02 July 2007

My rap on buoyancy as related to scuba diving

It's been just a few days over a year since my blog's inception. Over this time period I've noticed a trend in the way that people find my website. Most are through search engines where people are seeking answers to all sorts of questions relating to buoyancy. Many of them scuba related -
buoyancy dive, buoyancy diving, etc. Many of them not - neutrally buoyant pets?!?, neutral buoyancy in fish, buoyancy of plywood. Since the title of my blog is "Neutral Buoyancy" it pops up as one of the first few results in the search engines. Now I've been putting this off but I feel the time has come to post something about buoyancy to slightly appease the masses.

Merriam-Webster's definition of buoyancy (at least the definition relating to this subject) is as follows -
"Main Entry: buoy·an·cy
Pronunciation: 'boi-&n(t)-sE, 'bü-y&n(t)-
Function: noun
1 a : the tendency of a body to float or to rise when submerged in a fluid b : the power of a fluid to exert an upward force on a body placed in it; also : the upward force exerted"

PADI's open water dive manual goes in to quite detail about buoyancy (as it should). Here is some of what they say -
Have you ever wondered why a large steel ocean liner floats, but a small steel nail sinks? The answer is surprisingly simple: The ship's steel hull forms a shape that displaces - pushes aside- much water. The same amount of steel reshaped into a giant nail would sink, of course, like the regular sized nail. This demonstrates that whether an object floats depends on both its weight and how much water it displaces - its volume.
You can state the principle of buoyancy this way: An object placed in water is buoyed up by a force equal to the weight of the quantity of water it displaces.
That means that if an object displaces an amount of water weighing more than its own weight, it will float. If an object displaces an amount of water weighing less than its own weight, it will sink. If an object displaces an amount of water equal to its own weight, it will neither float nor sink, but remain suspended in the water. If an object floats, we call it positively buoyant; if it sinks we call it negatively buoyant; and if it neither floats nor sinks we call it neutrally buoyant."

When we learn to dive we learn to control our buoyancy to stay positively buoyant at the surface and neutrally buoyant under the water. We do so using a buoyancy control device (BCD or sometime just called BC) (more on mine here - Scuba Gear Part IIII) and weight. The BC has an air hose coming from the tank of compressed air on our backs. With a few simple controls we add air to expand the BC vest and thus increase buoyancy or release air and decrease buoyancy.

But there's more to it then this. Think about all the miles of air above you, from here to space. This air, the atmosphere, as thin and wispy as it seems exerts a certain amount of pressure on our bodies. Standing at sea level this is called one atmosphere, or one ata of pressure. Water is many times more dense then air and thus the distance to equal this pressure is much shorter. Every 33 feet/10 meters that we dive under the water equals one atmosphere of pressure. On most of our body this has really no effect, (well it has a lot of effects but I'm not going to get that detailed here) as most of our body is composed of liquid and a liquid is incompressible, with the exception of certain air spaces like the lungs, inner ear, etc. In our lungs we just continue to breath in a greater volume of air to equalize the pressure. Under 33 feet/10 meters our lungs hold twice as much air as they do at the surface in the same amount of space.

Now back to buoyancy. The air in our BC has been compressed to take up half the space and so we need to add air to the BC to stay neutrally buoyant at that depth. These are generally very subtle adjustments made with the BC. To much and you will start to float back to the surface. Likewise we need to release air from the BC as we ascend to the surface. Failure to do so and you will end up shooting to the surface as the air in your BC expands. An uncontrolled ascent to the surface is something you don't want to have happen in scuba diving. I'm not going to go into all that here though, this post is just a little about buoyancy. If you are interested in scuba diving I strongly recommend training. Serious injury can occur if you are not properly trained. Scuba is extremely enjoyable when properly trained and a course is not that difficult. That said once neutral buoyancy is achieved you will tend to rise and fall with each breath. If fact buoyancy of just a few feet can be controlled in this manner. Shallow breath, deep breath, etc. But, of course, never hold your breath while scuba diving.

The other tool we use is lead weight. A certain amount of weight is needed for the initial decent from the surface. This is based on many factors. Body weight, and composition - fat is more buoyant then muscle. Exposure suit - the thicker the wet suit the more buoyant it is. And the type of water you are diving in. Objects and humans are more buoyant in salt water then fresh water. I for example in Mexico, using the same wet suit and equipment need about 12 pounds of lead to dive in the ocean and only 2 - 3 pounds to dive in the fresh water cenotes. Another point to know/remember is that your tank will be more buoyant at the end of your dive so it may be a good idea to check buoyancy with a nearly empty tank and add weight as needed.

Buoyancy in cold water as opposed to warm water should only be affected by the fact that you have to wear a thicker exposure suit which in turn makes you more buoyant.

As I understand there is also a slight difference in buoyancy while diving at high altitude such as a mountain lake. This is due to the thinner atmosphere allowing your wet suit to expand slightly more then at sea level.

Other questions people have typed into search engines to find my blog -
- the buoyancy of a ferry? Hey I'm not an engineer.
- how stingrays are affected by buoyancy? Yeeeah, I'm no biologist either.
- neutral buoyancy in fish? Ditto. I think they have some kind of buoyancy bladder.
- neutral buoyancy of plywood. Eeeeeee . . . I believe all wood is positively buoyant but maybe under enough pressure . . .
- neutrally buoyant pets?!? I hope whoever typed this is looking for aquatic pets.
- buoyancy as relating to other types of fluids. Ya got me man.

Also see - Diving one atmosphere above sea level.

Thus ends my rap on buoyancy.