Surface pressure

[Storker]

When I got my OWD cert, I was taught to surface with at least 50 bar left in my tank. No matter whether I dived a 10L, a 12L or a 15L. I gotta admit, I stretched that "rule" a little bit. As long as I arrived at my safety stop with 50 bar left, I was good. Or at least, I thought so.

Now, 50 bar in a 10L tank is 500 surface liters, while 50 bar in a 15L tank is 750 surface liters. So that rule didn't make sense. Over in Imperial country, you're supposed to surface with 500 psi left in your tank. That's about 35 bar, and for an Al80 (11 liters water volume), it's about 350 surface liters.

A standard SPG has an accuracy of some 10-20 bar. So, if your SPG tells you that you have 50 bar left, worst case you might have only some 30 bar left. For a 10L tank, that's some 300 surface liters or about 10 minutes of gas. On the surface, so perhaps 50% less at safety stop depth. If your Imperial unit SPG has the same error, you might be down to almost zero minutes.

Is it safe to arrive at your safety stop with only 500 psi/35 bar left in your tank? Frankly, I don't think so. And I think you should start thinking about min gas calcs.

 

[Gone for diving]

My LP72 at 500psi has 15 cuft of air.
I dive doubles so I have 30cu ft.
(Almost half a single LP72 )

Lower pressure tanks the ratio is different.
10-20bar on gauge seems high.
That's 145-290psi.
My mechanical gauge is not out that much...

Also with hoseless transmitters, they definitely will not be out that much.

 

[John C. Ratliff]

A "safety stop" is just that, a stop which places more safety into your dive plan. The safety stop wasn't a part of diving until almost the 2000s. Before that, and before SPGs were used, we used a J-valve that gave us a notification at 300 psig (20.4 bar), and headed to the surface. We did thousands of dives that way, safely. However, we did not get close to the "knife edge" of the no-decompression limits either. That was about 13% of a standard steel tank, or about 64.7 cubic feet of air divided time 0.13 equals 8.4 cubic feet of air (238 liters at 20.4 bar).

Instructional agencies are seeing in the last 30 years (I've been diving since 1959) recreational divers using bigger tanks, holding more air, and divers going into technical diving (depth/time/overhead hazards), and so instituted the "safety stop." This allows off-gasing at shallow depth before surfacing, and is building into the dive another safety factor.

The amount of air left in the tank at the end of the dive, 500 psig for some areas, is also so that the charter boat knows that there has been no possibility of water entering the tank during or after the dive. It does give a safety factor too. So there are many reasons for the air remaining in the tank, but in an emergency, that air is there to be used.

The photo shows U.S. Navy Underwater Swimmers School divers making their deep dive for qualification, using twin aluminum (special for the U.S. Navy) 90 cubic foot tanks, a manifold with a 500 psig reserve in one tank, and an Aquamaster regulator. The instructor, in white tanks, was wearing steel 72 cubic foot tanks. The diver at the ladder was not making a safety stop, but getting ready to ascend to the surface and get out of the water. There were no safety stops, but again, we adhered to the no-decompression limit for this dive, in 1967.

SeaRat

 

[Vicko]

If I'm working with a dredge in 3m depth I'm perfectly fine surfacing with the tank walls bowing in.
If I have a 60 minute deco obligation I'm perfectly fine surfacing with enough gas to loose all my deco stages and not get bent.
It really depends on to many factors, like you said min gas calculations are a good idea.

 

[CuzzA]

Safety stops are optional. So if we're talking about no stop diving, as long as I can get to the surface at less than 30 ft. per minute, all is well. Most dives where I finish off the tank I surface with about 300 psi. AI should not have inaccuracies like an analog SPG.

