Pony/Redundant Bottle Size / Doing the Math / SAC Rate Assumptions question
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I am a little unsure what you mean by your average SAC rate? You could be using a computer program that calculates a SAC for each dive based on profile and total air used, and then averaging that number over all dives.
Or, you could compute a SAC by taking amount of air used divided by length of dive and getting a number which you then average over all dives.
Or maybe something else. If you are doing the second, then that is not so good if you have some shallow dives and some deep dives since you will underestimate what you need for deep dives.
I am not so excited by the first one either. If you are interested in safety I would think you would want to base your emergency supply on you highest SAC rates.
Or you could just do what I did. I know a quarter of an AL80 would get me up from where I dive and toss in a safety stop so I got an AL19.
Or, you could compute a SAC by taking amount of air used divided by length of dive and getting a number which you then average over all dives.
Or maybe something else. If you are doing the second, then that is not so good if you have some shallow dives and some deep dives since you will underestimate what you need for deep dives.
I am not so excited by the first one either. If you are interested in safety I would think you would want to base your emergency supply on you highest SAC rates.
Or you could just do what I did. I know a quarter of an AL80 would get me up from where I dive and toss in a safety stop so I got an AL19.
flots am
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Your average SAC rate is irrelevant unless you have an "average" emergency.
My average SAC rate runs between .4 and .7 depending on equipment and conditions.
My stressed "swimming into the current for 5 minutes then out of air" SAC rate is closer to three.
Your mileage and what you plan on being able to handle, may vary.
OP
Basking Ridge Diver
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Hatul - I agree for me personally - if I have an OOA / OOG issue I am not hanging around to sort things out - I am heading for the surface with a buddy or my redundant air source. At the surface - on the boat or shore I will fix the issue or call the day.
Steve_C - I was using my dive computer to give me my SAC rate for each dive last year and I average those dives together to give me my "average SAC rate". My thought process was that an OOA/OOG situation is not a catastrophe unless I don't have a buddy or redundant air source to lean on. So I am wondering how high a SAC rate needs to be if this is not a life or death scenario. I am assuming I am switching regs which is not a huge deal but certainly can be stressful if a stage is free flowing.
flots am - in your example of swimming into the current are you looking at a worst case scenario? Being down current and swimming into a strong current before you follow a line up to get back to your entry point? I was thinking regardless of where I am - if I am OOA / OOG I am heading to the surface with or without an anchor line and will deal with the outcome on the surface. Otherwise I could see needing to sling a much larger air source to include any worst case scenario. I guess that is where my head is at - what level of stress and what scenario is appropriate without being over the top. I don't plan on getting completely entangled in lines or being in a huge ripping current - I would call the dive on those conditions.
Thanks.
Steve_C - I was using my dive computer to give me my SAC rate for each dive last year and I average those dives together to give me my "average SAC rate". My thought process was that an OOA/OOG situation is not a catastrophe unless I don't have a buddy or redundant air source to lean on. So I am wondering how high a SAC rate needs to be if this is not a life or death scenario. I am assuming I am switching regs which is not a huge deal but certainly can be stressful if a stage is free flowing.
flots am - in your example of swimming into the current are you looking at a worst case scenario? Being down current and swimming into a strong current before you follow a line up to get back to your entry point? I was thinking regardless of where I am - if I am OOA / OOG I am heading to the surface with or without an anchor line and will deal with the outcome on the surface. Otherwise I could see needing to sling a much larger air source to include any worst case scenario. I guess that is where my head is at - what level of stress and what scenario is appropriate without being over the top. I don't plan on getting completely entangled in lines or being in a huge ripping current - I would call the dive on those conditions.
Thanks.
flots am
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Only you can guess at how hard you'll be breathing when you run out of air.
My example was actually from a deco class, but could easily apply to a recreational dive. Say you're @ 120' and have been swimming into the current for some reason (maybe just to get to the other end of the wreck) and . . .
- your buddy is maybe 20' ahead of you, up-stream, and you're are sucking pretty hard, can't quite catch up, and then discover you're out of air.
- your buddy is a moron and is almost out of air and you then discover you're out of air.
Do you have enough in your pony bottle for a safe ascent?
That's the real question.
This is also a case of training and practice and situational awareness. Having been in this position (during training).
flots.
-hh
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* Can't leave immediately due to Entanglement or Overhead
* Don't want to leave immediately - Find Buddy (get his attention - prevent lost buddy search)
* Preferred Exit requires traverse at depth to find Anchor Line for Ascent (especially in currents)
* Easy way to add conservatism to the minimum requirement calculation.
* Easy way to model how reality is more complex: EG, an extremely high SAC for those 30 seconds = normal SAC for 5 minutes.
And so on.
-hh
AfterDark
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I don't get this stress factor. Why would stress be a expected when carrying a redundant gas supply? I've had a couple of OOG incidents and I can tell you that not having a redundant system was very stressful, being at 70FSW and same ocean buddied during one and solo and entangled during the second I can relate to stress during OOG situations.
If I'm diving my ID's and have a reg malfunction on one the change over to the other and the ascent would be stress free. I'd even take the time to shoot my SMBw/dive flag and make a SS because I would have more than enough gas to do it, stress free.
If you dive ID's you don't need to ponder what size pony you need, you don't need one. Unless like me you carry one for your buddies OOG situation, in which case 19cuft better be enough.
If I'm diving my ID's and have a reg malfunction on one the change over to the other and the ascent would be stress free. I'd even take the time to shoot my SMBw/dive flag and make a SS because I would have more than enough gas to do it, stress free.
If you dive ID's you don't need to ponder what size pony you need, you don't need one. Unless like me you carry one for your buddies OOG situation, in which case 19cuft better be enough.
OP
Basking Ridge Diver
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I ended up with this -
I wanted a RAS to be small enough to take on every dive but big enough to handle my acceptance of some risk but not every emergency that could cause me harm.
I took into account that I am a recreational diver and have not gone below 120 feet and don't find a reason for me to go below that depth, I have a reasonable SAC rate and built in a safety stop at 20 feet for 3 mins for my contingency.
After playing with volume formulas and various depths for me a 19 cu pony seems to work for my 95% of diving situations. Plan the dive and dive the plan while using my head to be safe.
I wanted a RAS to be small enough to take on every dive but big enough to handle my acceptance of some risk but not every emergency that could cause me harm.
I took into account that I am a recreational diver and have not gone below 120 feet and don't find a reason for me to go below that depth, I have a reasonable SAC rate and built in a safety stop at 20 feet for 3 mins for my contingency.
After playing with volume formulas and various depths for me a 19 cu pony seems to work for my 95% of diving situations. Plan the dive and dive the plan while using my head to be safe.
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