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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.