Thermal clothing for diving: Heat when you need it – and the battery stays outside the drysuit
In winter, our lakes often offer fantastic visibility. However, when the water temperature is four degrees, even the best base layer will eventually become uncomfortable. Electric heating systems have therefore become standard kit for many divers. But heat affects more than just our comfort: it alters the absorption and release of inert gases. And an electrical fault can become a serious problem underwater within seconds.
A recent article by Stratis Kas in InDEPTH explores these connections. An accident previously described on the Dekoblog illustrates just how real the dangers can be.
Why the timing of heat is crucial
Blood circulation influences how quickly nitrogen and helium enter the tissues and are subsequently removed. Warmth promotes blood circulation, whilst cold causes vasoconstriction, particularly in the body’s periphery.
During the no-decompression limit, increased blood flow can raise the uptake of inert gases. During ascent, however, it aids the release of the absorbed gases via the blood and lungs. The combination of warmth at depth and cold during decompression is therefore particularly unfavourable: initially, gas absorption is facilitated, whilst subsequently, the elimination of these gases is impeded. Studies by the US Navy emphasise that the timing of heat exposure can significantly influence the risk associated with decompression. However, this does not justify a blanket reduction in decompression time for divers using heating.
It is precisely this unfavourable pattern that can arise if the heating is set to a high level during the bottom time and the battery later runs flat. The heat is then lacking, of all times, during the long, low-activity decompression phase.
The heater complements cold protection – it does not replace it
The drysuit and undergarments must be chosen so that the entire dive can be completed safely and without hypothermia, even in the event of a complete failure of the heating system. This includes all decompression stops and any return journey from a cave or wreck. The decisive factor is a failure at the worst possible moment.
Active heating must never be a prerequisite for safely completing the planned dive. Anyone who, without a functioning heating system, cannot manage the remaining decompression without suffering from hypothermia is inadequately insulated for that dive. In that case, warmer base layers or a correspondingly shorter dive are required.
Cold impairs our ability to act
Hypothermia is not just a problem during decompression. It can severely impair our psychomotor functions: thinking, perception and purposeful action become increasingly difficult.
This directly affects our diving skills: operating valves, switching to a different regulator, opening a boltsnap or deploying a buoy. Well-rehearsed movements become slower, less precise or can no longer be performed reliably. At the same time, judgement and problem-solving abilities may deteriorate. A situation that would otherwise be manageable can thus become dangerous.
Even experience and training do not protect us from the cold restricting our ability to act. Adequate thermal protection is therefore essential for being able to draw on one’s own abilities underwater at all.
Skin burns during a winter dive
In December 2022, an experienced 35-year-old diver undertook a night dive in a Swiss lake with two buddies. The water temperature was 4 °C and the maximum depth was 27 metres. Underneath his dry suit, he was wearing a heated undersuit with connected heated gloves. Power was supplied via an external battery pack attached to his harness.
After about 30 minutes, he switched on the heating. Within a few seconds, he felt a burning pain on both shoulders. He switched the system off immediately and disconnected the cable from the battery pack. The team aborted the dive whilst still within the no-decompression limit.
After getting out of the suit, second-degree burns with blistering were visible on his shoulders and upper arms. Parts of the thermal undersuit had melted.
The forensic examination revealed a cable break on the right heated glove and thermal damage to the supply cable. An overcurrent protection device was missing. The thermostat only switched off at 70 °C instead of the specified 45 °C. The battery itself functioned perfectly. The injuries healed completely with conservative treatment.
My clear recommendation: No batteries in a drysuit
This was not a battery fire. However, it demonstrates how quickly an electric heating system in direct contact with the body can become dangerous. At least the external power supply could be disconnected immediately.
I strongly advise against carrying batteries or rechargeable batteries inside a dry suit. An overheating battery inside a sealed suit is virtually impossible to reach or remove whilst underwater. Furthermore, if decompression is required or you are in a cave, the direct route to the surface is blocked.
My conclusion: the power source must be located on the outside, easily accessible so that the power can be cut off if necessary.
Staying cool at depth does not mean freezing
Moderate heating during the bottom time must under no circumstances be interpreted as an invitation to become hypothermic. What matters is the diver’s condition, not just the water temperature displayed on the computer. This does not reveal how well the peripheral tissues are being supplied with blood or how much the body has already cooled down.
During the no-decompression period, a neutral to slightly cool state is sufficient, provided it is neither uncomfortable nor impairs the diver’s ability to act. During ascent and decompression, cooling should be avoided. If more heating is required, it should be increased gradually and in good time. First allowing the body to cool down significantly and then heating it up to the maximum is not a good strategy: rapid warming of gas-saturated tissues can promote bubble formation.
Conclusion
During the no-decompression period, stay just cool enough that it does not become uncomfortable – during decompression, make sure you do not get cold. Cold also impairs our psychomotor abilities and thus our ability to apply our diving skills safely.
Passive insulation must be sufficient to complete the entire dive safely and without hypothermia, even in the event of a complete heating failure.
And for me, one thing is quite clear: the battery stays outside the drysuit.
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