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mitchy
04-17-2010, 12:39
My friend just bought a used AL63 tank. How much air will he have compared to an AL80?

Does he have 3/4 the amount? Will it last 3/4 as long?
Assuming at a particular depth an Al80 lasts him 25 mins (to say 800psi), what can he expect to get out of it time wise?

Rileybri
04-17-2010, 13:04
Well he will have 17 cf less gas than with his 80 that is for sure. I would gess he will use it much more rapidly as he is more focused on his smaller cylinder draining on him at depth unexpectedly fast, and causing an OOA thus causing even more hyperventilation. :smiley36:

DivingCRNA
04-17-2010, 13:13
The 63 means 63 cf at it's service pressure. The 80 means 80 cf at it's service pressure.

They aren't covering this kind of thing in open water any more?

inventor
04-17-2010, 13:21
The 63 means 63 cf at it's service pressure. The 80 means 80 cf at it's service pressure.

They aren't covering this kind of thing in open water any more?

Nope:smiley5:

mitchy
04-17-2010, 14:06
The 63 means 63 cf at it's service pressure. The 80 means 80 cf at it's service pressure.

They aren't covering this kind of thing in open water any more?

Thanks for the sarcasm. I do understand there is 17 cf less; I guess I incorrectly worded that past of my question. I am unsure exactly how volume relates to pressure. When I start a dive with 3000 psi, and start an ascent at say 800 psi, does that mean I have used 22/30 of my air, or is pressure not related this way to volume?

DevilDiver
04-17-2010, 14:06
The 63 means 63 cf at it's service pressure. The 80 means 80 cf at it's service pressure.

They aren't covering this kind of thing in open water any more?

What's Open Water? Is that like E-Learning?

DivingCRNA
04-17-2010, 14:13
The 63 means 63 cf at it's service pressure. The 80 means 80 cf at it's service pressure.

They aren't covering this kind of thing in open water any more?

Thanks for the sarcasm, although I notice you weren't able to answer the questions I asked. I do understand there is 17 cf less; I am unsure exactly how that relates to pressure. When I start a dive with 3000 psi, and start an ascent at say 800 psi, does that mean I have used 22/30 of my air, or is pressure not related this way to volume?

I can answer. It means he has 17 cf less of gas in his tank than in an AL 80 and needs to plan accordingly. 3000 psi does not always mean 80 cf. You breath gas in volume, not pressure. The volume you breath varies with SAC and depth.

I simply do not have time today to describe all the details of gas management to you. My guess, however, is that your "friend" does not have nearly a big enough tank with an AL63 if they only get 25 minutes out of an AL80.

mitchy
04-17-2010, 14:20
The 63 means 63 cf at it's service pressure. The 80 means 80 cf at it's service pressure.

They aren't covering this kind of thing in open water any more?

Thanks for the sarcasm, although I notice you weren't able to answer the questions I asked. I do understand there is 17 cf less; I am unsure exactly how that relates to pressure. When I start a dive with 3000 psi, and start an ascent at say 800 psi, does that mean I have used 22/30 of my air, or is pressure not related this way to volume?

I can answer. It means he has 17 cf less of gas in his tank than in an AL 80 and needs to plan accordingly. 3000 psi does not always mean 80 cf. You breath gas in volume, not pressure. The volume you breath varies with SAC and depth.

I simply do not have time today to describe all the details of gas management to you. My guess, however, is that your "friend" does not have nearly a big enough tank with an AL63 if they only get 25 minutes out of an AL80.

25 mins at 150 feet is okay, isn't it?

DMWiz
04-17-2010, 14:44
Actually most AL80 with a 3000psi fill have 77 cubic feet of gas, where a 63 filled to 3000psi has 63 cubic feet of gas.

ianr33
04-17-2010, 14:52
My friend just bought a used AL63 tank. How much air will he have compared to an AL80?

Does he have 3/4 the amount? Will it last 3/4 as long?
Assuming at a particular depth an Al80 lasts him 25 mins (to say 800psi), what can he expect to get out of it time wise?

Well,in your example he used 2200/3000*80 = 59 cu ft of gas

When the Al63 is down to 800 psi he will have used 2200/3000*63 =42 cu ft

That will take 42/59 * 25 = 20.8 minutes.

