Optimal Pony Bottle Size for Failure at 100ft?

[chrisjur]

I have been utilizing a 30cu ft. pony bottle that I "stole" from a friend a while back, but am evaluating the purchase of a new pony.

A bit of background: I frequently dive deep-ish wrecks where most of our time is spent between 75 and 100 feet. While I do not venture out on my own, I do often dive in teams or am separated from a buddy at a distance that would be too far for him to acknowledge my situation or provide me with emergency air. I do not perform wreck penetration alone, under any circumstances.

So, the question that I'm asking is what size pony would be optimal to address a failure that occurs at 100ft.? - which I think is a reasonable depth to work with for this exercise given my type of diving.

I've worked out the math below, but I would greatly appreciate any feedback regarding anything that I got wrong, or any poor assumptions that I am making.

More assumptions and background info:

1. This will be used as a true pony bottle by definition and standard practice - it will only be used in emergencies and never to otherwise extend bottom time
2. I am calculating this based on a hypothetical catastrophic failure at 100ft depth in a non-overhead environment
3. Used for recreational diving only (e.g. no deco obligation)
4. Assuming 20% reserve is required for the pony bottle so that I'm not breathing it dry
4. My average SAC is 0.6 cu ft./min over the past 77 dives according to my air-integrated Garmin MK2i. However, for these calculations, I will use 1 cu ft./min since I will probably be huffing and puffing my ass off after the sh*t hits the fan at 100 ft.

Calculations:

1. Failure occurs at 100ft
Depth: 100ft
Duration: 3 mins
Assumption: After switching to pony bottle, I will probably need 2-3 mins to compose myself and think about my situation
Gas consumption: 12 cu ft.
Formula: 4ATA x 3mins x 1.0cu ft/min SAC = 12 cu ft. gas consumed

2. Ascend to 15ft for a safety stop
Duration: ~3 mins
Assumption: 85ft of ascent at a conservative 2 seconds/ft. = 170 seconds
Average Depth: Let's say 50 ft.
Gas Consumption: 7.5 cu ft.
Formula: 2.5ATA x 3 mins x 1.0cuft/min SAC = 7.5 cu ft. gas consumed

3. Safety stop
Depth: 15ft
Duration: 3 mins
Gas consumption: 4.5 cu ft.
Formula: 1.5ATA x 3 mins x 1cu ft./min SAC = 4.5 cu ft. gas consumed

4. Final exit and swim to the boat
Depth: Surface
Duration: 5 mins
Gas consumption: 5 cu ft.
Formula: 1ATA x 5 mins x 1 cu ft./min SAC = 5 cu ft. gas consumed

Total Gas consumed from all stages above: 29 cu ft. gas consumed

Therefore, the recommended pony tank size is 40 cu ft.
(With a 40 cu ft. tank, keeping 20% reserve would equate to 32cu ft. of usable gas, which would be suitable for the above scenario, with some headroom.)

Is my logic correct here? Any other insight would be greatly appreciated.

Thanks so much in advance.

 

[Lorenzoid]

Well thought out. Kudos. I didn't look at your numbers, which I assume is what you wanted (sorry), but I can see your thought process is on track.

 

[VikingDives]

I have been utilizing a 30cu ft. pony bottle that I "stole" from a friend a while back, but am evaluating the purchase of a new pony.

A bit of background: I frequently dive deep-ish wrecks where most of our time is spent between 75 and 100 feet. While I do not venture out on my own, I do often dive in teams or am separated from a buddy at a distance that would be too far for him to acknowledge my situation or provide me with emergency air. I do not perform wreck penetration alone, under any circumstances.

So, the question that I'm asking is what size pony would be optimal to address a failure that occurs at 100ft.? - which I think is a reasonable depth to work with for this exercise given my type of diving.

I've worked out the math below, but I would greatly appreciate any feedback regarding anything that I got wrong, or any poor assumptions that I am making.

