NITROX in a Pony Tank

[skdvr]

I am getting ready to take the NITROX course in a few weeks but if I do not ask now I am sure I will forget by then. I have been looking at buying a Pony Tank and was curious about NITROX. If I am diving NITROX in my main cylinder should I also have NITROX in the pony, or does it really matter?

Thanks
Phil

[Aussie]

Originally Posted by skdvr

I am getting ready to take the NITROX course in a few weeks but if I do not ask now I am sure I will forget by then. I have been looking at buying a Pony Tank and was curious about NITROX. If I am diving NITROX in my main cylinder should I also have NITROX in the pony, or does it really matter?

Thanks
Phil

G'day Phil,

I recommend when you get a pony cylinder that you get it O2 clean and labelled ready for Nitrox.
I idea is that you can put in a 20.9% Mix (which is Air) whenever you want, as long as its good clean air (filtered more). You then can change to what ever mix you need at some other time.

I think it depends on what diving you are doing. I can not see the problem with having the same mix in your pony as your main cylinder if your doing rec diving. It would make sense if you had to use your pony to have the same mix as you computer would already be set to the mix of the main cylinders.

Aussie

[CompuDude]

I've seen quite a bit of argument on this one, actually, with good points made by each side.

There is a strong argument for always having plain air in your pony, because in theory, you won't be on it long enough to really trash your computer's off-gassing profiles if you were on Nitrox before switching, and you'll never need to worry about MOD limits.

Yet there is also a strong argument for always matching your back gas, be it Nitrox or air, for a similar reason.

Finally, there is an argument for having Nitrox in your tank, because the very emergencies you're likely to need it most, Nitrox will aid the off-gassing.

On the flip side, there is a danger to having too rich a mix in your pony. If you have 40% stored in there on general principle, and end up having problems while at 130' while diving air, you could in theory have some issues due to exceeding the mix's MOD (by quite a bit, in that case).

So it's a somewhat tough call.

Me, I'd just keep it simple and always have plain air in it... if it was simply being used as a pony and not a deco bottle (which is an entirely different topic).

[ScaredSilly]

I figure it this way - if I have a pony bottle and I use it things have gone tits ups with my primary gear and my partner. So I am going to be out of the water for a while. Now while ideally I would perfer that my pony have the same mix so that gas loading was the same as my main supply so as not to confuse my computer or other gas calculations but is the say 10 minutes of air going to really screw up my loading? Probably not but it might. You would need to be the judge of that.

Now also when diving were I might want the bottle will be for traveling - most places are still air only so that is what you will be getting - i.e. you can have whatever Nitrox you want along as it is 20.9% O2. And if they do have Nitrox I might ask for it or I might not.

[meesier42]

Compudude is right, there are lots of opinions on this one, and I don't know if there is a "right" answer.

I look at my pony as an emergency "oh sh*%" air supply and if I have to use it the dive is over, which means I am heading up. If the computer is a little off due to this, I guess I really don't care. As I am probably done for the day anyway as I figure out what went wrong, how I fix it, and most importantly how do I prevent it from happening again. Which should give me plenty of time to off gas and reset before the next dive anyhow. So for me, I just fill my pony with air.

[skdvr]

Thanks for the responses. I certainly will only be using it as a Pony and not a deco bottle (Yet anyway). I can see the points both ways. I am still not sure what I would do, but I think I am leaning twards air. I think that like all of you said that if I am in a situation where I have to use it the dive day is probably over so the little bit of difference between where my computer thinks I am at and where my body actually is will not make much of a difference. I think that if I ever did want to have nitrox in it I would make it match what I am wearing on my back and I would not just keep a certain mix in it. I think that I would do air or match. I do not know enough about this stuff yet and I really appreciate the input.

Thanks again
Phil

[paintsnow]

see, with air i would want 32% which has a MOD of 130 ft

now if you manage to go lower then that on accident and need your pony, well
i would try to do a quick accent to 130 and then start breathing off the pony.

if i have nitrox in my main tank i would want to match it

in reality the best solution is one with nitrox and one with air

but you just have to decide which is best for you depending on how well you watch your depth and most importantly how big your pony is

if i have a 40 i would probably fill it with air and suck it dry at 15 feet

but again the 32% wouldn't hurt

the only reason i could see for not using 32 other then tech is diving in an area with a lot of downcurrents

sorry for my thoughts being all over the place

[skdvr]

Originally Posted by paintsnow

see, with air i would want 32% which has a MOD of 130 ft

now if you manage to go lower then that on accident and need your pony, well
i would try to do a quick accent to 130 and then start breathing off the pony.

if i have nitrox in my main tank i would want to match it

in reality the best solution is one with nitrox and one with air

but you just have to decide which is best for you depending on how well you watch your depth and most importantly how big your pony is

if i have a 40 i would probably fill it with air and suck it dry at 15 feet

but again the 32% wouldn't hurt

the only reason i could see for not using 32 other then tech is diving in an area with a lot of downcurrents

sorry for my thoughts being all over the place

When you say the best solution is one with nitrox and one with air, why is that? And, which one with air and which with nitrox?

Why do you say that if you have a 40cf pony that you would suck it dry at 15ft.? Is that to get the longest SS possible?

Phil

[in_cavediver]

CompuDude summed it up nicely. I will add though, for an emergency ascent, the time spent on the pony with air vs nitrox setting of your computer is so minimal, its in the range of the error of the deco algorithm.

Simply put, your computer is attempting to model what your body is doing. It has no direct cooeralation to what is actually happening in your body, just a 'good guess'. If you are really worried about, spend an extra minute at 15-20'.

As for the opinion about breathing 32% at say 150', its po2 is around 1.75ata. Well above what you'd like but given the choice of breathing or not, I'd take breathing. (mind you, you'd be ascending anyway so the exposure is very short). It wasn't that long ago that a PO2 of 2.0ata was considered OK. (for the record, I like a bottom PO2 of 1.2-1.3 max). Don't overthink it. Gas is life underwater and the best case is to have an appropriate gas, next is any gas because the last choice is breathing water.

My opinion on the ideal gas has more to do with availability and max depth than anything else. Air is probably the nicest choice but sometimes premix nitrox is as cheap or more so.

[texdiveguy]

Recreational divers: Keep it simple and SAFE--AIR!