Why the 50bar/700psi rule in the first place?

[String]

50 bar and i think america uses 500psi (35bar).

Firstly i think for standard shallow recreational type diving that amount of air in theory should be more than enough to get a diver and his buddy from wherever they are to the surface with no real issues even on a small tank (10l or so). It maybe a simple way of drumming into people a sensible reserve figure for every dive without adapting it to tank size.

I agree its misleading in that it isnt getting people to think about the actual quantity of gas in the tank - 50bar in a 15l tank is a lot more than 50 bar in a 10l etc so the smaller the tank the smaller the reserve.

That said, certainly BSAC training specifies using rule of thirds for air planning right from the start. I assume other agencies are the same. Obviously for dives where you just swim around a reef and come up under a bag 1/3s is overkill - you simply want a reserve to get you and an OOA buddy to the surface safely without having to worry about returning to a starting point or shot line.

 

[gregor1234]

The 500 psi back in the boat is not a really bad idea as someone suggested as long as you understand that it applies to certain circumstances. (But it so happens those circumstances make up 90% of the diving that new OW divers should be doing.) Any divers i've talked to are taught the rule of thirds in OW along with the "500 back in the boat" to use as a convenient rule of thumb with AL 80s (that the majority of newly minted divers will use).

As such it is a reasonable rule of thumb. The arechetypal profile taught is to start with 3000, turn on 2000, and end up back at the anchor chain ready to ascent with 1000. For depths 100 ft or less, you'll return back to the boat with 500 or more. And, if your buddy winds up OOA even at the very end of the dive just before ascending, you and he will still have enough to make a safe controlled ascent because as a new OW diver you're supposed to be no deeper than 60 ft anyway.

If you plan to arrive back at the boat with 500, that 500 is your reserve for ascent from <100 ft in case of an buddy OOA at the end of the dive.

Now, what about other conditions where you would want a greater (or smaller) reserve? What about if you're drift diving in a heavy see or using doubles? Penetrating a cave ? going below 120 ft? Deco diving? ? Well a new OW diver shoudn't be doing any of those things. The "500 back in the boat" only applies to certain situations but it is those situations that describe most of th diving the novice diver is doing.

 

[roakey]

The 500 psi back in the boat is not a really bad idea as someone suggested as long as you understand that it applies to certain circumstances.

Sorry to disagree, but it's a horrible idea. The key is that it tells you NOTHING about when to terminate the dive and start up. It's a guessing game. You guess when to turn, and you either guess right and have 500 psi, or you guess wrong and come up with -100 psi... Real gas management is knowing at what point, based on your gas pressure WHEN to turn the dive and/or begin your ascent.

The example I used in the rock bottom discussions (you folks should really use the search function and find this rather than rehasihng the same stuff here) is that telling someone to "be back on the boat with 500 psi" is just as useful as giving someone directions like "turn right one half mile before the big red barn..."

And it can easily be shown (and is, find the rock bottom discussions) that in many cases having 500 psi back on the boat is insufficient gas to get you and your buddy back to the surface safely in an OOA situation.

Here you go, I've done the search for you... Please read these, they have some very good information:

http://www.scubaboard.com/showpost.php?p=401869&postcount=65
http://www.scubaboard.com/showthread.php?t=90264&highlight=rock+bottom

There are other rock bottom posts, it's just that these are a good start...

Roak

 

[TSandM]

A month ago, I attended a gas management seminar done by NW Grateful Diver (Bob). These were EXACTLY the issues he covered in the class. I had never thought about the fact that 500 psi was a different amount of air for each diver, depending on the size and type of tank, although it was obvious as soon as Bob mentioned it. I had never thought about (and never been told to think about) how much air I would actually need to get me and a buddy back from depth safely . . . and the calculations are sobering.

Unfortunately, the seminar, which was well publicized, was attended by precisely two of us. So other divers either don't know how important it is to know this stuff, or don't think it's important enough to get to a teaching session where it's all laid out clearly.

Gas management wasn't even touched upon in my OW or AOW classes, except to lay out the "500 psi back to the boat" concept. It wasn't even mentioned as something you would want to explore further on your own. I find that distressing.

 

[PavoDive]

I agree with the general idea that the 500psi/BB rule is bad, dangerous and that it teaches nothing but lazines. But then, I invite you to see how many OW divers know how to use the tables or the wheel two months after they finish their course. Some things are responsibility of the Instructor, to that I agree, but a lot more things are responsibility of the diver. Instructors (of any sort and discipline) can guide a diver towards responsibility; but guranteeing that somebody will be absolutely responsible is a task beyond the capabilities of the instructor, much like guaranteeing a responsible child is somewhat beyond the capabilities of any parent. Being responsible requires the student to make a decision. His/her instructors, DMs, buddies can influence, advise, suggest making the decision, but can't actually make it.

Put the guy to do calculations, and it's likely that skill will be off his brain in a couple months. And what would you have then?? nothing... zero... nada! Even worse, you'll have a guy who doesn't have any guide -however inaccurate- to base his decisions on.

You may say that that kind of people shouldn't be certified. I strongly agree, but that doesn't change the fact that they are being certified, have been certified and will be certified. Matter of fact, I think they are waaaay more than we are!
I-do-three-dives-in-a-year divers are more or less the common factor, so you have to figure out something that will remain in their brains and that will keep 'em out of harm's way. For me it's the rule of thirds that makes more sense than the senseless 500psi/BB, but I don't really expect them to proeficiently do rock bottom calculations, at least not after three years of 3-dives-in-a-year.

