Recreation diver:pony bottle?

[dsteding]

Check how many psi you go through on an ascent from max depth, under a non-stress condition

Shouldn't you check how much psi you go through when stressed? I think that would be a better baseline for a minimum size, even with lots of practice with it, I bet any diver would see their RMV/SAC increase a fair amount in such situations.

Another option is to figure out your working RMV by measuring gas consumption when swimming hard, and using that as a proxy for being stressed.

The link Lamont posted likely has the math on this, he's posted good treatises on this subject in the past. I do know that my rock bottom for me and my buddy to get to the surface from 100 feet, with stops, is about 70 cubic feet (assuming a stressed RMV of 1 for each of us), so if I were to sling a bottle as a backup, it would be a 40 cubic foot bottle for just myself. Just think that 1 minute of stressed breathing at 100 feet will consume 4 cubic feet of gas . . .

 

[EvaL]

Hi Simon,
After nearly 170 dives, I've decided it's time to get a pony (just because my equipment hasn't gone wrong doesn't mean it won't do so forever). I'd say the size depends on how deep most of your dives are and your breathing rate (and how much your breathing increases under this condition, which can be stressful). My recommendation for a diver breathing normally in this situation is:
If diving 20m (60ft) or less, 6 cu. ft (no safety stop);
30m (90ft) or less, 13 cu. ft;
40m (120ft) or less, 19 cu. ft.
My diveshop won't carry pony bottles less than 19 cu ft. because the instructors believe that anything less won't get divers safely to the surface. I recently purchased a 19 cu ft. pony (Luxfer Al) even though I usually don't dive beyond 30m (90ft) because I need to factor in the extra air I need for the safety stop for deeper dives. It's a little bulkier than I would have liked (13 cu ft is ideal travel size), but it offers more air and safety, and I can live with that.
Dive safely.

 

[markfm]

Dsteding --
Adding margin is certainly a good thing, whether using some of Lamont's (and others') excellent writeups as a starting point or by other means. I just personally prefer to work based on my own consumption rather than an approximation -- my actual burn rate, with pad, gives me what I actually need to bail.

Practice, as with most things, is key.
I try to practice things on a regular basis. Practice, building things to the point where reactions become automatic, drilled in, seems the best way to keep the stress level down.

I like to think of the classic, mask remove-replace. For me, and many I've seen, it's a stressful thing at first. Keep practicing and it becomes a non-stress item. Eventually you get to the point where you're doing R&Rs unconsciously, to clear fogging or blow your schnozz underwater

My pony is a standard item, for me. Grab my primary reg or kick it out of my mouth, and I switch to my secondary. No air in my reg, I reach for the pony.
Should I always be in arm's reach of my buddy? Yup. Does it always happen? Nope.

I don't disagree that more gas is always good to have, just that as in most things it's a risk-benefit trade. My 19 is small enough, for me, to actually travel with, and anything larger would frankly be a hassle. My Instructor has a 30 and rarely takes it anywhere -- just a bit too cumbersome.

Likewise, I surely don't disagree with people who indicate they always carry a pony below a certain depth, and won't solo beyond another given depth -- they've determined their own, personal, comfort zone, to be respected.

Lamont definitely has a good write-up, and someone else did a long one a while back, also good, reaches roughly the same conclusions relative to gas planning (minor variances are understandable).

 

[rjack321]

I do know that my rock bottom for me and my buddy to get to the surface from 100 feet, with stops, is about 70 cubic feet (assuming a stressed RMV of 1 for each of us), so if I were to sling a bottle as a backup, it would be a 40 cubic foot bottle for just myself. Just think that 1 minute of stressed breathing at 100 feet will consume 4 cubic feet of gas . . .

70cf? Show me the math buddy.

I agree that's its much more than 19cf in most instances. But its not 70cf unless you have a serious problem leaving the bottom in a timely manner (90 seconds or less).

 

[fire_diver]

A pony bottle is one of those things you never need, unless you need it bad. How big a bottle you need depends on a lot of factors. For an ascent from 130 ft with a safety stop, I carry a 6 cu ft. Many people chose a larger bottle. YMMV

Just curios if you have done the calculations to determine if that 6cf is really big enough?

FD

 

[jpcpat]

Just curios if you have done the calculations to determine if that 6cf is really big enough?

