Not Being Cold
Larry "Harris" Taylor, Ph.D.
This is an
electronic reprint of an article that appeared in Great Lakes Diving (Feb/Mar.
1995, p. 18, 20-21). This material is copyrighted. All rights retained by the
author. This article is made available as a service to the diving community by
the author and may be distributed for any non-commercial or Not-For-Profit
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Diving is fun! Being cold is not! Perhaps the greatest deterrent to diving is the cold. Cold is a physiological stressor. With the possible exception of the death of a loved one or relationship, there is perhaps no greater stress than the cold. It is possible to dive in cold environments and enjoy it. All that is needed is a bit of understanding of the nature of heat and cold (the absence of heat), the manner in which we delay the inevitable loss of heat, and a little bit of coin to purchase the necessary protection.
Heat is thermal energy; the sum of the kinetic energies for all the random movements of all molecules contained within a substance. All substances can be thought of as heat reservoirs. The amount of heat within this reservoir is subject to change. Food intake or external heat sources such as the sun, extremely hot water, or radiant heaters can add heat to the body. Heat loss from a variety of mechanisms (discussed below) can remove heat. The body, unless overwhelmed by the environment or disease, usually keeps our internal heat reservoirs at an appropriate level.
Heat can be envisioned as a "fluid" in motion. It ALWAYS moves from the warmer object to the colder object until the temperatures of the objects touching each other are at the same temperature. This means that when you enter Lake Superior, the heat "fluid" leaves your body in an attempt to raise the temperature of Lake Superior to your body temperature. Since the mass (size of the heat reservoir) of Lake Superior is so large compared to the mass of your body, the movement of heat from you to the water will cause no significant rise in temperature of Lake Superior, but the attempt to warm the entire lake will drain significant amounts of heat from you. This heat drain can result in lowering of the body's temperature (hypothermia). Severe hypothermia can be life threatening.
A measure of the rate at which heat flows from the body is termed thermal conductivity. Some values of interest to the diver are listed below:
This means that a diver loses heat into the water approximately 25 times faster than into dry, still air at the same temperature. So, the obvious means to stay warm is to stay dry.
A diver loses heat in several ways:
Conduction: the primary heat loss mechanism in diving. This is the loss of thermal energy by direct contact between substances; a molecular transfer of energy. Heat moves from the warm diver to the cooler water. Water is denser than air (has more molecules per unit volume); thus more molecular collisions per unit volume occur with the result of more heat transfer. Water will remove heat from a diver 25 times faster than dry still air of the same temperature. If the diver is breathing gas at a temperature below body temperature, the diver will lose heat as the breathing gas is warmed by body heat. The denser the breathing gas mixture (i.e., the deeper the dive), the greater the heat loss due to respiration. Note that heat loss in respiration is related to the density of the breathing mix. This means helium mixes (much less dense than air) will be less heat robbing than air at the same depth.
Convection: is associated with conduction. As a volume of cooler fluid in contact with a warm body is heated, it expands and circulates away from the contact point. The area vacated by the warmer fluid is filled with cooler fluid and more heat transfer occurs. Convection increases the conductive heat loss. A wet suit functions, in part, by limiting the conductive and convective heat loss by restricting the flow of the volume of water immediately surrounding the diver.
Evaporation: the loss of heat associated with a change in state of liquid water to water vapor. A significant amount of energy is required to change liquid water to water vapor. A diver breathes a very dry gas. The lungs need a humidified gas. As the diver breathes, water is evaporated along the respiratory tract to humidify the breathing mixture. This can result in significant heat loss from the diver. Additionally, much heat can be lost from the evaporation of water on a diver's clothing (for example, a wet suit) on a breezy day.
Radiation: the loss of heat energy by direct emission of heat energy (infrared radiation). Although this means of losing heat is of critical importance in the vacuum of outer space (minimal air molecules for the process of conduction and convection to occur), heat loss to the submerged recreational diver is small.
The loss of heat from a diver immersed in cold water CANNOT BE PREVENTED. It can, however, be slowed. The purpose of insulation, generally gas trapped in some physical matrix (like nitrogen bubbles in a wet suit or air in the dry suit underwear), is to slow the loss of heat from the diver to the water by imposing a physical barrier. Thus, heat must move through the insulation on its way from the diver to the water. The better the insulation, the longer it will take the body heat of the diver to move into the water.
Wet suit compression associated with the increased pressure of descent not only decreases buoyancy, it reduces the volume of the protective insulation (gas trapped in rubber material of the wet suit). This is why wet suits offer less thermal protection in deeper water.
The Layer Concept Of Thermal Protection
Heat ALWAYS moves from the warmer body to the colder material. This process cannot be prevented. It can, however, be slowed enough to allow the diver time to enjoy a cold water dive in relative comfort. The rate of thermal loss will depend on diver size, the temperature of the water, the amount of time in the water, amount of physical exertion and the nature of the insulation system employed. Modern thermal protection systems are composed of three distinct layers. They are the "wicking" layer, the insulation layer, and the protective outer layer.
The "Wicking" Layer
skin conducts heat away from the body much faster than dry skin, it seems
obvious that a prime consideration in thermal protection is to keep the skin
dry. This is accomplished with the use of a layer of synthetic material.
