Rule of Thirds & Shallow Rec diving

[PfcAJ]

The idea of surfacing with ~500psi on a regular al80 is pretty reasonable. It just so happens that 500psi is what you'd end up with if you started your ascent when you hit rock bottom (more or less, tank dependent, yadda yadda. its close).

The problem with such a simple thing is that most folks aren't presented with the tools of HOW to end up on the surface with 500psi. When do you begin your ascent? Do you need to ascend sooner if you're deeper? What about if I use an lp104 vs an al63? What if I want a longer safety stop? Deep(er) stops? What if I'm diving from an anchor? Or drifting? Or shore diving?

"Surface with 500psi" is a fine idea. Its just too simple and lacks the depth needed to foster understanding.

 

[DaleC]

Maybe we are missing something.

In the real life example I posted (#140) describe how you go about getting to the surface with 50b when you are descending 100' and want to see the whole boat deck (360'). How do you figure that out, without some form of calculation? How do you know travel, ascent and SS volumes? How do you know what size of tank you need?

Not how do you guess. How do you know.
That's what some of us are struggling with.

 

[Foxfish]

The idea of surfacing with ~500psi on a regular al80 is pretty reasonable.

Your numbers sound too low to me.

 

[GCullen94]

Foxfish where are you getting "your numbers sound too low to me" guessing again? too low for what? what depth? what kind of ascent profile?

What if it was a 30 foot reef dive? 500PSI perfect.
What if it was a 100 foot wreck dive/wall? 500PSI is cutting it

 

[Storker]

how do you define a gas planning strategy?

I don't know how I define a gas planning strategy, but my gas planning strategy is pretty darned simple:

"Let's see, we'll jump off the boat, paddle over to the cliff and descend along the wall. How deep? No point in going deeper than 25m, there ain't much life below 25. OK, 25 is max depth. And 22 is planned depth. *checks RDP* 25min @22m, that's group L. 25min @ 25m, that's N. Riight. *checks personal rock bottom table* . You've got a 15L, right? I've got a 15L, too. Minimum pressure at 25m is 80bar. OK, latest turnaround is 25 minutes or 80 bar. *makes note on slate*.

How deep on the return? What? OK, thanks! Hey, buddy, the skipper says there are some pretty cool soft-coral fields at 12-15m. Riight. *checks table* *mutters* N, then to 15m, that's 16m on the RDP, add 25min... 67 minutes, that's group V. We're good on the no-deco. Minimum pressure... *checks rock bottom table* ...60. *makes note on slate*

OK, let's review: Descend along the wall to 22m, keep at around 22, don't go deeper than 25. East for 25 minutes or until 80 bar. Ascend to 12-15m, max depth 15, west for 25min or until 60bar. If we go shallower than 10, we can stretch it to 40bar. 3 minutes safety stop at 5m. We good on that? Great!"

...


"Hey, skipper? We're diving towards east, max depth 25m, max time one hour. I'm gonna shoot my dSMB from the safety stop. Planning to come up pretty close to the boat, but we're not sure about the current, so we may be a little bit off. Just look for the dSMB. Got that? Great, see ya!" *splash* *splash*

 

[Foxfish]

Foxfish where are you getting "your numbers sound too low to me" guessing again? too low for what? what depth? what kind of ascent profile?

What if it was a 30 foot reef dive? 500PSI perfect.
What if it was a 100 foot wreck dive/wall? 500PSI is cutting it

I know it because it does not comply with what divers are taught in their OW and AOW which is to surface with 50 b or 725 psi. Do you think that is wrong?

 

[D_B]

The figure I've heard given here is 500psi

 

[GCullen94]

I know it because it does not comply with what divers are taught in their OW and AOW which is to surface with 50 b or 725 psi. Do you think that is wrong?

What agency are you referring to here? I was taught 300psi + depth x10 in OW. however, in my AOW we covered how to calculate min gas, this was through PADI. When I took GUE fundies we were taught min gas. There's a huge difference in 500 PSI in an al 63 and a HP130.

If you haven't noticed by my previous posts and others... Yes I do think your way is wrong.

The problem with a hard rule of "be back with 50b" is it is too conservative for shallow, too liberal for deep and can cause people to skip safety stops if they are a little low because they are afraid of getting in trouble by the DM for coming up with less than what they were told to be back with. Which is safer, coming up with 50B and no stop after a 30M dive, pushing NDLs or coming up with 40B and taking a stop?

 

[Foxfish]

I note that you've calculated that you need 110 b (1600 psi) to ascend safely from 30 m (100 ft) with an al80 (about 11 L). That seems very conservative to me. Maybe you have a very high air consumption rate?

 

[Storker]

I note that you've calculated that you need 110 b (1600 psi) to ascend safely from 30 m (100 ft) with an al80 (about 11 L). That seems very conservative to me. Maybe you have a very high air consumption rate?

It fits pretty well with my calculations. 15.5 l/(bar·minute) or 0.55 cu.ft gives 90 bar as minimum pressure for an 11L at 30m. Add 20 bar to avoid sucking your tank totally empty in an emergency, and you're up to 110.

It may be because I still haven't quite reached 50 dives in my log, but I don't see 15.5SLM as a "very high air consumption rate", especially in cold waters and with a dry suit. My rates are currently between 14 and 16, with 13.5 as my all-time low.