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

[Rick Murchison]

as just a plain bad idea

I agree - it's a really bad idea! My point was more along the lines of: being such a bad idea, why is it still being taught?

no gas management taught in OW and most instructors are to lazy to add it

I actually wonder why the agencies themselves don't alter the standards to reflect a safer approach.

it's a horrible idea. The key is that it tells you NOTHING about when to terminate the dive and start up.

The question becomes "Is there a place in diving for the recreational, casual, sight-seeing diver? Is there a place for the diver who doesn't want to do any real gas planning - who won't do any gas planning beyond having a single number branded in his brain?"
You may think not... but in reality those are the divers that provide the bulk of the money to the entire industry; those are the divers that make it possible for the rest of us to dive.
So, let's take a look at "500 psi back on the boat" and its efficacy for the casual recreational warm water underwater sightseer. While doing that I'll sort of run through how I present the "500 psi back on the boat" rule to my open water classes in the context of gas planning while at the same time answering the students' question "Why the '500 psi' rule."
Like the 130' depth limit, the origins of the "500 psi" rule are murky, but... by happy coincidence it turns out to be a pretty good one.
What kind of dive, and what kind of diver is the rule set for?
The basic parameters are "Recreational depth and within the doppler NDL." That means 100' max depth and a direct ascent to the surface always an option.
In order to have that 500 psi back on the boat, you have to know what you need to leave the bottom with, and to know that, you have to know how much gas it'll take for you to get to the surface - and so you need to know your SCR.
(insert a lesson on calculating SCR here)
So for our example, let's assume a SCR of .8 CFM, which is liberal enough to cover just about anyone. (obviously, if you have someone whose SCR is greater than .8CFM that student will need a little extra instruction and a special rule)
To get to the surface from 100' at 30 feet per minute, you'll need 200 seconds, and your average depth will be 50 feet. So so you'll need 200/60 X (50+33)/33 X .8 CF for the ascent. That's 7.1 CF.
You also want to allow for a 5 minute safety stop, so that'll use up another 5 X (15+33)/33 X .8 CF or 5.8 CF.
Therefore you'll use 12.9 - say 13 - CF for your ascent and safety stop. Since 13 CF is about 500 psi in an AL 80 (magic, ain't it?), you can see that you'll need to leave the bottom with 1000 psi to get back on the boat with 500.
Now, what if you have to share air? If you could maintain your same .8 CFM breathing rate then planning for a 500 psi reserve will be just enough for two of you to make a normal ascent and safety stop, too. Or, if you skip the safety stop - a perfectly acceptable thing to do in an emergency like an OOA situation - you can go as high as about 1.5 CFM each (that's huffin' & puffin' pretty good) with your SCR and still have enough gas to make a 30 fpm ascent with an OOA buddy, all the way to the surface.
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.
As for instructors being "too lazy" to teach gas planning - perhaps there are some, but I think that it's more accurate to say most are simply delivering the goods the customer wants. And the customer is, in the end, "always right."
Rick

 

[PavoDive]

Dj, welcome aBoard, I can see this is your very first post!

Now, please let me point some things:

The problem with measuring the volume of air in your tank is the fact that the volume in the tank never changes.

That's absolutely right, the volume of the tank never changes (an AL80 will have the same volume full or empty). What we are talking about here is the "equivalent volume of air left in the tank", and that one DOES change (See why we called it "volume"?? it's waaay shorter!)

However, the information that is truly important is how many more minutes of air you have at the current depth and breathing rate.

That is true if, and only if, you can be sure there will be no problems in the remaining time of the dive. Your breathing rate will surely change if a problem arises, and it will ******in' change if both, you and your buddy, are stressed and breathing from the same source. So current depth and breathing rate sometimes aren't enough, I'd suggest you check the links that roakey posted.

Teaching volume management during OW would be overwhelming for any student with lots of other things to worry about. You simplify the situation by standardizing tank size (Al 80) and teach the student to recognize and deal with the quantity their gauges are measuring directly - pressure. You wouldn't want an OW student, who can't even stay neutral, to be doing calculations on a table or slate underwater, trying to convert their current tank pressure into a breathable volume.

Indeed I would want a student to be able to do those calculations. However, as I stated in a previous post, I think it's rather unlikely that skill will remain sharp in the student's brain for more than two years [months?? weeks? Oh my, I'm scared!]. Standardizing tank size is really a solution to the problem as you said, in the sense that it would take off the burden of the calculations (and the guessing game). The problem is that differences in tank sizes and pressures aren't just a marketing plot of tank manufacturers. Different tanks can be (and are) used for different purposes, and the chance that you come upon a tank that's not an AL80 exists (if you go diving overseas, for example), and it's a good idea to be prepared to face that with a solid approach, not just air-left-guessing.

I just took my OW this summer and I definitely would have been overwhelmed by it, and I'm an engineer. I understand the idea that 500 psi on the surface is different than 500 psi at 100 fsw

I pretty much understand that feeling, as I think all and every one who was on a first dive at any point on his life would. Now, you're right, 500psi on the ground are different than 500psi at 100', but it is also important to understand the fact that 500psi in a 80cu.ft tank are different than 500psi in a pony bottle. I think the later is the point that we're trying to make in this thread.

Gas management, or at least SAC rate calculation, is being taught to me for my Deep Diving class as part of my AOW that I'm currently taking through SSI. Deep Diving is where this concept most applies and the student should be advanced enough to be able to include this task along with other concerns.

It's good to hear you're learning to calculate your SAC, and even better if it's really gas management (can you suggest to your instructor that he taught your proper gas management?). And while SAC is important, it certainly isn't all that there is to be. What would you gain if you knew your car makes xyz miles a gallon, if you don't know how many gallons the car's tank is, or even worse, how many gallons are actually left in the tank??

