Air management question

[MXGratefulDiver]

We always tell students and divers that 700 PSI is the point at which you signal your DM to let them know you are low on air and time to ascend to your 3 minute safety stop. No need for math while trying to maintain buoyancy at the end of a tiring dive after so long out of the water.

... it surprises me that an IDC instructor with a tech background would give that sort of advice. It'll generally work with a few caveats ... like you mustn't be very deep, your air consumption rate is "average" or better, and nothing goes wrong during ascent that requires you to share air with your buddy ... but we're not supposed to be training people on the assumption that nothing will go wrong.

It makes me wonder how often your DM's are required to deal with OOA emergencies, or how often you have students and clients dealing with LOA situations that, by definition, mean they're cutting their safety margins too thin.

I prefer to train my students to understand how much gas they need for a planned dive before getting in the water. Since there aren't any fill stations below the surface, it helps maintain a certain confidence knowing ... because they planned for it ... that they have enough air to complete the dive. I also prefer to train my students how to think while maintaining buoyancy control ... it helps a great deal if something unexpected happens that they have to deal with.

... Bob (Grateful Diver)

 

[GrumpyOldGuy]

Rock bottom calculations are the gold standard for recreational divers. There are plenty of spread sheets you can download to calculate. This is really important for new or returning divers as you don't have the experience burned into you brain. For deep dives it becomes critical.

The 100psi for every 10 feet is a rough rule of thumb. It assumes you and you buddy have good breathing control and you don't need any time to mess around, its an immediate ascent. I was actually taught 100/10 + 300-500 PSI. It you need something simple, it works. It is not the gold standard though, a rule of thumb.

Terminal values like 500 or 700 PSI on the boat are incredibly crude. Suitable only as a lowest common denominator for vacation divers in a nanny diver situation in modest controlled conditions.

 

[Jim Lapenta]

JumpPilot...welcome to Scuba Board and welcome back t diving.

SBers have presented you with a few good ideas. I like the Rule of Thirds and use it personally quite often. When I teach a Open Water course here is the formula we use.

Finding Turn Pressure
Starting Pressure: e.g. 3000 psi
minus
Reserve (500 psi) , ascent & safety stop (300 psi): 800 psi
leaves
Remaining pressure:3000 - 800 = 2200 psi

Start Pressure: 3000 psi
minus
Half remaining pressure:2200 / 2 = 1100 psi
leaves
Turn pressure: 3000 - 1100 = 1900 psi

This formula works pretty well for simple dives that a new OW diver might plan. Also with experience comes personal adjustments to meet your needs, e.g. the Rule of Thirds.

What ever formula you decide to try, plan you dive and dive your plan...then make adjustments and do it all over again...many time.

Happy Bubbles,

~Oldbear~

This is good. Especially for new and newer divers. I see so many who forget about the reserve and think thirds means 1000,1000, and 1000. Including some instructors which I really don't understand.

 

[jumppilot]

Thank you everyone!

 

[Freewillow]

Rock bottom calculations are the gold standard for recreational divers. There are plenty of spread sheets you can download to calculate. This is really important for new or returning divers as you don't have the experience burned into you brain. For deep dives it becomes critical.

The 100psi for every 10 feet is a rough rule of thumb. It assumes you and you buddy have good breathing control and you don't need any time to mess around, its an immediate ascent. I was actually taught 100/10 + 300-500 PSI. It you need something simple, it works. It is not the gold standard though, a rule of thumb.

Terminal values like 500 or 700 PSI on the boat are incredibly crude. Suitable only as a lowest common denominator for vacation divers in a nanny diver situation in modest controlled conditions.

Would you be kind enough and indicate where such a Xcell spredsheet can be found, or send me one by MP. Thanks

 

[00wabbit]

I use an ap called scuba calc on my phone. It has a section called minimum ascent pressure. You input sac rate, tanks size and some other variables and it will give you a list of rock bottom pressures every 10ft. Then just write those down on your slate for your max depth and a few above and below.

No no need for pesky math underwater if you plan ahead on the surface.

 

[GrumpyOldGuy]

Would you be kind enough and indicate where such a Xcell spredsheet can be found, or send me one by MP. Thanks

http://www.scubaboard.com/forums/basic-scuba-discussions/335496-rock-bottom-calculator-excel.html

ScubaBoard search functions suck, Google works much better.

Here is the one I started with..

 

[gfaith]

Would you be kind enough and indicate where such a Xcell spredsheet can be found, or send me one by MP. Thanks

Be sure to read that whole thread and not just assume the first Excel file will be the one you should use. Kevrumbo attached one on the second page that includes 2 divers (you and your buddy).

 

[Kharon]

... Remember, you have to have enough gas to get you and your buddy, both presumably stressed, to the surface, taking into account any required deco, and you also need to allow a little time to sort things out before you start up. ...

Not if you dive solo. One of a number of reasons I think diving with a buddy is more dangerous than diving solo (as long as you are fully trained, fully skilled, and fully equipped).

 

[Basking Ridge Diver]

Or you (or you and your buddy) carry a Pony Bottle - two divers having catastrophic failure at the same time is not realistic. But you need to decide your own limits, risks and what is your risk tolerance.
After all - it is your decision and your life - make an informed decision for you.