Pony/Redundant Bottle Size / Doing the Math / SAC Rate Assumptions question

I have been reading the board for Pony Bottle discussions lately. A lot of information has been shared - I am thinking this is really an assumption question on how to determine Pony Size.

I use an excel sheet to show my volume of gas needed to ascend from various depths with a safety stop (3 minutes at 20 feet) built in so I can estimate my needs in an OOA/OOG situation (Volume = SAC*Pressure*Time) - I have separate tab for planning my dives.

The question is this - I have been using my average SAC rate for complete dives over the course of a year to determine my SAC in regards to Pony size and volume needed. If I am really only ascending and doing a safety stop from depth in an OOA/OOG situation - isn't my average SAC Rate an already high estimate? I calculated my SAC as including descending, bottom time with various conditions over the year and ascending with a safety stop - or start to ending a dive. I barely exert myself while I ascend up (I don't inflate my BC to ascend) and certainly not while I am doing my safety - so I would expect in general my SAC for this portion of the dive would be much less than my average for a complete dive.

I currently use my avg SAC Rate over the year and double it (distress factor = 2 times my Avg SAC) to give me a feel in a panic situation what I might use. If I am ascending and doing a safety stop - do I need to double my average SAC or is this over inflating the need for additional volume? Or is an average SAC a reasonable factor in determining Pony volume and not to double it? I am not prone to panic in situations and I have been diving a long time on single tanks with and without buddies - now that my son is OW certified - I am rethinking the need.

Recreational diver - air only - no decompression dives - Northeast Ocean and lake diving.

as its not a good idea to plan with too little, I'd say to base it on a stressed rate until out of the water.... if you are OOG, there is going to be stress...

I loved sheet 2 on the spreadsheet by the way

What you're doing is not bad at all considering you double it. But you don't need to be that complex and do it for a whole year.

At the beginning of each dive year I take an extra tank and leave it on a platform at the training quarry, but you can do this anywhere. AFTER a normal dive, but before surfacing, go to the platform and calculate your SAC rate 3 ways. First, breathe from the tank while resting (deco rate) for 5 minutes and note start and end pressures. Then do the same for 5 minutes while swimming leisurely around the platform at constant depth (swimming rate). Then finally grab hold of the platform and kick like hell for one minute as hard and as fast as you can (emergency rate).

Calculate your 3 SAC rates and take your pick when deciding how much pony/bailout gas to carry. Doing it at the beginning of the season also adds a safety factor since your SAC rate sucks most then.


Please pardon any typos. Sent from my iPhone using Tapatalk

tomfcrist:

I loved sheet 2 on the spreadsheet by the way

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Yeah, not my project. It was there when I got it from here on SB back a few years ago. Never removed it, always forget its there.

I think you need to assume that a true out of air, bailout situation is going to be stressful. You can likely be WAY above your normal SAC initially if you just experienced an "Oh my God, I almost died" moment... Hopefully you calm yourself immediately and get down to something near your normal SAC, but you could be at 2-3x your average SAC for a several minutes without being aware of it, and you may even have an elevated respiration rate until you get safely back on shore or the boat!

Your average SAC is a good way to start thinking about pony bottle size, but I'd "pad" that number to account for stress and other unforeseen events.

Best wishes.

Basking Ridge Diver:

I currently use my avg SAC Rate over the year and double it (distress factor = 2 times my Avg SAC) to give me a feel in a panic situation what I might use. If I am ascending and doing a safety stop - do I need to double my average SAC or is this over inflating the need for additional volume? Or is an average SAC a reasonable factor in determining Pony volume and not to double it? I am not prone to panic in situations and I have been diving a long time on single tanks with and without buddies - now that my son is OW certified - I am rethinking the need.

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You can more accurately estimate your stressed SAC rate, by being stressed when you measure it.

Go to the pool, record your tank pressure, and swim as hard as you can against the wall for several minutes until you're really sucking pretty hard. Then measure your tank pressure again, check the elapsed time and do the math.

You might not be freaking out underwater because you ran out of air, it's entirely possible that you ran out of air because you're freaking out. Or swimming into the current.

2 CFM isn't out of the ordinary. It could even be higher depending on your personal physiology and what's going on.

Basking Ridge Diver:

The question is this - I have been using my average SAC rate for complete dives over the course of a year to determine my SAC in regards to Pony size and volume needed. If I am really onI currently use my avg SAC Rate over the year and double it (distress factor = 2 times my Avg SAC) to give me a feel in a panic situation what I might use. If I am ascending and doing a safety stop - do I need to double my average SAC or is this over inflating the need for additional volume? Or is an average SAC a reasonable factor in determining Pony volume and not to double it? I am not prone to panic in situations and I have been diving a long time on single tanks with and without buddies - now that my son is OW certified - I am rethinking the need.

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I use a reasonably conservative calculation. I include a minute on the bottom to "sort things out", the entire ascent at 30 ft/min, a 3 minute safety stop, all at twice my average SRMV (last 360 dives).

From 130 feet, this is just over 17 cu ft of gas for me. I dive with a 19 cu ft pony. Of course, on a no deco dive, I can speed up at least the early ascent and ascend directly to the surface without the safety stop if I need to. Even with the same time on the bottom, this reduces the gas requirement to a little over 10 cu ft.

---------- Post added March 21st, 2014 at 06:01 PM ----------

rhwestfall:

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Nice spreadsheet, consistent with my own calculations

rhwestfall:

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Now why would anyone hang around at depth for 5 minutes when you have a gas supply problem? I'm going up right away. Perhaps allow 30 seconds at depth but not more.

My general rule for solo diving is 13 cu ft pony for depth 60 ft or less and 19 cu ft for deeper dives within sport limits. Especially when shore diving I want to minimize the amount of weight I need to carry and still be safe.