FD

I've done practice ascents using this setup, and it worked, just barely. As I read this thread, it occurs to me that In an OOA emergency, I'm liable to be using a bit more air, so more capacity from that depth is in order. Anyone have a 13 for sale

 

[jkaterenchuk]

At least one training agency (ANDI) recommends use of a RBS(redundant breathing system) from the start of training. They recommend the following RBS air volume -

Max Depth Reqd Volume
66 fsw 13 cu ft
100 fsw 20 cu ft
130 fsw 30 cu ft

You can calculate these numbers for yourself by determining what volume of gas you would breath per minute in a stressed condition (they use 25 liters per minute) and then determining what this volume would be per minute at the maximum depth. Calculate the time it would take you to ascend from the deepest point in a dive to the surface without exceeding the maximum ascent rate for your agency and not omitting a safety stop. Multiply these two number togeather to arrive at the volume of gas required in the RBS tank. For additional contingency it is assumed that the volume per minute at maximum depth does not change.

Having had a OOS at 110 fsw and having to breath off a malfuntioning buddies Octo while doing an ascent has changed my attitude. I carry a 30 cu ft pony on every deep dive. I personnally sling the bottle on my left side and instead of mounting it to the primary tank. Over time I have gotten use to it being there and do not notice it any longer.

 

[dsteding]

70cf? Show me the math buddy.

I agree that's its much more than 19cf in most instances. But its not 70cf unless you have a serious problem leaving the bottom in a timely manner (90 seconds or less).

Rjack-my 70 cubic feet was to get both my buddy and myself back to the surface (rock bottom). My point is that I'd chop that in half if I was using a pony for myself (and carry a 40 as a result).

Okay, here is the math. This is off the top of my head, I actually put together an excel spreadsheet to do these calculations, but it is on my computer at home.

Working RMVs were measured for both myself and my dive buddy the evening before, they were admittedly high, and higher than the 1.0 regularly used (I think around 1.8, Bob has this amazing ability to move really fast underwater, and we were following him like tired puppy dogs).

So, plug that into the following scenario:

100 feet, problem happens, one minute to sort it out. 4 ATAs*1 min*Combined RMV of 3.8=15 cubic feet of gas.

Ascend to 70 feet, taking one minute, average depth is 85 feet=13 cubic feet of gas.

Ascend to 40 feet, taking one minute, average depth is 55 feet=10 cubic feet

Ascend to 15 feet, taking ~half a minute, average depth is 27.5 feet=~4 cubic feet.

Three minutes at 15 feet is 16 cubic feet.

Ascend to surface is ~3 cubic feet.

This is a standard NAUI 30 fpm ascent with a stop at 15, I bet a GUE profile of stops every ten from 50-0 would keep you deeper longer and use a bit more gas.

Regardless, I'm up to 61 cubic feet with no reserve (so a bit less than the 70 I mentioned off the cuff earlier). IIRC we had a rock bottom at 2000 psi for a 130 for this particular dive, which is about 74 cubic feet. Granted, this is an EXTREMELY conservative calculation of RB, but given that we were (1) doing our first deep dive together and (2) had high working RMVs and (3) like to build some conservatism into our gas planning, I think it was perfectly appropriate.

Would it be appropriate now to lower the combined RMV? Maybe. It depends on who I am diving with and how comfortable I am in handling emergencies with them.

 

[jkaterenchuk]

Rjack-my 70 cubic feet was to get both my buddy and myself back to the surface (rock bottom). My point is that I'd chop that in half if I was using a pony for myself (and carry a 40 as a result).

Can you explain the reason for carrying a tank for both you and your buddy? Typically, each diver would carry an RBS for themselves or they could hand it off to their buddy if needed. Also, most ponys are setup with only a 1st stage, 2nd stage and SPG not a octo. I am not saying what your doing is wrong but rather just interested in your reasoning.

 

[dsteding]

Can you explain the reason for carrying a tank for both you and your buddy? Typically, each diver would carry an RBS for themselves or they could hand it off to their buddy if needed. Also, most ponys are setup with only a 1st stage, 2nd stage and SPG not a octo. I am not saying what your doing is wrong but rather just interested in your reasoning.

Sorry-not being clear. My RB calculation above was to clarify my 70 cubic foot RB calculation, which would be the amount of back gas (in my main tank) that I would reserve to get me and my buddy back to the surface. That scenario assumes my buddy has a catastrophic loss of air at that depth. I'll then have enough air to air-share and get us both to the surface in a slow, controlled manner.

You are right, pertinent to this conversation is the amount of air an individual would need to get themselves to the surface . . . splitting that 70 cubic feet for that particular dive profile would give me what I would carry in terms of a pony size-I'm splitting it precisely because I am assuming that would be just my pony to get me to the surface (although the way I dive, I am my buddy's "pony" if you will, and he/she is mine).

Does that make sense?