Log - August-2-2003 by Robert McCarthy Previous | Next Expedition Expedition | People Who wants to go swimming? Robert McCarthy The ocean water temperature is 2.75oC, which is about 37oF. The day is fairly calm, sunny, and a balmy 43oF, considering we’re positioned at 78o 23’ latitude. Perfect swimming weather? Hardly, but the coast guard sent 6 SCUBA divers out on a small boat today to dive for clams. CLAMS? Yes, clams, and not for dinner tonight either. Dr. Robie MacDonald and Mary O’Brien, both with the Institute of Ocean Sciences, are also on this small boat to help locate the clams for the divers. Their research is to retrieve some clams at different water depths, and analyze their shells. That story can wait a few days, but today I want to talk about SCUBA diving. SCUBA stands for Self Contained Underwater Breathing Apparatus, and not just anybody can harness a tank to their back and jump in. You must take an extensive training class which includes testing, to even qualify to take an “open-water” test. Most SCUBA diving classes are held partly in a swimming pool and partly in a classroom. But to become certified, you must leave the pool and pass a series of performance tests during either a lake or an ocean dive. I became certified when I was a high school student, and I remember having to study for the classroom tests. Why do you need to study? Just stick the regulator in your mouth and breathe normally, right? What could go wrong? PLENTY! Pressure is a big concern for SCUBA divers. We already discussed that for every 10 meters of water you descent, the pressure is increased by another atmosphere. These divers will dive to a maximum depth of 30 meters today, so their bodies will feel the effects of 4 atmospheres of pressure. The regulator is the key to SCUBA diving. So what is a regulator and how does it work? The tank that you wear on your back contains air at very high pressures. A filled tank usually holds about 3000 psi (pounds per square inch) gauge pressure, which is a pressure of approximately 204 atmospheres. Each tank holds the equivalent of about 80 cubic feet of air. Think about a large box 4.25 feet on each side. That’s how much air is contained within a filled SCUBA tank. Another way of illustrating this is if you took that box filled with air down to a depth of 2040 meters. Just like the Styrofoam cups that compressed, that box would occupy the same volume as a SCUBA tank. With all that pressure, you could not breathe directly from this tank. Plus, when you are down 30 meters, your body is feeling the effects of 4 atmospheres, so the pressure of the air going into you mouth must also be at 4 atmospheres. Your lungs couldn’t expand enough to breathe in air at only 1 atmosphere. This is interesting, because we are probably familiar with a snorkel. The length of a snorkel is only about 18 inches long. So why not just make longer and longer snorkels? If you tried to breathe through a garden hose while on the bottom of a swimming pool, you would find that you couldn’t. The pressure pushing in on your chest and lung muscles is too great to allow you to take a breath. So the deep-sea diving suits with the diving helmet had to be supplied with compressed air from the surface. So how can you take very high-pressure air from the tank, sense the ambient water pressure at the depth you are diving, and supply the correct pressure air to your lungs? With the use of a regulator. One end of the regulator goes in your mouth, and the other end is attached to the air tank by a hose. When you breathe in through the mouthpiece, you create a partial vacuum in the regulator. The outside water pressure is now greater than the inside pressure of the regulator, so the water pressure pushes in on a watertight diaphragm. This rubber diaphragm then pushes on a release mechanism, thereby opening up a passageway for the air from the tank. This air is at the same pressure as the ambient water pressure. Therefore, the deeper you dive, the more compressed air you are inhaling, so the quicker you run out of air. When you exhale, the increased pressure in the regulator pushes out on the rubber diaphragm, shutting off the flow of air from the tank. This pressure inside the regulator builds up enough to open the exhaust valve. And the process is repeated for the duration of the dive. Plus, if you need air at any time, all you need to do is manually push in on the diaphragm (called the purge valve), and air will flow out of the mouthpiece. Divers must always watch their tank pressure (to gauge how much time is left on their dive), and watch their depth. If you breathe highly compressed air for too long, your body absorbs the air bubbles in your blood. If you come up to the surface too fast, the tiny air bubbles in your blood expand too fast. This is called the “bends”, and you must either dive back down or get to a decompression chamber to recreate the high atmospheric pressure of the dive. Then they gradually restore the pressure in the chamber to atmospheric pressure. Thus, divers are always calculating their dive depth and dive time at that depth, to determine if they need to decompress on the way up and for how long. So, enjoy your dive! I’ll update you on the clamming operation soon.