Extension for Lower Grades
Topic: Diver's Decompression
- Learn about the physics and physiology of scuba diving.
- Study the principals applied in the U.S. Navy Decompression tables.
- Work decompression problems typical of those worked by military and Professional divers prior to executing dives.
During the program, underwater filmmaker Howard Hall comments on the use of a special underwater breathing apparatus known as a "rebreather." He states that the rebreather allows him to remain submerged for many hours at considerable depth without having to "decompress."
What is decompression? Literally, decompression refers to any reduction in pressure. For example, when an airplane ascends into the sky, the cabin decompresses, often noticeable to passengers as a clogging sensation in the inner ears. Normally, the sensation goes away when the ears suddenly "equalize." That is, the ground-level air pressure trapped behind the eardrums releases and becomes equal to the thinner, high-altitude air pressure outside the eardrums. The movement of air from a location of greater internal pressure to one of lesser external pressure is decompression.
As a diver submerges, pressure increases in direct proportion to the depth. This pressure is caused by the combined weight of the surrounding water and the atmosphere above, and is called ambient pressure. At a depth of 70 feet (the working depth mentioned in the program), ambient pressure is equal to more than three atmospheres (three times the atmospheric pressure at sea level, or 3 x 14.7 psi). In order to overcome this pressure and fill his lungs with vital air, the diver must breathe air supplied to him at a level equal to ambient pressure. But a potential problem arises:
Air (a mixture of approximately 20 percent vital oxygen and 80 percent inert nitrogen) supplied to the lungs under pressure dissolves in the bloodstream. The oxygen component of the air is used by the body, and waste carbon dioxide is exhaled. But what happens to the nitrogen component of the air mixture? Under normal atmospheric conditions, it has no effect. But under pressure, it dissolves in the blood stream and in tissues and remains there after the diver begins to ascend. If the diver ascends too quickly, the air expands-remember the airplane passenger's ears?-and equalizes with the decreasing ambient pressure. Nitrogen bubbles form in the blood stream and the tissues, causing the infamous "bends."
How does a diver avoid the bends? By avoiding prolonged exposure to depth and pressure, or, by decompressing when necessary.
Activity: Plan a Decompression Dive
Time Needed For Activity: One 45-minute period
Target Grade Level: High School
Any textbook or reference to diving for sport, military or professional divers will do. All contain basically the same tables based on U.S. Navy data. A recorded text that is easy to come by is The New Science of Skin and Scuba diving.
- note pads
- photocopies of decompression tables taken from text
Caution: The instruction of scuba diving techniques is an activity strictly reserved for professionally trained and licensed instructors. This classroom activity is designed only as a theoretical written practice applying principals used by diving instructors. Conducting this activity in no way substitutes for actual training intended for use by divers or would-be divers.
- Find the 70-foot depth level on the decompression table.
- Note the "no-decompression" time limit for this depth (50 minutes). This is the maximum amount of time a diver would be able to remain at this depth without having to decompress before safely ascending to the surface.
- Let's say the demands of the diver's work (underwater filming) will cause him to exceed this time limit. He estimates it will take exactly 50 minutes to film the footage he needs, but he must take into account the additional few minutes he may spend on the bottom. He must also build in a safety factor in case of emergency. He therefore plans for a dive of one hour's duration at a maximum depth of 70 feet.
- Find the 60-minute Bottom Time for a dive to 70 feet. Note that he must now make a decompression stop lasting one minute at a depth of 10 feet in order to avoid the bends.
- After this decompression stop, he may surface. Note that his Repetitive Group will be "K." This refers to the amount of nitrogen that remains dissolved in his tissues. It is low enough now for him to safely leave the water, but it is still considerably higher than normal ("A"-level nitrogen).
- The diver now plans to take a short break aboard his boat while he reloads his camera and exchanges his low air supply for a full one. This break lasts about 20 minutes. Looking at the Surface Interval table, find the letter "K." Under this Group Designation, find the surface interval bracket into which 20 minutes will fit (0:10 - 0:28). Moving through the table from this point, note that the diver is still designated a "K" diver at the end of his short break. This indicates that his nitrogen level remains at or about the level it was when he left the water. (He would have to remain out of the water for a total of 12 hours for the nitrogen level to return to normal!)
- Moving on to the Repetitive Dive Table, note that if the diver returns to 70 feet as a "K" diver, he must decompress for more than an hour before resurfacing. This is probably longer than an ordinary air supply would last the average diver. His fail-safe option is to postpone the dive until his nitrogen is reduced to a safe level.
- Have students plan similar dive profiles of their own using the decompression tables. Encourage them to experiment with a variety of depths and bottom times. Evaluate results. Who gets "bent" and who doesn't?
Consider the vocation of diving. Discuss with students the demands of working underwater for prolonged periods. Consider stress factors viewed during the program such as depth, pressure, cold, seasickness, limited visibility, and dangerous animals. If there are students with skin or scuba diving experience, have them discuss it. Consider inviting a professional diver or diving instructor into your class to help conduct this unit. A demonstration of diving equipment is also highly recommended.
Extension for Lower Grades:
Conduct a demonstration of decompression. You will need two identical 1-litre bottles of carbonated soft drink, stored at room temperature. Begin by jostling one bottle slightly for a few moments. Explain that this bottle represents a diver working underwater. Then open the bottle quickly. Have students note the results as the gas escapes suddenly from the fluid in a cloudy stream of bubbles and foam. Explain that the bubbles represent nitrogen in the diver's blood. The bottle has just experienced "the bends" due to sudden decompression.
Now, after handling the second bottle, set it down and wait for a few moments. Have the students note the behavior of the bubbles. Then, very slowly twist the cap until the gas begins to escape, then stop. Leave the bottle to rest momentarily and then repeat the exercise. Continue releasing the gas a short burst at a time until you can completely remove the cap without hissing or sudden discharge of gas. These brief intervals represent stage decompression, the same technique used by divers as they gradually decompress from depth to the surface in order to avoid the bends.
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