Air is light, air management is not to be taken lightly.

 

[Kim]

I agree with the general idea that the 500psi/BB rule is bad, dangerous and that it teaches nothing but lazines. But then, I invite you to see how many OW divers know how to use the tables or the wheel two months after they finish their course. Some things are responsibility of the Instructor, to that I agree, but a lot more things are responsibility of the diver. Instructors (of any sort and discipline) can guide a diver towards responsibility; but guranteeing that somebody will be absolutely responsible is a task beyond the capabilities of the instructor, much like guaranteeing a responsible child is somewhat beyond the capabilities of any parent. Being responsible requires the student to make a decision. His/her instructors, DMs, buddies can influence, advise, suggest making the decision, but can't actually make it.

Put the guy to do calculations, and it's likely that skill will be off his brain in a couple months. And what would you have then?? nothing... zero... nada! Even worse, you'll have a guy who doesn't have any guide -however inaccurate- to base his decisions on.

You may say that that kind of people shouldn't be certified. I strongly agree, but that doesn't change the fact that they are being certified, have been certified and will be certified. Matter of fact, I think they are waaaay more than we are!
I-do-three-dives-in-a-year divers are more or less the common factor, so you have to figure out something that will remain in their brains and that will keep 'em out of harm's way. For me it's the rule of thirds that makes more sense than the senseless 500psi/BB, but I don't really expect them to proeficiently do rock bottom calculations, at least not after three years of 3-dives-in-a-year.

Air is light, air management is not to be taken lightly.

I can see your point. 500psi/BB is absolutely better than nothing at all. On the other hand you don't know what you don't know (including what to search for on SB! eyebrow ). As TSandM pointed out - if it doesn't get a mention in OW, then how does a new diver who would like to be more responsible know what they should further investigate? Unless they have access to such a source as SB or other such forum - or they hang out with a much more experienced crowd - how do they know? For me it's obviously of much more importance to know how much actual air I have - rather than what pressure it's at. I honestly believe that this should at least get a mention at OW.

 

[Marek K]

I agree that divers, from the very beginning, should understand something about air/gas management... if only the fact that 50 Bar or 500 psi means a different amount of air/gas, depending on tank size; and that ending a dive with that amount means you have to turn, and then begin your ascent, with more that that.

But everything can't be taught in OW; it's arguably a pretty steep learning curve if it's done right already. (Regarding which I'm sure many would be very happy to argue! )

Keeping in mind the "maximum recommended" depth for an OW diver, as well as other conditions (no overhead or penetrations), I agree that students don't really need to know excruciating details of gas management. That can come in AOW, and continue from there.

Maybe more would if they were taught to think in terms of volume rather than pressure right from the beginning?

Kim, that's an interesting viewpoint from a part of the world where tank size is measured in liquid volume!! eyebrow Maybe you metric types need to go back to measuring tanks by cubic feet at rated pressure!

No?

--Marek

 

[Kim]

Kim, that's an interesting viewpoint from a part of the world where tank size is measured in liquid volume!! eyebrow Maybe you metric types need to go back to measuring tanks by cubic feet at rated pressure!

No?

--Marek

Why? liter capacity X bar tells me exactly how many liters of air I have - I can do that in my head anytime during the dive. If I know my SAC measured in SLM (surface liters/minute) then the calculations become rather easy. I think Imperial is much more complicated for anything less than a full tank. :05:
The main point is though - if I don't do the calculation because nobody ever told me why I should then it all becomes immaterial - especially if the only thing that I've been told is important is the pressure reading. eyebrow

 

[Kim]

That can come in AOW, and continue from there.

But it doesn't, does it? (at least not with the agencies that train the most divers.)

I trained with PADI up to Rescue and never heard any of this stuff. The first time I heard anything at all was doing an IANTD Overhead course (that taught me Rule of Thirds and gas volume/SAC rate). I actually only realized HOW important it was and how to calculate rock bottom after joining this board and reading about it here. I was a little shocked to say the least.

 

[Marek K]

Why? liter capacity X bar tells me exactly how many liters of air I have - I can do that in my head anytime during the dive. If I know my SAC measured in SLM (surface liters/minute) then the calculations become rather easy. I think Imperial is much more complicated for anything less than a full tank.

Ha!! I figured that would get you going!

The main point is though - if I don't do the calculation because nobody ever told me why I should then it all becomes immaterial - especially if the only thing that I've been told is important is the pressure reading.

Agreed. It has nothing to do with Imperial vs. Metric. And I'm not sure it needs to be a "calculation" per se at the beginning; it need to be, in OW at least, an awareness of what's going on.

Originally Posted by Marek K
That can come in AOW, and continue from there.

But it doesn't, does it? (at least not with the agencies that train the most divers.)

I got my AOW in 1986. Hard to remember what was covered then, but it sure didn't include gas management. Or a lot of other concepts that are apparently covered now... apparently still not gas management?

My point is, I don't think OW students need to be taught rock-bottom and such. That's not what you're saying, though, is it? I agree with you that some awareness needs to start there, and be developed progressively.

--Marek