プロピレングリコール
In the past the fluid of choice was a 50/50 mixture of propylene glycol because it is non-toxic unlike ethylene glycol (which is used in car radiators). Propylene glycol is viscous, and would eventually gum up some parts in the loop(s), so it has fallen out of favor. Today , the most common transfer agent is a mixture of water and ethyl alcohol (ethanol).