What pressure to end a dive? A debate!

[redacted]

Depending on the amount of balancing employed in your balanced 2nd, that is to be expected. An unbalanced 2nd will be more sensetive to tank pressures below IP.

 

[HowardE]

Thats still something ive not experienced with any of my reg sets. During deliberate depletion and testing i get little or more likely no warning at all before air just stops flowing.

what does your gauge read? Is it zero? Have you checked your SPG against another SPG on the same tank to make sure the SPG reads correctly at low PSI (or BAR)? Maybe your regs are different than every one else's?

 

[Rick Murchison]

Hey, is there an echo in here?

Probably Thought I'd read the whole thing but I must have missed one (or more) of yours, Mike. Sorry...:14:
Rick

 

[LI Diver]

Ok can someone enlighten me?? If you have two different 80 cu ft tanks and 500psi left in them,don't they have the same volume of air regardless of what the max pressure for the tank is??? The original post did not say different size tanks?
Granted the percentage will be less but won't you have the same number of breaths left in each???

 

[Rick Murchison]

Thats still something ive not experienced with any of my reg sets. During deliberate depletion and testing i get little or more likely no warning at all before air just stops flowing.

How big a tank are you using for your tests? The smaller the tank, the fewer breaths with noticeable difference (higher resistance than normal). With a 72 or 80 you should get pretty good warning, especially shallow. I once ran an 85 down at 115' just to see, and picked up the difference about five or six breaths before the "final plunge" in pressure.
Rick

 

[The Kraken]

End the dive with an indicated pressure that will allow both you and your dive partner to make a normal ascent to the surface while executing any and all stops that may be required should either partner have a catastrophic equipment failure.

the K

 

[MikeFerrara]

Ok can someone enlighten me?? If you have two different 80 cu ft tanks and 500psi left in them,don't they have the same volume of air regardless of what the max pressure for the tank is??? The original post did not say different size tanks?
Granted the percentage will be less but won't you have the same number of breaths left in each???

No.

The volume per unit of pressure in an LP tank is greater than in an HP

 

[The Kraken]

And not all 80 cubic feet tanks are 80 cubic feet.

An aluminum 80 is what, 77.4 cubic feet?

At a service pressure of say, 3000 psi, the aluminum 80 at 500 psi would contain 12.9cubic feet of gas.

A HP steel 80 with a service pressure of 3442 psi would contain 11.62 cubic feet of gas.

And let's suppose you both are diving aluminum 80's with a nominal volume of 77.4 cubic feet at the rated service pressure.

There are some aluminum 80's that have a service pressure of around 2600 psi. So, if both of you drained your tanks down to 500 psi, one of you would have 12.9 cf of gas and the other would have 14.9 cf of gas.

the K

 

[LI Diver]

No.

The volume per unit of pressure in an LP tank is greater than in an HP

Ok I believe you, but why? Is the 80 cu ft a measurement of how much air you have at a given pressure? If so what is it??
Ok never mind at the rated pressure obviously.So therefore if you have 80 cu ft at a lower pressure the tank would have to be bigger right???

 

[spectrum]

If you have two different 80 cu ft tanks and 500psi left in them,don't they have the same volume of air regardless of what the max pressure for the tank is???
Granted the percentage will be less but won't you have the same number of breaths left in each???

NO and NO

Divide the rated pressure by the cubic foot capacity to see how many PSI equal 1 cubic foot of air. Repeat for the other cylinder.

Divide 500 by those numbers and compare the results.

Cubic feet is the measure of air, PSI is just a cylinder specific artifact. If they are different as you say then there will be differing numbers of breaths for the same PSI reading.

For reasonably close cylinders like an AL80 and a NEUTRAL ALUMINUM 80 it will be accademic since you are reading an analog gauge down in a less than accurate range.

Pete