DEPTH Blog

Drysuits provide the greatest form of passive thermal protection for the diver. They are designed as one piece suits with a waterproof zipper for entering and exiting and have attached boots and seals at the diver’s wrists and neck to provide a dry internal environment. The suits are normally designed so a wide variety of insulating undergarments may be worn beneath them. These undergarments trap a layer of air providing the primary protection against cold. Too much air trapped in the drysuit can create buoyancy problems because the air forms a “bubble” that will move inside the suit, but some air is needed in order prevent the suit material from compressing and catching skin in the folds and causing a suit squeeze. A suit “squeeze” can be uncomfortable but is avoidable by adding a small amount of air to remove any suit wrinkles.

Cave diving is a specialized form of diving that can be performed in both inland freshwater caves and oceanic “blue holes.” To scientists, caves offer new laboratories for research. In cave diving, the emphasis should be placed on developing the proper psychological attitude, training in specialized techniques and life-support systems, dive planning, and the selection of an appropriately trained buddy diver.

Divers have used air as a breathing gas since the beginning of diving. Its principal advantages are that it is readily available and inexpensive to compress into cylinders or use directly from compressors with surface-supplied diving equipment. Air is not the “ideal” breathing mixture for diving because of the decompression liability it imposes. Since decompression obligation is dependent on exposure to inspired PN2 (nitrogen partial pressure), this obligation can be reduced by replacing a portion of the nitrogen content of the diver’s breathing gas with oxygen, which is metabolized by the body. This is the fundamental benefit of enriched air nitrox (Nx) diving (Wells 1989). Historically, the two most commonly used nitrox mixtures in NOAA have been 32% and 36% oxygen. Once called NOAA Nitrox 32 (NN32) and NOAA Nitrox 36 (NN36), such mixtures are now identified using a more general nomenclature as Nx32 and Nx36. The remaining gas in nitrox mixes is considered to be nitrogen, even though it may contain other inert gases like argon. “Nitrox” is a generic term that can be used for any gaseous mixture of nitrogen and oxygen, but in the context of this chapter, the implication is that nitrox is a mixture with a higher concentration of oxygen than that of air. Using such oxygen enhanced mixtures can significantly increase the amount of time a diver can spend at depth without incurring additional decompression when compared to air diving.

Performing repetitive dives requires a the use of a dive plan. The diver must know what the no-stop dive time limits will be for the dives prior to descending so as not to incur additional decompression obligations. A planned dive schedule will work assuming the diver adheres to the maximum depth and time parameters defined before descending; however, this does not always occur. There are many reasons why divers may find themselves deeper than planned. Some of these might include: higher than normal tides while working on a specific site, down-welling currents, the need to descend deeper to pick up tools or experimental apparatus that may have been dropped, the unexpected need to provide assistance to divers who are working at deeper depths (either on a routine or emergency basis), or perhaps just plain inattention of the divers.

Thermal Stress Irrespective of Ambient Temperature
Hypothermia is not a problem exclusive to frigid environments—it can occur irrespective of ambient temperature. Similarly, divers may also suffer extremes of hot and cold thermal stress simultaneously during the same dive. There have been documented cases of severe heat exhaustion in arctic waters by commercial divers as a result of wearing thick, occlusive drysuits, aggravated by dehydration from breathing dry compressed gas and perspiring from prolonged underwater swimming or heavy underwater work. Perspiration from excessive or from pre-dive overheating can also cause the diver’s drysuit underwear to lose insulation, thus predisposing him to hypothermia.