Definition of turn pressure?

[DaveDog]

Saw this term use what does it mean?

 

[JimC]

The pressure reading of the minimum gas required to safely get you and another person (buddy) back to the surface or another "guarantied" breathable gas source.

 

[Rick Inman]

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.

 

[countryboy]

..back on the boat with 500 psi...

Did I win??

Just kidding.. yes it was surface swim.. yes I know tank factor and gas management / dive planning...

Let's see the answers...

 

[DaveDog]

When figuring this do you want 500psi when you surface.

 

[MSilvia]

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.

Wow... a game on Scubaboard that's actually scuba related. Who'da thunk it?

Well, assuming we were planning to snorkle out to the white buoy and back, we don't have to worry about how much gas it'll take to get us back to the beach, just how much we need to ascend safely while sharing gas. Assuming we've both practiced the b'jeebus out of that scenario and can do it without increasing our RMV rate due to stress, and that the ascent line is right near the octopus' home (within a 1 minute swim), and also that we'll ascend at 30'/minute with a 3 minute safety stop at 20 fsw, we need to calculate the gas requirement for 2 minutes at 100' (a minute to get squared away, and a minute of ascent), 1 minute at 70', 1 minute at 40', and 4 minutes at 20' (3 minute stop, plus ascent to the surface).

The ATA for each of those depths is:
100 = 4.04
70 = 3.13
40 = 2.22
20 = 1.61

Multiplying those numbers by our combined SAC of 1.3, and the number of minutes at each depth, we get the following gas requirements for the ascent:

100 feet for 2 minutes = 10.51 cubic feet
70 feet for 1 minute = 4.07 cubic feet
40 feet for 1 minute = 2.89 cubic feet
20 feet for 4 minutes = 8.38 cubic feet

That gives us a total gas requirement for safe ascent of 25.85 cubic feet, which means we absolutely must start our ascent as soon as either one of us has used up 66.15 cubic feet of the 95 we each started with. With a service pressure of 2640psi, each cubic foot of gas in the tank accounts for 27.79 psi. If we need 25.85 cubic feet, that translates to needing 719psi in the tank for both of us to ascend and have a near-empty cylinder at the surface. Personally, I don't want an empty cylinder under even the worst situations, so I'd probably say we need to start thinking about heading up at around 1000 psi, and definately be starting up the ascent line with 800psi minimum.

Of course, if you want to be back on the beach with 500psi after air sharing, that changes things somewhat.

 

[H2Andy]

When figuring this do you want 500psi when you surface.

not necessarily. you may want a larger (unused) reserve under certain
circumstances-- for example, a thirds dive (1/3 out, 1/3 back, 1/3 reserve).
you could end up with 1,000 psi at the end of the dive.

of course, if you have that much air, nothing prevents you from conducting
an impromptu "mini" dive to use up the air. i would keep this "mini" dive shallow
and close to the exit point.

i would exit with at least 200 to 300 psi on the tank, but that's to keep pressure
in it and prevent rust, etc. also, pressure gauges aren't all that reliable at
low volumes, so 500psi makes sense as the "lowest reliable reading" you
can count on.

short answer: i wouldn't plan for less, but i might plan for
more, depending on the dive

 

[TheRedHead]

Of course, if you want to be back on the beach with 500psi after air sharing, that changes things somewhat.

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

This is rock bottom, not turn. Your couldn't calculate turn without knowing the distance covered in the dive and taking into account for currents.

Anyway, no one answered the OP's question. Turn pressure is the amount of gas needed to make your way to your exit point expressed in psi, i.e., when I hit 1500 psi I'm going to turn around and head back to the boat.

 

[MSilvia]

How about this one:

The plan is to do a giant stride entry off the rock at high tide, swim out until it's about 20 feet deep, descend, and spend the rest of the dive descending a gradual slope toward a max depth of about 65 feet until we hit the turn pressure, at which point we'll swim back and exit.

Diver one is wearing an AL80 with a service pressure of 3000 psi, diver two is wearing an LP95 like the one in the first example, with a service pressure of 2640. Diver one has a SAC of .8, and diver two doesn't know his SAC.

What are the turn pressures for each diver?

 

[do it easy]

OK , I'll play:

I figure, conservatively, 23 cf to surface from 100'. This is both buddies breathing from 1 tank, and doing a 3 minute safety stop. I calculated all the ascent gas consumption as if it were at 100', the ascent is 30 ft/min, and the 3 min stop at 15'. This all translates to 634 psi. I'm also assuming that the divers are close the buoy and are not swimming out and back.

Here's the big butt!!!! At 100', I think PADI's RDP NDL is 20 minutes? for air, so the divers will be limited by the NDL time, rather than the rockbottom pressure. After some quick calculations, both divers should ASCEND after 20 minutes, or roughly when the .6 SAC diver hits 1306 or the .7 SAC diver hits 1084, which ever of these three limits occurs first.

I could be wrong about a lot of things here, so check my calculations...