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Monday, October 6, 2014

A Story from the Testing Lab: Calibration Standards

Many years ago we received a typical phone call from a customer that was struggling to get their 10-year-old brightness tester to calibrate. We talked them through the calibration procedure. They seemed to understand the steps and process. They calibrated, check linearity and translucency. When they went to check the calibration it was not right. We talked them through diagnostics looking at lamp voltage. They had a spare lamp. So, they installed it and refocused the new lamp. Calibration was still off. Finally, they checked the different voltages on the instrument, but this did not give us the solution to the calibration problem.  Throughout all of this, the customer insisted that we needed to visit the site and get this rectified.

We scheduled the trip and sent a technician to the site. They were armed with all sorts of tools, parts, calibration standards and strategies to allow them to attack this problem instrument head on.  Upon arriving at the facility, the technician sat with the instrument and read the set of calibration standards they had brought with them. The calibration was even farther off than the customer had indicated.  The technician calibrated the instrument with the new standards and checked the calibration for linearity and translucency. The instrument was in perfect calibration.

After this was completed the technician asked the customer to see the calibration standards they had been using. The customer went to a drawer in the testing lab and removed a set of paper calibration standards from an envelope. She explained that they had carefully cared for the standards since they came with the instrument OVER 10 YEARS AGO.

It was obvious to the technician that the paper calibration standards had yellowed and become frayed over the 10 years of use. Those calibration standards were no longer usable.

The Paper Industry has a unique, but extremely reliable calibration process and hierarchy. This is based on using paper calibration standards as a transfer standard from the Master instruments. However, since the standards are paper, they will age and require replacement on occasion. The life of the standards is based on how well the standards are maintained (see TAPPI T 1219 Storage of paper samples for optical measurements and color matching). Subscriptions are available on a monthly or quarterly basis.

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