BUOYANCY: DENSITY AND THE DIVER
Buoyancy is a major part of scuba diving. The sign of a good diver is one who has mastered their buoyancy. Being able to go up and down safely, and hover in one place takes a lot of practice and experience. The buoyancy of a diver is caused by the diver's density. What is really happening when a diver adjusts their buoyancy?
THE FIN PIVOT (Courtesy of Diver Training Magazine)
Styrofoam floats and granite sinks in water. Why is that? It is all about density. Water has a density of one. Man made it come out that way because everyone has access to water. Anything that floats on water has a density of less than one. Anything that sinks in water has a density of more than one. If you found a piece of granite, no matter how large it was, has a density of 2.7 you would know it would sink in water. Divers have a density of less than one sometimes. At other times their density is more than one. It would follow that a diver sometimes has a density of exactly one and can remain suspended in water. Incidentally, we are talking about pure water. If seawater is to be considered the density of that is about 1.03.
How does one find the density of an object. There are two things that have to be known: How much does the object weigh, and how big is it? If the weight is divided by the size the density is determined. The math would look like:
Density = Mass/Volume.
That's it in a nutshell.
Divers have different densities. The following are a list of factors that affect the density of a scuba diver:
How do you change your density (buoyancy)? You are scuba diving at the bottom of a lake and want to go to the surface without kicking. If you made your density less than one you would move upward. There are only two possibilities. Either you could change your mass (weight) or your volume (size). Looking at D=M/V, if you made your mass less, or made your volume more the density in the equation would have to go down. You could make your mass less by dropping your weight belt. Up you would go and down would go poisonous lead. Without a weight belt your density would suddenly get less than one and you would be in an uncontrolled free ascent. There are only a few ways to change your mass underwater other than the removal of the weights. But, it is easy to change the volume! You can easily make yourself bigger by inflating your lungs, dry suit, and/or BCD. The latter is the most convenient, and that is what is done to control buoyancy.
The ultimate in proving you have your buoyancy is demonstrated by these two divers in the Deep-Six pool. If you are able to hover, without touching the bottom or the surface, you have achieved the mark of an excellent diver.
The average 170 pound male with inflated lungs has a volume of approximately 2.8 cubic feet. The weight of seawater is 64 pounds per cubic foot. (Freshwater is 62.4 pounds per cubic foot.) The density of the male is (170 lbs./2.8 cu ft) is 61 pounds per cubic foot. Since seawater is 64 pounds per cu ft, it denser than the male. If that male jumped into the sea he would float with a force of 3 pounds per cu ft, or 8.4 pounds. That would lift his 9 pound head out of the water. Calvin (Divemaster) weighed 150 pounds. He went in the pool, let most air out of his lungs and weighed 5 pounds. He lost 145 pounds. 145 pounds of pool water was pushed up. Calvin's volume is (145/8) 18.1 gallons or (145/62.4) 2.4 cubic feet.
Before a diver goes down with a scuba they should do a "buoyancy check." This check should be done every time the diver changes into a new diving pattern. For example, if the diver gains or loses weight, changes the size or thickness of the exposure suit, goes from fresh to salt water, etc., then a new check should be done prior to diving. It is easily done. The diver should put on all the gear they are to wear diving except the BC and the tank and regulator. They should enter the water (fins are a safety must!) and fill their lungs completely. Without moving the arms, hands, or legs, and holding the breath, the diver should float to a level where the water covers the top of the mask while they are looking forward. They now have perfect buoyancy for skin diving. If scuba is to be used an extra 3 pounds should be added to the weight system to compensate for the decrease in density due to the loss of air from the 71.2 or 80 cu ft. tank. (Note: If another type of scuba tank is to be used the additional weight requirement may change due to the density of the tank. Check the chapter on Scuba Cylinders for more details.)