CT-Rich
Contributor
In the 1980s there was a thermos like device and air from the 1st stage would go through a coil in hot water before going to the 2nd stage regulator. Since I never saw one in person, I’ll assume it was a dud. Skin Diver Magazine never met a piece of gear they didn’t love….It's possible, but a self contained system that uniformly distributed heat similar to what astronauts wear would be very expensive and high maintenance.
Electric heaters have far fewer potential failure points.