Basic Math Formula to calculate reserve, safety stop, etc etc

[Steve_C]

A better question might be why is PADI talking about diving thirds when a OW diver should never be in an overhead environment? (where a free ascent to the surface for whatever reason is not possible)

Because there are lots of scenarios where a free ascent is possible but is not the most desirable choice (offshore diving from a boat, avoiding boat traffic, etc.). For that matter, personally I dislike surface swims in general and much prefer to be able to return under water.

It is not important whether you call it thirds or whatever. But having a plan and a reserve to safely complete that plan is good. I usually start with a thirds plan for the type of diving I do. When I am safely back to the anchor line, the needed reserve to get me and buddy to boat plus safety stop is less and we will use some of it poking around the anchor area.

 

[JohnN]

I do.....all diving is deco.

Fascinating, so you teach planned decompression to OW students ?

The point I was attempting to make (obviously to obliquely) was that training for scenarios you shouldn't be in can encourage divers to get into those scenarios.

Wouldn't an OW student be better served to understand what an overhead environment is and taught to avoid it until their skill and training is appropriate?

 

[RJP]

Hello all,

I am new in this and I am reading though the PADI Open Water Diver manual to get certified.

But, i am kind of lost in "how to calculate the different scenarios of reserve, safety stop and turn around point" Need some guidance.

I have this question:
My buddy and I are planning a shore dive. We're descending onto a very gradual slope that begins at 5 meters/15 feet, so our descent and ascent will be gradual part of swimming out and back underwater. We have similar cylinders filled up to 200 bar/3000 psi. We plan:

*50 bar/ 500 psi reserve
*20 bar/300 psi for our safety stop
*To turn the dive when we've used one-third of the air available to use on the dive.

This means we should head back when either of our SPGs read

a. 70 bar/800 psi
b. 145 bar/1900 psi
c. 157 bar/2270 psi
d. 170 bar/2500 psi

Thanks for your help,
G

There's the correct PADI test answer, which was given above. And - while I doubt either you or your buddy has an SPG with sufficient markings, much less precision, to reliably read 157bar/2270 PSI - the rationale for understanding how to calculate such numbers makes sense.

Then there's the real world, where there's really A.) no way to "calculate" an appropriate turn pressure based on the info provided, and B.) no reason to make diving more complicated than it needs to be.

As others have pointed out, when you turn a dive depends largely on the dive, the depth, the conditions, etc. For instance, if you dive heading out WITH the current and turn and swim back INTO the current at 1/3 tank... you might not like the result. For instance I'd be much less conservative on a shore dive in Bonaire as compared to a wreck dive off NJ.

In your scenario - from both a "real world" and a "simple calculation" standpoint - I wouldn't regard 500psi as "a reserve" but rather how much air should be left in your tank when you exit the water. A true reserve should be sufficient gas to get you AND YOUR BUDDY out of the water from any point in the dive, recognizing that - even if you're not in an overhead environment - sometimes doing that at depth makes more sense than a direct ascent to the surface right then an there... even though that option should always be available to an Open Water diver. And if the dive does go all pear-shaped... I don't care if the dive op wants me to keep 500psi in my rental tank to avoid doing a VIP... so I don't need to factor that into my calculation.

Accordingly, my simple rule of thumb for turning recreational dives is 1,000psi for my way out, 1,000psi for my way back, and 1,000psi for you.

I can calculate that. My buddy can calculate that. My SPG can display that. And if we get back to the exit point and both of us have 1,000psi left... we simply extend the dive.

 

[Keith.M]

Fascinating, so you teach planned decompression to OW students ?

The point I was attempting to make (obviously to obliquely) was that training for scenarios you shouldn't be in can encourage divers to get into those scenarios.

Wouldn't an OW student be better served to understand what an overhead environment is and taught to avoid it until their skill and training is appropriate?

The fact the question that the OP was referring to was planning a dive by turning at 1/3 of available air had nothing to do with overhead diving.

You are the only one that put that into the conversation.

The take I get from you is that you are just another of a long line of bash PADI wannabes.

Have fun!

 

[JohnN]

The take I get from you is that you are just another of a long line of bash PADI wannabes.

Have fun!

Enjoy the Kool-Aid

 

[rongoodman]

SAC rate x avg depth x # of divers x ascent time
That is the cuft to bring 2 divers to the surface (convert cuft to psi)

Never mind--I misread your formula.

 

[Adobo]

The fact the question that the OP was referring to was planning a dive by turning at 1/3 of available air had nothing to do with overhead diving.

You are the only one that put that into the conversation.

The take I get from you is that you are just another of a long line of bash PADI wannabes.

Have fun!

I think he brought up overhead environments because 1/3s is usually discussed in the context of diving overhead environments. It is fair to ask, why in an academic exercise for open water divers is that example used? But I guess that is a topic for a different thread.

---------- Post added November 10th, 2014 at 10:35 AM ----------

Because there are lots of scenarios where a free ascent is possible but is not the most desirable choice (offshore diving from a boat, avoiding boat traffic, etc.). For that matter, personally I dislike surface swims in general and much prefer to be able to return under water.

Isn't that why you would take the usable gas for the dive and divide it by two? In other words, use half of your usable gas to go out underwater and then use the other half to come back in underwater.

 

[namerg]

Thank you all. Obviously there are some comments that go over my knowledge. Not sure if I am freaking out or not.. But, do you really have to do all those calculations before scuba diving ? Or, what is the simple rule of thumb ? You have to know, how deep you are going to be ? Then based on depth, do you take a 10000 or more psi's with you ? Sorry, sorry for my stupid questions. I am totally new on this.

 

[Steve_C]

Isn't that why you would take the usable gas for the dive and divide it by two? In other words, use half of your usable gas to go out underwater and then use the other half to come back in underwater.

And so your buddy needs some air

And your are coming back and the viz has dropped and it take a few extra minutes to find the anchor line,

And somebody gets a cramp and you need to stop for a few minutues.

And .......

Depends on how you define usable gas I guess

 

[Basking Ridge Diver]

I am totally new on this.

Don't over analyze - yes you can just jump in and have fun. Be sure you and your buddy have the same plan (when are you going to turn and return?) - then go dive and continue to check on each other during the dive.
Worst case you both surface and swim back to shore...

Sometimes it is information overload - apologies if I caused any heartburn...