Deep-Six Underwater Systems, Inc.
"Add Depth to Your Life"

Table of Contents

1 Pressure and Gases
2 The Face Mask
3 The Snorkel
4 The Fins
5 Weight Systems
6 The Knife
7 The Wetsuit
8 Pressure and Water
9 The Ear and Pressure
10 The Sinus and Pressure
11 The Stomach/Intestine and Pressure
12 The Lung and Pressure
13 Barotrauma caused by External Air Spaces
14 The Buoyancy Compesation Device (BCD)
15 The Scuba Cylinder
16 The Scuba Cylinder Valve
17 The Regulator
18 Density and the Diver
19 The 4 Gas Laws
20 Hand Signals
21 Carbon Monoxide Poisoning
22 Hyperventilation
23 Nitrogen Narcosis
24 Diver's Flags
25 Sound Underwater
26 Color Underwater
27 Decompression Sickness
28 Breathing Oxygen
29 Deep Diving
30 Thermoclines
31 Thunderstorms
32 Underwater Life
33 Open Water Dives
34 The Final Examination
35 The Environment
36 Advanced Course

8 - Pressure and Water

     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 the 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 at1 atm due to the 15 miles 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. 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 we flow out when a knife wound occurs. (Things flow from high pressure to low!)

     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 quart in size. When you take it to 33' it should be 1/2 quart. Check it out doing Boyle's math:

          1 Atm (beach) x 1 quart should = 2 Atm (at 33') x 1/2 quart

                 1x1 = 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 quart = 2 Atm (33') x WHAT?

                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, for example, your ears start to hurt when you go to the bottom of a swimming pool. In the ears there are gas spaces that are being squeezed smaller as one goes deeper. This crushing effect causes discomfort and pain. There are other air 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!

     One further note: Pressure underwater increases and decreases most rapidly when you are near the surface. Going down 10 feet in a swimming pool is a much greater pressure change when compared to going from 30 to 40 feet underwater. Descending from the surface to 33' changes the pressure from 1 atm. to 2 and that represents a doubling of the pressure. If you went down 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) to equal what happens from the surface (1 atm) to 33' (2 atm).

     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:

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! 

Copyright Information about this text, DIVING WITH DEEP-SIX is as follows: Copyright 1996 - 2007 by George D. Campbell, III; President. All Rights Reserved. This file may be posted on Electronic Bulletin Boards for download, but may not be modified, printed for distribution, or used for any commercial purpose without the author's written permission. is using this material with the permission of Deep Six. The full version is available at:
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