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Heated Undersuits and Decompression

 As Summer starts to fade away and the nights start drawing in the water temperature starts to drop too but do you do if you want to keep diving? Drysuits and Undersuits are much more effective at keeping you warm underwater because they maintain a layer of insulating air around your body. But even a conventional drysuit and undersuit combo can only keep you warm to a certain point so what then?

With modern technology, you can dive with a battery-powered undersuit that has heating coils built into it to keep you warm; much like an electric blanket that you wear under your drysuit.

Apart from the obvious warmth and comfort what are the pros and cons of Heated undersuits? And how do they affect your decompression?




Most heated undersuits are just traditional undersuits but with heating elements woven inside. Even without the heated elements most of these undersuits still act as a traditional undersuit so it isn’t all or nothing and if the battery runs out you’ll still stay warm. If you read the last section in this blog then having conventional insulation as well as a heated function is a huge benefit in keeping yourself warm and safe in the water.

You can also wear a lot of the undersuits out of the water as well if your boiler breaks during the winter or you just want to save on the heating bill. Normal blankets can be awkward to do anything in but undersuits are made to allow you to move and stay warm.

Warm Hands

One of the biggest dangers of cold water immersion is the loss of dexterity and the ability to perform complex tasks. Studies have proven that when your hands and extremities get cold, complex tasks become difficult and even impossible because your muscles and nerves aren’t working correctly. While diving if you need to operate a boltsnap or a pinch clip and your hands are cold, your success rate will drop and time taken to complete a task will increase. This increases your chances of an incident. In 10°C water muscle strength can fall to as much as 1/4 so demanding work can be much harder too.

With heated gloves or even a heated suit, warming your body and bloodflow you’ll stay warmer for longer and maintain dexterity as if you’re diving in much warmer waters. Without external heating, your body is the only thing that can produce heat so warming yourself up during the dive is hard.


When your body gets cold it reallocates blood flow from the extremities to your core where all your important organs are. This is why your hands tend to feel the cold first. While a good method of maintaining bodily functions, vasoconstriction can cause decompression complications. Slower blood flow through your extremities keeps saturated gases in your body and slows your desaturation rate that your dive computer will be unaware of.

By keeping your extremities and body warm with a heated undersuit your body reduces vasoconstriction and if warm enough vasodilates, increasing your blood flow around your body so you decompress more evenly and safely, reducing your chances of decompression illness. Just be sure not to over-warm because this can increase the rate of decompression to a dangerous rate. More on this below.


 As most of you may have noticed, when you go diving you often need to pee more than normal. There is a scientific reason to this and it’s basically another one of your body’s ways of keeping warm. Your body produces urine in response to being cold which will dehydrate you and again slow your decompression. By ditching fluids there’s less volume your body has to warm up to keep warm.

Feeling the urge to pee on a dive is a natural occurrence and it’s mainly down to your skin feeling cold while your core is still quite warm. This in-balance tells your brain that it’s cold outside and it starts producing urine. Keeping your core and your skin warm will reduce the body’s urge to pee and keep you better hydrated so you can decompress more efficiently. The better hydrated you are then the more efficient your body will work at decompressing.

Breathing Rate

Coldwater has some surprising effects on the human body. Some effects like the Mammalian Diving Reflex can help by reducing heart rate to prepare you for a dive but Cold Water Shock rapidly increases your breathing rate and can cause hyperventilation. In and around the surface of the water is a dangerous place to hyperventilate.

Jumping into cold water with insufficient protection will send your body into shock but if you’re nicely insulated then it won’t get to that stage. Keeping warm while on a dive will help keep your breathing rate as slow as possible so you don’t rush through your gas.


 After a cold dive, many diver’s first response is to warm their extremities on a radiator or a hot drink. By rapidly increasing the temperature of certain tissues locally speeds up the decompression in that area that has shown to create silent bubbles. Less serious than some alternatives but silent bubbles can cause further complications.

If you don’t feel cold at the end of a dive then your first reaction is less likely to be to rapidly warm yourself up. If you feel warm post-dive then you are less likely to rapidly decompress certain areas and decompress at a natural rate.




The Cost

Heated undersuits have a higher price compared to traditional undersuits because of the extra parts and craftsmanship that goes into them. On top of that, you may also need to invest in a separate battery if one doesn’t come with the suit and external batteries will need some kind of special valve to get the power inside of your suit.


External and Internal batteries both have their pros and cons. External Batteries are great so that if anything should go wrong you can always disconnect the battery. The downside to this is that you need to run the power cable through the suit which can compromise the integrity of the suit and another potential leak point.

Internal batteries have no risk of causing the drysuit to leak but are inaccessible once you don your suit. Should your drysuit leak then you need to be careful of the battery inside your drysuit from coming into contact with seawater and reacting.


Some heated undersuits use a chemical reaction to produce heat but these can be hazardous. Once you start the chemical reaction it just works at its rate so you have little control over how warm it gets and you can’t just stop the reaction when you get too warm. These types of suit are typically not designed for scuba diving so they haven’t been tested at pressure or in contact with saltwater or enriched gas mixes which can have hazardous reactions to the chemicals used so they are not recommended for scuba diving.



Decompression is the main consideration when diving with a heated undersuit because as much as you decompress faster as you ascend you also on-gas faster. So when considering a heated undersuit you also need to consider the effect it has on your decompression, not just your comfort.

Decompression and thermal stress deserves it’s own topic here and there are plenty of scientific studies online you can read up on so if you are considering a heated undersuit for particularly challenging dives then I’d recommend doing some research on the subject first. 


Warm-Warm sounds like the best option, where you’re warm throughout your dive, however, studies have shown that divers who wear hot water suits throughout the whole dive have a higher rate of decompression sickness. If you’re warm during both the descent and ascent phases of the dive (Warm-Warm) then you do have a higher chance of a DCS compared to other profiles.

Warmer body tissues absorb dissolved gases faster than cold ones so it all skews your decompression tables and if you on-gas fast at the beginning of the dive and then your heating stops so you become cold then you off-gas slowly, which is a dangerous combination.


Warm-Cold is probably the worst scenario because you are absorbing plenty of gas into your tissues when they are warm but when it comes time to decompress your tissues are less efficient, trapping the gas inside and increasing your chances of DCS. This is effectively wearing a warming undersuit and it malfunctioning or running out of power during your dive before or as you ascend.

Studies have focused on this aspect of externally heated undersuits and found that on dives where you’re warm at the beginning but cold at the end (Warm-Cold) increase your chances of DCS whereas cold at the beginning and then warm at the end (Cold-Warm) showed much lower rates of decompression illness, even when bottom times were extended.


Cold-Warm is the best profile for avoiding DCS. If you’re cool when you descend and for your max depth then you’ll be less saturated as you start to ascend. During the ascent and stops phase if you then turn on your suit then you will off-gas faster.

The best model for avoiding DCS is Cold-Warm but a rapid temperature increase during the ascent phases of the dive can cause localised issues mentioned earlier so it’s best to gradually warm during ascent.


More information can be found when you search for thermal stress and decompression online but as a general rule; heated undersuits have their benefits to keep you functioning correctly but use them strategically and consider your decompression while doing so.