With Winter upon us the water temperature around Sydney is dropping and I already experienced water below 16ºC at Shiprock over the June long weekend.  As the water temperature drops it gets much harder to stay warm for an entire dive and we end up cutting our dives short due to cold rather than running low on air.  When I check my dive logs for past years I see that the water temperature in Sydney is frequently below 16°C most of the time between June and November and I even have a dive in late November 2010 when it got down to 13.2°C.  I remember how cold I was on that dive.   What can we do to stay warm?

The problem of staying warm in water is due to a bit of physics.  Water is a good conductor of heat and so heat is drawn away from our bodies and lost in the water.  Air, on the other hand, is not such a good conductor and can actually works well as an insulator.  This is why being in water at 15ºC feels so much colder than air at the same temperature.  In water we lose body heat quite quickly.  In the air we just have to put on a few layers of clothes which trap warm air around our bodies greatly reducing the rate at which we lose heat.

There are three may ways to combat the heat loss in water, or at least slow it down long enough that we can feel warm for an entire dive. They are:

• Wet suits
• Semi-dry suits
• Dry suits.

Wet Suits

We’re all familiar with wet suits and they successfully keep us warm if the water is not to cold.  They work in two main ways.  First, the neoprene in the suits is a good insulator because it has tiny gas bubbles trapped in the fabric.  The gas acts as an insulator.  Second, the wet suit traps water between the body and the suit so that the water warms up by absorbing body heat.  If the flow of water in and out from under the suit is restricted, the trapped water stays quite warm.

Ideally a wet suit should be tight fitting to reduce the amount of trapped water and also to reduce the amount of water flowing in and out.  If you lose weight, the suit may become too loose and be less effective for insulation.

There are two main problems with wet suits.  First, water still flows in and out of the suit and so the warm water is lost and replaced with cold water.  Secondly, gas bubbles in the neoprene compress with depth reducing the insulating properties of the neoprene.   Additionally, the more you move the more water will flow in and out.  If you move more slowly, for example while taking photos, there will be less flow.  Unfortunately, less movement also means less body heat generated so you still get cold.

A thicker wet suit, for example, 7mm instead of 5mm, will provide more insulation but as the water gets cooler even this may not be enough.  Additionally, a thicker wet suit is more buoyant and so more weight is required.

Another alternative to improving the insulation from a wet suit is to wear an insulator vest under the suit.  This provides an extra layer of insulation and helps to trap water under the suit better.  St George Underwater has a range of vests which may help.

Ultimately, a wet suit just isn’t going to cut it in very cold water.  The water temperature at which you’ll still be comfortable in a wet suit will vary from person to person.  It also appears to vary with age.  Last Winter I did OK with water down to around 15ºC in a two piece 7mm suit with an insulator vest underneath.  This year in the same gear I’m already struggling at around 16ºC.  Most people will struggle in a wet suit if the water temperature is below 15ºC.

Semi-dry Suits

A variation on the wet suit is the semi-dry suit. It works on the same principle as a wet suit but has seals on the ankles, wrists and neck to prevent or at least greatly restrict the ingress of water.  The zipper also seals restricting water flow.  Water will still get into the suit but there is less of it and almost no flow in and out.  This makes it a bit easier to stay warm, or at least stay warm longer.  The seals are similar to those on a dry suit and need to be quite tight and it is important to have a good fit.  Like a wet suit, a semi-dry needs to be tight fitting.

Dry Suits

Dry suits are seen as the ultimate method for staying warm and are even effective in arctic conditions.  Dry suits have very tight seals around the wrists and neck and the boots are built in.  A proper fitting dry suit will not let any water in at all.  By staying dry, a dry suit provides much better insulation because a layer of air is trapped around the body.  Additionally, you can where one or more layers of clothes under the dry suit to provide extra insulation.  This makes dry suits more flexible as you can add or reduce under garments to suit the water temperature.

