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

[roakey]

So much to respond to...

Bottom line... for the average tropical sightseer diver, "500 psi on the boat" is not a "horrible idea" at all, but rather a reasonable rule of thumb that is supported - not negated - by gas planning.

Nope. It's a horrible idea. Note that I'm not saying that being back on the boat with 500psi is a horrible idea, but considering that as "gas planning" is a horrible idea. Horrible in that it ISN'T gas planning.

You need to know when to END the dive, when to start your ascent or it's not gas planning. Taking wild guess at depth to see how close you can come to "500psi back on the boat" is NOT gas planning -- that's my point.

Ron Brandt had an interesting reply in that it appears (the way I read it) at least one Cozumel operator is offering simple gas planning:

There are dive opps in Cozumel that either want you to end at 700 and one I dove with wanted us to end at 1000!

Note the use of the word "end" -- I take that to mean "you start you ascent when you hit this pressure" -- you may not agree with it, but *thats* gas planning!

PhinDiver states:

...For me it is 500 psi ON THE BOAT, not start ascending with 500 psi. Which means, I start ascending with more psi depending on the depth of my dive. Usually, i start going up on shallow dives at about 900 to 800 psi.

Though PhinDiver *thinks* their gas planning is "500psi on the boat" it isn't -- "Usually, i start going up on shallow dives at about 900 to 800 psi." is their gas planning -- based on when to end the dive, not pressure back on the boat. With his 1000+ dives he knows when to end the dive to have 500 psi back on the boat. But his gas planning is not "500 psi back on the boat" -- it's really the 800-900 psi ON SHALLOW DIVES (which implies he knows how to tweak this on deeper dives).

Now, the trick is, without 1000+ dives, how do you teach an OW student when to turn the dive?

1) If you don't give 'em a number, they will return with the tank empty.

Well, if you really only want to teach them one number (I'm assuming an AL80 here) and assuming they stick to the 60 feet or less depth for an OW diver, tell them to begin their ascent with no less than 850 psi in their cylinder. From 60 feet with a safety stop this will give them about 500 psi "back on the boat" at a SAC of 1cf/min (if I did my calculations right). Now tell them that if they need to swim back underwater to an entry point, they need to add to that 850psi number -- heck, tell them that you’ll teach them that in AOW if you want to!

The point is, the number they should have burned in their brains is when to start their ascent. Not having that, they really have no idea when to start their ascent whatsoever, and who knows what they'll come up with?

As an added bonus, they'll no longer be goal driven for 500 psi on the boat -- embarrassed if they have 100, and annoyed if they have 1000 -- they will have simply turned the dive when they should have (say, 850psi) and whatever they have when they get out is what they have -- who cares what the number is!

I've been on several dive charters where the "500psi rule" is expressed as "Be at safety stop no later than 500psi."

This has the same problem -- it's an "after the fact" number. It's turning right 1/2 mile before the big red barn.

This concept is amazingly simple for cavers -- we know when to turn the dive, we'd have a lot of deaths if our gas planning was "500psi back at the entrance" -- when do we turn around? Do we have enough gas to get our buddy out alive? To answer those questions we come up with a pressure of when to *turn* the dive. At that point the dive's over and we come out and I personally don't give a hoot what my pressure is when I surface outside the cave -- the only important number is when we turned.

In conclusion, by all means give the beginning diver a single number if need be, but make it the number at which the ascent begins. De-emphasize what number you should have "back on the boat" -- that's completely unimportant (just make sure the "begin ascent" number is well chosen to assure the "back on the boat” number is sufficient).

In addition, drill into the beginning OW student that that "begin ascent" number (such as 850psi) is sufficient for them to share air with an OOA diver and that both they and the OOA diver can perform a safe, unhurried ascent to the surface including a safety stop. Will they end up with less than 500 psi back on the boat? You bet -- but if you eliminate that number completely form your curriculum as a goal, they won't care and they'll be safe and less stressed.

Roak

 

[Charlie99]

100psi/10' means you should ascend when you have 1000psi in a 100' dive, right??

A spare air tank is rated to 3000psi . Now, will you wait 'till your spare air hits the 1000psi mark to head to the surface?? why? why not?

Ahhh! so SIZE MATTERS, RIGHT?

Hmmmm. A "rule of thumb" is meant to be a general guide, applicable to normal situations. I do believe that most people would consider a spare air as the main tank as somewhat unusual, and might, just might possibly suspect that the rule of thumb might not apply in that case.

 

[PavoDive]

Hmmmm. A "rule of thumb" is meant to be a general guide, applicable to normal situations. I do believe that most people would consider a spare air as the main tank as somewhat unusual, and might, just might possibly suspect that the rule of thumb might not apply in that case.

Charlie that's true, but what I explained holds true when you change from an AL80 to an AL50. It's just that the "50+ breaths @ surface" made it a lot easier to explain --and get.
I've seen people "who does not suspect" that the rule of thumb does not apply when they dive an AL50 or an AL63.

Gio.

 

[GreenDiverDown]

I like the "turn right one half mile before the big red barn" analogy.

It's not difficult to teach OW students the basics of gas management right from the start...(BTW) I believe that Rick is doing just that by teaching his students how the 500 psi rule is derived.

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.
Roak

What is the rockbottom for 0 depth?

Considering that rockbottom continually decreases as the divers ascend, you really don't even need the 500!

I understand that you are talking about a direct ascent. But, you can still be true to the concept and practice of rockbottom while climbing on deck with much less than 500...even from an initial level that is quite deep.