Although the ending pressure is the same he will have less gas in reserve,maybe not enough for a safe ascent in the event of an emergency
(I'm assuming here that tanks are 3000psi service pressure and have used nominal volumes. An Al80 is really something like 77.4 cu ft at 3000psi )

And 25 minutes at 150 feet on an Al80 would need an excellent SAC rate. Maybe you can figure that out for homework :smiley2:

DivingCRNA
04-17-2010, 14:54
The 63 means 63 cf at it's service pressure. The 80 means 80 cf at it's service pressure.

They aren't covering this kind of thing in open water any more?

Thanks for the sarcasm, although I notice you weren't able to answer the questions I asked. I do understand there is 17 cf less; I am unsure exactly how that relates to pressure. When I start a dive with 3000 psi, and start an ascent at say 800 psi, does that mean I have used 22/30 of my air, or is pressure not related this way to volume?

I can answer. It means he has 17 cf less of gas in his tank than in an AL 80 and needs to plan accordingly. 3000 psi does not always mean 80 cf. You breath gas in volume, not pressure. The volume you breath varies with SAC and depth.

I simply do not have time today to describe all the details of gas management to you. My guess, however, is that your "friend" does not have nearly a big enough tank with an AL63 if they only get 25 minutes out of an AL80.

25 mins at 150 feet is okay, isn't it?

If he is carrying in the neighborhood of and hour's worth of gas for deco....

The way these questions are being posed I remember a line from Star Trek IV, The Voyage Home.

" It would be impossible to discuss the subject without a common frame of reference.":smilie39::smilie39::smilie39:

mitchy
04-17-2010, 16:10
My friend just bought a used AL63 tank. How much air will he have compared to an AL80?

Does he have 3/4 the amount? Will it last 3/4 as long?
Assuming at a particular depth an Al80 lasts him 25 mins (to say 800psi), what can he expect to get out of it time wise?

Well,in your example he used 2200/3000*80 = 59 cu ft of gas

When the Al63 is down to 800 psi he will have used 2200/3000*63 =42 cu ft

That will take 42/59 * 25 = 20.8 minutes.

Although the ending pressure is the same he will have less gas in reserve,maybe not enough for a safe ascent in the event of an emergency
(I'm assuming here that tanks are 3000psi service pressure and have used nominal volumes. An Al80 is really something like 77.4 cu ft at 3000psi )

And 25 minutes at 150 feet on an Al80 would need an excellent SAC rate. Maybe you can figure that out for homework :smiley2:

Thanks for the reply. While I do know how much each tank holds, I was unsure as to actual volume of air consumed. I wasn't sure if having 1/3 the original psi meant 1/3 of the original air volume left, although I assumed it probably was. I guess by hypothetically grabbing numbers for comparison sake I was confusing the situation. I now understand that 3/4 the air means 3/4 the bottom time as compared to the larger tank.

ianr33
04-17-2010, 17:03
Remember you still need to think about your Rock Bottom gas reserves though.
1500 psi in an 80 is 40 cu ft . That should be enough to get 2 divers safely back to the surface from 120 feet or so. (You have to figure it for 2 divers in case one has a catastrophic loss of gas)
1500psi in a 40 though is only 20 cu ft. Thats likely not enough gas for both of the divers in the example above to survive.

Its an excellent exercise to run the numbers and see what pressure you need to reserve for a safe ascent from any particular depth.
My rule of thumb,using an Al80 is to ascend when the pressure is 10 times depth +300.
e.g. from 100 feet turn pressure is 1300 psi
from 70 feet it is 1000 psi and so on.

Thats actually fairly conservative ,which is not a bad thing, and its easy to remember.

scubadiver888
04-18-2010, 08:30
My friend just bought a used AL63 tank. How much air will he have compared to an AL80?

Does he have 3/4 the amount? Will it last 3/4 as long?
Assuming at a particular depth an Al80 lasts him 25 mins (to say 800psi), what can he expect to get out of it time wise?

Mitchy,

Each manufacturer could be slightly different but here is a list of specifications for Catalina aluminum cylinders: http://www.xsscuba.com/tank_cat_alum_specs_.html. You should notice that the AL80 is actually 77.4 cubic feet (cf) of air at 3000 PSI.