More assumptions and background info:

1. This will be used as a true pony bottle by definition and standard practice - it will only be used in emergencies and never to otherwise extend bottom time
2. I am calculating this based on a hypothetical catastrophic failure at 100ft depth in a non-overhead environment
3. Used for recreational diving only (e.g. no deco obligation)
4. Assuming 20% reserve is required for the pony bottle so that I'm not breathing it dry
4. My average SAC is 0.6 cu ft./min over the past 77 dives according to my air-integrated Garmin MK2i. However, for these calculations, I will use 1 cu ft./min since I will probably be huffing and puffing my ass off after the sh*t hits the fan at 100 ft.

Calculations:

1. Failure occurs at 100ft
Depth: 100ft
Duration: 3 mins
Assumption: After switching to pony bottle, I will probably need 2-3 mins to compose myself and think about my situation
Gas consumption: 12 cu ft.
Formula: 4ATA x 3mins x 1.0cu ft/min SAC = 12 cu ft. gas consumed

2. Ascend to 15ft for a safety stop
Duration: ~3 mins
Assumption: 85ft of ascent at a conservative 2 seconds/ft. = 170 seconds
Average Depth: Let's say 50 ft.
Gas Consumption: 7.5 cu ft.
Formula: 2.5ATA x 3 mins x 1.0cuft/min SAC = 7.5 cu ft. gas consumed

3. Safety stop
Depth: 15ft
Duration: 3 mins
Gas consumption: 4.5 cu ft.
Formula: 1.5ATA x 3 mins x 1cu ft./min SAC = 4.5 cu ft. gas consumed

4. Final exit and swim to the boat
Depth: Surface
Duration: 5 mins
Gas consumption: 5 cu ft.
Formula: 1ATA x 5 mins x 1 cu ft./min SAC = 5 cu ft. gas consumed

Total Gas consumed from all stages above: 29 cu ft. gas consumed

Therefore, the recommended pony tank size is 40 cu ft.
(With a 40 cu ft. tank, keeping 20% reserve would equate to 32cu ft. of usable gas, which would be suitable for the above scenario, with some headroom.)

Is my logic correct here? Any other insight would be greatly appreciated.

Thanks so much in advance.

I too agree this is well thought out. If you haven't looked at rock bottom gas planning, that might be worth a peak. I can't tell from your post if you're assuming that you'll used more air under stress than you normally would, but that's a safe assumption if you haven't factored that into your planning.

I missed the 0.6 to 1.0 on the first pass. I'd suggest factoring in higher consumption at the beginning and more normal consumption by the time you hit the safety stop.

 

[rhwestfall]

0.6 normal to 1.0 excited isn't a big multiplier, but the round up to a 40 gives you lots of cushion...

 

[TerryC]

I use the same calculations when teaching Solo and Deep courses.
Your thinking is spot on.
40 cu ft i think is an ideal pony size.

 

[lermontov]

if you doing a wreck dives and want to spend more time looking around then why not just take an extra tank and do a sidemount set up - you could even use 2x7l which will give you more gas than 11 and provide redundancy- if you considering going down the tech route than buy tanks/gear with that in mind

IF not the 6l pony is perfect

 

[johndiver999]

I'm not sure anyone can criticize you for your decision or your calculations, but I will try.. LOL.

Overall your assumptions are very, very conservative (vvc). For example, assuming you cannot slow your breathing down while you ascend and do a safety stop is, maybe not giving yourself enough "credit", but what do I know.

The assumption that after you have a catastrophic failure, that it will take 2-3 minutes to just start your ascent, is vvc, IF you can ascend directly. If you would typically have to swim laterally to an anchor line etc., then that assumption is perfectly reasonable.

The assumption that you will need the pony bottle to swim at the surface for a long time is also vvc.

Adding another 20% for additional contingencies is also vvc, IF you have a means to ensure the bottle is completely full before the dive (i.e., a whip for example).

