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" Video by Doug Weinstein (surface) and Richard Bailey (underwater) A while back, I reported here on rebreather technology, but for most divers, traditional open-circuit scuba is still the way to go. Modern scuba gear is relatively cheap, rugged and reliable. Nonetheless, things do go wrong, and although divers generally have both a regular and a backup second-stage regulator (the part that goes in your mouth), those both connect to the same first-stage regulator and the same tank of air. Equipment failure (like a primary O-ring blowout that can empty a tank in seconds) is unusual, but still not unheard of. And sometimes divers just don&amp;#039;t notice that they&amp;#039;re diving low on air until it&amp;#039;s too late. The response is a buddy system: Run out, and you can use your diving partner&amp;#039;s backup regulator to share air from his or her tank until you make it back to the surface. There&amp;#039;s no substitute for having a buddy around, and you&amp;#039;re supposed to stay close. But buddies can get separated, especially when visibility is poor or currents are strong. If the air goes kaput on you then, you&amp;#039;re in big trouble. That&amp;#039;s where Spare Air comes in. It&amp;#039;s a 3-cu.ft. mini-tank with its own built-in regulator that holds just enough air to let you make a safe ascent. Think of it as a reverse parachute for divers. When I mentioned it on my blog a while back, some readers emailed that it&amp;#039;s not worth buying because 3 cu. ft. isn&amp;#039;t much air. So I decided to give it a try. I breathed the Spare Air cylinder empty twice: once at 35 ft. down and once at about 60 ft. At 35 ft. it provided two and a half minutes of air--so much that I was getting pretty bored by the time it ran out. At 60 ft. it only lasted about a minute and a half. That means that at depths below 100 ft. it would last less than a minute. (The regulator always delivers air at the same pressure as the water around it. The deeper you are, the more air the regulator delivers with each breath, since it has to balance the greater pressure of the water at depth. So the deeper you are, the faster your air runs out. This applies to all scuba systems.) Less than a minute isn&amp;#039;t long, but of course if you ran out of air you&amp;#039;d start swimming toward the surface, meaning that the extra air would last longer on the way up. Three cu. ft. (the Spare Air folks say it&amp;#039;s equivalent to 57 breaths at the surface) is enough to let you ascend gradually, facing less risk of decompression sickness (&amp;quot;the bends&amp;quot;). Plus, dive boats generally hang a &amp;quot;hookah&amp;quot; regulator off the side to a depth of 15 ft. for those running low; make it that far, and you can even manage a full safety stop before exiting the water. Breathing with the Spare Air is easy, and you get a bit of warning (about two or three breaths&amp;#039; worth) when it&amp;#039;s about to run out, as breathing gets harder and the tank gets noticeably cooler from gas expansion. It fits easily on your buoyancy control vest with a holster that allows for easy deployment. A small pin-type gauge tells you if it&amp;#039;s full or empty, and Spare Air is available in both regular and Nitrox varieties. Some divers carry 15-cu.-ft. &amp;quot;pony&amp;quot; tanks in case their main rig fails, and those are a lot more capable. But they&amp;#039;re also a lot more trouble to set up--and to travel with--while the Spare Air (disassembled so the TSA folks can rest easy) fits easily into a suitcase or regulator bag. Like a lot of emergency gear, the one you have with you is worth more than the one in a closet at home, and Spare Air is easy to bring along. Now, I dive with it myself. Glenn Harlan Reynolds is the blogger behind Instapundit and a contributing editor to Popular Mechanics, for which he writes the regular &amp;quot;Thinking Ahead&amp;quot; column--and the occasional gadget review. Click here for an archive of his writing for PM. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/adventure/sports/a2693/4254403/>