The principle used to measure hardness is based on measuring the resistance force of the penetration of a pin into the test material under a known spring load. The amount of penetration is converted to hardness reading on a scale with 100 units.

• Whenever possible, a Durometer measurement should be taken at least 12 mm (approx. 0.500″) in from the edge of a sample, as the hardness characteristic of any sample tends to change at the edges.
• Whenever possible, the thickness of a sample should exceed 6 mm (approx. 0.250″). If the sample is thinner then this, then the sample can be backed with a sample of the same material to increase the thickness.
• Test Coupons or test blocks (rubber or plastic) should not be used for calibration verification. As all samples are subject to characteristic changes over time along with temperature and lighting conditions, the rated Shore value when supplied as new can and will vary with age. Proper calibration verification requires independent confirmation of the internal spring forces along with the verification of the pin displacement vs. the indicated value on the scale.
• As per the ASTM D2240 Standard, readings below 10 and above 90 are not to be considered reliable and should be discarded. Therefore, it is important to select the approapriate Scale that will provide results between 10-90 units.