When I teach my beginning open water scuba diving course, one of the things we talk about during the educational portion are the myths associated with scuba diving.
One of the biggest myths is that, "you can run out of air underwater." Here is why the perpetuation of this myth is unfounded. Unless it is your goal to actually breath all of the air out of the scuba cylinder, we will contend that no one wants to run out of air. As a NAUI Instructor, I promote diving with a buddy until the time comes that your training provides you with the requisites to attempt diving on your own. Solo diving will be left for another discussion.
So, what do I mean with that statement. Let's look at the SPG. The submersible pressure gauge is a tool used to aid the diver determine how much air is left in the cylinder. Believe it or not, there was a time when divers did not use pressure gauges. The scuba cylinder had a valve on it that when in the "off" position, left a little bit of air in the cylinder. Once the diver felt the resistance of getting low on air or ran out, they pulled a lever which opened the valve the rest of the way providing them the additional air required for an ascent. As you can imagine, the "J" valve fell out of favor. If the lever was in the "on" position all along and unbeknownst to the diver when they pulled the lever and there was no air, their only options were buddy breathing (one regulator, two divers) or an emergency ascent to the surface.
So, what do I teach my students. I teach them that, "you will never run out of air." When I say this to them, I ask them to tell me what they think that means. I usually get two interpretations. The first is that this diver, the one I teaching at this moment will not run out of air. As if they have a technique that makes it possible for them to survive underwater without needing to breath. Well, we know that can't happen. No one has that ability. Next, I hear that the diver believes that there is so much air in the scuba cylinder that it will never run out -- that it will never become empty. Well, as we know, that is not true either. It is possible for the cylinder to be breathed down to nothing and it is possible for all the air to escape from the cylinder (accidentally or on purpose).
This is what I teach. The SPG tells the diver how much gas is in the cylinder at that particular moment. Has anyone ever told you that all SPG's have a margin of error? Some more than others, but usually a few percentage points. Electronic pressure gauges are the same but the margin of error is higher. That's something to keep in mind. Also, just because the SPG reads 3,000psi doesn't mean there is 3,000psi in the cylinder. Most divers check the SPG when assembling their gear, but what about the moment they enter the water and after the cylinder cools? What about once you reach your desired depth or dive site? What would it mean if you looked at the SPG and it still read 3,000psi? What about 300psi? If you develop confidence as your dive career progresses, you should be able to look at it and determine that it is not working properly, tell your buddy, and swim safely and surely to the surface.
Mechanical failures are rare. Even when they do happen, what the diver does then can make the difference between life and death. Divers learn to never hold their breath, ascend slowly, and some even learn to relax well enough to extend bottom times and improve their buoyancy. Learning about air consumption rates, where you are, being ever present and aware of your equipment and your environment usually takes a little longer.
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