Recreation diver:pony bottle?

[jkaterenchuk]

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?

Yes I now understand clearly. What size tanks do you and your buddy carry on your dives so that you always have this 70 cu ft available for an emergency?

 

[rjack321]

Got it, 1.8cf/min SAC is the discrepancy.

Here's my Rock bottom:
100 ft = 4 ATA
my SAC = ~0.5 to 0.6 normally, 1.0 stressed. (I know this is about max since I was huffing and puffing on our 150ft shrimp dives in current back in May and that's what I used.)

100ft = 1min
ascend to 50ft = 2mins
50 = 1
40 = 1
30 = 1
20 = 1
10 = 1

average depth, 45ft ~ 2.5ATAs

2.5 x 2 cf/min x 8min ascent = 40cf

I don't do 100 ft dives with people who's SAC is way off mine, obviously this is only conservative if their stressed SAC is 1. Now that I know your normal SAC is 1 cf/min I think I'll be bumping these numbers up a tad on Sunday, although at Edmonds you're never far from the surface.

 

[dsteding]

Yes I now understand clearly. What size tanks do you and your buddy carry on your dives so that you always have this 70 cu ft available for an emergency?

Depends on the depth of the dive, and one of the things about this particular dive profile (a shore dive where we swam out, descended to 60 feet, took about 8 minutes to get down to ~100 feet, and then spent ~8 minutes at depth, followed by a slow ascent upslope, total dive time was 40 minutes or so) is that your rock bottom (RB) decreases as you get shallower. So, we left the 100 foot mark at about 2200 psi and stayed above our rock bottom for the rest of the dive. I think at 60 feet the same calculation yielded about 1500 psi, and we could essentially extend our dive time by shallowing and staying "ahead" of our RB for the depth we are at.

But, to answer your question, that particular dive was on a single HP 130, my buddy was carrying a single HP 119. The instructor and observer were on single 95s, but also had RMVs that were half of what ours were at the time. I'd do it now in doubles . . . for the extra air and the redundancy.

One of the best rules of thumb I've heard (for newer divers at least, and I fall into that category) is to not dive deeper than the cubic feet of your tank. So, don't take an 80 past 80 feet, and dive 119s or 130s if you are going deeper than 100 feet . . . it works fairly nice especially for these kinds of profiles where you are deep first, and then shallow up-slope for the rest of the dive.

 

[dsteding]

Got it, 1.8cf/min SAC is the discrepancy . . . I don't do 100 ft dives with people who's SAC is way off mine, obviously this is only conservative if their stressed SAC is 1. Now that I know your normal SAC is 1 cf/min I think I'll be bumping these numbers up a tad on Sunday, although at Edmonds you're never far from the surface.

Yeah, that calculation is from back in August, on dive 20 or so for me. Then, my resting SAC was 0.9 (resting after chasing Bob around Cove 3 measuring my working SAC). My SAC has come down since then (0.7 or so in singles, about 0.7-0.9 in doubles, still getting used to those), so it could be time to revise my thinking on my RB calculations a bit . . . I'd still use a SAC of about 1.5 for myself as a stressed SAC.

 

[darkpup]

Richard,

You may also want to check his actual SAC rate on Sunday. I bet it's lower than he expects it is.

I about fell out of my chair when I first read the 70cf for a 100' dive. The Emergency SAC of 1.8 cf per minute helped to understand the numbers though.

~ Jason

 

[darkpup]

Yeah, that calculation is from back in August, on dive 20 or so for me. Then, my resting SAC was 0.9 (resting after chasing Bob around Cove 3 measuring my working SAC). My SAC has come down since then (0.7 or so in singles, about 0.7-0.9 in doubles, still getting used to those), so it could be time to revise my thinking on my RB calculations a bit . . . I'd still use a SAC of about 1.5 for myself as a stressed SAC.

For what it's worth, 70cft is a bit more gas than Richard and I plan for RB on a 20 minute dive to 150'. Obviously the al40s filled with 50% help our calculations a bit, but 70cft is a lot of gas.

We usually keep our wreck penetrations limited to 80cft of gas divided into 3rds on top of RB. That gives you about 26cf of gas for the in bound leg of a penetration. Then again, chasing Bob around would definitely increase my SAC rate. I'll have to remember to bring a scooter to Washington in January.

~ Jason

 

[lamont]

I think the lesson there about the 1.8 SAC is that if you're breathing that hard you need to consider pull and glide, drifting deco, bringing a scooter or not chasing after bob -- and practice efficient frog kicking...

 

[rjack321]

Darkpup you're back! BTW, I got cylinders and a wing for you in Jan, just bring your BP, suit and small stuff. I assume you'll have time for at least a couple dives...

I was also a bit surprised at needing a stage for RB, that's why I asked for some numbers to see what was going on.

Doug, if you want we can do some SAC calculations during lunch with Jim on Sunday. No Excel spreadsheets required

 

[dsteding]

Richard,

You may also want to check is actual SAC rate on Sunday. I bet it's lower than he expects it is.

I about fell out of my chair when I first read the 70cf for a 100' dive. The Emergency SAC of 1.8 cf per minute helped to understand the numbers though.

~ Jason

I wouldn't be surprised if it was. I spent last weekend with Steve White in a pre-fundies seminar, we worked on trim, buoyancy, and kicks almost exclusively (with some s-drills thrown at us for good measure). My guess is that it has gone down a bit more as my efficiency has greatly improved.

Sorry for making you almost fall out of your chair. The point I was trying to make (and hopefully it has been clarified) is that newer divers are the ones that really need to understand gas management. They are the ones that are more likely to suck a tank dry in an emergency. I almost fell out of my chair the first time I made that calculation--especially because I regularly hear about newer divers going deep with 80s--but that is what it would take based on the measured emergency RMVs of me and my buddy at the time.

While the standard RB calculation using a emergency RMV of 1.0 works for experienced divers, actually measuring it, which we did on one dive in Bob's AOW class before planning the deep dive, will give a newer diver a much better picture. I try to keep a running track of my RMV for the dives I do, it is going down (until I got into doubles, then it went back up).

A lot of times it seems that the people asking the "what size pony" questions are those that are newer divers. Good for them to consider what size they need . . . but they may also fall into the category of someone with an emergency RMV greater than 1.0, especially if they are diving cold water with heavy exposure protection. Just something to consider in figuring out appropriate pony size. A 19 may work better for the calm, collected, experienced diver to bail themselves out from 100 feet (half of Richard's RB calculation is 20, so it arguably cuts things too short), but a newer diver maybe should carry a bigger pony for the same purpose.

 

[dsteding]

I think the lesson there about the 1.8 SAC is that if you're breathing that hard you need to consider pull and glide, drifting deco, bringing a scooter or not chasing after bob -- and practice efficient frog kicking...

All very true Lamont. Bob is wicked efficient underwater, and at the time (in early August), I was far from it. The dive I am referring to is the 3rd of his AOW class, we spent it in Cove 3 at 33 feet, first was five minutes in one direction chasing Bob (who usually moves slow, but can really motor when he wants) and then coming back "resting" which was kinda-sorta resting given the workout of the first five minutes.

I only realized how far I was from being efficient after spending this weekend in doubles working with Steve.

It would be an interesting exercise to do the same thing now, I know it has come down a fair amount.