Redundant Air Source Diving

[R1Adam]

Hi guys,

I'm a PADI AOW EANx diver with about 25 dives under my belt. I'm going away in a weeks time and will get another 10 - 20 dives under my belt and will also be doing my PADI deep and wreck specialities.

When I get back to the UK I'm going to be attending some trips out with my Local Dive Club. I got a schedule of planned trips whilst I was in the shop today and was talking to one of the instructors. He said that any dive over 24M will require a redundant air source.

Now as I'm totally new to the concept of using a redundant air source, I'm looking for some advice about them. All I know at the moment is that it consists of a pony cylinder, 1st & 2nd stage reg and pressure gauge which is either attached to your cylinder or carried in a sling.

What is a redundant air source used for? Is it just there for an emergency situation or is it actually used on the dive? Also, I dive using nitrox so what would I fill the pony with? Would I have to match the gases in both cylinders? I have a Suunto D6 and I'm not sure if it's capable of gas switching, I think it is!

I have dived before to depths greater than 24M and have not used a pony so what do you think the reasoning is behind their requirement to use a redundant air source below 24M?

If the pony is only used for emergencies, would you turn on the air at the surface before each dive? If this is the case, surely after numerous dives you'd start to loose pressure from the cylinder due to pressurising the regs and testing, therefore using up air. If the air is not turned on at the surface then I'm thinking that this could cause problems underwater in the event it's needed.

Your advice would be greatly appreciated.

Thanks

Adam

 

[Garrobo]

For an emergency. Turn it on at the surface. You won't lose that much air. If it drops pressure just fill it again. I don't know for sure but I expect that you could surface with air although you are using a different mixture for the dive. 24 meters ain't that deep anyway.

 

[MXGratefulDiver]

Redundant air source is strictly for emergency use ... you do not use it unless needed, and it is not to be factored into your gas consumption (aka "rock bottom") calculations.

As you noted, there are two basic ways to carry it ... tank mounted and slung. Tank mounted is more convenient, but offers two primary limitations ...

- most people cannot reach the tank valve, in the event that you attempt to deploy it only to realize you forgot to turn it on.

- you cannot unclip and hand it off to a buddy in the event that your buddy runs OOA.

Both of these considerations make slinging a pony a better option. Slinging means that you put a "harness" on the bottle, with two clips that attach to your BCD at the chest and waist D-rings. This puts the bottle under your arm where, if the harness is properly rigged, it basically "disappears" while diving.

If you are tank mounting, you must turn on your bottle prior to the dive and leave it on during the dive. Another potential drawback to this approach is that if you have a leaking o-ring or valve you will not notice it (although if your buddy is on the ball, he or she will).

If you are slinging, you can pressurize the second stage and turn the bottle off unless is it needed. The fact that the tank valve is right there where you can easily reach it makes this a preferable option, to avoid the potential for a valve or o-ring leak to lose gas.

In some respects, the purpose of a redundant bottle is to reduce or eliminate the need for reliance on your dive buddy in the event of a catastrophic failure that causes you to lose access to your primary air supply. Remember, as long as you are able to breathe, you can fix almost any other problem underwater. As you go deeper, the option to surface quickly in the event of a failure is reduced dramatically. The bottle gives you a level of self-sufficiency that your dive buddy rarely will.

It is not necessary to match the gas in the pony bottle to your primary supply, although it might be desireable. Remember, if you have to deploy it, you are going to be making as direct an ascent as possible ... so NDL isn't really a concern.

... Bob (Grateful Diver)

 

[R1Adam]

Thanks Bob,

That's a really helpful and informative reply. It sounds to me like slinging the pony is certainly the best way to go. I'm assuming that they require me to use one because the vis can be quite bad in these waters at times. If you loose sight of your buddy then you are self sufficient and can make a safe ascent!

