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

[namerg]

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

 

[Keith.M]

You are starting with 3000 psi.

You have reserved 800 psi as non available air for your dive plan so that leaves 2200 psi.

2200/3 = ~734 psi

3000 - 734 = 2266

So from what is presented, you would select c.

 

[namerg]

Good. But, how did you come up with /3 ? why 3 ? Thanks for your help.

 

[Basking Ridge Diver]

If you are shore diving -
How deep do you intend to go? Will you stay above 40 - 50 feet?
At 50 feet how far from shore do you think that will will be? 100 yards or more?
What is your tank size? AL80 or different? You mention similar but not the cu feet.
I am assuming this is a lake/pond/quarry and not the ocean - what is the bottom like - are there hazards such as tree branches and snag points?
Has anyone dove here before - you or your buddy? Have others dove here where you can gain some insight on the dive?
For me these would have a bearing on my answer.

*** you told the above poster that you would turn at thirds - he is dividing by 3 based on what you told him...

 

[Keith.M]

Good. But, how did you come up with /3 ? why 3 ? Thanks for your help.

That was part of the scenario

*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.

Turn the dive at one third of the air available. So once we calculate how much air is available, then you divide that by 3.

 

[JohnN]

You are starting with 3000 psi.

You have reserved 800 psi as non available air for your dive plan so that leaves 2200 psi.

2200/3 = ~734 psi

3000 - 734 = 2266

So from what is presented, you would select 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)

 

[Keith.M]

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)

I think the real value here is learning to plan.

As you become a more experienced Open Water diver, you will make adjustments to this because of your experience.

I mean really, if you dive that plan perfectly, you are going to end up exiting with around ~1200 psi.

Is that really going to be your goal or are you looking to be able to use that extra 700 on bottom time.

 

[JohnN]

I think the real value here is learning to plan.

As you become a more experienced Open Water diver, you will make adjustments to this because of your experience.

I mean really, if you dive that plan perfectly, you are going to end up exiting with around ~1200 psi.

Is that really going to be your goal or are you looking to be able to use that extra 700 on bottom time.

OK, then why not introduce decompression theory and planning into an OW course.

 

[decompression]

OK, then why not introduce decompression theory and planning into an OW course.

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

---------- Post added November 10th, 2014 at 09:39 AM ----------

Calculate "rock bottom"


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)
Seems complicated but do it a few times and it's easy.

 

[namerg]

That was part of the scenario

*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.

Turn the dive at one third of the air available. So once we calculate how much air is available, then you divide that by 3.

Thanks Keith.M