DIVING SCIENCE DEEP ST. MATTHEW’S PARISH SCHOOL Buoyancy Buoyancy Buoyancy is an upward force on an object immersed in a Misconceptions fluid (i.e. a liquid or a gas), enabling it to float or at least to 1. Objects Float appear lighter. Because They Are Light Weight: While it is true that many light objects float, there are many examples of light objects that sink. Pebbles sink but they are much lighter than a big ship that floats. 2. Objects Float Because They Are Full Of Air: Air actually adds mass so it could make an object more likely to sink. An example is a Archimedes discovered that the pushed aside is the displaced wa- scuba tank. When the tank is buoyant force is equal to the ter. Larger objects will displace empty it will float, but when weight of the displaced fluid. In more water than smaller objects. it is full with a lot of air, it will sink! (1 cubic foot of air the diagram above you can see Key Concepts: weighs .08 pounds, so an 80 that the object weighs 3 pounds 1. If the water displaced less when it is placed in the water. cubic foot tank would weigh weighs more than the object, 6.4 pounds more when full) You can also see that the object then the object will float. forced 3 pounds of water into the .08 lb. x 80 cu. ft. = 6.4 2. If the water displaced bowl. The water had to move out lbs.. weighs less than the object, of the way when the object was then the object will sink. submerged. The water that is DIVERS AND BUOYANCY CONTROL A scuba diver needs to be able to adjust neutral buoyancy. Diver’s use weight belts buoyancy. When swimming on the surface, to add negative buoyancy, and they use a a diver needs to have positive buoyancy. BCD (buoyancy compensating device) to add This means that the diver floats. To leave positive buoyancy. The BCD is similar to a the surface to explore below, a diver wants life jacket, but it can be inflated or deflated to to have negative buoyancy. During the make adjustments. When the BCD inflates, dive a diver will not want to be scraping it increases the volume of the diver which along the bottom or struggling to stay displaces more water. More displaced water down. The diver will want to achieve means a greater buoyant force. BRUCE HARLAN PAGE 1 DIVING SCIENCE DEEP ST. MATTHEW’S PARISH SCHOOL When calculating the pressure for different depths, don’t forget Pressure to include the 1 ATA of pressure The deeper a diver de- from the atmosphere. scends, the more pressure is felt. At the surface, the Effects diver experiences pressure due to the weight of the at- of mosphere. At sea level, this pressure is usually about Pressure 14.7 pounds per square inch. Since 14.7 is a bit of an The pressure from a awkward number to work fluid presses in on all surfaces. with, divers usually refer to A diver will experience pressure the pressure from the at- all over the body. The body is mosphere as 1 atmosphere mostly liquid, so divers do not (or 1ATA) of pressure. normally feel pressure. (Liquids can not be compressed). There are places in the body that are air spaces, such as the Pressure and Depth lungs and ears. Since water is much heaver than air, it takes only 33 feet of sea wa- ter to equal the pressure of 600 miles of atmos- phere. That means that a diver that descends to a depth of 33 feet, has doubled the Areas where pressure is noticed: pressure from 1 • The ears. ATA at the surface • The lungs. to 2 ATA. Every additional 33 feet • The sinuses. of depth will add • Artificial air spaces such as the another ATA of mask and BCD. pressure. Note: BRUCE HARLAN PAGE 2 DIVING SCIENCE DEEP ST. MATTHEW’S PARISH SCHOOL DEALING WITH PRESSURE: A diver at a depth of 66 feet would have 3 ATA of pressure pushing in on her lungs. That is over 44 pounds for every square inch. Because of this pressure , the diver would not have the strength to inhale a breath. This is where the technology of SCUBA comes in. The regulator measures the pressure of the environment and then it delivers air at the same pressure. The diver at 66 feet would be breathing air delivered by the scuba tank and regulator at 3ATA. Now the diver will be able to inhale. Since more depth = more pressure, then more air is used at depth. A tank would last 3 times as long at the surface