A pony bottle is a redundant air supply used in SCUBA diving. It is a small bottle that looks like a miniature fire extinguisher, and usually carries about 15-30 seconds worth of air. Some of the larger pony bottles carry a couple of minutes worth. Carrying a redundent air supply system is one of those Good Ideas, particularly for some types of dive plans. While the statistical odds of having a mechanical failure in your air supply system are actually pretty low if you keep your gear maintained, scientific studies have shown that out-of-air (OOA) incidents experienced by recreational divers aren't as uncommon as we would hope. Perhaps ironically, these same studies show that for the most part, OOA's are caused by diver error - ie, poor planning and/or not adequately monitoring their air supply!

The Buddy System is still safe, and good practice, but is not something you want to completely rely upon. Assuming your diving buddy's tank is your spare air assumes a few things.
  • that you're diving with a buddy

  • that your buddy is close by when you need him (too far is too late)

  • that the buddy is competent enough to share air once you get to him

  • that the buddy is willing to share his air with you. There's no rule that says that a buddy must put himself in danger to rescue his partner. If the OOA diver has already panicked, its probably good wisdom for the buddy to stay away and not help - there are incidents of double fatalities in the accident reports, too. However, many OOA reports tell us that the buddy may not get much choice in the matter: the OOA diver simply MUGS him, yanking his primary regulator right out of his mouth.
  • and finally, if he has enough air left for the two of you to safely terminate the dive. Afterall, if you ran out because you wern't paying attention, the odds are probably pretty good that he's low too.

Carrying a pony bottle with you ensures that your "spare air" is...
  • with you
  • close by
  • as competent as you are
  • lacking free will, so its always there, willing to help you
  • it always has the amount of air you expect. (ie, whatever size you decided to buy, so long as you keep it filled and calculate the difference for depth)

If you are PADI or NAUI Certified, you probably already know all this already (hopefully not the hard way). If not, I seriously recommend always bringing a pony bottle with you when you go on a dive. Especially if it's recreational diving.

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