Different mixes and pony bottles

I currently dive air but plan to take the nitrox course in the next month or so. I usually dive AL80s but for deeper longer dives I will do an HP100 and a 40cf pony bottle. Nitrox fills are a bit incovenient for me so i will likely only get them for deep dives etc. Since I don't use my pony except in an emergency, I keep it filled so its always ready. Will I need to empty it and switch what it is filled with to match my back gas for dives with nitrox or should I just keep it filled with air or just keep it filled with nitrox? I would prefer not to have to upgrade to a two gas computer although that too is a possibility. Thanks.

Hi.... this topic of sorts has just been covered in a recent thread here....BUT its always a question newer diver ask and is a good one!

There are 2 schools of thought on the question of whether you should match the contingency gas in your bail out pony bottle to that in your back gas cylinder/s.

There are divers that will tell you yes you do need to do this for various reasons.

The fact is really NO you do not need to carry nitrox in your contingency pony even on those dives were you are using nitrox as your back gas.

AIR is the best gas for a recreational pony bottle, has several advantages and no negatives. So you can make your life much easier by simply keeping a current fill of AIR in that pony bottle....also take the time to practice with it and check it's functioning each dive.

As to nitrox being used only for 'deep' dives.....this is a misconception many divers have.....your nitrox course will clear that up......but nitrox is a gas more friendly to shallow extended range and mid level depths in the recreational profiles.

Have fun and be safe!

if you're just going to use it for emergency redundancy (i.e. not as a deco bottle) you could have either air or nitrox in it. Remember if you have nitrox, put in a % where the MOD will be at a depth you are diving (i.e. you don't want a really high % of O2 that would be toxic at your max depth). [you will learn about it in your course]

if you just have air, that is also fine - remember you are really just needing the gas for a safe ascent and not to continue your dive.

no need to empty and refill as long as whatever gas in it fits with your planned dive profile.

I understand that nitrox isnt only for deep dives, its just that getting the fills are out of the way for me so im not going to do it for all my shore dives, just the boat dives i go on which in general are deeper than the shore dives. Also, will I need a two gas computer if I have two different gases?

diveandclimb:

I understand that nitrox isnt only for deep dives, its just that getting the fills are out of the way for me so im not going to do it for all my shore dives, just the boat dives i go on which in general are deeper than the shore dives. Also, will I need a two gas computer if I have two different gases?

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If you're just bailing out on your pony bottle... you shouldn't need a multi-gas computer - whatever you put in your 40cf stage bottle. If you're diving to 80 FSW (on air) and have 36% in your stage... are you continuing your dive, then on 36% and enjoying a better NDL? Or are you just switching and ascending to the surface (making appropriate stops) with the assumption that you have air?

If you're really bailing out.... you (in this case) wouldn't need to tell your computer you switched. Continue on an air profile, and make the ascent.

I know my example is simplified, but if you're planning dives that exceed the NDL, then we shouldn't be having this discussion in its current form, and then you should consider a multi gas computer, or know how to plan staged decompression dives.

diveandclimb:

I understand that nitrox isnt only for deep dives, its just that getting the fills are out of the way for me so im not going to do it for all my shore dives, just the boat dives i go on which in general are deeper than the shore dives. Also, will I need a two gas computer if I have two different gases?

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I would recommend a computer that will allow you to do either AIR or NITROX dives.....known in the industry as a NITROX computer. This is not the same as a 2 gas or even a 3 gas computer that allows the diver to switch underwater during the dive from one mix to the other as in a diver say using a different mix for planned decompression dives. If your plans are simply basic recreational NDL profile AIR or NITROX diving a basic NITROX computer will handle those needs fine.

I just am using the pony for bailout (obviously or it would be a stage/deco bottle). My current computer can do air or nitrox it just hadn't occured to me that I could just use the nitrox during bailout as if it were air. Thanks. PS: Since I'm not stupid I know that continuing the dive using the pony after a primery tank failure is dumb.

First you need to describe what 'deep' is. It is relative for most, for example 'deep' to me is 200 fsw, for some of the CCR divers that I frequently dive with this is 'recreational' since it is within Normoxic limits and 'deep' to them is 460 fsw. OK, back to your question.
Nitrox (EANx) LIMITS your depth, and does not allow you to dive deeper. This has to do with the partial pressure of oxygen (PO2). Max on PO2 that you want to breath for a short time is 1.6. So lets look at the two common EANx mixes that you will learn about in your class.
EANx32 (32% oxygen, 67% nitrogen, 1% other but for argument sake, 68% nitrogen). This will give you a maximum operating depth (MOD) of 130 fsw (PO2, 1.6)
EANx36 (36% oxygen, 64% nitrogen) gives you a MOD of 110 fsw (PO2 1.6)

As you can see the higher the oxygen the lower the MOD. The benefit you get from higher O2 is less Nitrogen, thus less narcosis and less formation of bubbles. The less Nitrogen the more the gas acts like you are more shallow, termed EAD, equivalent air depth. You want your EAD to be 130 or less, I like my gas to be 100 or less.

How 'deep' do you consistently dive? Do you get narked at this depth? Have you felt what being narked feels like? If you really want to go 'deep' you can now see why you can't just replace the nitrogen with oxygen because it would cause an O2 hit. So, this is why you need to replace it with something else, Helium. Once you decide to go deep, take a 'recreational trimix class' IANTD has one. This keeps you to a depth of 140 fsw but allows you the benefit of a low EAD and a clearer head while at depth.

Now to your BO bottle (40 cuft). If you are staying above 130ft, fill it with EANx32. Even if your computer is set to 'Air' you are getting the benefits of higher O2 and less saturation of tissues as you start your ascent. (I would not use EANx32 for the bottom gas due to building up your OTU, but the BO is fine to switch to at depth and start your ascent).

stu_in_fl:

if you're just going to use it for emergency redundancy (i.e. not as a deco bottle) you could have either air or nitrox in it. Remember if you have nitrox, put in a % where the MOD will be at a depth you are diving (i.e. you don't want a really high % of O2 that would be toxic at your max depth). [you will learn about it in your course] .

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Umm...What good would it do to mark this bottle if it's for bailout? You can't properly verify the mix anyways in an OOG situation, there's simply not enough time. Not to mention, the pony should always have your deepest mix, or you're carrying a bailout that's not useful when you need it most.

Pony's are an excellent way to develop bad habits for this very reason. What happens when you go OOA and are slinging a deco bottle?

ucfdiver:

Pony's are an excellent way to develop bad habits for this very reason. What happens when you go OOA and are slinging a deco bottle?

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????????

What does pony bottle usage have to do with developing 'bad' habits when your dive profile requires you to carry a deco bottle?? You can even PM me your explaination as not to steer from the threads real topic.

My recreational experience in the use of a pony bottle only played to shorten the mechanical learning curve of carrying a deco bottle.

This thread has nothing really to do with deco bottle procedures---a totally different set of protocols well beyond the scope of this thread.