The relationship between air and pressure is linear. So 77.4 / 3000 or 0.0258 cf / PSI. Therefore if your friend uses 2200 PSI (3000 - 800) for a 25 minute dive on an AL80, they used 2200 * 0.0258 cf of air or 56.76 cf of the 77.4 cf. This means they have 20.64 cf of air left. Their consumption rate is 56.76 cf in 25 minutes or normalized to 2.2704 cf / min.

An AL63 is actually 63 cf of air. Therefore 1 PSI for that cylinder is going to be 63 / 3000 or 0.021 cf / PSI.

If they want to end the dive with 800 PSI (or use 2200 PSI) it would be 2200 PSI = 46.2 cf. At 2.2704 cf / min we would have just over 22 minutes.

On the other hand, if you wanted to end the dive with 20.64 cf of air left in the cylinder then you want to end with 983 PSI (20.64 / 0.021). This means you want to use 63 - 20.64 or 42.36 cf. At a consumption rate of 2.2704 cf / min you would have 42.36 / 2.2704 or just over 18 minutes.

The rule of back on board the boat with 500 PSI is a general rule and not always the best rule. It assumes you are using an AL80. The last calculation is the safer rule. You want a certain about of time left in you cylinder. The amount of cubic feet of air and how long it will last for you is directly related. The PSI needs to be calculated for each cylinder.

Your friend ends his 25 minute dive on an AL80 with 9 minutes of air left in the cylinder. If he wants to do the same with an AL63 he will dive for 18 minutes.

mitchy
04-18-2010, 09:34
Thanks for the info. I'll pass it on, and then discourage him from using the tank when diving with me! It's the first tank he bought at a local scuba swap, and didn't realize in wasn't at Al80. He uses air faster than me, so I definitely don't want him using the smaller tank.

fire diver
04-18-2010, 14:30
Thanks for the info. I'll pass it on, and then discourage him from using the tank when diving with me! It's the first tank he bought at a local scuba swap, and didn't realize in wasn't at Al80. He uses air faster than me, so I definitely don't want him using the smaller tank.

diving with him isnt a problem. Just switch tanks, you use the 65 and he uses the 80. This should offset his higher SAC rate. Head to the surface when the first diver reaches 1000psi.

mitchy
04-18-2010, 15:48
Thanks for the info. I'll pass it on, and then discourage him from using the tank when diving with me! It's the first tank he bought at a local scuba swap, and didn't realize in wasn't at Al80. He uses air faster than me, so I definitely don't want him using the smaller tank.

diving with him isnt a problem. Just switch tanks, you use the 65 and he uses the 80. This should offset his higher SAC rate. Head to the surface when the first diver reaches 1000psi.

I had actually thought about that. We would probably be ready to surface at the same time.

Flatliner
04-18-2010, 16:15
Or he could sell me the AL63 for one of my kids to use and buy an HP100...

UCFKnightDiver
04-18-2010, 19:51
Thanks for the info. I'll pass it on, and then discourage him from using the tank when diving with me! It's the first tank he bought at a local scuba swap, and didn't realize in wasn't at Al80. He uses air faster than me, so I definitely don't want him using the smaller tank.

diving with him isnt a problem. Just switch tanks, you use the 65 and he uses the 80. This should offset his higher SAC rate. Head to the surface when the first diver reaches 1000psi.

What about gas matching? :)

ianr33
04-19-2010, 09:39
Thanks for the info. I'll pass it on, and then discourage him from using the tank when diving with me! It's the first tank he bought at a local scuba swap, and didn't realize in wasn't at Al80. He uses air faster than me, so I definitely don't want him using the smaller tank.

diving with him isnt a problem. Just switch tanks, you use the 65 and he uses the 80. This should offset his higher SAC rate. Head to the surface when the first diver reaches 1000psi.

What about gas matching? :)

Whats that ? :smiley36:

Lets look at the numbers:

1000 psi in an Al 63 is 1000/3000*63 =21 cu ft

Now lets think about these 2 divers being at 100 feet ,both tanks are down to 1000 psi and they are just about to start ascending when a reg freeflows/hose bursts/Oring pops etc.

