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This article is from the Scuba Diving FAQ, by njs@scifi.maid.com (Nick Simicich) with numerous contributions by others.

14 I'm thinking of getting a redundant breathing system,

in case I have a hose failure or run out of air, and can't find my
buddy. I've heard about something called "Spare Air", and also "Pony
Bottles". Should I buy one? Or is there something better?

First off, carrying a redundant breathing system is a good idea. There
are a couple of important questions.
1. What are the types of redundant systems, and how much do they
cost?
2. How much air do you need to be safe in case of a problem?
3. How likely are you to carry your redundant system with you
when you dive and vacation?

Types of redundant systems.

What sorts of redundant systems are there? First, by "redundant
system" I'm referring to a system that will continue to work no matter
how catastrophic the failure of your main system. Thus, I won't
consider a Y valve a redundant system because of the fact that a burst
disk could rupture or an O-ring could fail and exhaust your entire air
supply, or, that because of an error or a bad gauge, you could exhaust
your entire air supply. The three most frequently used redundant
systems are
1. the bailout bottle,
2. the pony bottle and
3. the independent twin tank.

Some British BCs have a small air bottle attached to the BC. With
proper training and practice, it is possible to use this air for
breathing. But since this isn't a straightforward regulator system, we
won't discuss it here either.

The bailout bottle is available in sizes as small as 1.2 cu ft, and as
large as 3 cu ft. The best known brand is "Spare Air". The bottle has
a regulator that must (for older models) be switched on before use.
Bailout bottles can cost between $200-$300. The ones sold at a
discount by mail order houses are typically smaller bottles of older
design.

The pony bottle is a smaller spare tank that is actually a small
standard scuba bottle, and attaches to a standard regulator. Many
people use an inexpensive regulator on their pony bottles. You also
need some sort of mounting system. Pony bottles can cost between
$250-$350 depending on the regulator selected, the size of the pony,
and the care you take while shopping. You can get a 13 cubic foot pony
(in 2000 PSI and 3000 PSI models), a 17 cubic foot pony, a 30 cubic
foot pony, a 40 cubic foot pony, and some other sizes.

The independent twin tank is a second tank which is the same size as
your first tank, and which has its own regulator. Since the two tanks
fit into a single double tank bracket, they may look like a set of
doubles, but, in fact, they are two separate tanks. The independent
twin tank is a good option for certain specialty diving, like wreck
penetrations or extreme deep diving, but I won't discuss it further
here. Costs vary widely depending on how much the mounting costs, the
type of tank, and so forth.

 

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• prev: 13 Can I dive in contact lenses (contacts)? Is it safe? Will I go blind?
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• next: 15 How much air do you need to be safe?

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