Comparison of Standard and Microtests for International Rubber Hardness
Abstract 1. Results obtained with an electronic microtester and the standard testers for International Rubber Hardness agree within about 1 IRHD unless there is a pronounced hardening near the surface or other nonuniformity of the compound. 2. The precision of test is not quite as good with the microtester as with the standard tester, but the precision obtained with the microtester can probably be improved by using a visual indicator in place of the audio indicator for detecting the null position. 3. The values of hardness are more dependent on thickness of the specimen at low levels of hardness than at high levels. The ratio of thicknesses to be used with the standard and microtesters appears to be 4:1, rather than 6:1, in order to obtain comparable results. 4. Some compounds exhibit a pronounced hardening near the surface or other nonhomogeneity, which causes high and erratic values of hardness, particularly with the microtester. 5. The incorporation of the microtest for rubber hardness in ASTM Method D 1415 is recommended. It is particularly useful for measuring the hardness of small rubber parts, for which there is no satisfactory test at the present time in the Book of ASTM Standards.