surface, the pressure decreases, and the air expands to its original volume. When the bucket gets to 10 meters (33 feet) of seawater or 2 ata, the pressure has doubled on the bucket. The volume of air in the bucket has decreased to half of the original surface volume. When the bucket descends to 20 meters (66 feet), which is 3 ata, the pressure has tripled on the bucket and the volume of air in the bucket will have decreased to a third of the original surface volume (figure 4-15). This inverse relationship between pressure and volume is known as Boyle’s Law, which is named for the scientist who first recognized the relationship. Knowing the name of the law is not as important as understanding the concept of the relationship between pressure and volume. When one increases, the other decreases, if it can. Now consider a situation where you have the ability to maintain the volume of air in the bucket during its descent. As you take the bucket to depth, you add air to the bucket through a hose. You keep the volume constant by adding enough air to match the original volume. You take the bucket to 10 meters (33 feet), fill it with air, and raise it to the surface. As the pressure decreases from 2 ata to 1 ata, the volume of air Chapter 4- Diving Science Bucket is still full of air, although excess air volume has vented off. Compressed air expands and overflows bucket as pressure is reduced. Compressed air expands and overflows bucket as pressure is reduced. Compressed air is added to open system at depth. doubles. The bucket cannot hold this increased volume, so the excess air bubbles out around the rim of the bucket. If you took the bucket to 30 meters (99 feet), filled it with air, and then raised it to the surface, a volume of air equal to 3 buckets would escape and the volume equal to one bucket would remain (figure 4-16). Closed System The effects of pressure on a closed system can be much more dramatic. If you take a sealed plastic bag filled with air down to depth, the bag becomes smaller and smaller as the air compresses and the volume decreases. When you return the bag to the surface, the air expands and the volume increases back to its original size (figure 4-17). Now imagine that you take the same bag down to depth, open it, fill it with air back to its original volume, and close it again. When you bring the bag back to the surface, the air expands, but it cannot escape because the bag is sealed. The bag will expand slightly, but will finally burst to release the expanding air (figure 4-18). The only way to prevent the bag from bursting is to vent Diving Science 103 VOLUME 1 0'-1 ata VOLUME REDUCED TO 33'-2 ata VOLUME REDUCED TO 66'-3 ata VOLUME REDUCED TO 99'-4 ata OPEN SYSTEM – DESCENT FIGURE 4-15. AS THE PRESSURE ON AIR INCREASES, THE VOLUME DECREASES. VOLUME x 1 0'-1 ata VOLUME x 1 33'-2 ata VOLUME x 1 66'-3 ata VOLUME x 1 99'-4 ata OPEN SYSTEM – ASCENT FIGURE 4-16. IF YOU FILL AN OPEN BUCKET WITH AIR WHILE AT DEPTH, THE AIR EXPANDS AND BUBBLES OUT OF THE BUCKET AS YOU BRING THE BUCKET BACK TO THE SURFACE.
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