There are two main types of dry suits: membrane and neoprene.  Membrane suits (also called shell suits) are basically just a layer of membrane (rubber, nylon etc.) which simply provides a seal around the body.  The suit itself doesn’t provide insulation or buoyancy.  Insulation is dependent on the under garments and buoyancy from the trapped air.  You can probably get by carrying less weight with a membrane suit.

A neoprene dry suit sort of combines a wet suit with a dry suit.  The suit still provides a sealing outer layer but the neoprene provides both insulation and buoyancy.  You may still need to wear under garments but can usually get by with less.  The extra buoyancy may mean you need to carry more weight.

As a dry suit has air trapped inside the suit it needs to be hooked up with a low pressure air feed from your regulator.  As the depth increases the air inside the suit gets compressed and so more air needs to be added, just like with a BCD.  As the depth decreases, the air expands and needs to be purged.  This needs to be considered when diving with a dry suit.

Unlike wet and semi-dry suits, a dry suit is loose fitting to allow for under garments and a certain amount of air.  The wrists and neck need to be very tight fitting to ensure no water gets in.  Ideally the dry suit should also be the right length and also the right girth so it allows just enough air and not too much.  If the suit is too long, for example, there well be rolls in the outer layer trapping more air and making the suit more buoyant. Trying to get rid of that air will result in other areas of the suit not having enough air and the suit will press tightly on your body reducing the amount of insulation.

While you may get lucky and find an off the rack dry suit that fits well, in many cases a custom made suit is necessary.

Diving with a dry is a bit different than diving with a wet suit.  The air in the dry suit provides buoyancy and you have to be mindful to add or dump air as your depth changes.  The suit also provides buoyancy in different places to what you might be used to and so it may take some time to get your balance correct.  One particular thing to be careful with when diving with a dry suit is air getting trapped in the legs forcing your legs above your head.  Most dry suits have automatic valves to help combat that problem but they will need to be adjusted correctly.

Dry Suit Trial

Last Winter I had a trial of an Apollo 4mm neoprene dry suit from St George Underwater.  I did 5 dives in all with water temperature ranging from 16.3ºC down to 14.9ºC.  This was an off the rack suit and as I have very small wrists. I had a lot of problems for the first couple of dives keeping dry with water leaking in around the wrists.  I pretty much solved the problem by using Apollo Bio-Seals (separate items sold at the shop) around my wrists and neck.  These are basically elastic straps worn on the wrists and neck under the seals on the dry suit and I found them to be very effective.  I was completely dry for the last two dives, which was lucky as the water temperature was 15ºC and 14.9ºC respectively.

I had no problems at all adjusting to using the dry suit and found it very comfortable and quite warm, especially when I stayed dry.  The only other issue I had with the suit was the fit.  The suit was a but long for me and so I had rolls in the legs, arms and torso.  This trapped air in some parts of the suit and meant I didn’t have enough air in other pasts so the suit was pressing against my body making it feel cooler.  Despite this problem I was still warmer in the dry suit than I would have been in my two piece 7mm suit.

While the dry suit was warmer and more comfortable I did find a few disadvantages:

• The zipper goes across my shoulders at the back so I always needed someone’s help to get in and out.
• The suit was very buoyant so that I need to carry the same amount of weight as with my two piece 7mm suit.
• When the suit got wet on the inside it took a lot of time to get it dry.

The advantage of being warm and comfortable for the dive more than makes up for those minor disadvantages.

When I buy a dry suit, and I will almost certainly buy one at some point, I will get it custom made to ensure good seals on my wrists and neck and also to make sure the length is correct.  I would also request a front zipper.

Try a Dry Day

How do you know if a dry suit will work for you?  As I said earlier, dry suit diving is different from a wet suit.  I know some people that just didn’t get used to it.  The best idea is to try one so you know if it works.  This is especially true to determine if you can get by with an off the rack suit or you’ll need one custom made.
St George Underwater is having a

Try a Dry Day

on

Sunday 24th June 2012.

Call the shop and book your spot to come out with Apollo and try diving in a dry suit.  You’ll get a good idea how it feels, how different it is, and most importantly how much warmer it can be diving in cold water.  You’ll also know if an off the rack suit will work for you.