On moving effortlessly underwater....
Most divers associate being underwater with moving in three dimensions, gliding smoothly and without effort over pristine marine environments and historic ship wrecks. In truth though, the reality is somewhat different. Regardless of the environments in which we end up diving in, attaining and maintaining neutral buoyancy is the most difficult, most important and most misunderstood technique in diving.
Why do we wear weight?
I often hear people say that they are very "floaty". Can we dispense of this myth straight away please? Imagine you are in a swimming pool, without a wet or dry suit. Do you float or sink? In the many thousands of divers that I have observed, I have yet to find anyone who really floats. The vast majority of people are neutrally buoyant at best, and most need to "tread water" in some form in order to stay afloat. Note this applies to everyone; ectomorphs and endomorphs, large and small, skinny and overweight.
Buoyancy is the difference between the weight of water displaced by the diver versus their weight. Almost universally, humans displace less water than their weight and hence sink. (As an aside, **limbs don't float either**-no one has "floaty" feet).
To take this one stage further, if an individual attempts to remain afloat whilst wearing a steel cylinder (or a full aluminum one) and a deflated BCD, they will sink.
So, we do not need weight to offset any physiological propensity of an individual to float. Why then do we need to wear weight in order to get underwater?
Exposure protection = trapped gas.
In order for exposure protection to work, it needs to insulate. Gas is a more efficient insulator than water, so in some form or another all diving suits use it to insulate. In the case of a wetsuit, the neoprene contains bubbles of gas (in the best ones, nitrogen) which serve to insulate a warm layer of water trapped between the suit and the diver's skin. Perhaps more obviously, drysuits insulate using air trapped in the suit to achieve the same effect.
In both cases, this gas creates buoyancy. Hence we need to wear weights to offset the inherent flotation caused by wearing insulation. Logically, the more gas in the exposure protection, the more weight the diver will have to wear. Equally, the more gas in the diver's insulation, the warmer the diver should be.
To be more scientific, for each litre of gas in your exposure protection, you need to add a kilogram of weight. If you wear 8 kilos of weight, you are offsetting 8 litres of gas and so on.
How much weight do we need?
Well, given the above, this should be relatively simple to calculate. You simply need to figure out how much gas is contained within the insulation system. However, practical reality makes this more complicated. Bluntly put, each individual's buoyancy varies. As we breathe in, we effectively become less dense and hence more buoyant and when we breathe out, less buoyant.
Breathing effectively creates a shift in our buoyancy. So the only way to test this is by getting in the water and trying it.
To test for this, you simply need to find out how much weight you need to begin your descent. This is directly proportional to the volume of gas in the exposure suit and in our lungs. Of course, we can decrease this by simply exhaling, or beginning our descent with nearly empty lungs.
Crucially however, there should be an absolute minimum volume of gas in either BCD or drysuit. Ensuring that both are empty is critical.
So if you vent all the air from your buoyancy and exposure protection, and then breathe out, you should start to gently descend. To test for this, you should also be able to gently float while holding a normal breathe and then sink slowly as you breathe out.
Whilst body size makes little difference to buoyancy characteristics, bigger people's wet or dry suits contain more gas, so they will need slightly more weight than a smaller person.
What about the "weight of air?"
Many texts will suggest that you add "weight to compensate for the weight of the air you consume during a dive". This is somewhat misleading. With some types of cylinder, there is a buoyancy shift as the cylinder empties. For example, the popular Luxfer 11.1 litre or 80 cu. ft. cylinder goes from 1.1kg negative when full to 1.8kg positive when empty. Typically, steel cylinders do not have any buoyancy shift that will affect buoyancy so this does not need to be be factored in when checking for the amount of weight to be worn.
Why is being correctly weighted so important?
1. It allows you to control descents.
Being correctly weighted allows you to control your descent. Being able to use a minimum amount of air in your BCD or drysuit and your breathing to control your rate of descent allows for pin point accuracy and slow, safe descents.
2. It slows down your ascent.
The benefits of correct weighting actually slow your ascent. When overweighted, you have to add more air to your BCD/drysuit in order to become neutrally buoyant (at 1l of gas per extra kg!). As you ascend, this air expands and this can result in an uncontrolled ascent. If you cannot hold a stop at 5m, this is due to overweighting not lack of weight. Remember to get underwater, you have to be wearing sufficient weight to do so.
3. When using a drysuit, the air does not distribute itself throughout the suit, but travels around the suit as a bubble. This will travel to the "shallowest" part of the suit. Hence it causes instability. This can be manifest as tipping, swimming with the hands, ascending inverted and swimming in poor trim. If your feat are floating it is actually a sign of overweighting. Adding ankle weights is actually compounding the problem.
4. It reduces air consumption.
Overweighted divers often end up in a poor position underwater which creates drag and causes silting and environmental damage. We tend to wear weights around our waists in some form. Too much weight pulls our bottom half down, which is further exaggerated by all the air that is used to offset this migrating up towards the head end. When you are diving, you should be able to tuck you chin into your chest and look down the length of your body and not see your legs or feet.
5. It allows for better and easier neutral buoyancy.
We recently had two students come to do Drysuit Adventure Dives with us. Their dive school had them in membrane type drysuits on their Open Water course with 18 and 20 kg of weight each. Apart from finding diving very difficult, they simply could not experience neutral buoyancy with this amount of weight. After doing weight checks, we established that they actually needed 7 and 8kg in order to be correctly weighted. Their experience of being underwater was transformed.
Contrary to poplar belief, you do not need more weight with a membrane type drysuit that you would need in a thick (5mm) semi dry. In fresh water in a membrane suit, the average adult needs no more than 8kg of weight. Smaller adults and children will need less, down to around 4kg.
Under very cold conditions, when wearing a lot under the drysuit for warmth, you may need to add more.
A two piece 5mm semi dry will need 9-10 kg in fresh water, again on an average adult.
A 3mm one piece in fresh water will need no more than 2kg.
To convert the above numbers for salt water, typically add 2 kg. Salinity makes little difference, for example the Northern Red Sea is one of the most salty seas in the world, yet it is only 4% more saline than the others. This would account for a 4% difference in weighting only.
To emphasize, these are guidelines, not rules. The only way to find out precisely how much weight you need is to carry out an effective weighting check in the water.