ending pressure: different size and pressures

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smoothfin

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Messages
51
Reaction score
1
Location
Port Washington WI (USA)
# of dives
500 - 999
Maybe someone can give me some feedback on these calcs, or some insight into how you personally determine this: if you want to end your dive with a certain amount air in your tank, then you have to take into account the amount of air the tank holds and its psi.

example:
If a diver using an 80 cf tank at 3000 psi always ends the dive and begins ascending when his or her tank pressure reads 800 psi, then the tank at that point should be holding about 21.3 cf of air, using this calc:

(80/3000) * 800 = 21.3333

now if the diver switches to an 85 cf tank at 2640 psi, and wants to end the dive and begin ascending when the tank holds the same amount of air, then he or she should end the dive when the psi is 663 (rounded up), based on this calc:

(2640/85) * 21.3333 = 662.5872

(you'd probably round that 663 up to 700 to make it easier to read on a pressure gauge)
 
You've got the calcs right, you're not leaving a lot of reserve though.

In advanced diving you'd plan to dive X feet for Y minutes and know that R = Rate of breathing or RMV which is measured in cft/min. I use 1 as a planning RMV.

Calculate R * T = CuFt required for dive time * 1.5 (Rule of Thirds) = Total to carry
 
You have the math right and more importanlty you recognize that a certain psi in one tank will not represent the same volume in another type of tank. Many divers never even figure that out.

As to whether a certain reserve is enough gas or not depends on the specific situation. I would argue thaqt 500 psi is more than adequate in many recreational situations (shallow no deco dives in non over head environments with ample buddy support) and I would also argue that in some situations a 1/3 reserve may be far less than adequate (deep deco dive in an overhead environment with a hoover buddy that you may have to support on the way to the surface or first deco stop and/or in the event you lose one or more of your deco gasses and have to complete the deco on back gas.)

The important thing is that you understand the relationship between psi and volume and can work the problem either way to identify what psi you need to retain a certain volume in reserve.
 
smoothfin:
example:
If a diver using an 80 cf tank at 3000 psi always ends the dive and begins ascending when his or her tank pressure reads 800 psi, then the tank at that point should be holding about 21.3 cf of air, using this calc:

(80/3000) * 800 = 21.3333

Just remember an AL-80 is actually a 77cf tank. I believe stamps on all other tanks indicate the real capacity at rated pressure.
 
Make it easy on yourself. A tank rated as 80 (77) cu ft at 3000 psi holds 2.5 cu ft/ 100 psi. 77/80 * 100 = 2.56

My double 104's come to just under 8 cu ft/ 100 psi and my wifes double 100's are about 6 cu ft / 100 psi.

You can easily memorize these figures for the tank sizes that you and a buddy use (or jot some common tank sizes down in your wet notes for quick reference) and quickly do your gas matching in your head....in the water (especially handy when the whether is hot and the water is cold).
Obviously it's nice when every one can use the same tanks but my wife wants nothing to do with my big 104's and I hate her HP 100's

simplify, simplify, simplify!
 
DA Aquamaster:
You have the math right and more importanlty you recognize that a certain psi in one tank will not represent the same volume in another type of tank. Many divers never even figure that out.

As to whether a certain reserve is enough gas or not depends on the specific situation. I would argue thaqt 500 psi is more than adequate in many recreational situations (shallow no deco dives in non over head environments with ample buddy support) and I would also argue that in some situations a 1/3 reserve may be far less than adequate (deep deco dive in an overhead environment with a hoover buddy that you may have to support on the way to the surface or first deco stop and/or in the event you lose one or more of your deco gasses and have to complete the deco on back gas.)

The important thing is that you understand the relationship between psi and volume and can work the problem either way to identify what psi you need to retain a certain volume in reserve.

The important thing is that you reserve enough for you and a budy to get back, up or out whichever is appropriate.

With identical sized tanks it's just a matter of determining turn/ascent pressure.
With different sized tanks, you calculate the turn/ascent pressure (could be thirds quarters sixths or whatever) for the small tanks and then the person with the larger tanks limits themself to a volume equal to that as usable prior to turn.

When done that way you are assuming that the same amount of gas will be used by each diver on the way out as on the way in but it doesn't matter that one diver consumes more than the other. Compensation is built in because each diver is allowed the same amount. When it's gone, you turn and as long as no one cheats on the turn pressure ther will be enough to get them out if you need to share. Of course if there is trouble and you're sharing gas, breathing rates may very well go up and swimming speed may be different which is one reason that 3rds is the most liberal gas plan used for overhead dives.

The gas needed for ascents and decompression is different in that the ascent is time limited and not necessarily the same speed (almost always slower) as the descent so breathing rates do come into play. The GUE rock bottom that's taught in the DIRF for recreational no-stop diving assumes that 1 cu ft/minute is adequate for each diver even under stress. That's less than some divers (a minority) will use but the stops they have figured in are just safety stops so they could be skipped and you could bump up the ascent speed a little. In either case, for this reason, the back gas needed for ascent/decmpression is backed out prior the turn volume calcs (3rds or whatever) on overhead dives.

Why worry about all this for recreational dives? Because some recreational dives are still deep enough to require a significant amount of gas for the ascent and in some cases it's very desireable (even mandatory) to get back to the entry point before ascending (maybe because of heavy boat traffic or something. The techniques for making sure that there's enough gas to do that is exactly the same as it is for a big cave dive or a deep staged decompression dive only without as many stops to calc individually (thank goodness for software).
 
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