Performances of quenchants have been enhanced and maintained based on their cooling characteristics as determined by specific test systems. A rotary arm–type test system with a small ball probe has been developed for this purpose by making prototypes. The unique concept, derived mainly from a circular motion of a small ball probe in quenchants, was proposed by Tawara in 1941. The prototypes have been realized by current heating, measuring, and mechatronic techniques. The material used for the probe has changed from nickel alloy to platinum to resolve the discoloration and thermal aging problems on the probe surface. The performance of the prototypes has been verified by systematic tests using specific quenchants under some cooling conditions.