What is Heat?
In thermodynamics, heat is energy transferred from one system to another as a result of thermal interactions. Energy exchanged as heat changes the internal energy of each system by equal and opposite amounts (full article).
Heat transmission or transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy or heat between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation and others.
The fundamental modes of heat transfer are:
Conduction. The transfer of energy between objects that are in physical contact.
Thermal conductivity is the property of a material to conduct heat and evaluated primarily in terms of Fourier’s Law for heat conduction.
Convection. The transfer of energy between an object and its environment, due to fluid motion.
The average temperature is a reference for evaluating properties related to convective heat transfer.
Radiation. The transfer of energy by the emission of electromagnetic radiation.
A good way to imagine what radiation is that you can feel heat without touching the object emitting the heat.
Diving and Heat Loss
Water compared to air is a very good heat conductor. Water will transfer heat 24.17 times faster than air. The thermal conductivity of water is .598 (W/mK) and the thermal conductivity of air is .0259 (W/mK) watts per meters kelvin (at 20 °C).
Water removes body heat much faster than air. For the purposes of dive theory we simplify that water conducts heat 20 times a better than air. Water is 770 times more dense than air and 3.200 times more heat is required to raise to same temperature in water than it is in air of the same volume.
Any water that is cooler than the body temperature (even tropical waters with 28 °C to 30 °C) has the potential to eventually chill a diver or even induce hypothermia. If the water temperature is lower the heat loss in a diver will be faster.
Wetsuits trap a thin layer of water against a diver’s body. While the diver still gets wet, his body heats up the trapped layer of water to nearly body temperature. If the wetsuit fits properly the layer of water does not circulate away from the diver’s body and prevents more rapid heat loss.
For diving in colder waters (colder that 16 °C) a drysuit should be used. A drysuit traps air instead of water and insulates even better. It also allows the diver to wear undergarments that will keep him or her warmer during diving.
Physiology & Hypothermia
The environment exposes human physiology to a wide temperature range so our physiology has temperature maintenance mechanisms. The normal core body temperature is critical to the normal chemical processes continuously taking place in
the human body. A deviation above or below core temperature more than a few degrees for more than a short period may be life threatening.
Human body has physiological responses to protect against a drop in core temperature. This means that temperatures that we find comfortable and warm in the air quickly become uncomfortable and cool in water. Without an insulating exposure suit the average diver will be dangerously cold in half an hour in 16 °C water.
Hypothermia is reduced body temperature that happens when a body dissipates more heat than it absorbs. In humans, it is defined as a body core temperature below 35 °C.
If a diver looses a significant amount of heat during diving hypothermia may occur. The typical symptoms of hypothermia include shivering, numbness in the extremities, blueness in the fingers, lips and toes, etc. A diver experiencing these symptoms should end the dive, get out of the water and seek warmth.
Advance or extremely advanced hypothermia can occur when then diver ignores uncontrollable shivering and lets his body continue to cool.
At a certain point the body heat loss is so severe that the temperature regulation mechanisms fails and the body core temperature drops even more.
The diver stops shivering, sluggish thinking and amnesia start to appear. Inability to use hands and stumbling are also usually present. Cellular metabolic processes shut down.
As the temperature decreases, further physiological systems falter and heart rate, respiratory rate, and blood pressure all decrease. Advanced hypothermia leads to unconsciousness, coma and death when body temp reaches around 30 °C.
Advanced hypothermia is a medical emergency and requires immediate emergency care.