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     If you will recall, the pressure of the atmosphere may be measured in pounds per square inch (psi) or in atmospheres (atm). There are 14.7 pounds of air on every square inch at sea level, or 1 atmosphere. Again, the pressure is caused by billions of moving molecules bombarding each square inch. Most of those molecules are nitrogen (78%), and the rest are oxygen (21%).

     A diver is ready to enter the water and is standing at the beach with the ambient (surrounding) pressure at 1 atm due to the 62 miles (Karmen Line) of air overhead. He or she enters the water and descends to 33' (34' in fresh water) where the pressure is 2 atmospheres (29.4 psi). One of those atmospheres is caused by the air and the other from the water. Divers refer to the total pressure (2 atm.) as, "absolute pressure." When one refers to the pressure caused only by the water it is called, "gauge pressure." Therefore, the absolute pressure at 33' in seawater is 2 atm, and the gauge pressure is 1 atm. because the air is ignored. Likewise, the gauge pressure at 66' would be 2 atmospheres and the absolute pressure would be 3.

     The average male human being has about 2800 square inches on the outside of their body. If one considers there are 14.7 pounds of molecules pounding each square inch,  the total pressure works out to be 21 tons. We should be crushed to death. Fortunately there are molecules inside our body that push out with an almost equal force. We are stabilized and don't even know it. In reality, the internal molecules found in the blood and tissues push with a greater force outward because of the power of the pump (heart). That is why the flow is outward when a knife wound occurs. (Things flow from high pressure to low.) Blood pressure, like 120/80, is measured in millimeters of mercury. Sea level pressure is 760 mm, which equals 14.7 psi. So, 120 mm is about 2.4 psi and 80 mm is 1.6 psi. When your heart pumps it creates a pressure of 2.4 psi, and when it is "resting" the pressure drops to 1.6 psi.

     While we are at it, let's see how depth under the sea is related to millimeters of mercury (Hg). At sea level the average atmospheric pressure is 29.91" Hg. That is equivalent to 760mm or 76.0cm. (Check a ruler that has inches next to mm or cm just to see the 760mm is next to 29.91".) If 33' of seawater is 1 atmosphere then 33' of seawater equals 760mm Hg. It follows that 1' of seawater would equal 23mm Hg (760/33=23), and the heart pressure at its maximum equals about 5' of seawater.

     Now, if you filled a plastic bag with water and took it down in the sea there would be little change to the shape of the bag. As you found out with the hypodermic syringe, fluids are barely compressible. If you filled that same bag with air it would get smaller as you descended into greater pressure. In fact, the bag would be 1/2 the size at 33' because the pressure there is double (2 atm absolute). If you went further down to 66', where the pressure is 3 atm., the bag would be 1/3 of the size, and so on.

     Robert Boyle stated the above in a mathematical way: P1V1 = P2V2. That's Boyle's Law and is very important for divers. The affect of Boyle's Law can kill a diver. How does the law work? The P1 is the pressure at the first location such as sea level (1 atm.). The P2 is the pressure at the 2nd location such as 33'. The V1 is the volume of the gas space, such as our plastic bag, at the first location, and the P2 is the volume of the gas space at the 2nd location. To make it clearer, let's say the plastic bag at the beach is 1 liter in size. When you take it to 33' it should be 1/2 liter. Check it out doing Boyle's math:

          1 Atm (at the surface) x 1 liter should = 2 Atm (at 33') x 1/2 liter

                 1 x 1 = 2 x 1/2

     If you didn't know one of the numbers in the above equation you should be able to figure it out. If you didn't know how big the bag would be at 33' you would have math that looked like this:

          1 Atm (beach) x 1 liter = 2 Atm (33') x WHAT ANSWER WOULD GO HERE?

                1 x 1=2 x ?    The question mark is a number you should now be able to figure out.

     Your body is composed mainly of fluids and solids. If we were made totally of fluids and solids scuba diving would present far fewer problems. We have gas spaces in our bodies and they act like a plastic bag filled with air when we go down to greater pressures. That is why your ears begin to hurt when you go to the bottom of a swimming pool. There are gas spaces in the ears that are being squeezed smaller as one goes deeper. This crushing effect causes discomfort and pain. There are other gas spaces that also shrink and expand as the diver goes down and up. The list includes the middle ears, sinuses, stomach, intestines, and the lungs. The mouth, nose, and throat are open to the outside and are not usually affected by Boyle's law.

     It is important in skin and scuba diving to keep the pressure inside body air spaces the same as the pressure on the outside.

     Pressure underwater increases and decreases most rapidly when you are near the surface. Going down 10 feet in a swimming pool results in a much greater pressure change then going from 30 to 40 feet underwater. Descending from the surface to 33' changes the pressure from 1 atm. to 2 atm. and that would double it. Descending another 33' to 66', the pressure would increase from 2 to 3. That is not another doubling. You would have to go from 33' (2 atm) to 99' (4 atm), or 66', to equal what happens from the surface (1 atm) to 33' (2 atm). So, the upper 33' doubles the pressure but it takes 66' to do the same thing when deeper.

     As a diver descends the pressure of the breathing air gets greater. As stated before, the pressure of the air entering the diver's mouth at 33' is double that of the air breathed at the surface. There are twice as many molecules of oxygen and nitrogen going in and out of the diver's lungs. If the diver descends further the number of molecules increase and that makes breathing more difficult. It is similar to sucking water verses oil through a straw. At 132' the density of the air is five times the surface density. Five times the number of molecules must move out from the tank and through the regulator. Five times the number will enter the diver's lungs with each breath. Five times the number will be exhaled. The extra effort may be quite noticeable.

     You should be able to figure out what the water pressure is for any depth. From memory, you probably know the answer to the question, "What is the pressure of the water at 33' (fresh: 34')?" You would say 2 atmospheres, correct? If the question was for 66" (68'), you would answer 3 atmospheres. Now, what if you were asked for the pressure at 57' would you be able to figure it out? Try to work out a simply equation for doing this math. Use 33' and 66' in the search because you already know the answers for those two. The answer is below (BUT DO NOT LOOK AT IT BEFORE FIGURING THE ANSWER OUT).

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Depth and pressure problem answer:  Depth/33' = Gauge pressure. Add 1 for the atmosphere to get the absolute pressure. So, Depth/33' + 1 = Pressure of the water in atmospheres.

Using 57":    57'/33' = 1.73;    1.73 +1=2.73. The pressure at 57' is 2.73 atm.

Note: Use 34' instead of 33' if it is fresh water.

     The deepest lake in the world is Lake Baikal in southern Siberia. It is 5,390 feet. What would be the absolute pressure at the bottom of the fresh water Lake Baikal in psi?

     The deepest depth of the ocean in the Marianas Trench near Guam. It measures 35,839 feet. What would be the pressure at the bottom of the salt water Marianas Trench in psi? 

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