There is very little that is absolutely right or wrong in thermal protection. Cold tolerance is an individual consideration, relative to multiple factors, including skinfold thickness, body-mass-to-surface-area ratio, metabolic rate, activity during the dive and even one’s attitude toward cold-water diving. When it comes to selecting the right thermal protection for a dive in temperate to moderately cold water, there is really only one important factor: What do you need to stay comfortable? But as temperatures get more extreme, the only practical thermal protection is a drysuit.
Staying warm in extreme conditions is more than an issue of comfort. Cold stress has a string of ill effects that can impair dive safety. A cold diver may not be able to focus on the dive, making it difficult to effectively monitor his status or that of his buddy. Increased air consumption can dramatically curtail available underwater time, and compromised strength and flexibility can adversely affect response time in the event of an emergency.
Drysuits offer a range of thermal protection that can go far beyond wetsuits. They eliminate contact with water and allow the diver to alter thermal protection by adjusting the layers of insulating garments worn under the suit. The three common styles of drysuits — shell, neoprene and crushed neoprene — have their pros and cons.
- Shell suits are thinner and lighter, but they offer little inherent insulation beyond the undergarments. Shell suits have an advantage for repetitive ice diving if the suit is to be left unprotected in the cold. Ice crystals are shaken off as the suit is picked up for donning.
- Standard neoprene drysuits are relatively inexpensive, but the thermal protection of the neoprene decreases as the suit compresses with depth.
- Crushed-neoprene suits are the most expensive, but they offer a greater degree of stability across dive depths. However, neoprene suits, standard or crushed, are more difficult to don for repeated use in sub-zero conditions. The flattened suit has to be thawed to relax for donning, usually a few inches at a time while you try to stay out of the wind, an often difficult task!
Each layer of the drysuit ensemble contributes to the effectiveness of the thermal protection — barrier, insulation and wicking. The suit itself is the barrier layer, barring water entry to eliminate convective heat flow. The innermost layer is known as the wicking layer. This is usually a close fit or skintight layer of synthetic material such as polypropylene that keeps moisture away from the skin. The midlayer insulation reduces conductive heat loss by trapping air. The insulation is generally made from synthetic materials, commonly Thinsulate or other high-loft material.
Handwear is an important consideration in cold-water diving and a critical consideration in the frigid extremes of ice diving. The small mass-to-surface-area ratio of the hands means that they lose heat at a rapid rate, and the body’s natural defense to heat loss — aggressive vasoconstriction — can impair both comfort and dexterity.
Choosing the correct hand protection is often a dilemma, as the bulk associated with greater thermal protection will compromise dexterity. The key is to balance the protection against functionality. There is little point to having warm hands and an inability to equalize middle-ear pressure, adjust buoyancy, work a camera or complete any other required task. Wet mitts offer advantages in dexterity and low risk of catastrophic failure. Dry gloves offer the advantage of dexterity but at the cost of comfort. Dry mitts offer great comfort with poor dexterity and an intolerable state should mitt flood occur.
Making the Transition
Establishing proficiency in drysuit diving requires more than putting on a new suit. While it is recommended that drysuit divers wear buoyancy compensators, the drysuit should be the primary compensator system. This is logical since air must be added to the suit to eliminate squeeze during descent. Adding and dumping from one system is easier than using two. The buoyancy compensator is best reserved as an emergency backup underwater and for use at the surface when lift requirements may exceed the suit’s ability to hold air.
Adjusting buoyancy underwater takes some getting used to. The air in a drysuit can move through the entire length of the suit, and buoyancy changes due to attitude (body position) are more pronounced. These shifts are easily controlled by experienced and properly weighted drysuit divers but can be disconcerting to the overweighted novice. The image of all the air rushing to the feet and of fins being “blown off” in an uncontrollable inverted ascent creates a dire picture for the uninitiated. In truth, such problems are easily avoided with reasonable skill and attention, but until a diver has developed those skills it is best to have the simplest dive plans.
Drysuit Leaks
While it would be great for a drysuit to be forever dry, this may not be the case for even the best suit. Seals may be poorly set, valves may be temporarily obstructed, and materials can fail. A minor or slow leak in cold water will increase the degree of thermal stress, but the risk of progressing from minor leak to catastrophic leak that affects buoyancy control is probably quite small. Zipper failure is the greatest concern in a typical shoulder-entry suit because the gas bubble in the suit would be lost. The failure of a front-entry zipper, on the other hand, would be less important since the gas bubble would remain high in the suit. Even if the suit fills with water to the chest, the buoyancy of the remaining air would be retained. It is a myth that water in a drysuit will drag a diver down. Remember that water floats in water.
The impact of a significant drysuit leak may be put in perspective by the documented case of a diver with a failed front-entry zipper who completed a 43-minute dive in 29°F (-1.9°C) water in the Antarctic (Pollock, 2007). The diver was definitely cold and his concentration was affected, but the leak was not debilitating, and his core temperature did not drop below the 95°F (35°C) threshold that defines hypothermia. The diver surfaced with an even greater appreciation for the protection provided by drysuits. Thus, as a practical matter, repairing leaks is important, but more for comfort and to maintain the ability to carry out a dive plan than for personal safety.
Reference
Pollock NW. “Scientific diving in Antarctica: history and current practice.” Diving Hyperb Med. 2007; 37(4): 204-11.
© Alert Diver — Q4 Fall 2009