Divers will be rather excited. Their SAC goes to 1.0 (could actually go a lot higher but lets say 1.0 )

They take a minute to figure out the problem and share air.Thats going to take;
1.0 (SAC rate) X 4 (ATM) X 2 (Number of divers) = 8 cu ft gas.

Being well trained they start to make a perfect 30 ft/min ascent to the surface. That takes just over 3 minutes (100/3)

Average pressure along the way is (4 +1)/2 = 2.5ATM

the Ascent will use:
1.0(SAC Rate) X 3 (Time to Ascend)X2.5 (Av.ATM)X2 (Number of Divers )
= 15 cubic feet

Total gas used to sort problem and ascend =8 +15 = 23

Gas remaining on surface MINUS 2 cubic Feet !!!!!!!!!

In real life the SAC rate could go well over 1.0.( The ascent rate could go too fast (maybe get bent) or too slow (run out of gas even earlier)

If the dive is only to 30 feet then no problem,but just saying "ascend at 1000psi" does not cut it in my book.

fire diver
04-19-2010, 11:13
Thanks for the info. I'll pass it on, and then discourage him from using the tank when diving with me! It's the first tank he bought at a local scuba swap, and didn't realize in wasn't at Al80. He uses air faster than me, so I definitely don't want him using the smaller tank.

diving with him isnt a problem. Just switch tanks, you use the 65 and he uses the 80. This should offset his higher SAC rate. Head to the surface when the first diver reaches 1000psi.

What about gas matching? :)

Whats that ? :smiley36:

Lets look at the numbers:

1000 psi in an Al 63 is 1000/3000*63 =21 cu ft

Now lets think about these 2 divers being at 100 feet ,both tanks are down to 1000 psi and they are just about to start ascending when a reg freeflows/hose bursts/Oring pops etc.........

You are over-complicating this basic question. Remember, we arent talking about rock-bottom gas calcs. This was a question about tank volumes and diving times for dis-similar tanks and SAC rates.

The solution I gave is the simplist solution. Nothing here has said the affore mentioned divers are going to be at 100 feet until they hit 1000 psi. the vast majority of dives are much shallower.

Plan your dive....Dive your plan.....Make your plan SIMPLE.

UCFKnightDiver
04-19-2010, 11:17
Thanks for the info. I'll pass it on, and then discourage him from using the tank when diving with me! It's the first tank he bought at a local scuba swap, and didn't realize in wasn't at Al80. He uses air faster than me, so I definitely don't want him using the smaller tank.

diving with him isnt a problem. Just switch tanks, you use the 65 and he uses the 80. This should offset his higher SAC rate. Head to the surface when the first diver reaches 1000psi.

What about gas matching? :)

Whats that ? :smiley36:

Lets look at the numbers:

1000 psi in an Al 63 is 1000/3000*63 =21 cu ft

Now lets think about these 2 divers being at 100 feet ,both tanks are down to 1000 psi and they are just about to start ascending when a reg freeflows/hose bursts/Oring pops etc.

Divers will be rather excited. Their SAC goes to 1.0 (could actually go a lot higher but lets say 1.0 )

They take a minute to figure out the problem and share air.Thats going to take;
1.0 (SAC rate) X 4 (ATM) X 2 (Number of divers) = 8 cu ft gas.

Being well trained they start to make a perfect 30 ft/min ascent to the surface. That takes just over 3 minutes (100/3)

Average pressure along the way is (4 +1)/2 = 2.5ATM

the Ascent will use:
1.0(SAC Rate) X 3 (Time to Ascend)X2.5 (Av.ATM)X2 (Number of Divers )
= 15 cubic feet

Total gas used to sort problem and ascend =8 +15 = 23

Gas remaining on surface MINUS 2 cubic Feet !!!!!!!!!

In real life the SAC rate could go well over 1.0.( The ascent rate could go too fast (maybe get bent) or too slow (run out of gas even earlier)

If the dive is only to 30 feet then no problem,but just saying "ascend at 1000psi" does not cut it in my book.

Also if the diver that's a real hoover breathes down the 80 quickly and then has a failure, the diver with the 63 probably won't have enough gas to air share to the surface