Overall the cumulative effect of layers of vvc assumptions gives you a result that is pretty extreme. If you WANT to carry a 40 cuft and you want to be super conservative, then the calculations and assumptions look fine to me. Nothing wrong with trying to be safe!

I personally, feel very comfortable with a 13 at that depth, but I have direct access to the surface and will assume that I begin the ascent immediately. I also, kinda assume that the first part of the ascent might be a tad fast and/or I might skip breathe a little.

I've personally used a 6 cu-ft pony bottle deeper than 100 feet, but if the bottle is a little low and you have any complications, that is cutting things very close. I used one (for real after foolishly sucking the main tank down to close to zero) in 130 and switched over, started my ascent, then dropped something and watched it fall away to the bottom and I knew there was no way I could chase it with the 6 cuft tank and it was lost forever. I sure wished I was smarter (and had a larger pony bottle) that day.

People always ask what size, but very few are willing to make the calculations, so the discussions always seem to be less than objective and go in circles forever. It is great to see someone state all their assumptions.

 

[scubadada]

Hi @chrisjur

I'm glad you do any calculations, most don't or can't even do that.

I use a minute at depth, a normal ascent, a 3 min SS, all at twice my average RMV. For a no stop dive you could easily skip the SS, perhaps even increase the ascent rate.

 

[chrisjur]

if you doing a wreck dives and want to spend more time looking around then why not just take an extra tank and do a sidemount set up - you could even use 2x7l which will give you more gas than 11 and provide redundancy- if you considering going down the tech route than buy tanks/gear with that in mind

IF not the 6l pony is perfect

Yes, exactly what I'm thinking...just not there yet. But thanks for validating some of the other thoughts lodged in the back of my brain...

 

[chrisjur]

I'm not sure anyone can criticize you for your decision or your calculations, but I will try.. LOL.

Overall your assumptions are very, very conservative (vvc). For example, assuming you cannot slow your breathing down while you ascend and do a safety stop is, maybe not giving yourself enough "credit", but what do I know.

The assumption that after you have a catastrophic failure, that it will take 2-3 minutes to just start your ascent, is vvc, IF you can ascend directly. If you would typically have to swim laterally to an anchor line etc., then that assumption is perfectly reasonable.

The assumption that you will need the pony bottle to swim at the surface for a long time is also vvc.

Adding another 20% for additional contingencies is also vvc, IF you have a means to ensure the bottle is completely full before the dive (i.e., a whip for example).

Overall the cumulative effect of layers of vvc assumptions gives you a result that is pretty extreme. If you WANT to carry a 40 cuft and you want to be super conservative, then the calculations and assumptions look fine to me. Nothing wrong with trying to be safe!

I personally, feel very comfortable with a 13 at that depth, but I have direct access to the surface and will assume that I begin the ascent immediately. I also, kinda assume that the first part of the ascent might be a tad fast and/or I might skip breathe a little.

I've personally used a 6 cu-ft pony bottle deeper than 100 feet, but if the bottle is a little low and you have any complications, that is cutting things very close. I used one (for real after foolishly sucking the main tank down to close to zero) in 130 and switched over, started my ascent, then dropped something and watched it fall away to the bottom and I knew there was no way I could chase it with the 6 cuft tank and it was lost forever. I sure wished I was smarter (and had a larger pony bottle) that day.

People always ask what size, but very few are willing to make the calculations, so the discussions always seem to be less than objective and go in circles forever. It is great to see someone state all their assumptions.

Thanks so much. In my head, I knew that I was very conservative, but you have convinced me that I'm probably "vvc" as I think about it more. Appreciate the insight.

If I cut that bottom time down from 3 to 2 mins and increase the ascent a bit, that could easily give me about 7cu ft. back, which brings me into a range comfortable for a 30 cu ft. tank. 40 cu ft. still might be the best for me, but certainly gives me something to think about.