I had a primary second stage fail once upon a time. I switched to the
octopus and continued the dive. I had a 13 cf pony to cover other
eventualities.

I've had 2nd stages breathe wet enough that it was irritating enough
that I disassembled them while underwater and fixed the problem...
Usually just a diaphram that wasn't seated correctly... To prevent
freeze ups, I dive in warm water -- works *every* time...

Failure of a properly maintained regulator designed for the kind of use it
gets is very rare. By far, most failures are either because the regulator
was not maintained or because it was not suitable for the way it was used,
in very cold water, for example.

Part of the answer to how to deal with regulator failure is inherent in the
statements above. The best way to deal with them is to avoid them in the
first place. The other parts of the answer are alternate air source
(octopus), redundant gas supply (pony bottle) and dive buddy.

Lee

All of the above, and if you dive in freezing conditions you should use a
regulator designed for these conditions.  When a regulator fails it almost
always fails in an open position and free-flowing.  It's very easy to breath
from a free-flowing regulator (when trained how to do it) but you will go
through your air very quickly so you should ascend immediately but safely if
possible.  If you have a buddy, which you should have, grab their alternate
supply and ascend.

If you use your alternate regulator and your primary is free-flowing I would
still go to a buddy.  If that's impossible keep a sharp eye on your air
supply as you ascend.

BTW, I'm a newbie and I've only experienced a regulator failure during a
certification dive -- and we created the problem.

Well, there are different sorts of reg problems.

The most common one is a mild leakage from either the first stage or
the second stage.  Annoying but not really a big deal - my own regs
dribble a bit of air for ages.   Can be fixed by servicing the reg and
checking the IP to make sure it is within manufacturer specs.

Another problem is a second stage freeflowing - which has happened to
me a few times during shallow water training (we do confined off a
beach).  You can breathe off the freeflowing reg as you ascend, or if
you can reach your tank valve, you can turn it on/off in sync with
your breathing (not really recommended except as a last resort).

A more serious problem is a first stage problem which can cause it to
flood - usually a problem with the diaphragm or the piston seat.  I've
had this happen twice in the last 10 years: one was a brand new reg
(my second time using it), and once was a reg that was in the middle
of its useful life (a few years old, serviced 2 month prior).   2
common causes - either you wait far too many years before servicing
your regs (which can cause problems with sealing or internals breaking
down), or there is a defective part (perhaps an O-ring that wasnt made
correctly).  If the latter, you're pretty SOL as there is not much any
reg technician can do to id that upfront.

Here, you really dont have too many choices except (i) hope you have a
redundant air source - either a different tank+reg set, or a buddy or
(ii) hope you can make an emergency ascent.   The problem is that
while most people can do an emergeny ascent from 20m - or even 30m -
if starting with full lungs, a flooded reg means that you'll be
starting with empty lungs.  So stay close to your buddy or work on
your apnea skills.

Generally speaking, the best in-water practice is to stay with your
buddy.  Play a game I have my OW students do during dives 3 & 4:  at
any random point, stop breathing and try to swim to your buddy and get
their air.  If the urge to breathe overpowers you, the two of you need
to rethink your buddy skills.

Vandit

IMHO if your primarary fails you have 3 sources of air:-

- your secondary
-your buddies secondary
-your buddies primary

if all else fails a emergency swimming ascent ( blow bubbles  ) and you
won't be dead ( but maybe with blood like a beer bottle ) but hopefully
someone can figure out the nitrogen situation.........  Hey lets go down
again with plenty of air and sit on a deco bar.

My buddy's mouthpiece detached from the reg. This is a bit vicious as
you only find out about it after you exhale, and there's a risk of
"breathing water". But other than that, the secondary is there, and
your buddy should be close enough anyway (but the notion of "close
enough" varies depending on whether you have a proper air supply or
not: I thought I was close to her, but she felt differently... weird)

Funny you should mention this.  I've been diving for quite a while and
never had a regulator problem until just a few weeks ago.  Like many
of the others here have said to you, I don't dive in freezing temps
and I do have my regs serviced at my LDS.  A few weeks ago while on a
shore dive, my first stage blew.  I was with 3 other divers and we
were close to the exit point , which was a ladder to climb up a 10 ft
cliff to the beach.  Since we just spotted some squids and we all had
air left, we were watching the squids, not ready to end the dive and
were in about 25 ft of water.  All the sudden, my primary started free-
flowing very hard in very rapid spurts.   It actually hurt my teeth to
try to maintain it in my mouth.  The others said the rapid pop-pop-pop
noise was so loud that it sounded like machine gun fire underwater and
got their attention immediately.  Puzzled, I took the reg out of my
mouth and smacked it.  It didn't stop so I switched to my octopus but
the octopus did the same thing.  My buddy came over next to me by the
time I tried my octopus and when he saw that failed too, he shoved his
primary in my mouth and we made a controlled assent.  I turned my tank
off as soon as I could to stop the racket .  Later that day, I was
talking with a fellow diver who is a dive shop owner and as soon as I
described the situation,  he asked,  "Was it a Mares reg by chance ?"
I replied yes and he told me that Mares are "famous" for doing that.
I could have made it to the surface fine without my buddy's help but
needless to say, I was a bit shaken by it because we had been down to
80 ft during that dive.  Good thing it happened at the end of the
dive.