Poor Man's Air tank

Important Note: This is intended for casual, shallow, open-water use only! When ascending, be sure to exhale to avoid air embolisms in your bloodstream -- although the water is shallow, it's better to be safe than sorry!

If you enjoy swimming and looking at things underwater, you've probably experimented with soda bottles and such underwater. Sure, they can hold air, but you can't really do that much with 'em: if you go under with a bottle in your mouth, you can't really breathe from it because the bottled air resists your efforts to suck it in, like a vacuum effect. The solution is simple; how you use this, uh, engineering marvel is the interesting part. Here's what to do:

Steps to Making a "poor-man's underwater air tank":

1. Find a used plastic soda bottle.
   • Soda bottles, particularly Coke bottles, work well because they have a good "lip" at the end and make a good seal with your mouth. A good seal is kinda important....
   • Size is up to you. We tried varying sizes from a 16-oz. Coke bottle up to a one-gallon milk jug. The best sizes for an unweighted swimmer appear to be 16 and 22-oz. bottles and the one-liter bottles. Also, the larger bottles tend to crinkle up underwater (more on this later), so the smaller, sturdier bottles might work better.
2. Make two holes in the bottom of the bottle. The larger the holes or the more numerous, the easier it will be to draw breaths from the bottle, but also, the more loss will occur from moving about, etc. For me, two holes about 4mm across worked well. The location is important: although a hole in the side of the bottle will also work, air leaks out more easily that way (since, considerations of pressure and vacuum aside, water is just plain heavier than air). --And that's it. Pretty simple, huh? The tricky part is in knowing how to use it -- or knowing how to use it most effectively.
   

   How to use your brand-new Po' Man's Air Tank:

   Some of this will depend on whether you are weighted down with something, or free-diving. We'll assume that you're testing the bottle out in a pool's deep-end, and can hang on to something (a poolside ladder, etc.) to control your buoyancy. Wearing a face mask (over your eyes and nose, leaving your mouth clear) is strongly recommended, but not absolutely necessary once you've gotten the hang of breathing right.
   • If you're under 16, get an adult to approve what you're doing and to watch you before proceeding!
   • At side of pool, inhale deeply, put bottle in mouth, and with the bottle held vertically slip underwater. Larger bottles will require a hand on them to keep them under control. Tilting the bottle some is okay, but the farther away from vertical you tilt it, the more air can be lost (remember, the holes should be in the very bottom).
   • Depending on whether you're weighted or not, swim or pull yourself down to the bottom, or wherever you're going. So, now you're several feet underwater with a bottle in your mouth. Hold your first breath for a while, but not so long that it becomes painful. Keep the bottle reasonably vertical.
   • Okay, time to breathe. Your bottle should still be fairly full of air at this point. Exhale slowly through your nose until you've made some room in your lungs. If you're wearing a mask, the air you exhale will bubble out from under the mask. That's fine. Then, breathe in from the bottle. Water will flow in through the holes at the bottom. This is fine; that's why the holes are there! Remember, the bottle is still mostly vertical, so the water is flowing in at the bottom while you breathe air out from the top. Way-cool!
   • Hold your breath again until you feel the need for another breath, then repeat the process. Depending on how big your bottle is and how deeply you inhale (and a bit on how deep you are), you should be able to get at least two decent breaths off the bottle, and in some cases, five or six. Some kids using a self-weighted gallon milk jug told me they were able to draw over six breaths, and were underwater for over three minutes, but I didn't see this myself.
   • Both the bottle and yourself will become less buoyant as you take more breaths. If you're not weighted this will probably give you a free hand for working underwater, etc. If you are weighted, keep in mind that you'll have to work that much harder when it's finally time to surface.
   • Keep an eye on the air level in your "tank." Don't take any chances; if you're not sure if there's a full breath left, breathe in sips or just head back up for air.
   • When ascending, exhale the remaining air out of your lungs to avoid what professional divers call "the Bends." This can be painful and even fatal, so be sure to exhale, no matter how shallow the water is!
   • Although you can exhale into the bottle and then in from it, it doesn't benefit you as much as exhaling through your nose and breathing in from the bottle. This way, the bottled air is still fresh and not mixed with the "stale" air from your lungs.
   • Once you get the hang of it, you can get a little more adventurous, like diving without a mask, diving deeper, staying down longer, etc.
   • Larger bottles, like 2-litre soda bottles and gallon milk jugs, do hold more air, but the drawback is that, as you breathe air from them, they'll crush up pretty badly until holes start showing in the sides after repeated use. They're also much more unwieldy than smaller bottles and harder to submerge.
   

   Cautions:

   
   Experiment and get familiar with this system before trying anything really risky. I recommend tryouts in a pool away from others and right near a side-ladder until you get really comfortable with it. Always dive with a friend or a partner; kids should not try this without an adult's consent and supervision. Always exhale your remaining air while you ascend. Note that if you're down fairly deep, and you exhale completely and then something dislodges the bottle from your mouth, you're a long way from the surface with no air. So, exhale slowly while you ascend and always be aware of your surroundings, other swimmers, etc.

   This is a pretty kewl (and very cheap) way to spend time in a pool or shallow lake or river. Have fun and keep it safe!

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