Pony Bottle, On or Off

[BobbyT22]

Extremely well put. Never heard it expressed quite that way before, and I learned something. Thanks.

It isn't a dumb question at all.

Just realize that many questions (and answers) are impacted by the diver's environment - for example, you may dive where gill nets are in use. If you don't, then you really aren't familiar with the dangers that that particular hazard poses to recreational divers. So...questions and answers can be environmentally dependent.

Most divers who use pony bottles plan to use them based on some personal scenario. They base what they buy/carry on some pre-identified potential need.

Say for example, a guy who thinks "I spend a lot of time between 90 and 110 fsw, and thats a long way down. If something happens that knocks my dive plan all to hell, say I get entangled in a net or something and use more of my backgas than I planned, I want a completely redundant air source that will get me to the surface regardless".

So the guy takes his respiratory minute volume, and he calculates that in an emergency he may burn gas at a rate of approximately 1.3 cubic feet a minute, which at 100'+ or so for an average diver in trouble may not be far off the mark.

Then he figures he'd need 2-3 minutes at ~100 fsw to unscrew whatever issues he (or someone else) is having; 3 minutes to ascend from 110'-100' to 10' at a rate of 30 fpm (because he pushed his bottom time); 3 minutes safety stop at 10'; and 1 min to ascend slowly from 10' to the surface. So he plans to be on the pony or bail-out bottle for a total of 10 minutes or so. Given the potential for things to get strenuous at depth, he's calculating a straight consumption of around 13 cubic feet minimum. Depending on how deep he goes, and how much task loading actually occurs at depth, gas consumption could be greater than that, but thats why he wants the bail-out bottle in the first place.

The point is that he's planned to have a finite amount of gas on-hand to respond to a worst-case scenario that is pertinent to his specific environment, etc.

When you leave your tank turned on, lots of odd things can occur that will cause you to lose gas. Things get bumped. You assume some odd attitude in the water, and X presses against Y, and you inadvertantly lose gas. Maybe not much - and you're aware of the loss, but its lost gas nevertheless. Maybe - if you happen to be deeper than 100' - you could conceivably lose quite a bit of gas in very little time. And its your bail-out gas...that you've planned on to respond to specific things that may occur in your own specific environment.

The reason guys re-charge their hoses on deep dives is because they lose the internal intermediate pressure between the second stage and the first stage, when the bottle is turned off. Why? Stuff happens. Things get bumped.

You keep bottles turned off because the gas in them has been calculated to meet specified needs. If X happens, you need Y amount of gas, and Y - Z (that bubbled out when your regulator got bumped and began flowing) is not what you had in mind.

Ultimately, if the short amount of time that it takes to turn a valve on to access a bottle is a problem for a diver, that diver is diving close to or beyond their limits. It simply doesn't take that much time to turn on a bottle. Divers who sling tanks ought to be able to remain calm for the matter of seconds it takes to turn on a sling tank and do a gas switch.

When you weigh the possibility of losing gas - possibly a lot of gas depending on how quickly the diver is able to respond (with whatever else is going on) - against the time it takes to turn on a bottle, generally most reasonable guys will conclude that it makes sense to ensure that you have the gas you calculated that you'd need when you need it. Which forms the basis for the argument against leaving your tank on throughout the dive.

Obviously there are other schools of thought on this matter - this is just one guy's answer to your question.

But the next time you put your regulator on your tank, after you charge your hoses, turn your tank off and time how long it takes you to turn it back on.

Don't you think you'd be able to maintain your presence of mind underwater for the matter of seconds it takes you to turn your tank on - if the alternative was possibly losing gas you were counting on?

Just a thought.

Dive safe,

Doc

 

[DaleC]

It isn't a dumb question at all.

Just realize that many questions (and answers) are impacted by the diver's environment - for example, you may dive where gill nets are in use. If you don't, then you really aren't familiar with the dangers that that particular hazard poses to recreational divers. So...questions and answers can be environmentally dependent.

Most divers who use pony bottles plan to use them based on some personal scenario. They base what they buy/carry on some pre-identified potential need.

Say for example, a guy who thinks "I spend a lot of time between 90 and 110 fsw, and thats a long way down. If something happens that knocks my dive plan all to hell, say I get entangled in a net or something and use more of my backgas than I planned, I want a completely redundant air source that will get me to the surface regardless".

So the guy takes his respiratory minute volume, and he calculates that in an emergency he may burn gas at a rate of approximately 1.3 cubic feet a minute, which at 100'+ or so for an average diver in trouble may not be far off the mark.

