In relation to dive compressor's, according to a master diver and retired air compressor rebuilder, cool down running with no pressure(all relief valves open) reduces the temperature inside the compressor. Hot air holds more moisture, duh.... Moisture can build up in a split second in the heat inside while just shutting down compressor. You let it cool down running, less heat, the air inside holds less water. Tons of dive compressor's in novice hands in places like Florida with high dewpoints, have to have full rebuilds in less than two years due to corrosion. The rebuilder told me it was nothing more than people not letting their compressors cool down running with all valves open, he rebuilt hundreds of Bauer's ruined by novices with less than 2-4 years use and mostly under 20 hours runtime. Vast majority from Fl and south east US . Not being commercial operations, a hobby diver's compressor may sit months without being used, if shut down without cool down in a humid environment, it will have standing water in the cylinders, not may have, but WILL have. Rebuild time in short order that way. Not so much in drier climates, but still possible so it is best practice to let high pressure compressor run a cool down with all valves open. I was told to run mine a minimum of 1.5 minutes after every use with valves all wide open based on using mine indoors in an HVAC controlled environment just to be safe. 1.5 minutes is an eternity waiting for it....but I do. I'll listen to the professional who did it for a living for decades.
