Bantin's Blog - Using Your BC.

Be warned, this is a particular hobby-horse of mine.

Dame schools were an early form of a private elementary school in English-speaking countries in Victorian times and they were later considered inadequate because they often relied on the older children teaching the younger ones what they had previously learned. In this way misinformation was often compounded and no new ways of teaching could evolve using this method.
Because diving is taught in the same way as those Dame Schools, a lot of technique gets passed down from generation to generation and in that way it does not evolve. A classic example is that of the way we release air from our BCs during and ascent. You've probably noticed that your BC comes with a selection of dump valves but only one inflation valve and conventionally this is positioned at the end of a long corrugated hose.
This was because when BCs were first invented they had no direct-feed of gas from the diver's tank. They had to be fed orally, the diver taking out his regulator and blowing some air into the oral inflation valve when required. Similarly, during an ascent, excess was released through the same valve, holding it high above the rest of the BC. To enable both of these manoeuvres, the oral inflation valve was positioned at the end of a flexible corrugated hose. Most BCs available today still feature this hose although they don't actually need to. After all, a drysuit is used in the same way, adding air during a descent and releasing it on the way back up, but without the need for any corrugated hose attached to the suit. A hose-fed inflation valve and a dump valve are all that's required. If the corrugated hose were essential, all drysuits would have them.
Indeed, there are some BCs available today that are equipped just like a drysuit with only an inflation valve and dump valves and no corrugated hose yet they have not proved popular in the marketplace. Why not?
Diving instructors were not born underwater. They all had to learn to dive in exactly the same way you did. The first ones to learn with a BC were taught to blow air in via the oral inflation valve and release it in the same way. At that time it was the only option. Once they became instructors they taught their trainees in the same way.
Once direct feeds were available, they were eventually adopted but old school instructors did not want to forego the original method of use complete with the corrugated hose. Those they taught, who went on to become instructors themselves, compounded this phenomenon.
Fast forward forty years and instructors are still teaching trainees to dump air using the oral inflation valve, raising the corrugated hose to do so. Why? Because that is how they were taught themselves.
So what about those dump valves that most BCs also come with? As usual, there will always be some misinformation offered to make the excuse for not using them. Some say that if you pull the cord on a shoulder-mounted dump valve it releases all the air from the BC in one quick whoosh. That isn't true. Try it. They are as progressive in their action as the older method.
In fact, many of these valves are so designed that they release the air without letting much water back in the other way. That's not what you can say about using the oral inflation valve.
Some divers end up with their BC full of water. Even if you do, by fully inflating the BC while floating at the surface and pulling the lower dump valve, the one located near your kidneys, you can squeeze most of this water out before you need to climb a boat ladder. Remember, a litre of water weighs a kilogram once you are airside and that's more weight to carry and climb the ladder with.
Don't be afraid to use the dump valves of your BC. They are positioned in the right place (the lower one is useful when making a quick head-down descent from the surface and you need to lose surface buoyancy) and are designed for the job. As I said earlier, there are some BCs available that eschew the corrugated hose altogether, treating it as something of an anachronism.
So what is the oral inflation valve actually intended for? If you are floating at the surface and need to top-up the air in your BC, you can do this by mouth just as you might top-up the life jacket that is stowed under your seat in the aircraft you went on holiday in. Of course, there are some kit manufacturers who pander to the resistance of some divers to take on board modern developments in kit and these provide BCs with only a corrugated hose and no dump valves. They still work just as you could even use an inverted plastic carrier bag for buoyancy control. It's just not as convenient!

Happy diving - John Bantin

Bantin's Blog - Choosing a BC

CHOOSING A BC

You may have read my blog explaining buoyancy control and why you need a BC. Actually, you need a BC for three distinct purposes. The first is to enable you to wear your tank on your back, or even side-slung as is a modern fashion with a few divers who want to emulate intrepid cave divers.

The second is to give you support or flotation at the surface. When inflated, you need it to float you comfortably in a vertical position. In fact, the earliest BCs were called Adjustable Buoyancy Life Jackets although with no permanent flotation this was something of a misnomer. The third reason is to allow you to use it for buoyancy control.

Ask on a diving forum what sort of BC you should buy and the inevitable answer will be "a wing-style BC with a stainless-steel backplate, because you're going to need it one day." They assume you are going to eventually become a technical diver.

It's true to say the majority of dives done world-wide are with single tanks so telling you to get something suitable for use with multiple tanks can be very misleading. People who get into multiple tank technical diving are very much in a minority and even they will be using inappropriate equipment if they use such a BC for an ordinary leisure dive while away on holiday.

A conventional jacket-style BC is far more appropriate for this. Why? Because such an item usually comes with one or more pockets for carrying ancillary kit such as a surface marker buoy or safety sausage and other items such as a reef hook or a small underwater lamp.

A lot of nonsense is often talked about attitude in the water, citing the idea that a wing-style BC will let you swim in a better horizontal position. The truth is that the air in either style of BC will always migrate to the highest place under water, which is directly behind you shoulders in both cases. The difference comes when you fully inflate the jacket-style BC because once the top of the buoyancy cell is inflated, the lower part that is positioned towards the front underwater also inflates and that should sit you upright in armchair comfort while you wait to be picked up by your dive boat.

So what's the point of a wing?  A wing-style BC can have an enormous buoyancy cell that will be adequate to float you and several heavy steel tanks. At the same time it offers an uncluttered chest area so that a diver can hang all the regulators that go with those tanks, while the metal backplate offers more locations to stow kit.

