Gear config, Long primary

[GreenDiverDown]

... This is also why you won't see a DIR diver doing a 100fsw dive on a single Al80...there just ain't enough gas to do the dive safely.

This seems really odd to me. Does DIR really teach this? How deep would I expect to see a DIR diver an a single 80?

 

[FreeFloat]

This seems really odd to me. Does DIR really teach this? How deep would I expect to see a DIR diver an a single 80?

This is also why you won't see a DIR diver doing a 100fsw dive on a single Al80...there just ain't enough gas to do the dive safely.

No? Hm. I thought in my DIR class we went over rock bottom limits for dives up to and including 100fsw.... in fact I believe we wrote Rule of Thumb limits into our Wetnotes....... JimC am I remembering wrong?

Granted, you sure wouldn't have much of a dive at 100' before you reached turn pressure and began the ascent - but that's not to say it couldn't be done...........

 

[Soggy]

This seems really odd to me. Does DIR really teach this? How deep would I expect to see a DIR diver an a single 80?

Kinda off topic, but let's do the math....

Figure a stressed breathing rate of 1 cft/min for 2 divers, given an OOA at 100ft....

Plan on at least a minute to handle the emergency at the bottom, then we go 30ft/min to our first deep stop at 70ft and then go 10ft/min from there to the surface. So, our total ascent time is at least:

1 @ 100ft
1 min to 70 ft
7 mins from 70-0 ft

9 minutes total ascent time.

The 1 minute at 100 ft will require 2 cft/min * 4 ATM = 8 cft
The 1 minute to 70 ft will require 2 cft/min * ave 3.5 ATM = 7 cft
The 7 minutes from 70-0 will require 2cft/min * ave of 2 ATM * 7 minutes = 28 cft

So, we require a rock bottom of 8 + 7 + 28 = 43 cft just to get off the bottom, which leaves us only 34 cft for the descent and bottom portion of the dive. That's not much at 100 ft, especially if you end up on a dive that requires thirds (like in a shipping channel where you must return to the anchor point).

Given a bottom sac of .6, that gives us basically 14 minutes at depth....not worth it to me when I've got an NDL of around 40 minutes.

I personally, wouldn't go below 80 ft or so on a single 80, especially in cold water.

 

[cornfed]

Granted, you sure wouldn't have much of a dive at 100' before you reached turn pressure and began the ascent - but that's not to say it couldn't be done...........

That's the catch. It's not that it's implicitly unsafe but more likely not worth the hassle. To safely make the dive you have to keep it pretty short.

 

[Soggy]

That's the catch. It's not that it's implicitly unsafe but more likely not worth the hassle. To safely make the dive you have to keep it pretty short.

Yeah, I guess that's what I meant.

 

[GreenDiverDown]

Without a doubt, gas management and planning are very important...especially as one approaches the deeper depths.

The amount of air in a single Al80 might be and often is marginal for these deeper dives. I personally prefer to use my doubles at these depths. But I wouldn't rule out...and have done years of... dives to these depths with a single 80.

But let me continue your cyber-diving...just for fun...

My rock bottom for a dive to 100 feet typically is 1000 psi when using an Al80. That gives me 2000 psi at depth. With a DCR of 94 psi/min @ 100ft (that's equal to a SAC of .6 BTW) I have 21 min.

I may or may not use all of that time. More often than not I would multi-level and re-calculate my rock bottom as I progressed shallower.

Its easy to have a great dive on an Al 80. But its easier to have a horrible one. In fact, I would wager that most folks diving to 100 on their 80 have no idea of the risks and have not been taught how to plan these dives appropriately.


I just didn't think your comment on DIR and 100 feet and "just not enough gas" was accurate.

 

[Soggy]

My rock bottom for a dive to 100 feet typically is 1000 psi when using an Al80. That gives me 2000 psi at depth. With a DCR of 94 psi/min @ 100ft (that's equal to a SAC of .6 BTW) I have 21 min.

How did you come up with 1000 psi?

I determined that rock bottom for this dive was 43 cft, which is around 1600 psi in an Al80, which leaves you 1400 for the dive. I wouldn't do that dive with you with that plan, because your gas is my gas, and if I goober it up, you might not have enough gas to get us both to the surface without blowing off stops. We might get lucky, but I don't plan on luck.

 

[Potapko]

Kinda off topic, but let's do the math....

Quite a bit off the original topic but I for one thank you very much. Quite a wake-up call! This has never come up on any dive I have done yet with any shop. All that is calculated is personal air needs and personal consumption rates. These rec guys regularly go to this kind of depth with no more reserve than this! Beginning to see why you people disagree with the typical rec diving config.

That's it! I need serious training!

 

[FreeFloat]

Even before I understood the concept of rock bottom, I used an arbitrary figure of 1200 psi as a 'turn' pressure for boat dives in the 100' range........ or 1000 psi for 80' boat dives. Very interesting how close it comes.

Of course, due to all the rounding, rock bottom is typically "padded" enough to be quite conservative ergo safe. If buddy's tank were to spontaneously fail at depth we'd bot still make it out alive.

Thanks Soggy

And yes, I wasn't arguing that single tanks have no place below 100' - just that I don't recall anything said to that effect in teh DIR class. I think the general perception was more along the lines of "this is the minimum gas you'd need to conduct this dive safely - if a 30+ boat ride and 30 minutes gearing/ungearing is still worth it for such a short dive....."

 

[JimC]

How did you come up with 1000 psi?

Well, not all of us would agree on your accent profile, and thus rock bottom allocation. Aperently Stephen dousn't, I don't. I know thats not the way we determined RB in my DIR-F.

On an AL80, my RB on a 100ft dive is somewhere in the 1200 range. Granted if I were diveing to 100ft I'd be in my dubs.

No? Hm. I thought in my DIR class we went over rock bottom limits for dives up to and including 100fsw.... in fact I believe we wrote Rule of Thumb limits into our Wetnotes....... JimC am I remembering wrong?

Rule of Thumb for Rock Bottom was:
1000psi from 100-60 feet on an AL80
800psi from 60-0 on an AL80

I feel toward the deeper end of the 100 scale its a to little tho.