Surface pressure

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WRONG.

30m/100ft is 4 ATA

Each 10m/33ft is one Bar/ATA plus the one Bar/ata on the surface
I already made that exact point. But, 100 ft / 33 ft/atm = ~3. The math is correct.

SeaRat
 
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),

I already made that exact point. But, 100 ft / 33 ft/atm = ~3. The math is correct.

That's not what he's saying CAT uses the average depth of the ascent for the calculation. So starting pressure (4ATA + 1ATA (surface))/2=2.5ATA average pressure for the ascent; this isn't exact but is rather a conservative and easy-to-use calculation for calculating minimum gas (rock bottom)
 
The reason for the dive operators is that these are rental systems, and if you take a rental system to zero psi, you have the chance of water inside the tank. This means a visual inspection is required in order to put the tank back into service. If you see water inside the tank, the rental regulator needs an overhaul too, so you cannot put them back immediately into service. Hence, come back with 500 psi in your tank. We (the dive operator) then knows there is no possibility that the regulator or tank has been compromised with seawater.

SeaRat
Oh certainly. I wasn't questioning the validity of the request to surface with >500 psi... and respect for the dive ops rules is something that should be drilled into every OW student. The only reason I mentioned the "every dive briefing I've heard..." was to respond to the original post, which asked about a "rule" to start a safety stop with 500 psi. Wanted to point out that there is an important difference between what the OP was suggesting and what I've heard on every dive boat.
 
Sorry, it is 100/33 +1 = 4. Atmospheric pressure + 3 atm underwater.
My post:
John C. Ratliff:
I already made that exact point. But, 100 ft / 33 ft/atm = ~3. The math is correct.

Your math is incorrect, in that you have not labeled it correctly. The correct labeling would note that this is absolute pressure, not gauge pressure. Gauges are zeroed at one atmosphere. So to make your equation correct, you would need to write it in this manner:

100 ft / 33 ft / atmosphere + 1 atmosphere absolute = 4 atmospheres absolute

'Sorry, but I'm just being old school, U.S. Naval School for Underwater Swimmers correct. The photo shows the U.S.S. divers at one atmosphere, absolute. :wink:

SeaRat
 

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That's not what he's saying CAT uses the average depth of the ascent for the calculation. So starting pressure (4ATA + 1ATA (surface))/2=2.5ATA average pressure for the ascent; this isn't exact but is rather a conservative and easy-to-use calculation for calculating minimum gas (rock bottom)
tmassey:
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 theaverage depth from the bottom (so for 100 feet that’s 2.5 atmospheres),

Okay, crofrog, who the heck is CAT? And, average depth for a max depth of 100 feet would not necessarily be 2.5 atmospheres. That, in my mind, would be dependent upon time spent at various depths. For example, if most of the time were spent at 33 feet depth, with a brief trip toward the end down to 100 feet, which I have done in my time, when my buddies were wearing twin 72s and I was wearing a twin 42 setup (our jump tanks vs twin 72s). Or, if most of the time were spent at 100 feet, then the average would be different.

SeaRat
 
Okay, crofrog, who the heck is CAT? And, average depth for a max depth of 100 feet would not necessarily be 2.5 atmospheres. That, in my mind, would be dependent upon time spent at various depths. For example, if most of the time were spent at 33 feet depth, with a brief trip toward the end down to 100 feet, which I have done in my time, when my buddies were wearing twin 72s and I was wearing a twin 42 setup (our jump tanks vs twin 72s). Or, if most of the time were spent at 100 feet, then the average would be different.

SeaRat
It’s very well explained in the YouTube video I linked

It’s GUE mnemonic for calculating minimum gas.
Consumption: default Surface consumption rate of 0.75cuft/min*2 divers
Average pressure: of the ascent (simplified): (max ata + 1ata) /2 = average ata
Time: 1minute for problem solving + 10ft/min

So for a 100ft dive
1.5cuft/min * 2.5ata * 11min = 41.25cuft round up to 42.

The average ata and ascent rate are conservative to build safety margin into the calculation.


The thing you keep missing is it’s the average for the ascent, and it’s simplified for a direct ascent to the surface. These things change once overhead (physical or deco) are involved
 
Crofrog,

Thanks for the clarification. There was nothing in the text explaining that it was an estimate for ascent only; my assumption was that it was for calculating either decompression or no-decompression limits, not the gas needed for an ascent. GUE is a very different organization, which developed its own acronyms and methods.

For me, having dived continuously since 1959, a 10 feet per minute ascent rate seems really, really slow. It’s as if the diver is not swimming at all, and doing it all by breath control.

SeaRat
 
That's not what he's saying CAT uses the average depth of the ascent for the calculation. So starting pressure (4ATA + 1ATA (surface))/2=2.5ATA average pressure for the ascent; this isn't exact but is rather a conservative and easy-to-use calculation for calculating minimum gas (rock bottom)

Crofrog,

Thanks for the clarification. There was nothing in the text explaining that it was an estimate for ascent only; my assumption was that it was for calculating either decompression or no-decompression limits, not the gas needed for an ascent. GUE is a very different organization, which developed its own acronyms and methods.

For me, having dived continuously since 1959, a 10 feet per minute ascent rate seems really, really slow. It’s as if the diver is not swimming at all, and doing it all by breath control.

SeaRat

The 10ft/min is to build in conservatism; and yes you’re not swimming up. The min deco ascent for gue has you go 30ft/min to 50% or your max depth and than 10ft/min to the surface this is normally done as 30 seconds moving 30 seconds stopped.
 

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