Definition of turn pressure?

[MSilvia]

Isn't the 500 psi a reserve for the emergency your've more accurately accounted for in your calculations?

I think the intent of that particular rule of thumb is a combination of what you said and not wanting divers coming back with tanks full of water. In any case, "be back at the boat with X" doesn't give you much information about when to head back. IMHO, it would be much more helpful for a DM to say "Be back at the ascent line at 800 psi" or "Let me know you're low on air when you get to 1000psi", or something like that that's actually a useful guideline while still underwater.

Still, "return with 500psi" is so deeply ingrained in some divers' minds that they treat it like 500psi is somehow a magic number, even when it's arbitrary and not related to the requirements of the dive.

 

[TheRedHead]

Matt, you're right of course in it being a simple rule of thumb. If nothing bad happens in using rock bottom, by your calculations, the diver would return with more than 500 psi. If you add the additional 500 psi then you'd return with over 1300 psi and shortened your dive.

Turn pressure is not as neat because you can estimate how far you are going on a particular dive and apparent current, but the dive seldom works out as neatly. You might stop and look at something for a long time or may find a ripping current. I try to estimate turn pressure at the start of a dive, but I adjust it on the fly.

 

[MSilvia]

You might stop and look at something for a long time or may find a ripping current.

Whenever I dive in a current, I try to begin the dive swimming into it, so that the return is easier. I may push the turn time out a little, if I know I'll be able to relax and drift back. I figure it's better to work hard before getting to the turn pressure and then drift back to the line than it is to drift off with the current and realize you have to swim a mile upstream with your remaining gas supply in order to reach the boat.

Either way, it's good to adjust to changing conditions on the fly. Even when deco diving, when sticking to the plan is vitally important, I usually pre plan for what I have in mind, what I'll do if I abort early, and what I'll do if I overstay by up to 10 minutes. I print out all three plans and tape them to my wrist slate for easy reference. If I can't stick to plan A, plans B and C are right there.

 

[jeckyll]

There is a great set of posts by lamont in the Lessons Learned forum about this.

I ended up writing Rock Bottom times on my Dive light given that I use an AL80 and assuming a SAC or 1.0 for distressed divers (read: divers with a problem that requires both to breath of the same tank).

30 fsw - 700 psi
60 fsw - 1000 psi
100 fsw - 1300 psi
130 fsw - 1800 psi

These of course mean that at that psi you'd head up, do your stops and exit. To actually do the detailed calculations is honest beyond me at this point in my diving. I guess I could work out the details by churning through the math, but my profiles typically don't require it (due to the nature of the dives)

 

[countryboy]

The way I figured it... Using the higher sac rate (times 2 for an issue and to ascend w/ safety stop)..

Reader's digest version:

Start 2640 psi
Descend (consume 240 psi) = 2400 psi left (4 mins)
dive out consume 325 psi (4 1/2 mins out) = 2075 psi TURN PRESSURE
return consume 325 psi (4 1/2 mins) = 1750 psi
Ascend w/ issues include safety stop 3 mins 1400 psi = 350 psi remaining. (7 1/2 mins)

Very conservative on rounding, and using 1.4 sac rate (higher RMV * 2 divers for problems)

This would give a total dive time of ~20 1/2 mins..

 

[Rick Murchison]

Saw this term use what does it mean?

It simply is the pressure you plan to "turn the dive" on. This may or may not be "the amount of gas to get you and your buddy safely to the surface."
For example, on a leisurely reef dive, where time and deco status aren't a problem, I may brief it something like this:
We'll drop down to 60 feet on the wall and if there's a current, turn into it. If not, we'll turn right and explore in the 60 foot range until one of us reaches a turn pressure of 1500 psi, where we will head back to our entry/exit point and ease up to 30 feet or so. This will allow a leisurely, exploring swim back to the exit point where we will linger at about 15 feet for at least 3 minutes. That'll get us back to the exit and the surface with right at 500 psi each or better, and keep us well above that needed for an air sharing ascent from any point in the dive.
Turn pressure *may* be set at the minimum needed to get you and your buddy back - like the "rule of thirds" in a cave - but it certainly doesn't have to be.
Where you "turn the dive" may be a pressure, a time, a depth, a place, a decompression status, the completion of a task, or any combination of those factors. Under any circumstances "turn pressure" should be a part of your dive planning on every dive.
Rick

 

[Jason B]

Let's make it interesting.

The dive plan is to surface swim out to the white buoy, descend to 50', spend three minutes descending to 100' and look around for the octo that's supposed to be there.

Your SAC is .6 and your buddy's is .7. You're both diving LP95's filled to 2640psi. What's your turn pressure, and when you hit it, what do you do?

Thanks for playing.

Both divers are using LP95's filled to 2640. Easy.

Thirds = 800psi (both divers)
Turn pressure = 1800psi (both divers)
When the first diver hits turn pressure, signal "turn dive" and head home.


*EDIT: Brain Fart - I was reading this as turn pressure using thirds. That may not have been the intent.

 

[SeaHorseWoman]

Thank you Uncle Ricky. Now I think I understand what turn pressure is. I hope I get to the point/experience that I know what the rest of you are talking about. I like games.

How is SAC determined? I assume it is air consumption, but what does the S stand for?

barbaraSHW

 

[Charlie99]

I use "turn pressure" and "rock bottom" to mean slightly different things.

For me, rock bottom is the air I need to safely abort a dive at any point. That calculation assumes both divers breathing from one tank and in a relatively excited, high air consumption rate. It only gets me back to an acceptably safe point --- i.e. direct ascent where safe, even if it then requires a long surface swim.

Turn pressure is more related to air needed to get back to the boat or desired exit point. It asssumes normal air consumption.

Obviously, one turns the dive when either rock bottom or turn pressure is reached.

 

[Zoomer]

SAC rate or Surface Air Consumption Rate....

http://www.scubatoys.com/education/sac.asp