Performances of quenchants have been enhanced and maintained based on their cooling characteristics 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. Its 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 mechatronics techniques. Finally the probe material has been changed from nickel alloy to platinum for resolving 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 various cooling conditions.