Thermal issues affect the comfort, performance and decompression stress experienced by divers. The impact varies with the timing, direction and magnitude of the thermal stress. Thermal protection can be provided by a variety of passive and active systems. Active systems should be used with particular care since they can markedly alter inert gas exchange and decompression risk.
Increased decompression stress will be experienced by divers remaining warm during descent and bottom phases and cool or cold during ascent and stop phases. Decreased decompression stress will be experienced by divers remaining cool or cold during descent and bottom phases and warm during ascent and stop phases. Dive computers measure water temperature, not thermal status, leaving it to the diver to consciously manage thermal status and risk.
Diving is conducted in thermal environments ranging from tropical through polar. While physical comfort and concentration and performance issues are often perceived as the top priorities, thermal status can also play a critical role in decompression risk. Thermal effects can either increasing or decreasing the net decompression stress, depending on the timing, direction and magnitude of the effect.
US Navy test
The best demonstration of the fundamental relationships was provided by a study of 73 male US Navy divers (37±6 years of age; 27.6±3.1 kg·m-2 body mass index) completing a total of 484 person-dives in an ocean simulation facility.1
Divers were fully immersed and exercising at the substantial rate of approximately seven times resting effort (seven metabolic equivalents [MET]) in a wet chamber during simulated dives to a depth of 37 msw (120 fsw). The bottom phase was followed by a long decompression (87 minutes) to accommodate increased bottom time in the event that the rate of decompression sickness (DCS) stayed low during the study. The water temperature was held constant (clamped) for two phases - descent/bottom and ascent/stop.
Clamp temperatures were 36ºC (97ºF), described as ‘Warm’, and 27ºC (80ºF), described as ‘Cold’. Ultimately, the greatest decompression risk was experienced when the clamped conditions were warm for descent/bottom (promoting inert gas uptake) and cold for ascent/stop (impairing inert gas elimination). The lowest decompression risk was experienced when the clamped conditions were cold for descent/bottom (impairing uptake) and warm for ascent/stop (promoting elimination).
The surprising result of the US Navy study1 was the magnitude of the effect. The ‘Warm-Cold’ combination had a 30 minute bottom time and yielded 22% DCS while the ‘Cold-Warm’ combination achieved an extended bottom time of 70 minutes that yielded only 0.1% DCS. While the decompression phase of the study dives was long in comparison with typical operational dive profiles, the study clearly shows that thermal status can have truly dramatic effects. Given this, it is important for divers to have a reasonable understanding of thermal physiology.
Major avenues of heat exchange
There are four primary avenues of heat exchange important in the diving environment - radiation, conduction, evaporation and convection.
Radiation represents the electromagnetic energy radiating from any object to any cooler object separated by space (air or vacuum). Conduction represents the heat flow between objects in physical contact. Insulation represents the inverse of conduction, that is, the resistance to heat flow. Evaporation represents the heat energy expended to convert liquid water to gaseous state. Evaporative heat loss results from humidifying inspired gases and the evaporation of sweat on the skin. Convection represents the heat flow through circulating currents in liquid or gas environment.
The typical concern in most diving environments is the minimization of heat loss. Even tropical waters can produce substantial cold stress over long exposures. Radiative heat loss is a relatively minor concern in diving. Radiative barriers have been added to the inside of some wetsuits and drysuits, but probably with limited benefit.
Heat loss in water
Conduction is the primary avenue for heat loss in water. The heat capacity of water (density x specific heat) is >3500 times greater than air, yielding conductive loss rates 20-27 times greater than air. While ‘cold’ may be a bit extreme a descriptor for 28ºC water,1 it will produce substantial thermal stress for an unprotected diver since mean skin temperature is usually around 32ºC. Protection against conductive losses is gained through improved insulation. A uniform distribution of an excellent insulator such as a vacuum space would be best, but persistent loft is a challenge in drysuits since hydrostatic pressure shifts gas to the highest point of a suit during immersion, effectively reducing the insulation layer elsewhere.
Evaporative heat loss from the skin is not a concern in high relative humidity environments. A fully saturated environment exists during unprotected immersion or in a wetsuit. A fully saturated environment develops very quickly in a sealed drysuit.
Convective heat loss can vary substantially, depending on the stability of the near skin microclimate. Drysuits provide a stable environment, wetsuit provide a reasonably stable environment if the design and fit effectively minimize water circulation. Convective losses can be substantial in a poorly fitting wetsuit.
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