 

[tmassey]

<Rant mode=“on”>
“50 bar/500 psi on the surface” is the dumbest rule. First of all, as you’re talking about, it completely ignores variations such as tank size, service pressure, depth of dive, etc. Second of all, you won’t know until you’re out of the water if you successfully executed that. There is nothing in that rule that helps you to know when you should leave the bottom in order to have enough gas through the ascent. Yes, with experience you can learn at what pressure you need to leave the bottom in order to make it to the surface with the mystical “500 psi” on the surface, but why in the world would we create a “rule“ that does nothing but let people know they failed, rather than give them the proper instruction to be able to succeed every. Single. Time. Friends don’t let friends dive with “500 psi on the surface” as their gas plan. </rant>

OK, with that out of my system…

You are asking some wonderfully intelligent questions. Questions that all divers should be asking themselves. And when you think about that more deeply and take advantage of the experience of others, you will be left with one of two formulas: Rock Bottom or CAT. Two slightly different formulas that end up at almost exactly the same place to calculate how much gas you need to be safe to make it to the surface.

I’ll give you the very quick version of CAT: Use average consumption for two people (0.75 cu ft/min x 2), multiply that by the average depth from the bottom (so for 100 feet that’s 2.5 atmospheres), and then multiply that by the time it will take you to surface: One minute to work a problem, then 10 feet a minute to get up, so for 100 feet it’s 11 minutes. that would get the cubic feet you need to make it to the surface. Figure out what that pressure is, and make sure you leave the bottom with at least that much gas in your tank.

You might say, 10 feet a minute on the way up? In an emergency? I’m not taking that long. And you probably won’t. But the other two numbers are not fudged at all: there’s no extra factor for being excited, heavy breathing, current, all of that other stuff. Your only margin for error is that 10 feet per minute. So stick with the 10 feet per minute.

That means you need 42 ft.³ of gas to get two people up safely from 100 feet in the event of an emergency. That’s right: Literally half of an aluminum 80, and that’s only from 100 feet. It’s worse at 130 feet.

<rant> And that is why people still use the 500 ft.³ rule. It lets them stay on the bottom leaving too little gas to truly make it to the surface in an emergency, yet still feel like they were “safe”. Sure, if a CESA is part of your dive plan, you probably don’t have to leave that much gas. But a CESA should be part of no rational person’s contingency dive plan. Leave enough gas, and if an aluminum 80 doesn’t give you enough gas, use bigger/more tanks.</rant>

Seriously, this is an excellent question that shows a tremendous amount of deep thinking. Yes: what size tank you have changes what the pressure should be. What the service pressure of that tank is changes what the pressure should be. Using a formula that takes into account these types of details and gives you a number you can be very confident in is an excellent step forward in anyone’s dive planning.

(Numerous small dictation corrections: voice dictation and compressors don’t play well together.)

 

[crofrog]

 

[scubadada]

Noone has ever thought of this. This is a good thread Why Surface with 500 PSI?

I do much of my diving in SE FL, solo drift diving. For once, I'm with @CuzzA, I have no problems surfacing with around 300 psi

 

[John C. Ratliff]

…I’ll give you the very quick version of CAT: Use average consumption for two people (0.75 cu ft/min x 2), multiply that by the average depth from the bottom (so for 100 feet that’s 2.5 atmospheres), and then multiply that by the time it will take you to surface: One minute to work a problem, then 10 feet a minute to get up, so for 100 feet it’s 11 minutes. that would get the cubic feet you need to make it to the surface. Figure out what that pressure is, and make sure you leave the bottom with at least that much gas in your tank… (emphasis added, jct)

100 ft / 33 ft/atm = ~3 atm
If used in a calculation to determine the volume of air, I think you need to use the absolute atmosphere, so that you can get the volumes correct. For instanec, at 33 feet (10 meters) a diver is at 2 atmospheres absolute, and the volume is 1/2 the surface volume. At 66 feet, that would be 1/3 the volume; at 99 feet (30 meters) that would be 1/4 the volume. This also means that from 99 feet the air in a given space will expand to 4 times it’s original volume. Using absolute pressures makes the statements more clear.

I like your post, and appreciate your explaining what was happening.

SeaRat

 

[tmassey]

Notice that my post said average atmospheres, which indeed is 2 1/2. And yes: we need “ata”: absolute atmospheres. We need to get down to the volume of gas we are drawing on average with each breath.

I did also say it was the very quick version.

ETA: let me be explicit about this. No one should calculate safety critical dive parameters based on some random guy’s offhand post on the inter-webs…. Just don’t.