Misconceptions, misunderstanding and misguided marketing are part of the problem. For instance, the popular belief that humans lose most of their heat through their head could lead one to believe that all we really need to do is wear warm head protection. He used thermographs (infrared images) to illustrate the case. In Figure 1 the subject is wearing a complete dry suit and indeed almost all of the heat loss is through the head. However, the undressed subject in Figure 2 loses heat everywhere. Since heat loss is a function of temperature and surface area it is fairly obvious that the majority of the heat loss will not be through the head. Therefore, the factors that affect heat loss and gain are highly dependent on dress and environmental conditions.

Figure 1. Thermograph of fully-clothed subject.

Figure 2. Thermograph of unclothed subject.

He talked about passive and active protection against heat loss and showed that both can be provided with wet suits and dry suits. In wet suits, the quality of the foam, its thickness, the goodness of fit and the quality of the design and fabrication are the main factors that determine thermal protection of a wet suit. Aging of the material through ultraviolet exposure, repeated compressions and contact with contaminants will reduce the thermal resistance of the suits.

Most suits are foamed neoprene and there have been a number of marketing fads suggesting an increase in the thermal protection of neoprene. A recent craze is metal-coated neoprene. Manufacturers claim the metal coating reflects back body heat. However, without supporting evidence, the scientific community cannot see a basis for these claims. Conditions within a wet suit are not conducive to radiative heat transfer. The tight fit and moisture layer between the skin and suit provide no gap for radiation to occur and the temperature difference between the skin surface and inner surface of the suit are small so there is little to drive radiative heat transfer. In fact, with the close contact between the skin and the metal coating, it was even hypothesized that heat conduction may increase and that metal-coated neoprene might be colder. Measurements of thermal conductivity were made comparing neoprene material of the same manufacturer and type with the only difference being the metal coating. The experiments were done in water in a hyperbaric chamber to a maximum depth of 100 metres. The results, Figure 3, show that there was no difference between the two materials within the error of the measurement instruments. This supported the scepticism regarding metal-coated neoprene for wet suits. However, the gas layer in a dry suit may provide some opportunity for heat reflection in a dry suit. This has yet to be tested.

Figure 3. Thermal conductivity of metal-coated versus normal neoprene.

In dry suits, factors similar to wet suits affect the thermal performance of the suit, i.e., quality of materials, goodness of fit, design and fabrication. However, the thermal model is more complex because you are separated from the cold water by a gas layer. This allows for conduction, convection, possibly radiation and the confounding effect that water in the suit from perspiration and leaks can produce through evaporation, condensation and conduction. The best way to control these problems is through good underwear. Good underwear is incompressible to maintain the low conductivity of the gas layer, but it also has a low conductivity itself, prevents convection and manages moisture without being too thick. An example of effective underwear is that developed by DCIEM and Mustang Survival, Figure 4. The inner layer is a hydrophilic layer that wicks water from the skin. Next is an insulation layer of compressed Thinsulate Ò and then an outer hydrophilic layer to further wick water away. The final layer is a microporous liquid layer, namely Gortex Ò, fused to an outer shell. It is one thing to have good insulation like Thinsulate Ò, but to get the maximum value there must also be a way to manage the water in the suit.

Figure 4. Dry suit underwear construction for insulation and water management.

Another water management problem in a dry suit that the underwear cannot be expected to manage is urine. The Canadian Navy uses adult incontinence diapers. This may seem a bit silly at first until you need them and then you realize how well they work. They also work for both sexes. Other navies have been known to use condoms that drain through a valve in the suit. These are obviously only good for men and have been known to slip off when the user gets cold.

The main advantage of a dry suit is the gas layer that protects the diver. It has a low conductivity compared to neoprene and the thickness of the air gap can be maintained through a suit inflation system. The gas used is usually air but because argon has an even lower thermal conductivity than air (about 30%) there is a belief that it will provide substantially better thermal protection.

However, examination of the dry suit system even using simple thermal models suggest that switching to argon from air would provide at best a 25% improvement. Increasing the complexity of the model to better reflect reality suggests that the true improvement is more likely in the 5 to 10% range. A well controlled experiment would be needed to identify with confidence an improvement of this nature.

Then there is active heating of the diver. There are lots of different systems but the object for the Canadian Forces is to keep the diver's core temperature at 37 to 38 °C for a maximum of 4 hours. This means providing a diver in a dry suit with about 200 to 300 watts to keep up with heat losses. To date, open-circuit, hot-water suits are the only commercially-successful equipment to achieve successful diver heating. They are essentially a wet suit with hot water flowing through them. But they represent a brute force method of heating that is limited to surface-supplied applications from fixed or relatively large diving platforms.

Diver-carried heat packs may keep a diver comfortable during a short dive but do not have the power needed to keep a diver from becoming hypothermic over a long dive. Electric suits have been developed that operate from 12 Volt lead acid batteries. These can be used from small boats and provide a tethered diver with around 250 watts of heat. There are reliability issues with electric garments, however.

A closed circuit hot water system using electricity as a power source may be the better solution. In this system, a tube suit is worn next to the skin, then the underwear, then the dry suit. An in-water heater, powered from the surface by lead-acid batteries, circulates hot water through the tube suit to keep the diver warm.

Figure 5. Hot water heating system in development.

This type of system is under development at DCIEM, Figure 5. To make the system self contained, DCIEM is looking at fuel cell technology. An aluminum/oxygen semi-fuel cell is under development and the goal is to provide 500 watts for 6 hours. The results from the first prototype show the energy is there so now the detailed engineering needs to be done.