Easy Diving | Weights
Carrying the right lead is a basic skill far too many divers get wrong
Fact – a significant percentage of recreational divers are over weighted
Fact – most of them don’t realize it
Fact – it’s a problem that’s really easy to fix
For many divers, not necessarily limited to the novice category, the concept of weight distribution means having the same amount of lead in each pocket, and trim suggests the possible involvement of a hairdresser during the dive briefing.
Overweighting is one of the most chronic problems that I have to deal with as a dive guide, with issues that range from minor annoyance through increased air consumption, all the way to human champagne cork impersonations (you know, pop, bubbles, and whoooosh! Up they go!)
Even experienced divers may find they can shed a couple of kilos from their weight-belts (if not their mid-riffs), and maybe after that they find their air consumption improves and they don’t need to fork out extra for the twin-set and pony bottle every time they dive the house reef. Weighting is a concept that I think is often poorly taught, poorly understood, and poorly practiced, so let’s get back to basics
Five easy steps to help you conduct a buoyancy check at the surface:
1 Wear all your equipment
2 Enter water too deep to stand in and deflate your BCD completely
3 Hang vertical and motionless in the water while holding a normal breath
4 Add or subtract a small amount of weight until you float at eye level while holding a normal breath
5 As a final test, exhale. You should sink slowly
If you are correctly weighted and you try to descend while maintaining a normal breathing pattern, you will remain firmly planted at the surface: positive, negative, positive, negative, not going anywhere. In addition to this, inexperienced divers sometimes try to flap themselves downwards using their arms, which doesn’t work and just increases the diver’s workload making them breathe harder and therefore remain perpetually positive. The end product is a diver who may well be correctly weighted, but can’t actually sink.
This is where instructors and guides start throwing extra weight at divers to get them under the water, instead of taking a few minutes to explain some basic physics.
As divers out of the water, we are wearing heavy, bulky, cumbersome equipment, but underwater, weightlessness can only be achieved by understanding that this equipment – specifically the weights and the BCD – are part of a system that also contains the human body – specifically the lungs and the legs.
If you breathe out, you are negatively buoyant and will descend. If you breathe in, you’re positive, and won’t. If you kick your legs, you will propel yourself back to the surface. If you flap, well, you look a bit silly.
Relaxation is important. Focus on your breathing; long and slow breaths, get the hose up, remain vertical to decrease water resistance, make your weight check and then to descend, breathe out deeply. And for goodness’ sake, stop finning! Exhale all the air from your lungs to make yourself negative and hold it out for a few seconds. If you need to take a breath, make it small and shallow and exhale deeply again. Once you’re a couple of metres or so underwater, you can resume normal breathing… long and slow.
Don’t forget that a standard 12 litre aluminum cylinder will lose up to 2kg of weight between full and empty, so the most accurate weight check should be made with a light tank – around 30 to 40 bar. Regulations differ, but at Sinai Divers in Sharm where I work, we safety-stop at 50 bar. We don’t want floaters at sites like Shark Reef, which are heavy with boat traffic, and we therefore try to ensure all of our divers are correctly weighted before they venture into 800m deep water.
I mentioned human champagne corks earlier, in the same sentence as over-weighting. How is it possible that too much weight causes excessive floating?
The general physical principle is Boyle’s Law – that the volume of a gas is inversely proportional to the pressure to which it is subjected – meaning greater pressure equals smaller gas volume. This is what causes packets of peanuts to expand in an aeroplane, where the pressure is less than at sea level and why we have to equalize our ears when we dive. It also causes submarines to implode in Hollywood movies.
We know this from diving – we have to add air to our BCD to overcome the effect of the pressure that is exerted upon us as we descend. What we should not be doing is also adding air to our BCDs to overcome the effect of the weights we are wearing. To do so means we are over-compensating and even a small ascent will cause that extra air to rapidly expand and overwhelm the negative effect of the weight. This is what causes divers to rise uncontrollably to the surface.
Look for the classic signs of overweighting – give your BCD a squeeze underwater, at any given depth it should be almost empty. Assuming you are operating it correctly, adding air during descent should be nothing more than a short, sharp, pffft on the inflator button from time to time. Other things to watch out for are rapid descents, constantly bobbing up and down in the water, inability to remain horizontal, inflating and deflating all the time or having to inhale sharply and possibly breath-hold to prevent unwarranted head/coral interaction.
It is sometimes helpful to deliberately overweight divers just to get them underwater where they can hopefully start to relax. When I do this though, I stress to the diver that the extra weight is unnecessary and they should work on removing it over the next few dives, because while correct weighting is so vital to diving, sometimes people just need to spend some time underwater before the concept of buoyancy becomes meaningful!
Correct weighting means a diver will have a better position in the water – or is, at least, pointing in the right general direction – which makes control and streamlining easier, increases the efficiency of fin kicks and creates more relaxed and comfortable divers with better air consumption. Now it’s time to start looking at weight distribution, trim and different gear configurations – and as for the secret to using less air… that’s a whole new article!