General Metrology Forum

Correction Factors

 
James D Jenkins
Re: Correction Factors
by James Jenkins - Wednesday, March 18, 2009, 5:12 PM
 
I assume you mean a document that addresses the use of correction factors for sensors and other types of measurement instrumentation.

correction factor

numerical factor by which the uncorrected result of a measurement is multiplied to compensate for systematic error

NOTE Since the systematic error cannot be known perfectly, the compensation cannot be complete.

The above definition provides both the need to deal with correction factors carefully and appropriately to ensure adequate measurement quality and the awareness that where correction factors are used, they are a source for unknown error, or measurement uncertainty.

Where used, correction factors should be managed via a procedure that provides at least, in my opinion:

  1. A documented evaluation that determines if the drift of the measurement system, as exhibited by the difference between prior applied "correction factor" and the newly assigned "correction factor", is within the limits as stated in the process uncertainty budget. (Any out-of-limit results should be addressed according to your management system procedure for measuring device reliability management.)
  2. Verifiable implementation of the correction factor into the system prior to use.
  3. A documented evaluation that determines if the applicable uncertainty budgets need to be updated with the latest calibration uncertainty, as given for the "correction factor". (If the calibration uncertainty is equal to or less than what is budgeted, then updating is not necessary.)
In Summary: While use of correction factors require additional care to ensure proper implementation, their proper use decreases the overall uncertainty of measurement, thus improving measurement quality. The down side is that instrumentation, using correction factors, will changes like everything else with time. Meaning, the correction factor will correct for less and less error in the measurement result as time passes.

Hence the need for re-calibration and the assigning of new correction factors, along with trying to keep everything within budget or adjusting the budget to reflect the reality. As you can see, several things should be addressed in any procedure dealing with instrumentation that uses "correction factors".