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Monday, June 24, 2013

What is Specific Calibration?

In the late 1980's, customers in the kaolin industry were using the Technidyne Brightimeter S-4 and Micro S-5 to measure the brightness and color of kaolin.  These TAPPI geometry instruments had historically been used in the industry and provided slightly higher brightness results than the ISO or diffuse geometry instruments.  

Powder Press Apparatus
When measuring kaolin or other mineral pigments a sample suitable for measurement is made by putting the powdery substance in a 2" metal ring and pressing it against glass with a Powder Press Apparatus.  The resulting plaque is quite fragile, but must then be placed on the brightness tester and an average of 3-5 measurements is made.  However, since the TAPPI geometry instruments were oriented with the sample opening (for placement of the specimen) on the top of the instrument, the fragile plaque would break and some of the kaolin would fall into the optics of the instrument.

A procedure called specific calibration was developed. This transferred calibration from a top-loading TAPPI geometry instrument to a bottom-loading ISO instrument.  In this instance, if the plaque failed the contents of the ring would not fall into the instrument.  Also, the diffuse instrument geometry did a better job of averaging the variation across the face of the 2" plaque.

Specific calibration is achieved by measuring a series of plaques of a given grade on the TAPPI geometry instrument and then forcing the ISO geometry instrument to read these same brightness and color values.  Technidyne offers software exclusively for this purpose.  There are limitations to this process:
  • The specific calibration will only maintain is linearity over a small brightness range, therefore, if multiple grades of material are to be measured, several specific calibrations may need to be used.
  • You must have both geometries (ISO and TAPPI) to achieve specific calibration; Technidyne often supplies testing services to those companies that only have one instrument.
  • Extreme care must be taken with these sensitive powdery substances, and many measurements must be made to ensure a good calibration.

For more information on Specific Calibration contact Tom Crawford at Technidyne.