Loads of potential lift must be a good thing, mustn't it? As a rule-of-thumb, you don't need more lift than you have weight on your belt. That is especially true when using aluminium tanks popular at leisure diving centres abroad. A big wing, fully inflated, with tend to thrust you face down at the surface if used with a single tank. It's not the right tool for the job. A BC with, say, only 10kg of maximum lift can be quite adequate for this purpose.

If you like the idea of the uncluttered chest area, you can get a leisure diving wing-style BC with a more appropriate size buoyancy cell. It's a question of personal choice.

When you come to pack your bag to travel, it will be apparent that the BC might be the heaviest item of diving equipment you have. A good solidly built BC might be very hard wearing and great for diving locally but at this time you'll wish you had a more lightweight one. The cost of an extra BC for foreign holiday use can soon be recovered in the savings on excess-baggage charges. Whether conventional in layout or a wing, there are now many lighter weight BCs available and some can even be rolled up to form a compact shape in your bag.

Get the right tool for the job. Don't be bullied into getting the wrong BC just because a disinterested party on the Internet has given you a bad steer. When you visit Mike's Dive Store, explain exactly what sort of diving you need it for and the staff there will help you to choose what's appropriate for your needs. It's in their interest to make sure you are happy because happy customers are regular customers. There's a large range of different solutions to the BC choosing problem and it's important that you end up buying the right one - and if you do take up cave diving or feel you want to side-mount your tank/s, they'll be ready with the right solution for that too!

My next blog will be about actually using your BC under water. It includes a particular hobby-horse of mine.

Happy diving - John Bantin

Bantin's Blog - Buoyancy Control

Buoyancy Control

It amazes me that I still see divers who do not seem to have managed to master good buoyancy control.

Ships float because they displace a greater weight of water than they weigh themselves. That's because, although they are made of steel, they have a great amount of airspace within them. A man in a lifejacket floats for very much the same reason. Although, by and large, the human body is neutrally buoyant the additional flotation of the lifejacket tips the balance in favour of floating. Alas, our heads tend to be heavier than the water they displace, which means recently drowned people are usually found floating head-down so it's important to wear the lifejacket in such as way as to support the head.

As divers we contrive to be neutrally buoyant. That is to say we neither float up nor sink. We simply want to be able to go effortlessly in the direction we choose. Of course, some of our equipment is heavy whereas other parts would float it left on their own. The wetsuit, made of neoprene, if full of tiny bubbles of material that contain gas. Together, the aggregated effect of these floating and sinking parts usually results in a diver needing to add some ballast in the form of lead weights.

The biggest influence on our buoyancy is our own lung volume. It can vary as much as six-litres by simply inhaling or exhaling. By controlling how much you breathe in or exhale you can make adjustments to your buoyancy far quicker than fiddling with some external device. If you find yourself starting to float up, exhale before adjusting your BC. Similarly, if you find you are descending more than you like, initially inhale a larger lung volume.

When you learn to dive the instructor usually loads you up with more lead than you would otherwise need. This is because he needs you to kneel comfortably on the bottom while you learn new diving skills. New divers also tend to be a little anxious and that entails them breathing more heavily than they would do otherwise and big lung-volumes add buoyancy.

Good instructors teach buoyancy control. It's quite difficult to master this in the shallow water of a pool because the dramatic depth changes in the shallows has such a quick effect on the buoyancy of your suit and the expansion or compression of the gas in those tiny bubbles of neoprene. The same goes for any air in your BC. The net result is that few new divers master buoyancy control until they are diving in deeper water.

Too often new divers wear too much additional lead because that is how they originally learned. Dive guides the world-over spend their time convincing these over-weighted divers to shed some of this additional weight.

A BC or BCD (buoyancy control device) is used to maintain neutral buoyancy at any depth. Do you know why it's beneficial to use one? Would it surprise you to know that pioneers of diving such as those divers on Cousteau's team didn't bother? Well, they didn't need to because they eschewed wetsuits. It's the wetsuit that makes a BC necessary. With the thin wetsuits they used when they did use such things, these pioneer divers did all their buoyancy control by varying their lung volumes.

As you go deeper, the integral bubbles in the neoprene of the wetsuit get compressed and your suit loses volume (along with insulation properties). This means you displace less water and appear to get heavier. You add air to a BC to compensate for this. As you ascend, you must release this air gradually in the same way.

This brings us to drysuits. A drysuit has the ability to be fed with air just like a BC. You do this to keep its overall volume constant. If you contrive your weights so that you are neutrally buoyant in a drysuit just under the surface, you should be able to maintain that neutral buoyancy by keeping the suit at the same volume by adding small amounts of air as you go deeper and at any safe leisure diving depth.

You don't inflate the suit despite the valve being called the inflation valve. The air in the suit remains at the same volume but it gets denser as you go deeper, it gets compressed and you add more. Some say that air 'sloshes around'. That only happens if you started with too much air in the suit when adjusting your weights for neutral buoyancy just under the surface. The term 'inflation valve' is misleading. You should not inflate the suit while under water. You should only maintain it at constant volume.

Of course, the one variable in all this is the amount of gas you are going to use during the dive. Gas has a mass and a full tank can be around 2.5kg heavier than an empty one. So if you are doing a buoyancy check with a full tank don't forget to allow for this.

Get your buoyancy right just under the surface so that you neither float nor sink whilst horizontal, with normal relaxed breathing and the minimum amount of lead, so that enjoy neutral buoyancy throughout the dive. By the way, the only real reason you need a BC with a drysuit, apart from the additional safety offered by its obvious redundancy, is to give you surface support in the right part of your body rather as with a lifejacket.

Happy diving - John Bantin