Thoughts on using a stage as your pony..

[DeepBound]

I see a lot of posts advising people to never dip into their pony gas because they decide to extend a dive longer than their main gas allows them.

I've been doing a lot of dives where I carry a stage bottle that serves as my pony, but it contains more gas than needed to get me back to safety in case my main gas source fails. For example, A dive that would call for a 13cf pony, but I carry a 40cf bottle and plan to use the top 1500 psi, and reserve the bottom 1500psi for emergencies. Or better yet, I carry an 80cf stage bottle, and reserve the bottom 1000 for an emergency, but plan to breathe 2000 of it during the dive. In these scenarios I always make sure that neither gas source gets below the amount I need to exit if the other fails.

Are there any problems with this setup? Is this basically similar to using independant doubles? Or is it dangerous in some way that I haven't thought of?

DeepBound

 

[do it easy]

I think that you are basically using independant doubles. You have a redundant air supply and if you manage it, like you said, so that you have a minimun amount in each tank at all times, I don't think that it is is any worse.

The one warning is not to plan your gas contingencies based on PSI, but rather volume. It might be coincidental, but in your example, you would reserve 1500 psi in an AL40, but 1000 psi in an AL80. Then again, these might just be some numbers you threw out off the cuff.

 

[DeepBound]

I see what you mean, those numbers are rounded way up. How about if I take using my emergency gas as dive gas one step further.. here's an example:

I'm exploring a wreck and a normal safe ascent would require swimming back to the anchor line and ascending it to 15ft deep, doing a 3minute safety stop, then ascending the rest of the way. Assuming 1cf/min SAC as a conservative number, and using a depth of 3ATA/66ft , this adds up to:

1 minute to get a grip on myself: 3cf
3 minutes to get to the upline: 9cf
90 seconds to ascend to 15ft deep: about 3cf
3 minutes at the safety stop: about 4cf
30 seconds to ascend to surface: 1cf
for a total of 20cf.

This means that I need 1500psi in an al40 or 750psi in my main al80 tank to get back. I always add 200psi regardless of the tank size so that it doesn't get completely empty. So I would explore the wreck using my back gas until it gets to 950, then use my stage until it gets down to 1700, then end the dive - head towards the upline. In more detail:

40cf 80cf
1700psi 950psi End the dive
800psi 500psi Arrive at upline
575psi 383psi Begin safety stop
200psi 200psi Surface

Now, lets say that I find myself almost approaching the upline, and I know that I've reserved 12cf in *each* tank for the return to the upline. Lets say that the current was with me, and I wasn't too far from the upline, so I still have 950psi in my main tank and 1500psi in my stage. Instead of going up the line, I extend the dive until my stage is down to 800psi then switch back to my main tank until it's down to 500psi, making sure I'm at the upline by then. So basically I take my time going back to the upline and breathe the allocated air from each tank for the return to the upline.

Then at the safety stop, I can stay for 6 minutes if I want to, using up the safety stop gas from each tank.

I think that you are basically using independant doubles. You have a redundant air supply and if you manage it, like you said, so that you have a minimun amount in each tank at all times, I don't think that it is is any worse.

The one warning is not to plan your gas contingencies based on PSI, but rather volume. It might be coincidental, but in your example, you would reserve 1500 psi in an AL40, but 1000 psi in an AL80. Then again, these might just be some numbers you threw out off the cuff.

 

[do it easy]

Now, lets say that I find myself almost approaching the upline, and I know that I've reserved 12cf in *each* tank for the return to the upline. Lets say that the current was with me, and I wasn't too far from the upline, so I still have 950psi in my main tank and 1500psi in my stage. Instead of going up the line, I extend the dive until my stage is down to 800psi then switch back to my main tank until it's down to 500psi, making sure I'm at the upline by then. So basically I take my time going back to the upline and breathe the allocated air from each tank for the return to the upline.

This is poor form- plan the dive then dive the plan. If you want to plan some time goofing off around the upline, then fine, but I wouldn't add time to the plan.

Here is the hidden danger- breathing down your 2 tanks until you hit your "ascend" pressures would take you off PADI's NDL chart- assuming that you spent the whole time at depth and had a SAC rate of .6, you would breath 96cf in 53 minutes. Now the triple kicker- did you plan enough gas for deco? are you trained/experienced with deco dives? And yes, I assumed your SAC rate was .6- if it were higher, then your gas plan might "work", but then your 1cf SAC rate isn't too conservative.

 

[DeepBound]

THat's true... a couple of times I've recalculated how much air I needed on the fly and then run into problems because I got into wishful re-planning mode!

I could stay down that long using nitrox

This is poor form- plan the dive then dive the plan. If you want to plan some time goofing off around the upline, then fine, but I wouldn't add time to the plan.

Here is the hidden danger- breathing down your 2 tanks until you hit your "ascend" pressures would take you off PADI's NDL chart- assuming that you spent the whole time at depth and had a SAC rate of .6, you would breath 96cf in 53 minutes. Now the triple kicker- did you plan enough gas for deco? are you trained/experienced with deco dives? And yes, I assumed your SAC rate was .6- if it were higher, then your gas plan might "work", but then your 1cf SAC rate isn't too conservative.