For instance, a dive to 100 feet would have a RB in the neighborhood of 30 cuft. But as you ascend to the 30 foot level RB decreases to near 11 cuft....that's 400 psi in an Al80. Obviously, that diver could easily be back on the boat with less than 500 and still have used reasonable gas management principles.

 

[Charlie99]

Charlie that's true, but what I explained holds true when you change from an AL80 to an AL50.
I've seen people "who does not suspect" that the rule of thumb does not apply when they dive an AL50 or an AL63.

Do you really dive an AL50 very often? I avoid them like the plague, since my camband has a permanant kink at the standard 7.25" diameter and I'm too lazy to get it to behave with the 6.9" AL50.

I'll use AL63 in heavy surf or lava rock entry, but those are generally shallow dives. The other common use of an AL63 is as an alternative tank for a smaller person --- generally a person with lower SAC.

In real life, the variation in SAC is bigger than the size range of normal tanks. But then again, in real life, most divers don't know their SAC/RMV.
Ooops. I just let the secret out of the bag. The rule is an APPROXIMATION.

 

[Kim]

In the case of a Scuba student - unless he's enrolled in a commercial class or a class required by someone else - the student is the ultimate arbiter of what the class entails.

While I can see what you are saying I think that this is exactly part of the problem.

How can any student in anything know what is important to learn and understand before they have actually learnt it? I realize that this is simply a dumbing down to the level that the agencies can say..."We can teach you in XXdays for XXdollars" - but you only have to talk to a few resort DMs and hear the various horror stories about near misses to know that there are a lot of people who shouldn't be U/W in the first place. Sure, normally the DMs are on the ball enough to get their sorry butts out of the fire - so it doesn't get recorded - and everyone congratulates themselves on their 'safety record'.
IMO if you want people to take scuba seriously - then you should train them to take it seriously - and give them the tools and knowledge to progress. As one fundamental part of diving is about how much air you have - and how long it might last in different circumstances I see nothing wrong with the concept of stressing volume over pressure. In your post I see that you clearly DO teach these concepts - where the 500psi derives from, SAC rates etc. That's enough for me, as a diver trained like that will use the 500psi rule of thumb knowing that it's not cast in stone and that other factors COULD influence it. I don't expect OW divers to be able to do rock bottom calculations from the get go, but if they are aware of the principles it's up to them if they then choose to investigate further.
Unfortunately, many instructors don't seem to teach like that - and while the agencies might allow it - they don't demand it.

 

[PavoDive]

I'll use AL63 in heavy surf or lava rock entry, but those are generally shallow dives. The other common use of an AL63 is as an alternative tank for a smaller person --- generally a person with lower SAC.

In real life, the variation in SAC is bigger than the size range of normal tanks. But then again, in real life, most divers don't know their SAC/RMV.
Ooops. I just let the secret out of the bag. The rule is an APPROXIMATION.

I don't dive AL63 often, but have been to an <undisclosed location>'s operator in the caribbean, who carries several AL63 because they're preferred by most women and small men. In no few occassions, while diving there, I've had this kind of conversation:

PavoDive: (Talking to the lady who's ready to take the plunge to 120' on an AL63) Are you going to dive with that tank?
Lady: (looking amazed) Yeah. It's got 3000+. It has more than my husband's (pointing to her husband, who's diving an AL80), he's got only 2900. Why the question? :06:

But maybe you're right, all this is BS if divers don't know what their SACs are...

I hope we're not just preaching to the choir.

Gio.

 

[gregor1234]

I have the distinct impression that those who argue that the 500 psi rule of thumb is rotten are not objecting to the "500" itself or the conditions under which it is meant to apply, but to having any rule-of-thumb at all instead of teaching OW student how to do gas consumption calculations themselves.

In theory I can agree with teaching gas consumption calculations in OW but in practice you have to balance the measure extra safety conferred by that knowledge against the very real possibility that OW students will do the calculations incorrectly. It is not meant to be applied blindly but to be used by beginners until they get further knowledge. Our LDS teaches gas consumption in an additional "optional" lesson in the OW course (that everyone attends anyway) but doesn't require a demonstration of proficiency for certification.

The physics of the 500 PSI rule is just fine and Roakey's calculation s back this up. It is the reserve you should plan to have back at the boat, not at the bottom of the anchor chain. As such, it's an ample reserve for servicing your OAA-at-the-end-of-the-dive buddy in depths ranging nearly to 100 ft -well beyond the envelope recomended for the OW diver.

BTW I was taught that the J-valve trip pressure of 600 PSI was chosen because it gave enough reserve using the typical 71.2 cu ft tank to allow two buddy-breathing divers to ascent to the surface from 120 ft. (They didn't do 15 ft safety stops back then.) Pretty similar to the reasoning behind the 500 psi rule except the tank pressures and volumes are different and now there is a safety stop.

 

[GreenDiverDown]

While I'm definitely on the side of teaching gas planning in OW, I'll play the Devil's Advocate for just a moment...

The 500 psi rule ...as bad as it is...seems to be working.

I see very few OOG goof-ups...occasionally...but not all that often. Divers just seem to find their way back to the boat with gas to spare. There is little to no planning going on, but, somehow they do it. I think it's similar to how a poor golfer manages to work the way up the fairway and into the hole without planning an approach to the green.

Eventually, as a diver gains experience, he/she sorta subconsciously gets a feel for when they need to start moving up so that they will end their dive with this reserve. This serves them well on their typical shallow reef dive or guided dive. It's when they do a deeper dive or a dive that is significantly more challenging that their risk of goofing up increases. Even then, however, most seem to make it...they sure aren't dropping like flies.

Of course, they rarely consider that part of their gas belongs to their buddy...but what the hey!

 

[Eris Lucan7*]

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