Commonly used materials are called polypro, polypropylene, or Capilene. These
materials are "hydrophobic" (they will not absorb water), thus water from the
skin is "wicked" away from the skin, through the synthetic material into the
next layer away from the skin. This keeps the skin dry and heat is retained much
Anyone who has used these materials will note that they soon "acquire" a distinct odor. This is best removed by washing in a detergent made especially for breathable fabrics (available from outdoor outfitters.) It is important to examine the material after washing for odor. The garment MUST BE odor free. If you can smell detergent, then the detergent trapped within the material will absorb water and increase thermal loss. So, if odor is noticed, continue rewashing in warm water (no detergent) until no odor is noticed. Then, tumble dry with minimal heat. Insuring a clean, dry "wicking" layer is the first line of defense in protecting the diver from thermal loss. (Synthetic insulation is laundered in the same manner.)
Remember, the purpose of the layer next to the skin is to keep
the skin dry and minimize the accelerated heat loss associated with the direct
transfer of heat from body to water. This means the first layer MUST BE a
synthetic material. ALL natural fibers absorb water. Eventually, this absorption
of water reaches a point where the material becomes an excellent heat conductor.
Cotton is a particular No, No! Cotton is a fantastic summer fiber. This is
because cotton is a polar material and thus, readily absorbs water. A little bit
of air movement and the evaporation of this absorbed water provides a
comfortable cooling effect. This same cooling effect so appropriate during the
summer, will dramatically speed heat loss in cold environments. Fogery, in DEATH
BY EXPOSURE suggests that wearing cotton jeans during the winter in outdoor
activities can significantly reduce survival chances. So, to stay warm in cold
climates, keep the first layer next to the skin a synthetic fiber that will not
The insulation layer provides the primary resistance to thermal loss. Its purpose is to slow the unavoidable loss of heat from the body to the cold-water environment. The amount of insulation used and the material employed will depend upon a number of factors. Although there are a number of guidelines available, individual experience is still the best determination of insulation used for a given dive. There is NO UNIVERSAL insulation package that will suit all divers, over all types of diving, over the large temperature ranges one finds in the Great Lakes. Active divers will have several different types of insulation packages to mix and match to suit diving conditions and personal comfort levels. Remember that thermal comfort is an individual matter and it is possible to see a number of different systems on the same dive. Divers should determine thermal comfort on an individual basis.
The insulation in dry suit underwear is air trapped within the matrix of the insulation garment. Insulation effectiveness varies with the ability of the underwear to hold air. Basically, thermal insulation for dry suits falls into four types: wooly bears, open cell foam, type b marine thinsulate, and radiant barriers. We will examine each.
Wooly bears can be comprised of either natural (wool), synthetic (polyester, for example) or blends. They are called wooly bears because the insulation looks like fur. The fuzzy side is worn next to the skin. This underwear provides decent insulation at a reasonable price. It is often the first underwear purchased by a novice dry suit diver. When purchasing, pay attention to the number of fibers per unit area. This determines the insulation value (more fibers, more insulation, better heat retention). Avoid simply comparing weight. The weight comes primarily from the backing material and may give a false impression of the insulation value.
The underwear may shed. This fuzz can get into valves and compromise their function. The underwear will compress a bit at depth and lose some effectiveness. The major disadvantage of this type of material is that it loses thermal resistance rapidly when wet.
Open cell foam is very warm when dry. Its major advantage is that it does not compress at depth. It tends to be a little less flexible than other underwear types. Its major disadvantage is that the open cells provide a heat conduit when wet. Although an excellent insulator when dry, this underwear rapidly loses effectiveness when wet.
Type B marine thinsulate is, perhaps, the best available material. It is practically incompressible and absorbs no water. Thus it retains the majority (80 %) of its insulation capability, even when totally submerged. It is called thinsulate because the amount of material needed to provide the same amount of insulation is much less than when compared to wool or down. Thus, thinsulate garments can provide thermal protection while minimizing bulk. It is important to verify that the thinsulate used is type b. This is marine thinsulate. This material is non-compressible and extremely water resistant. Other thinsulates are not as effective in wet environments.
Radiant barrier garments prevent heat loss by radiation, These are extremely efficient insulators in the vacuum of space. However, heat loss by radiation is minimal in divers. To be effective in diving, these insulators must be combined with other materials to slow down heat loss by conduction or convection.
The Protection Layer
The modern (shell) dry suit has no inherent thermal protective properties. Its sole purpose is to protect the inner layers. As long as the outer layer protects the insulation from water, the insulation layer can work at its maximum potential. This protection function is most important. Divers should read and understand dry suit manufacturers maintenance guidelines. Proper maintenance insures the dry suit will perform to its specifications.
With a bit of knowledge and the appropriate equipment, it is possible to not only survive cold waters, but to enjoy the experience.
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Thermal protection is illustrated in the slide set on DrySuits
Larry "Harris" Taylor, Ph.D. is a biochemist and Diving Safety Coordinator at the University of Michigan. He has authored more than 100 scuba related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered one of the best recreational sources of information In North America.
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