Rock Bottom

[rongoodman]

I don't think the OW students have to be taught to calculate anything--they can memorize a couple of numbers. What they need to understand is that they're responsible for having enough gas to get themselves and their buddy safely back to the surface, and the amount needed increases with depth.

 

[amascuba]

I don't think the OW students have to be taught to calculate anything--they can memorize a couple of numbers. What they need to understand is that they're responsible for having enough gas to get themselves and their buddy safely back to the surface, and the amount needed increases with depth.

You could easily tell them to research 20 ft3 from 0 - 60 fsw and 40 ft3 from 61 - 100 fsw, but many open water students are not taught how to turn ft3 into psi. Given that, many of them wouldn't be able to look at their pressure gauge and accurately tell how many ft3 are available in their tanks.

If they DO understand that, then once their bottom times become longer, the more inefficient it is to just go by those two numbers, therefore really needing to understand how those numbers are derived so that they can come up with an efficient time and maximize their bottom time.

 

[DaleC]

I'm showing up back on the boat with +/- 500 psi.

How do you do that?

The funny thing is, looking back, I was never taught "how" to show up back on the boat with 500psi.

Being back on the boat with 500psi is like trying to solve following equation:

a + b = 500psi.

Thank you Lamont for illustrating a and b for me.

 

[Peter Guy]

One of the things I've always wanted to know was WHY do I want to "be back on the boat with 500 PSI?" What good is that?

BTW, to the poster who doesn't "believe" in Rock Bottom -- there is NOTHING in the RB calculations that says you have to be back on the boat with 1300 PSI -- remember, half that gas is yours to use as you see fit. And, of course, as you come up, once you get to around 40 feet, RB is the minimum of 500 PSI. Since half that is YOURS, you, in theory, would comply with RB rules by being "back on the boat" with 250 PSI -- see you actually get MORE useable gas by using the RB rules!!!

 

[amascuba]

The way that I've seen it explained is that it takes the average person 13 ft3 to ascend from 100 fsw to the surface in an emergency. 13 ft3 in an AL80 is 500 psi. That's all fine and dandy, but it doesn't account for your buddy, higher sac rates, nor ascending from deeper depths.

 

[Coldwater_Canuck]

The way that I've seen it explained is that it takes the average person 13 ft3 to ascend from 100 fsw to the surface in an emergency. 13 ft3 in an AL80 is 500 psi. That's all fine and dandy, but it doesn't account for your buddy, higher sac rates, nor ascending from deeper depths.

Even for the average person that seems a little low. On a good ascent from 70 or 80 I use about 500. I suppose I'm probably above average in SAC rate, but that is still a pretty big difference. Those 20 or 30 feet will probably add another 250 PSI or so. Not to mention your buddy and the (ironically) higher SAC rates associated with an OOA emergency.

For deep dives to be really prepared, I believe you really need some sort of redundant system. Because for me to feel safe to get me and a buddy up from 100 feet on an Al-80, I'd have to end at 2000 PSI to be on the safe side (on a panicked ascent from 100 feet, each using 1000PSI doesn't seem unreasonable), which is 99.9% of the time a huge waste of air since you'd surface with over 1200 PSI left. Now if you and your buddy both have 19 or 30 cf pony's, you now have 4 air sources and any 2 of them can fail and you'll still have enough air in the remaining 2 for a safe ascent. With redundancy, while I still aim to have 500 PSI on the surface (and consider this my "reserve" threshold if I am panicked or other factors mess up my SAC), I no longer need the reserve for my buddy. So I can leave 100 feet at probably 1200 instead of 2000

 

[Thalassamania]

Er ... PADI does not cover it, NAUI Instructors are free to do so if they choose to. Many so choose

 

[Sloth]

making this way to complicated..... just dive doubles and then RB is (almost) always 500 psi in rec limits

 

[mts0628]

Actually 500 psi has nothing to do with gas management. It was a throwback from the strictly steel tank days and the rationale was to keep at least 500 psi in the tank so that the chance of an empty tank would be minimized and thus less chance of rusting. Somehow it carried over to today's recreational diving and all tanks were told to be returned with 500 psi. Not safe for the diver in the gas management aspect (because there is none), but for the shops it reduced work of refinishing the inside of tanks.

 

[Brewone0to]

I know of a few shops that post a $$$ fee for rental tanks brought back with less than 500 psi.
They claim that it is for required VIS inspection on such a tank.
Some will even try to charge you for this if you bring your tank in to get a fill.