…One problem with maintaining heat (or cooling) in tethered divers is the losses in the tether itself...
The loss of an umbilical is an extraordinarily rare occurrence, in
saturation or surface supplied diving. Problems with interruption of flow, excessive, or insufficient heat from the system happens all the time. There is a rapid acting alarm system though… the diver lets everyone know immediately in the most unambiguous terms imaginable.
The problem is working divers operate 24/7 in a very hostile environment. Hot salt water supplied as a utility on most DSV (Diving Support Vessels) has become pretty reliable but managing the temperature reaching the diver +/- about one degree F. is a constant battle. There are a lot of variables and moving parts. It’s not laboratory or even factory floor conditions out there. It is very difficult for recreational divers to imagine how brutally rugged offshore oil and salvage work is.
… For an untethered diver a controller using well understood control principles can regulate the heat. The liquid path is shorter and therefore you don't need complicated feed-forward control techniques. The return liquid temp can be used in the feedback of the controller...
Where are you going to get the 100,000 BTU/hour plus required to heat the diver untethered? NASA, the world’s navies, and inventors trying to cash in on the offshore oil fields have tried to replace hot water suits for decades. Drysuits might be adequate for recreation and some science divers but are far too delicate for working divers, especially when hypothermia becomes a super time-sensitive life support problem.
The thermal conductivity of Helium is bad enough at about 6x higher than air. Controlling the comfort level with the lungs full of a media that transfers heat about 25x faster would be very difficult in real-world working conditions. DSVs that cost $100,000 to $500,000/day to charter can afford the most advanced thermal control technology available.
… The answer? Texting...
I’ll try that the next time I’m hanging off a hog line with a spud wrench in one hand, steadying myself with the other, and directing the crane to lower 5 millimeters. Should be interesting wearing heavy work gloves. Unless divers are doing complex and often very heavy work, it is safer and cheaper to send a ROV (Remote Operated Vehicle).
I forgot to mention these in my earlier post but there are a lot of other concerns with liquid breathing. Imagine 6-8 liters of fluid weighing around 17 Lbs in the delicate lung tissues. Falling out of bed or stepping off a curb would be enough to cause considerable damage. Will divers have to stay submerged 24/7 until they are “dried out”? It doesn’t sound like the physics of emptying and drying lungs quickly on a daily basis will be practical.
Then there’s the problem of eating and waste removal. It is difficult to feed people in hospitals intravenously without infections. It’s hard to imagine how you could pull it off in the ocean. It is also questionable if skin could handle continuous immersion is salt water that is close to body temperature. Six to eight hours a day in hot water suits makes skin easily abraded and susceptible to infection. Oh well, at least the divers should sleep well.
… Indirectly measuring gas levels, particularly N2 if that can be done, is a cheap solution but it won't work. The problem is that DCS is often localized and to prevent it you need to measure the N2 flow at that site...
The same is true for clots and emboli in medical settings. The objective is to look for markers. My understanding is dissolved diluent gas (Nitrogen and/or Helium) in the blood elevates before bubbles form that cause perceptible tissue damage or blood flow restriction. The idea is to monitor dissolved gas in the blood, not bubbles. Granted, tiny localized bubbles could form in very small areas but they will dissipate if the gas tension in the blood is low enough.
