Diving Nitrox with a pony bottle - theoretical

[MrBigfins]

Are you talking about a tank on your back, or a tank that you sling as an additional tank (stage or deco)?

Both back gas and "stage" (don't do deco as of yet so do not have any experience there) Although I don't see a difference in what data you would list on tank regardless of the use of the tank. If I am to use the tank immediately then date and initial are irrelevant to list on tank tape, when picking up tanks from a shop to use at a later date then the additional information of pressure and date etc are more useful.

I understand the concept of personal tanks having particular uses and gas mixtures assigned and permanently marked on tank for ease of sorting and gathering gear. That shouldn't change the procedure or information recorded when analyzing the gas mixtures regardless of the tanks intended use ie. Back gas, stage or deco

 

[tursiops]

Both back gas and "stage" (don't do deco as of yet so do not have any experience there) Although I don't see a difference in what data you would list on tank regardless of the use of the tank. If I am to use the tank immediately then date and initial are irrelevant to list on tank tape, when picking up tanks from a shop to use at a later date then the additional information of pressure and date etc are more useful.

I understand the concept of personal tanks having particular uses and gas mixtures assigned and permanently marked on tank for ease of sorting and gathering gear. That shouldn't change the procedure or information recorded when analyzing the gas mixtures regardless of the tanks intended use ie. Back gas, stage or deco

There are a couple of points you seem to be missing. If the tank is for your back, as your primary gas, then displaying BIG markings on it as to what is in it (esp an MOD) is not needed; presumably you have planned your dive and are diving your plan.
But if you are slinging a tank, then the markings are to prevent inadvertent errors in breathing the wrong gas at the wrong depth. If I am your buddy and see you switching to an MOD 30 FT tank at 50 feet, I will stop you. If you don't have BIG MOD markings on your tank, I have no way to be helpful.
So, you are right, there is no "difference in what data you would list on tank regardless of the use of the tank." But there is a difference in what you would additionally display prominently so others can see it, depending on the use of the tank.

 

[MrBigfins]

There are a couple of points you seem to be missing. If the tank is for your back, as your primary gas, then displaying BIG markings on it as to what is in it (esp an MOD) is not needed; presumably you have planned your dive and are diving your plan.
But if you are slinging a tank, then the markings are to prevent inadvertent errors in breathing the wrong gas at the wrong depth. If I am your buddy and see you switching to an MOD 30 FT tank at 50 feet, I will stop you. If you don't have BIG MOD markings on your tank, I have no way to be helpful.
So, you are right, there is no "difference in what data you would list on tank regardless of the use of the tank." But there is a difference in what you would additionally display prominently so others can see it, depending on the use of the tank.

Thank you, this was a well articulated explanation.

 

[PBcatfish]

The valve on your pony should only be open if you're breathing from it, so having a free flowing regulator seems unlikely to me.

Really? I keep my valve open in case someone has an out of air experience & needs a quick fix. The second stage on my pony is tuned a little heavy, so free-flow is not a common thing. Also, I carry it on my front, where I can get to it quickly, so if there was a free-flow, I would be able to get right on it.

Another reason I don't like having a pony at depth with the valve off is because that increases the changes of salt water getting inside my regulator if a purge button gets bumped or something like that.

 

[PBcatfish]

... I refuse to use the pony to extend dives as I have seen some do. ...

I understand that perspective, but I sometimes do invade the pony supply in a planned way. I sometimes dive with an aluminum 80, while others in the group dive on steel 100's. In that situation, I grab a 30 pony to boost my air supply. My game plan is to suck my main tank down to 500psi, then switch to the pony. When the pony hits 1,000psi, I go back to the main tank & head for the surface, keeping enough in the pony to get someone out of trouble if need be. I've done that about 10 times. I normally try to bring big enough tanks so that I will not need to pull that trick.

 

[PBcatfish]

Your analysis should include FO2, MOD, date, initials, and I like putting pressure on mine. The poster I was responding to was incorrectly assuming MOD decals like those on DIR-rigged stages was an FO2 marking. I'm just pointing out that FO2 goes on the analysis tape.

What you state is the way that I was originally told was correct. These days, I leave off the date & the signature. I mark Fo2, Intended MOD & contingency MOD. My tanks are unique in my group, so I am not likely to mix them up with others & I dive every week, so my fills don't get old. I like having both MOD numbers on there, in-case something happens & plan B becomes necessary. I like to have both MOD numbers fresh in my head when I ht the water.

 

[Diver-Drex]

I understand that perspective, but I sometimes do invade the pony supply in a planned way. I sometimes dive with an aluminum 80, while others in the group dive on steel 100's. In that situation, I grab a 30 pony to boost my air supply. My game plan is to suck my main tank down to 500psi, then switch to the pony. When the pony hits 1,000psi, I go back to the main tank & head for the surface, keeping enough in the pony to get someone out of trouble if need be. I've done that about 10 times. I normally try to bring big enough tanks so that I will not need to pull that trick.

if that’s your gas plan then I don’t see an issue. Just make sure you’re doing a rock bottom or min gas calculation and leaving enough gas in both cylinders for that. You’re leaving 500psi or 12cf in the big tank and then continuing the dive. If the small cylinder isn’t available is that enough to get you to the surface? 1000psi in a 30 is 10cf. Same question?

And to tie this back to the OP, in this case you should have the same gas in both cylinders. Or make the gas switch on your computer if the gases are different and your computer has that capability.

 

[PBcatfish]

if that’s your gas plan then I don’t see an issue. Just make sure you’re doing a rock bottom or min gas calculation and leaving enough gas in both cylinders for that. You’re leaving 500psi or 12cf in the big tank and then continuing the dive. If the small cylinder isn’t available is that enough to get you to the surface? 1000psi in a 30 is 10cf. Same question?

And to tie this back to the OP, in this case you should have the same gas in both cylinders. Or make the gas switch on your computer if the gases are different and your computer has that capability.

I normally take the same gas in both cylinders, although a ProPlus computer does give the option to use multiple mixes. I leave a little more in the big tank compared to the pony because the big tank also feeds the BC. 10CF is enough to get me up from my planned depth with enough left over for safety margin. The extra 2 cf in the big tank is in case I fumble & let too much out of the BC on the way up. That doesn't happen often, but I like to be prepared for the possibility.

 

[Stoo]

Once you get a pony you'll understand. You don't take a pony to the dive shop for a fill. You plug it into whatever tank you're diving with a transfill whip, and therefore it gets whatever your main tank has.

Umm, I've been at this a long time and never seen, or even heard of anyone filling a pony from a main tank. I suppose it might be useful if the pony is a little 13 cf jobby, but to fill a 30 or 40 cf tank would complete defeat the point of taking a pony. You'd end up with two "half" filled tanks.

Getting back to the OP, nitrox isn't regarded as a method to combat narcosis but even if there is some advantage, the difference will be negligible. And as someone else pointed out, if you have gone to your pony, then presumably the dive has gone sideways and you're on your way up which again, negates the narcosis issue generally.

 

[PBcatfish]

The little Spare Air cylinders (mostly 1.5-3cf, but sometimes 6cf) are intended to be filled off of primary tanks. They don't really put too much of a dent in the primary tank pressure.

I know a few guys that fill real pony's off of primaries, but not many.

One local shop charges me full rate to fill a pony, the same as if I was filling a 120, so I don't get my ponies filled there. I have another shop that charges by the cube, so those guys do all my top ups of big tanks & fill all my ponies.

Back when Sports Chalet was still in business, they used to fill ponies for free. They also would give no discounts when selling a tank that had a 4 year old hydro date. They are out of business for a reason. They just didn't understand what they were doing.