty. The lowest uncertainty levels can be maintained by sending your reference accelerometer to either a national laboratory or high quality calibration system/transducer provider with a laser primary calibration capability. A primary calibration can reduce the overall system uncertainty by 10-30%.  Regardless of which method of reference calibration and which method of component recalibration logistics is chosen, proper control of the measurement equipment and regular validation of the system performance are key to quality operation. Accredited (and quality minded laboratories) will also supplement with annual interlaboratory comparisons (ILCs) to generate round-robin accelerometer calibration data and comparisons to ensure proficiency and create an active learning feedback process between friendly, capable, quality laboratories. Check with your calibration system vendor to see if they offer a free or low cost service to administer ILC groups and provide data aggregation, comparative normalized error (E-sub-n) statistical analysis and blind reporting of the community data versus your specific test data. Each of these steps are key to developing and maintaining your control, confidence and quality as a calibration leader.

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