 

[krawlings]

I dive with a s30 pony and I sling it on my left side. The pony is primeraly for EMERGENCYS (OOG) .The deeper dives maltiplies the risk , And things can go from BAD to VARY BAD IN A QUICK INTANT . I believe that it can't hert to have to mucn air . If you don't need it then great but if you need it and it ain't ther you can't call a muligon , or do over . And yes the pony will slowly loose some air each time you check and or turn the tank all the way open for a dive . You should allways check your pony befor avery dive to make shore it is charged and how much is in the tank , and that it is in working order. You do not want to get in to a emergency situation and it is emty or not working . A bad time to find out.
I charge my reg. with air at the same time I check the tank pressure and check function .I then shut the valve and leave the reg. charged with air . so ther is one full breth and then it can be opened . I have had my pony reg start freeflowing when i jump in the water and I have to stop the flow . You loose a bit of air each time you jump in . I geuss it is up to the diver as to how and when he/she does this . It comes down to compfert and to how used the set up you are some like to leave it on the hole dive . That is ok to



GOOD LUCK DIVE SAFE KEN

 

[MXGratefulDiver]

Thanks Bob,

That's a really helpful and informative reply. It sounds to me like slinging the pony is certainly the best way to go. I'm assuming that they require me to use one because the vis can be quite bad in these waters at times. If you loose sight of your buddy then you are self sufficient and can make a safe ascent!

Quite so ... diving in bad vis challenges buddy teams.

Another important consideration is understanding gas management, and making sure that not only do you choose an adequately sized pony bottle, but that you are carrying sufficient gas in your primary tank for the dive you're planning ... thereby minimizing the potential that you'll need to deploy your backup during the dive.

If you haven't already seen it, there's an article on my web site that can help you learn more about how to do that ... Understanding Gas Management

... Bob (Grateful Diver)

 

[R1Adam]

I think charging the regs and then shutting off is a good idea as this prevents freeflow. One full breath should be enough to give you time to turn the pony back on and make the ascent.

Thanks for the link Bob, I shall have a good read of it! I've purchased myself a 15l cylinder which should be plenty for most dives I"ll be doing up to a max of about 32M once I've done my deep diving speciality next week.

 

[DandyDon]

I leave mine on, ready to go for anyone who wants to grab it and breath. It can be so reassuring.

 

[TSandM]

As Bob says, the further you get from the surface, the less possible a CESA-type escape from a gas emergency becomes. The deeper you are, the more desirable it is to be able to solve your own problems underwater, rather than ascend to do it. At some point, having a redundant system becomes very attractive. The options are carrying a smaller auxiliary bottle (aka "pony") or diving doubled tanks. Doubles are the strategy of choice of people who operate under a virtual overhead -- in other words, people who incur a decompression obligation that means that, no matter whether they could execute it or not, an emergency ascent is really not an option.

Pony bottles are the strategy of people who are not that deep or there that long, but who are concerned about their ability to provide some redundancy in the deeper recreational range, where counting on a buddy's availability, even if the buddy is trained and reliable, no longer seems satisfactory.

The better trained you are and your buddy is, and the better the diving conditions, the deeper and closer to deco limits you can go before feeling pressed to provide self-rescue. If you routinely dive in poor conditions or with unknown buddies, you may feel much more motivated to carry your own exit strategy. Once you have made that decision, look at the deepest dive you do, and calculate the gas you need to make an ascent from that depth at a normal speed, and make any stops you feel you should make. Assume a stressed gas consumption rate. That will give you the size of extra tank you should carry. How you carry it is up to you -- slinging a bottle is easy, and has the advantage of training you in a system that will scale nicely into staged decompression diving, should you contemplate that in the future.

 

[gcbryan]

If you decide to sling a bailout bottle you can either charge it and turn it off and the reasons for that approach have been given or you can keep it turned on.

The reasons for keeping it turned on are that unlike going for a deco bottle (where the concept of charging the bottle comes from) if you go to your bailout bottle you may be in a more stressful situation and not need the added stress of having to turn on the bottle.

I personally chose to turn it on at the surface and just check the pressure gauge a time or two throughout the dive.

There are good reasons for whichever way you choose.