Rock Bottom

[Gombessa]

During class instructions when we teach our students to plan their own dives we do teach this "rock bottom" theory. That being, if your SAC is X and you want to dive to Y, how long can you stay? The student manuals cover this topic and discuss that for every 10ft you are down you will need to keep a reserve of 100psi.

Hold on a sec. That's not "rock bottom." Basically, rock bottom is the amount of gas you keep in reserve so that, given your current depth, you can ascend with yourself AND YOUR BUDDY breathing off your tank(s), at a stressed SCR, while maintaining all of your safety stops. This reserve is "walled off" and is not to be included in your dive plan. So if you plan to use half your gas going out and half coming back in, you calculate your rock bottom, reserve it, and use only the remaining gas to determine halves.

Bob's site above (NWGratefulDiver.com) is truly a great resource for anyone who wants to learn more about gas management, as is Lamont's description (Rock Bottom and Gas Management for Recreational Divers).

As an aside, the 100psi for every 10ft doesn't take into account different tank sizes, and is in many cases is insufficient to serve as rock bottom.

 

[Splash-X]

.....

 

[Gombessa]

I edited my post shortly after it went live.

My orginal post did include details, 70ft, 500PSI on the boat, you should leave the bottom with 1200PSI (70ft = 700PSI and add the 500PSI)

With all due respect, that's still not what rock bottom is, and 1200psi (with what tanks, btw?) may easily be insufficient in a number of realistic circumstances to serve as rock bottom for a 70ft dive, especially with newer divers with typically high SCRs. Again, I would recommend to anyone the links above as excellent resources relevant to the discussion at hand.

Edit: Caught your edit late regarding AL80s (not exactly what I'd call HP, but that's neither here nor there).

http://nwgratefuldiver.com/articles/gas4.html directly gives an real-world example where 1200psi may be insufficient with AL80s on a 66ft dive. In brief, it may be insufficient if you know that you and/or your buddy's SCR is elevated under stress, if you know that you will need additional time at the bottom to sort out the problem before beginning your ascent, etc. All of these would call for adjusting RB up. But please read the link above, it's a good one! Once you know what RB is, if you want to disagree with and debate the principles or assumptions behind it, that's fine, but right now what you are arguing has absolutely nothing to do with rock bottom (again, no offense intended).

 

[Rainer]

I edited my post shortly after it went live.

My orginal post did include details, 70ft, 500PSI on the boat, you should leave the bottom with 1200PSI (70ft = 700PSI and add the 500PSI).

99.9% of all divers renting gear are getting 80AL HP tanks filled to 3000PSI so I don't see how "in many cases" it would be insufficient.

Teaching the basics of WHY you need to leave 100PSI per 10ft is key. Throwing in advanced formulas for new divers is going to only confuse them more.

I'd strongly suggest YOU read the linked article on RB. You don't seem to understand it at all.

 

[gcbryan]

There is no reason not to except that change (any change) is slow to come in the dive certification industry.

Once a diver learns rock bottom for their tank at the maximum depth they're likely to go to (on any dive) they don't usually have to do the math again except for more complicated dives. If you know the rock bottom for 100 fsw but you're only going to 60 fsw ...just reduce it a bit

 

[Bismark]

I edited my post shortly after it went live.

My orginal post did include details, 70ft, 500PSI on the boat, you should leave the bottom with 1200PSI (70ft = 700PSI and add the 500PSI).

99.9% of all divers renting gear are getting 80AL HP tanks filled to 3000PSI so I don't see how "in many cases" it would be insufficient.

Teaching the basics of WHY you need to leave 100PSI per 10ft is key. Throwing in advanced formulas for new divers is going to only confuse them more.

"advanced formulas for new divers"...............you have got to be kidding me.

And from your post you still don't understand what "Mininimum Gas" or "Rock Bottom" is.

 

[Coldwater_Canuck]

Some of us DO teach it ... I've been doing so for several years now, as have other instructors in my area. Then again, we're not training vacation divers, for the most part. Around here you're expected to be able to plan and execute your own dive by the time you've completed your basic OW class.

If you want to learn more, visit my Web site ...

... Bob (Grateful Diver)

That's great that you teach it, I can't help but feel my OW was taught too much by the book: which would be fine if the book wasn't written by someone on vacation in Mexico . I'm finally starting to feel (largely from reading a lot on the internet sadly) that I actually understand how to make a safe cold-water dive.

A good example of this is free-flows. I was just diving in the Great Lakes and was warned by multiple people about freeze-ups (a bit more of a concern than in Puget Sound since it is fresh water (easier to freeze) and the temperatures are less predictable and can be colder).

Some things I was told or read: don't inflate BCD while inhaling, inflate in short bursts, make sure your first stage is dried out, if it does freeflow then breathe from it to get to your buddy (or reduntant air) shut your tank off and use your backup air source until it warms up, having two people breathing from one reg in cold water increases chance of freeze-up, etc.

I didn't know any of this, but I figured I'd just forgotten. So I pull out the PADI OW book, look up free-flow. What does it have: one paragraph that says "breathe from the flow, surface, then shut off tank". Nothing about what causes and how to prevent a free-flow, nothing about freeze-ups, nothing about how to handle a free-flow when you don't have enough air to get to the surface ....

I think I'm starting to grow some of that cynicism all the DIR folks have: took me about 25 dives

 

[MXGratefulDiver]

It would be difficult indeed to teach everything there is to learn in a single class ... even the basics. Much of what is considered "local knowledge" comes from diving with more experienced divers. Some of the most important lessons I've ever learned about scuba diving were learned in this way (I was fortunate to have hooked up with Uncle Pug at a time when learning these things was critical).

And it never stops.

A couple of weeks ago I was down in the Channel Islands. An ordinary condition down there that we seldom have to deal with here is surge. We get current ... but the techniques you learn for diving in current don't necessarily apply in surge. You could tell the locals from the non-locals just by watching how comfortable they were with those conditions.

Local stuff like that rarely comes from a classroom. Some of it can ... and one of the things I've always liked about NAUI's approach to classes is that they do make allowances for instructors to add things to account for local conditions ... but most will be learned through bottom time and a good mentor.

... Bob (Grateful Diver)

 

[Coldwater_Canuck]

It would be difficult indeed to teach everything there is to learn in a single class ... even the basics. Much of what is considered "local knowledge" comes from diving with more experienced divers. Some of the most important lessons I've ever learned about scuba diving were learned in this way (I was fortunate to have hooked up with Uncle Pug at a time when learning these things was critical).

And it never stops.

A couple of weeks ago I was down in the Channel Islands. An ordinary condition down there that we seldom have to deal with here is surge. We get current ... but the techniques you learn for diving in current don't necessarily apply in surge. You could tell the locals from the non-locals just by watching how comfortable they were with those conditions.

Local stuff like that rarely comes from a classroom. Some of it can ... and one of the things I've always liked about NAUI's approach to classes is that they do make allowances for instructors to add things to account for local conditions ... but most will be learned through bottom time and a good mentor.

... Bob (Grateful Diver)

Well that's true, everything can't be taught in a single class. Maybe the problem is worldwide training standards for vastly different conditions. It's good that NAUI allows more discrtion, I'm not sure what PADI's stance would be, but my impression is PADI caters to vacation divers (and possibly all the other agencies).

I guess the issue I have is after OW you are supposed to be a relatively safe diver on your own, and I feel too much is being left out. Rock bottom times is one of them.

 

[Blackwood]

I guess the issue I have is after OW you are supposed to be a relatively safe diver on your own, and I feel too much is being left out. Rock bottom times is one of them.

What's a rock bottom time?