Everybody wants to have good measurements. To this end, many recommend a regular schedule of recalibration. While this sounds reasonable, it can actually degrade the quality of the measurements.

The key to getting the most out of a measurement process is to know when to recalibrate and when to refrain from recalibrating. This was beautifully illustrated by two of my clients.

Plant A

Like all process industries, Plant A lived and died by the values delivered by the in-house lab. The lab director wanted to provide good values, so he followed the manufacturer’s recommendation and had the calibration of his key analytical instrument checked every day using a known industrial standard. When the value he obtained didn’t match the accepted value for the standard, he would adjust the instrument by an amount equal to the difference between the accepted value and the observed value. Because they ended up recalibrating more than 80% of the time, the lab director was convinced that these adjustments were both necessary and correct. Figure 1 shows the observed values for 100 consecutive tests of the known standard for Plant A.

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