Specially designed head horizontally protruding, it can measure the hardness of tubular parts which can not be measured by general hardness testers, the minimum inner diameter of test piece is 25mm. Automatic "fool" type operation, only one key operation to get result, no need for artificial preload applying, which can avoid human error. For all Rockwell scales, hardness value is automatically corrected in high, medium and low three sections. According to GB/T230.2, ISO 6508-2 and ASTM E18.