What pony size?

[dumpsterDiver]

Say you have an "average" SAC of .7. Given a dive factor of 2 for stress that's an RMV of 1.4. Now factor in 2 minutes at depth (say 100 ft) to realize to situation, switch regs and catch your breath so to speak. Then about 1 minute to go to 40 feet and another 1 min or so to 15 feet. Now a 3 min safety stop plus a minute to surface. Thats...

1.4 (RMV) x 4 ata = 5.6 cf x 2 (mins) = 11.2 cf
1.4 x 2.8 ata = 3.9 cf x 1 (min) = 3.9 cf
1.4 x 1.75 ata = 2.45 cf x 1 (min)= 2.45 cf
1.4 x 1.5 ata = 2.1 cf x 3 (min) = 6.3 cf
1.4 x 1 ata = 1.4 cf x 1 (min) = 1.4 cf

Thats roughly 25 cf

Someone check my math...

I didn't check your math, I assume it is right, but lets look at it another way... You say you need 25 cu-ft to ascend from 100 ft. So we can assume your buddy needs the same. So.. if we follow your assumptions, then YOU must save a minimum of 50 cu-ft in your primary tank when diving in 100 feet assuming you are a good buddy diver. Right?

So you dive with an 80 cuft tank and have to save 50/80 of your air .. then you MUST ascend with 1875 psi in your tank. Do you REALLY start all ascents after using just 1100 psi? Does that sound reasonable (for when you are diving without a pony)? Or is it way, way to conservative?

I think it is way too conservative. The chance of an emergency is low, the chance that it comes at a time when you are entangled and can not ascend for 2 minutes is even less likely, the chance that all this will happen AND that you will also be bent if you skip a safety stop is getting ridiculously low.

These are, more or less, independent random events and you calculate the probability of all of them occurring simultaneously by multiplying their probabilities together. So is it 1/1000 chance a hose blows, a 1/100,000 chance you are entangled and a 1/10,000 chance that you will be bent on a recreational dive that skips a safety stop. The chance of that all that happening at the same time would be ... hell I don't know; add up all the zeros (they are made up anyway).

So you can make assumptions that are as conservative as you like, but for me.. I will take a small pony for no-deco diving. I will assume that I will not be entangled when the primary air source stops, I will assume that I can begin an ascent immediately (when the primary tank stops giving me air) and I will assume that in this very unlikely event that I will perform the initial portion of my ascent at probably 60 feet per minute and I will assume that the safety stop is optional and will be performed only as long as my limited air supply allows... I am comfortable with those assumptions... Drag a 30 cu-ft pony on every dive to 74 feet, but I won't..

 

[uncfnp]

I didn't check your math, I assume it is right, but lets look at it another way... You say you need 25 cu-ft to ascend from 100 ft. So we can assume your buddy needs the same. So.. if we follow your assumptions, then YOU must save a minimum of 50 cu-ft in your primary tank when diving in 100 feet assuming you are a good buddy diver. Right?

So you dive with an 80 cuft tank and have to save 50/80 of your air .. then you MUST ascend with 1875 psi in your tank. Do you REALLY start all ascents after using just 1100 psi? Does that sound reasonable (for when you are diving without a pony)? Or is it way, way to conservative?

I think it is way too conservative. The chance of an emergency is low, the chance that it comes at a time when you are entangled and can not ascend for 2 minutes is even less likely, the chance that all this will happen AND that you will also be bent if you skip a safety stop is getting ridiculously low.

These are, more or less, independent random events and you calculate the probability of all of them occurring simultaneously by multiplying their probabilities together. So is it 1/1000 chance a hose blows, a 1/100,000 chance you are entangled and a 1/10,000 chance that you will be bent on a recreational dive that skips a safety stop. The chance of that all that happening at the same time would be ... hell I don't know; add up all the zeros (they are made up anyway).

So you can make assumptions that are as conservative as you like, but for me.. I will take a small pony for no-deco diving. I will assume that I will not be entangled when the primary air source stops, I will assume that I can begin an ascent immediately (when the primary tank stops giving me air) and I will assume that in this very unlikely event that I will perform the initial portion of my ascent at probably 60 feet per minute and I will assume that the safety stop is optional and will be performed only as long as my limited air supply allows... I am comfortable with those assumptions... Drag a 30 cu-ft pony on every dive to 74 feet, but I won't..