Then he figures he'd need 2-3 minutes at ~100 fsw to unscrew whatever issues he (or someone else) is having; 3 minutes to ascend from 110'-100' to 10' at a rate of 30 fpm (because he pushed his bottom time); 3 minutes safety stop at 10'; and 1 min to ascend slowly from 10' to the surface. So he plans to be on the pony or bail-out bottle for a total of 10 minutes or so. Given the potential for things to get strenuous at depth, he's calculating a straight consumption of around 13 cubic feet minimum. Depending on how deep he goes, and how much task loading actually occurs at depth, gas consumption could be greater than that, but thats why he wants the bail-out bottle in the first place.

The point is that he's planned to have a finite amount of gas on-hand to respond to a worst-case scenario that is pertinent to his specific environment, etc.

When you leave your tank turned on, lots of odd things can occur that will cause you to lose gas. Things get bumped. You assume some odd attitude in the water, and X presses against Y, and you inadvertantly lose gas. Maybe not much - and you're aware of the loss, but its lost gas nevertheless. Maybe - if you happen to be deeper than 100' - you could conceivably lose quite a bit of gas in very little time. And its your bail-out gas...that you've planned on to respond to specific things that may occur in your own specific environment.

The reason guys re-charge their hoses on deep dives is because they lose the internal intermediate pressure between the second stage and the first stage, when the bottle is turned off. Why? Stuff happens. Things get bumped.

You keep bottles turned off because the gas in them has been calculated to meet specified needs. If X happens, you need Y amount of gas, and Y - Z (that bubbled out when your regulator got bumped and began flowing) is not what you had in mind.

Ultimately, if the short amount of time that it takes to turn a valve on to access a bottle is a problem for a diver, that diver is diving close to or beyond their limits. It simply doesn't take that much time to turn on a bottle. Divers who sling tanks ought to be able to remain calm for the matter of seconds it takes to turn on a sling tank and do a gas switch.

When you weigh the possibility of losing gas - possibly a lot of gas depending on how quickly the diver is able to respond (with whatever else is going on) - against the time it takes to turn on a bottle, generally most reasonable guys will conclude that it makes sense to ensure that you have the gas you calculated that you'd need when you need it. Which forms the basis for the argument against leaving your tank on throughout the dive.

Obviously there are other schools of thought on this matter - this is just one guy's answer to your question.

But the next time you put your regulator on your tank, after you charge your hoses, turn your tank off and time how long it takes you to turn it back on.

Don't you think you'd be able to maintain your presence of mind underwater for the matter of seconds it takes you to turn your tank on - if the alternative was possibly losing gas you were counting on?

Just a thought.

Dive safe,

Doc

Hi Doc, thoughtful post.

I can't argue against your reasoning even though I don't use my pony that way. I often believe that there can be more than one right answer to a problem. The key for me is knowing the advantages and limitations each offers and acting accordingly. In my case I leave the pony valve on for the duration of the dive. This is my reasoning:

1. It isn't deco gas and I don't need it for a non eventful rec dive the way a tech diver needs deco gas. If I lose all my pony gas somehow I can simply either a.) move shallower so a CESA can be employed or b.) call the dive.

2. I plan for one problem. If my back gas fails I have a functional pony, if my pony fails I have functioning back gas. I don't plan for a back gas failure and a pony failure.

3. Because I sling and leave my pony turned on I can see if it leaks and have an accurate SPG reading readily available.

4. I don't plan on my pony gas to solve problems at depth. I conservatively calculate my back gas reserve for that. I know that some people push their back gas limits because they think they have a pony but I don't.

5. I measure the risk of somehow losing a signifigant amount of pony gas against an error turning the gas on when needed.
If I am solo I will only need my pony in the event of a catostrophic gas loss at depth (extruded o ring, burst disk, uncontrolled 1st or 2nd stage freeflow, severed LP hose). When that occurs there is going to be a lot of confusion and air flow around my head so I will probably have to deploy my pony reg blind. I prefer to eliminate the additional task of locating and turning my valve on as well.

I also anticipate someone else needing my pony as well. When I dive I tell my buddies they can take any reg they want if they need it. Although I like to think I could control an OOA calmly I worry about a paniced diver grabbing my pony reg, not getting gas and then wrestling with my hands as I try to turn the valve on (or them loosening the pony first stage yoke knob by mistake).

Anyways, everyone has to make their own decisions about how they dive their gear, hopefully based on a full understanding of the risks and the pros and cons of the percieved solution.