Sure deep diving requires higher planning, and that also includes gas planning. But remember, OOAs can happen at any depth...

Dj, it's not difficult, and if you're an engineer, you've already got what is needed. Check roakey's links, there's an invaluable wealth of knowledge there...

Safe bubbling,
Gio.

 

[smokey braden]

i would like to welcome djsmocyc to this board and thank him for his contribution to this thread.

 

[Kim]

......... but I think that it's more accurate to say most are simply delivering the goods the customer wants. And the customer is, in the end, "always right."
Rick

This is probably the truth. It's a bit strange when the student tells the teacher what he needs to know though isn't it? It seems to be fairly standard in the US and areas influenced by the US that an AL80 is what you get. When that's the case it more or less works.
In Asia I've seen 12, 14, 15, and this weekend at a new site I've been told we'll get 10 liter tanks. As an AL80 is actually about 11 liters this can therefore swing between terminating dives unnecessarily early or coming up too short. Considering how expensive bottom time can be - and the consequences of running out of air for two people - I'd still call it a bad idea to teach this as the all encompassing number that you need to know.

 

[Charlie99]

...... 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.

When I asked my OW instructor about gas planning, he gave me three basic rules of thumb:

1) on a drift dive or wreck dive, head back up when you hit 100psi per 10' of depth.

2) On a shore dive or an anchored/moored dive where you can surface at anytime if needed, head up and back at 1/2 tank, or a little bit less if you've been wandering around in circles and the exit point is fairly close.

3) As for a dive where you can't surface at any point --- don't do the dive. (at least until you have a lot more experience and know the ramifications.)

Pretty simple rules. Perhaps oversimplified in some people's opinion, but they have served me well, and even my later, more sophisticated Rock Bottom Calculations come out pretty darn close to 100psi per 10'.


Charlie

 

[Rick Murchison]

It's a bit strange when the student tells the teacher what he needs to know though isn't it?

That depends.
If the class is to prepare the student to work for Company A, then Company A dictates the class objectives. 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. And most Scuba students want only to be able to go look at the pretty fishies with minimal cost and at an acceptably low risk. The industry, despite its warts, is doing a pretty good job of providing just that.
Rick

 

[Wristshot]

With all due respect to the learned folks that have expressed opinions here, there are some really simple reasons for what Rick M so aptly referred to as a "rule of thumb". [Again remembering that "rules of thumb" are for the masses and for average situations and for those that don't know enough to know when the "rules of thumb" don't apply.]

1) If you don't give 'em a number, they will return with the tank empty. Some places will open a tank and do a visual if the pressure (regardless of the volume) is below some threshold number. Some places will issue threats about what they will do if you return a tank below XXX PSI.

2) If you gotta give 'em a number, give 'em an easy one to remember. Is there any magic to 500PSI or 50BAR ? Probably not. (I like Rick's "murky" comment) Those numbers certainly seem simple. The fact that they are not equivalent bears out my belief that it is based on simplicity and convenience rather than a calculated numerical necessity.

3) If people have a target, they might acheive it. If they overshoot slightly, and return with 400 or 300, then no harm done (probably). If they don't have a "target" then how do they do their superficial rudimentary "planning" (and I include it in quotes because so many would argue that it does not qualify as planning).

If you know enough to pick apart the "rule of thumb" or if you understood all the posts included above, then chances are you are not contained within the target demographics for said "rule of thumb".

It is what it is, and that neither makes it good or bad.

Just my 10 PSI, and that brings me down to 780PSI, so it is time for me to ascend.


Wristshot

 

[markfm]

I did a google on Excalibur and can't find it. Do you have a url or source?

Mike

http://www.geocities.com/dbergis/freeware.htm

I've used it for years -- I like RPN, and the conversions are handy.

 

[Charlie99]

I've been on several dive charters where the "500psi rule" is expressed as "Be at safety stop no later than 500psi." In fact they specifically mentioned that they didn't care how much you surfaced or got on the boat with, as long as you hadn't drained it fully dry.

It doesn't take much pressure to keep water out of a tank. At 100psi your reg isn't going to deliver much air, but that would still be enough to keep out water well beyond recreational depths.

 

[PavoDive]

I certainly understand what Rick and Wristshot say, I see it also, don't give'em a number and they will come back empty, but that shouldn't be excuse for them not wiring (at least a bit) their brains.
Working with rules of thumb in pressures is OK as far as 1) you always work with the same size tank, and 2) everybody in the dive boat (where the DM issued the 500psiBB order) is diving about the same tank, same depth.

But the problem arises when the diver is exposed to a situation where the size of the bottles change (specially), and that's where their rules of thumb start flooding.
Let's see if I can make this clear for all those who are reading (and surely not posting), but still don't see the point of this "bunch of rock-bottom-calcs-religious-extremists":

When I asked my OW instructor about gas planning, he gave me three basic rules of thumb:

1) on a drift dive or wreck dive, head back up when you hit 100psi per 10' of depth.

2) On a shore dive or an anchored/moored dive where you can surface at anytime if needed, head up and back at 1/2 tank, or a little bit less if you've been wandering around in circles and the exit point is fairly close.

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

Hmmm... consider this (Disclaimer: I DON'T like spare airs, consider them unsafe, just because of the very reason I'll expose here):
A spare air tank is rated to 3000psi (I'm not sure about that, couldn't check in a quick google), but according to Scubatoys, a 3 cu.ft will give you only 50+ breaths at the surface. 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?

That's what we've been trying to say...

Gio.