DD. I didn't intent it as a suggestion for what the OP actual needs. Its just an example with a possible SAC/RMV and choice of ascents. The variations are almost endless. Its up to the OP to determine his needs and comfort level, plug his numbers in and decide if its right for him.

Its more the formula/calculation that I was demonstrating rather than a real life scenario...

I use a 13 cf.

 

[rhwestfall]

ah, the great pony debate. Haven't deen this one in what, a month?


.......and once they select a size, then one needs to:

1. decide how to carry it
2. decide if it should have EAN
3. decide if it should be carried valve open or closed

what else did I forget????

and still need to pick a reg....

feels like "Ground Hog Day".....

 

[chrpai]

ah, the great pony debate. Haven't deen this one in what, a month?


.......and once they select a size, then one needs to:

1. decide how to carry it
2. decide if it should have EAN
3. decide if it should be carried valve open or closed

what else did I forget????

and still need to pick a reg....

feels like "Ground Hog Day".....

For me... Al40, modified DIR rigging (cam strap) left side slung, air, valve closed. Reg is a Hog Bp2 with Duroc 2nd stage.

 

[GrumpyOldGuy]

For me... Al40, modified DIR rigging (cam strap) left side slung, air, valve closed. Reg is a Hog Bp2 with Duroc 2nd stage.

I don't think he was really asking, just pointing out in jest the gory details that tend to follow. He forgot some spare air bashing, maybe I missed it.

I don't think any of them have much bearing other than do they function for you. My stuff is made from recycled gear, it works for me.

 

[Max Speed]

If you carry four spare airs for a deep recreational dive you can switch between optimized nitrox mixes as you ascend...

Sent from my GT-P3113 using Tapatalk 2

 

[grey2112]

I just started using a 13cf Nitrox pony - my first dive with it was my first dive in my Deep Water cert - went to 94 feet at Blue Grotto. My instructor asked after the dive how it felt to have it on me - honestly, I loved it - it felt . . . right. The extra weight allowed me to trim my BC weights, and it was positioned on my main 80AL in such a way that it balanced out nicely and kept my buoyant ass down properly. To me, the 13 is perfect for my type of diving - mostly in 75 feet of water or less (90% of the time it will be in 40 feet or less), spearfishing in wrecks. It is an emergency supply for accidental OOA or to help another diver. I was honestly surprised by how much time I was able to get out of my pony when I tested it on my deep dive.

 

[rhwestfall]

I was honestly surprised by how much time I was able to get out of my pony when I tested it on my deep dive.

Yes, but cut that available time in half or more in a stressful situation.....

 

[flots am]

Your values are very conservative. Probably valid if you don't know what your SAC is and your a new diver, prone to stress.

Here is a similar set of numbers I would use for myself. My SAC is .4 in cold water, I still figure a stress rate but only to .7 cf/min

1.4 isn't all that conservative. If your "out of air" problem happens when you've been swimming into the current for a few minutes and are breathing pretty hard right before the added "excitement" of discovering you have no air, even 2.0 is not excessive.

Also, there are a number of possible first stage failures that cause an immediate and unexpected loss of all gas without warning, including a clogged dip tube, a clogged first stage inlet filter and on rare occasions, an exploding first stage (SCUBAPro Mk 10, I believe).

flots.

 

[Max Speed]

Just remember the major points:

1) a pony is only useful for strokes who are unable to plan their dives and stick with the plan.
2) good divers don't make mistakes.
3) your buddy will save you
4) it takes 2 minutes to figure out bad stuff and by then the pony is empty anyway
5) well maintained stuff never breaks

All of the above is true on the internet. Then you get on a real dive boat with experienced real divers and....surprise... everyone not on doubles has a....pony!

Pssst...the guys with really big pony bottles have figured out a way to keep internet divers off their case....they don't call it a pony, they call it SIDEMOUNT.

BTW- if you have survived driving a car it is because the human brain can diagnose and react to a problem in a couple of seconds...or less.

Sent from my GT-P3113 using Tapatalk 2