compared to 66 feet of depth. FREE DIVING: The World record No his choice. In Limits breath-hold most cases this The depth of dive is 172 meters or is a weighted 172m is 9 564 feet (Oct. 2005). sled, suspended times the pres- It was set by Herbert on a rope, that sure within a Nitsch who is pic- drags the free car tire. The tured to the right. diver into the lung of a free No Limits is the deep. A lifting diver is being deepest discipline in bag (balloon) compressed to Free Diving and al- brings him back the size of lows the athlete to to the surface. prune. use the equipment of Fill in the table (1 ATA = 14.7 psi) Depth (feet) Pressure (ATA) Pressure (psi) 0 33 66 99 132 564 BRUCE HARLAN PAGE 3 DIVING SCIENCE DEEP ST. MATTHEW’S PARISH SCHOOL Boyle’s Law Gases and Pressure Gasses are different than solids Boyle’s Law & Diving or liquids because they can be Boyle’s Law is the most compressed. The volume of a important science concept for gas decreases as the pressure divers to understand. It leads to on it increases. Robert Boyle the main rule in scuba diving: figured out the mathematical NEVER HOLD YOUR relationship between pressure BREATH! Look at the diagram and volume. to the left. Notice that the balloon expands as it rises to the surface. As the outside pressure P1 x V1 = P2 x V2 decreases, the air expands. This happens in the lungs too. If a P1 is the original pressure on the diver breaths from a tank at 33 feet gas. and surfaces while breath holding, the air in his lungs would expand P2 is the new pressure on the gas. to twice the volume! This would Vi is the original volume of the certainly cause gas. lung rupture V2 is the new volume of the gas. and a serious medical emergency. A Sample Problem: lung injury can A large balloon has a volume of 8 liters at the surface. What will happen in as the volume of the balloon be at 99 feet deep? little as 4 feet of depth. If We have learned that the pressure at the surface is 1 ATA and the pressure at 99 feet is 4 ATA. you breathe continuously P1 x V1 = P2 x V2 the expanding air can escape 1ATA x 8L = 4ATA x V2 as you exhale. (divide both sides by 4ATA) 2L = V2 Use what you have learned about depth, pressure, and Boyle’s Law to complete the table: Depth (ft) Pressure (psi) Pressure (ATA) Volume (L) 0 5 (Average adult lung capacity) 33 66 99 132 564 (Record free dive) BRUCE HARLAN PAGE 4 DIVING SCIENCE DEEP ST. MATTHEW’S PARISH SCHOOL Charles’ Law We have learned that pressure can change the thing that changes the volume of a gas? What about volume of a gas (Boyle’s Law). Is pressure the only temperature? In 6th grade we flew hot air balloons and discovered that heat makes air expand. So temperature does change the volume of a gas. Charles’ Law is simply the formula used to calculate the temperature/volume relationship. V1 = V2 T1 T2 The formula only works if the temperature scale used starts at absolute zero. Absolute zero is the theoretical lowest possible temperature. It has never been achieved. In the Celsius scale absolute zero is -273 deg. Scientists often use the Kelvin scale in which 0 deg. K equals absolute zero and -273 deg. C. To convert from Celsius to Kelvin, just add 273. Combined Gas Law (Boyle’s & Charles’) Because both pressure and temperature affect the P1 = P2 volume of a gas, the two laws can be combined. P1 xV1 = P2 x V2 T1 T2 We have to convert to deg. K by adding 273 to T1 T2 the temperatures: Sample Problem: A tank filled to 3000 psi at 3000 psi = P2 21 deg. C. The tank is left in a hot car and it warms to 60 deg. C. How will the pressure change? 294 K 333 K The tank is rigid and strong, so the volume is If we multiply both sides by 333 to solve for P2, not allowed to change. That means that we can we get about: remove the volume part of the equation. 3398 psi CHARLES’ LAW & DIVING Charles’ Law does not affect divers very much, but there are two things to keep in mind. Do not leave a full tank in a hot location. As you can see from the example in the problem above, over-heating a tank can cause it to go beyond its pressure rating. Tanks do not usually explode, but they do have a burst disk that will fail and cause the tank to release the air. Also, if a tank is filled to quickly at the shop, it will heat up.