Calibration block (calibration prism) is used as a tool for refractometers which cannot be calibrated by the distilled water. Such refractometers need also a special calibration liquid for which is necessary (by using the calibration block) to adjust the refraction to match calibration (zero) value.
It is suitable for models: RHN1-ATC, RBR82-ATC, RBR92-ATC.
If you do not want to buy the calibration block as a spare part it is not necessary to order it - it is a standard part of each our optical refractometer.