By Mark Powell
The image that many technical divers try to promote is that they are tough, macho divers. The shaved heads and stubble, combined with equipment that would make a Special Forces diver look inadequate, all add to this image. The message is that they can leap tall buildings in a single bound and endure conditions that no mere mortal could withstand. However the truth is a little different. In fact many technical divers spend as much time thinking about how to stay warm during their dives as they do planning the dive itself. The reason for this is that technical dives are more challenging than many recreational dives. They are typically deeper, longer, and colder. The intelligent technical diver recognises this and tries to protect against the cold wherever possible.
One of the main concerns of any technical diver is warmth, both from a comfort and a safety point of view. In general the deeper we go the colder it gets. A few years ago I was teaching an Advanced Trimix course in Turkey and between the surface and 80m there was a difference in temperature of 10 degrees. This meant that while we might have been comfortable on the surface you would get very cold at depth. Gloves intended to keep my hands warm at depth were uncomfortably warm during the decompression. A more common problem is that technical dives tend to be much longer than recreational dives and will usually involve long decompression stops. Even if there is no difference in temperature during the ascent it is common to get colder during the decompression. I don’t normally get too cold during the main part of the dive due to choosing the right undersuit, swimming around and the fact that my mind is occupied, but during decompression it is much more common for me to feel the cold. During long decompression stops you are not swimming around or moving to any great extent and your mind is not as occupied. It is during these long periods of mental and physical inactivity that I often feel the cold.
In addition to the comfort aspect of getting cold there are a number of very real safety issues involved. The cold can slow down your reaction speed and thought processes which, when combined with Narcosis can affect how quickly and effectively you react to problems. Cold hands can also quickly lose dexterity which can make it difficult to effectively handle equipment, operate computers, switch to decompression gas, etc.
Most importantly there is strong evidence that getting cold towards the end of a dive can increase the risk of decompression illness. During the early parts of the dive the diver is fairly warm and blood flows to all parts of the body. Nitrogen and Helium are carried by the blood flow to the whole body and the level of the inert gases dissolved in the body increases. Towards the end of the dive, during the ascent and decompression stops, the inert gas is normally carried by the blood back from all parts of the body to the lungs. However when we get cold one of the body’s reactions is to restrict blood blow to the extremities and concentrate it in the main core of the body in an effort to conserve heat. When the skin temperature starts to drop the body will reduce the circulation in your limbs, this is known as vasoconstriction. This has the effect of reducing the amount of blood flowing to the heavily inert gas-laden tissues in the extremities which in turn reduces the level at which Nitrogen and Helium off-gas from these extremities. In effect, being cold has slowed down the release of inert gas from these tissues. As a result the speed of off-gassing is slower than predicted and the excess Nitrogen and Helium in these tissues can cause decompression illness. Recent research has shown that this can cause a significant increase in the risk of decompression illness.
Due to the risk of decompression illness, suit floods and leaks are much more important to a technical diver. I know many sport divers whose suits regularly leak and for them it is merely an inconvenience however for a technical diver it can be a much more serious problem. Similarly a flooded suit can be cold, uncomfortable, inconvenient and irritating on a recreational dive but on a long decompression dive it can also be very dangerous. As the diver gets cold their decompression become less efficient. This means that at the very time they want to get out of the water they need to stay longer to make up for the inefficient decompression. Here it is a balancing act between the dangers of hypothermia and decompression sickness.
It is a combination of the comfort and the safety that makes technical divers so concerned about the particular design of undersuit they use. There are a range of undersuits that use different materials to keep the diver warm. In addition to keeping the body warm it is vital to keep your hands warm. As we have seen cold hands can cause problems with dexterity making it much more difficult to carry out certain tasks. For this reason a technical diver will also think long and hard about their choice of gloves for long cold dives. One option is to simply wear thicker gloves. The extra thickness helps provide better insulation for the fingers although the thickness of the gloves also results in reduced dexterity. The same problems apply with 3 finger mitts. These mitts have one compartment for the thumb, one for the index finger and a third compartment which contains the remaining three fingers. Having the three fingers together helps to keep them warm although the thumb and index finger get just as cold as in standard gloves. In addition having the three fingers together significantly reduced dexterity for some tasks and can make some standard hand signals difficult if not impossible. Again there is a trade off between warmth and dexterity. By getting cold we can lose dexterity but many of the gloves designed to keep our hands warm have the side effect of reducing our dexterity anyway!
Another option is dry gloves. These are gloves that are fully sealed against the drysuit and allow no water into the glove. In some designs the glove is linked directly to the airspace of the drysuit. This removes any problems with equalising the two air spaces but has the risk that a punctured glove can cause a complete suit flood. Other designs have a standard seal on the dry suit so that the dry gloves are a separate air space. In this case some mechanism is required to allow equalisation of the air space in the gloves so that the diver doesn’t get uncomfortable squeeze on their hands on the way down or excessive expansion of air in the gloves on the ascent.
The typical image of a technical diver is some macho tough guy however, as we have seen, the reality is that technical divers are more concerned about keeping warm and comfortable than most other divers because the consequences of getting cold can be far more serious on a decompression dive than on a recreational no-stop dive.
by Mark Powell