How much air is 500 psi?

[MikeS]

The 500 psi safety margin seems to pop up all over the place. I’ve not heard a good description of where that number came from. It does not seem logical that 500 psi would be appropriate for all tank types and sizes. So I asked myself; “self, how much air is 500 psi?”

The ultimate goal would be to know how much ‘breathing time” was in a 500 psi margin. In order figure that out I need to know the volume of air and the air consumption rate.

First to the volume of air. What would be the volume of the air contained in the tank at 500 psi, at normal atmospheric pressure (sea level) or 1 ATA. Lets look at an aluminum 80. It cannot actually hold 77.4 cubic feet, or it would be enormous, it holds the volume of air at 3000 psi that would be 77.4 cubic feet at 14.7 psi. From that we can calculate the volume of the tank to be about 0.38 cubic feet. (0.38 cf at 3000 psi = 77.4 cf at 14.7 psi) Someone please help me out here if this isn’t correct. So at 500 psi the tank holds the equivalent of 13.6 cubic feet at 14.7 psi. (0.38 cf at 500 psi = 12.9 cf at 14.7 psi)

To find out the breathing time we need to take into consideration the effect depth has on volume. Say we’re at a safety stop at 15 fsw. The pressure is approximately 1.5 ATA ((15/33)+1).

We also need to estimate a surface air consumption rate, this depends on the level of activity as well as the level of stress and will vary from person to person. For the sake of this exercise I will use the 0.5 cfm.

In conclusion 500 psi in an aluminum 80 would provide somewhere in around 17 minutes of air at 15 fsw. (12.9 cf /1.5 ATA / 0.5 cfm = 17.2 min).

If anyone can show me where I’ve gone wrong or confirm my calculation it would be greatly appreciated.

Thanks,
Mike

Edited to eliminate error casued by rounding tank volume from 3.8 to 4 cubic feet.

 

[Walter]

You are making this way too complicated.

3000/500 = 6
77.4/6 = 12.9

If you are using a standard steel 72 the numbers are:
2475/500 = 4.95
71.2/4.95 = 14.4

for my slim 72's the numbers are:
3300/500 = 6.6
71.4/6.6 = 10.8
since mine are twins, I actually have 21.6 cu ft at 500 PSI.

 

[redacted]

I saw an explanation somewhere that suggested an SPG could be a couple hundred psi off.

I try very hard to balance that 'rule' with my personal RULE that when I inhale on the regulator, I get gas. So far, so good.

 

[MikeS]

Walter,

Thanks for pointing out the easy way to get there. I edited the original post to reduce the error caused by rounding of the tank volume.

Mike

 

[mddolson]

Mikes wrote:For the sake of this exercise I will use the 0.5 cfm.

In conclusion 500 psi in an aluminum 80 would provide somewhere in around 18 minutes of air at 15 fsw. (13.6 cf /1.5 ATA / 0.5 cfm = 18.1 min)

Consider this Mike, at 15 feet your consumption rate is .75 cf/min.
However if your buddy runs out of air, (because he was stupid, forgot to watch his gauge, or had a free flow, whatever) you now have to support two divers. If he's anxious his consumption could easily by 1 or 1.5 cfm, so now you're using air at 2 to 2.25 cfm.

3 minutes on the safety stop = 6.8 cuft and drops your pressure to about 250 psi (half) .
You now need to get to the surface.
This will take another half minute at 30 ft/min. = 1 cu ft or down to about 200 psi. Your regulator will becomse stiff to breath when tank pressure drops below the intermediate pressure of 140 psi. So is 60 psi OK for any surface swim, or any other possible problems.

Mike D

 

[JimC]

Remeber, that many people forget. Its taught to be on the boat with 500psi, not turn the dive at 500psi. If your back on the boat with 500psi, from a 120 foot dive.. you probably turned the dive around 1000-800psi anyway. But, many people beleive that the 500psi is when they start to head up, not when they surface.


Working off that: 500psi is not a good mesure for your turn psi.

It 'works', for the most part, for your typical recriational dive with an AL 80. But then each dive should be diferent, and part of dive planning should include setting this number. Remeber, that gas is not just for you, its for your buddy aswell.

First figure your worst case situation. For a rec dive, at rec dive limits. Your at 130 feet, within NDL and suffer major gas failire and loose your supliy of gas. You and your buddy have to safly accend, preform a stop on your buddies gas. Thats what this 500psi is for, which unfortunatly is likely not enough at the limits.

All sorts of other factors play into this. Increased respiration from stress, current, etc will all add to this.

I figure it out by taking a SAC of 1.0 (added safty margin, both my buddy and I are under 0.5) doubling it to 2.0. Then work it back in steps.


3 minutes at 15 feet costs (2*1.5*3) 9cf

accent from 60 feet costs me 2 minutes, calculated at 60 feet for a bit more safty margin (2*3*2) 12cf (1foot/2sec)

accent from 90 feet to 60 feet costs 4cf (1foot/sec)

accent from 120 feet to 90 feet costs 5cf


So, my turn psg on an AL 80 for a dive to 120 feet is 30cf or about 1100psi. Granted that has a LOT of safty margin built in, but I for one would never want to have to try and pull it off on 500psi.

 

[MikeS]

Mike D

Good points. I think the key is that while I could probably get by blindly following the 500 psi rule it’s important to think about what it actually means in terms of “air time.” By doing so I will be better prepared to make intelligent choices during a situation as the one you describe. I think it would be better to make a two-minute safety stop and ascend slowly than make a three-minute safety stop and have to make an emergency ascent.

Mike

 

[Scubaroo]

I like how PADI (and probably others) teach 500psi in the US, and 50 bar in metric countries. 50 bar is about 700psi. That means they're teaching people in metric countries to surface with 40% more air than US divers.

 

[anni]

Yup that's true, here in Australia the golden rule is back on the boat with 50 bar, start up at 70 bar (most dives round here are 15m or so).

 

[mddolson]

MikeS :"it’s important to think about what it actually means in terms of “air time.”

I agree, You're making sene Mike.
I personally use an air integrated computer (Suunto Cobra)
and it calculates remaining air time based on current depth and rate of breathing and remaining tank pressure. This time is to the 500 psi reserve.

Dive safe

Mike D