Diving in Winter

With the onset of the winter season, the sea begins to lose the heat stored during the summer, becoming progressively colder. In this article, we will see what happens to our bodies and diving equipment when the temperature is low and how we can best prepare ourselves to dive safely in cold waters.

Heat production

Our body is “designed” to function within a narrow temperature range of around 37 °C. To maintain this temperature, we must continuously generate heat through metabolism.About 80% of the energy used during physical activity is converted into heat. Glucose is the primary source of metabolic energy; the majority of glucose in our body is stored in the liver as glycogen. In cold environments, glucose consumption is very high, and the only way to avoid becoming hypothermic is to compensate for consumption with an adequate diet that allows you to ingest a substantial number of calories. In addition to glucose, we need oxygen to produce heat; experiments have shown that an individual at rest increases oxygen consumption from 0.28 L/minute to 1.02 L/minute when the core body temperature decreases by 0.6 oC.

Heat loss in water

Heat is transferred according to four processes: radiation (e.g., when exposed to the sun), conduction (e.g., when we are in contact with a hot surface), convection (e.g., when we use air conditioning), and evaporation (e.g., when we sweat). Conduction and convection are the main modes of heat loss for the diver in immersion. On the other hand, the evaporative component becomes important once we emerge when we are wet and exposed to the wind. One gram of water evaporating from the skin’s surface extracts about 580 calories from the underlying tissues. This heat loss is sufficient to reduce the temperature of a liter of water by approximately 0.56 °C.

Since the first diving courses, they taught us that water is denser than air and conducts heat about 23 times more effectively than air. Consequently, our body immersed in water loses heat 4-5 times faster than when exposed to the same temperature in air.  To be more precise, the heat transfer coefficient of water (expressed in W/m2 /oC) is 44, while that of air is 2. This value increases if the water around our body moves, reaching 400 when we swim because the heat loss due to convection increases. For this same reason, at the same temperature, water is perceived as colder than air; in fact, the temperature defined as “neutral” (i.e., the one at which the heat produced by the metabolism and that absorbed by the environment are in equilibrium) is 26 oC in air but rises to 35 oC in water. If thermal protection is not used, the heat loss associated with immersion in water at 20 oC exceeds the capacity to produce metabolic heat, progressively leading to hypothermia. 

Table 1. Cooling factor as a function of water temperature.

Water temperature ( oC)Cooling factor ( oC/hr)
200.5
151.5
102.5
54.0
06.0

A source of heat loss that should not be overlooked is that due to breathing. On the surface, about 8-10% of the heat produced by our body is lost due to evaporation from the lungs and airways. This value increases considerably during a dive because the breathed gas becomes progressively denser, and the conduction heat loss component increases. For example, a diver who breathes a gas at 4 oC at a pressure of 30 atmospheres will lose all of his metabolic heat through the respiratory system. For this reason, professional divers who work in saturation at high depths use special warm water suits and often breathe heated mixtures.

Impact on divers

The first effect of cold-water immersion is the stimulation of cold receptors on the skin in the hypothalamus and spinal column. The activation of the thermoregulatory system triggers a series of reactions such as increased heart and respiratory rhythm, marked peripheral vasoconstriction with displacement of blood towards the central organs, and increased muscle tension (this explains the sense of “stiffness” that is often associated with feeling cold) and increased metabolism. These physiological reactions are mainly aimed at protecting vital organs by reducing heat loss from the internal parts of our body. Let’s see what impact these changes have on the diver.

Increased breathing rate: breathing becomes rapid but shallow with reduced efficiency. It’s just the opposite of what it should be during a dive.

Reduction of apnea: in water colder than 15 oC, the ability to remain in apnea is reduced to about 1/3 of normal.

Increased heart rate and metabolism: a significant effect is a corresponding increase in oxygen consumption with a noticeable impact on the duration of respiratory gases. In particular, the increase in metabolic oxygen consumption should be carefully considered when using rebreathers.

Peripheral vasoconstriction: The outermost parts of the body undergo a substantial reduction in active vascularization to transform into an insulating barrier to substantial internal heat loss. Unfortunately, this effect is associated with a marked decrease in blood in the extremities with loss of sensation and dexterity of the hands. Being exposed to water at a temperature of 8 oC causes a substantial loss of sense in less than two minutes and a loss of manipulative skills of 45% after 20 minutes. 15-minute immersion in water at 7 oC causes a 44% reduction in opposable thumb capacity. For a diver, the loss of dexterity can prevent him from performing a whole series of essential operations, such as checking the BCD and drysuit valves, to the detriment of safety.    

Centralization of circulatory flow: this effect stimulates blood pressure receptors, triggering a renal response with suppression of the anti-diuretic hormone to restore proper pressure by stimulating diuresis (up to 350 ml/h). This excessive diuresis can cause a reduction in plasma volume of up to 18%, resulting in increased blood density. Thicker blood increases the likelihood of clots and consequently exposes the diver to a greater risk of decompression problems.

Reaction time: cold causes psychological stress with a marked decrease in the ability to react to stimuli. In a way, feeling cold distracts by lengthening reaction times.

Cognitive function: A series of tests have shown a decrease in the ability to remember information after 50 minutes of immersion in water at 4.4 oC. A general reduction in cognitive function is associated with cold water exposure.

Impact on equipment

In addition to our bodies, the equipment loses heat when immersed in water. In a short time, cylinders, regulators, and BCDs will have the same temperature as the surrounding environment.

A problem can occur due to a physical phenomenon called “Joule-Thomson,” which cools an expanding gas. When the breathing gas contained at high pressure in the cylinders passes through the first and second stages of the regulator, it expands. If the ambient temperature is very low, the residual water vapor can turn into ice, causing problems for the regulator. Ice can also form around the first stage, which remains encapsulated. To limit these problems, there are specific regulators for cold water with a protective membrane that isolates the internal mechanism of the first stage from the surrounding environment, reducing the likelihood of ice formation.

Various materials, especially plastics and rubber, become rigid and brittle, lose elasticity, and are more exposed to mechanical stress when icy. For example, when very cold, o-rings can lose part of their sealing capacity and the neck and wrist seals of drysuits.

Battery life is shortened in icy environments; this must be considered when using dive lights and computers. More complex systems, such as rebreathers, are also more sensitive to low temperatures, and manufacturers often indicate clear limits regarding minimum operating temperature. 

Some precautions limit the impact of cold on materials:

  • Keep the equipment in a warm environment until just before use.
  • Use regulators specifically designed for cold water.
  • Take care to dry the regulators well before the subsequent use to avoid ice formation. In consecutive dives, where it is impossible to dry the regulators, protect them from the cold environment.
  • Avoid breathing in regulators before entering the water to reduce condensation that could turn to ice.
  • When diving, avoid using the purge button, e.g., to fill a lifting bag, because the high flow increases the Joule-Thomson effect, making it easier to freeze the regulator.
  • Avoid exposing materials to thermal shock; e.g., do not pour hot water on the frozen parts to thaw them.

Diving in winter and cold water is certainly possible and exciting, but you need to understand the potential risks and manage them properly. The cold has a marked impact on divers’ physiology and equipment. Once again, good preparation, adequate experience, and caution are the cornerstones for a pleasant and safe dive, even in cold waters.