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By Steve Weintz,War Is Boring

You get pretty cold pretty fast when you’re wet. Water absorbs more heat than air—and absorbs it 20 times faster. Without some kind of protection, people can suffer hypothermia even in warm tropical seas.

Originally published on Nov. 26, 2014.

There are several ways to stay warm in the deep blue, starting with wetsuits and drysuits and adding piped-in hot water or portable heating systems.

Surprisingly, the U.S. Navy is still looking for a solution to the cold-water problem for its divers. Its nuclear wetsuit didn’t quite work out.

For military divers, water’s chill factor complicates the challenges they already face. Cold affects energy levels, dexterity and thinking. Even simple tasks like turning a wrench or opening a hatch become draining in cold water.

Add extreme depths and the need for stealth and the complications become dangerous indeed.

Most divers today use neoprene wetsuits to keep warm. Neoprene is essentially a kind of very tough thick foam rubber. A wetsuit not only insulates a diver from the cold outside water, but also traps a layer of water next to the skin, where the diver’s body heats it.

Old-school hard-hat divers kept warm wearing woolen longjohns and knit caps inside their waterproof canvas suits and heavy copper helmets.

Modern drysuit divers use a similar system, wearing undergarments akin to outdoorsmen’s long underwear beneath their drysuits.

The deeper you go, the colder it gets—and the gas mixtures required for deep dives suck the heat from a human body. The helium in deep-diving gas mixtures conducts heat much more efficiently than ordinary air does. Beyond certain depths neoprene gets squeezed too much to insulate effectively. A drysuit or underwear isn’t enough to make up the heat loss.

Commercial and military divers going really deep most often use umbilicals that supply their breathing gas. These umbilicals include hoses that pump heated water into special undergarments to fend off the chill.

Umbilicals run to a surface vessel or a diving bell supplied from a ship. While essential for long deep-sea dives, they’re not always practical.

Keeping deep divers warm has challenged the Navy for almost half a century … and it’s still working on the problem.

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Above—the nuclear wetsuit. AEC photos. At top—a U.S. Navy diver. Navy photo

The nuclear wetsuit

During the 1960s the Navy’s SEALAB program, along with Jacques Cousteau’s CONSHELF missions, pioneered the concept of saturation diving.

Capt. George Bond and Capt. Walter Mazzone of the Naval Medical Research Laboratory proved that once a diver’s body was saturated with dissolved gases he or she could stay down for very long periods if provided proper shelter.

An aquanaut inside an undersea habitat required the same number of hours of decompression whether he stayed down a couple days … or a month.

The SEALAB program demonstrated that saturation diving worked in the field and allowed Navy divers to live and work underwater. Once Bond and Mazzone had a handle on the pressure problem, the temperature issue became the limiting factor.

The crews of SEALAB II found that the cold Pacific waters 200 feet down took a big toll on performance. Their neoprene wetsuits lost their insulating capacity as they got squeezed by the pressure. The helium they breathed sapped their body heat.

Towards the end of SEALAB II, even master divers had to force themselves into the frigid water to go outside the habitat.

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For SEALAB III, the Navy sought help from the Atomic Energy Commission to keep its divers warm 600 feet down.

In 1968 the AEC and its contractors developed a nuclear-powered wetsuit.

A small canister containing nearly a kilogram of plutonium-238 heated water through a heat exchanger. A battery-powered pump circulated through lots of tubes within a longjohn-like undergarment worn under a wetsuit.

Engineers built and tested a prototype, but without enough Pu-238 for a full-up trial. “The system in its present state is incapable of maintaining thermal balance in a diver at depth and its use under SEALAB III conditions would entail a grave risk of hypothermia,” researchers concluded.

Radiation exposure, though small, was significant enough to limit total dive time with the system.

And it was really expensive.

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RINI Technologies’ FDHS. Photos via

Faceplate Magazine

& Rini Technologies, Inc.

Chemicals and heat pumps

Saturation diving permitted deepwater oil rigs and deepwater clandestine missions. Many military missions require divers to cut the cord and swim freely. Salvage and repair divers may need to move about a ship or structure without fearing entanglement.

Special operations forces can rarely tether themselves with umbilicals—and sometimes work at great depths.

In the 1970s, Navy researchers looked into chemical heating systems. Magnesium reacts with water intensely, sometimes explosively, giving off a great deal of heat.

Engineers at the Naval Construction Battalion Center in Port Hueneme, California, developed a prototype heater consisting of metal plates soaked in a toothpaste-like slurry of powdered magnesium and iron.

When flooded with seawater the chemical reaction inside the reaction cell generated hundreds of watts of heat.

A diver could mount a thick flat box on his chest or backpack to hold the reaction cell, electronics and pumps. A heat exchanger passed the heat from the reaction cell to the diver’s tube-lined garment. Drawbacks included hydrogen gas bubbles from the heater unit and the need to refuel the device with slurry.

Not much more information is available about this system, which either worked and entered the black world … or didn’t and was forgotten.

Today the Navy is exploring another technology to keep free-swimming divers warm—heat pumps.

A heat pump is a sort of reversible refrigerator—run it one way and it cools, run it the other and it heats. A refrigerant expands into gas or condenses back to liquid, pulling or pushing heat into a secondary circulating fluid. For the diver this means yet another tube-lined set of longjohns.

The September 2014 issue of Faceplate Magazine includes news about RINI Technologies’ battery-powered heat-pump system small enough to strap onto a diver’s kit. The Free-Swimming Dive Heating System can warm a diver in water down to 30 degrees Fahrenheit and, unlike chemical systems, doesn’t vent gas or consume electrolyte.

With a heat-pump system the only factors limiting a dive become breathing gas, fatigue and battery life. Rebreathers which recycle breathing gas already extend dive times considerably. Standard chemical batteries can run the FDHS for up to three hours. Nuclear batteries could substantially extend the system’s run times.

The FDHS is still in development. As of the fall of 2014 it’s still facing another round of improvements and debugging. If it pans out, we’ll probably hear about it when Navy salvage divers and Seabees use it.

Whatever the special forces do with it will remain in the deep cold.

This piece was originally published by War Is Boring