Ho hum, Google can find 13,000 instances of Betriebsmedium, which some contributor to Leo has reasonably called operating medium. Wikipedia defines this as “In industrial engineering, a gaseous, vaporous, fluid or shapeless solid material that plays an active role in manufacturing processes ... “ Eeek. I just did a job for radiator valves where the client used Betriebsmedium, which is actually water. So I translated it as water, as I wasn’t sure what a British plumber would make of operating medium.

I was then entertained to read that calling a spade a spade is actually based on a mistranslation...