Max Depth vs. Cylinder Capacity

[elmer fudd]

Here's the tricky, your supposed to do it before you get in the water. If you're gonna go way into it you do your buddies too, calculating reserve in case they have to help you then you have your turn pressures. I know crazy.

But that's the part that doesn't make a lot of sense. On the surface we can take into account each others relative SAC rates and the nature of the dive and plan things out better than that. As a quickie way to make sure you don't run out of air on a dive that doesn't go according to plan, then, yes I can see using that method.

 

[sea2summit]

But that's the part that doesn't make a lot of sense. On the surface we can take into account each others relative SAC rates and the nature of the dive and plan things out better than that. As a quickie way to make sure you don't run out of air on a dive that doesn't go according to plan, then, yes I can see using that method.

Okay Check it out, lets say you have a buddy team diving AL80s to 120' with a SAC of .7.

-Now through some we can calculate under ideal conditions it will take us 4 min to go from 120' to the surface at a 30'/min ascent rate.

-Next we can calculate some basic stuff breaking our trip into ATA so from 120'-99' we'll call it 4 ATA so 5ATA x .7SAC=3.5cft/min, next 99'-66' 4ATA x .7SAC=2.8cft/min, next 66'-33' 3ATA x .7SAC=2.1cft/min, last 33'-surface 2ATA x .7SAC=1.4cft/min. So all that together is 9.8cft needed to surface (yes that's rough math but I could do it on a rocking dive boat).

-Okay so my AL80 has 3000psi so 3000psi/80cft=37.5psi/cft

-So the gas I need to get me from 120' to the surface is 37.5psi/cft x 9.8 cft = ~368psi in my tank + 175 so my reg will give me gas so lets call it 550psi I need to be no crap headed to the surface.

-So now my buddies reg free flows all his gas before he gets my attention to tell me to turn the dive so I need an additional ~368psi for him so now we are at ~900psi turn pressure.

Not to hard right? For sure don't want to be doing that math when I have 1500psi remaining and I'm at 120'.

 

[peterbj7]

I haven't tried to fathom out that last post! But please remember one thing - your SAC is not constant or anywhere near. When you're working hard, or stressed, it can easily rise by a factor of 7 or more, something that has caught out too many would-be rescuers and untrained public safety divers. SAC as being discussed in this thread is a guide only, subject to several major constraints.

 

[dumpsterDiver]

Rule of Thumb?

Today I wore a 149 cu-ft tank pumped to it's rated pressure of around 3,600 psi. First dive: quick drop to around 160 feet, shot a nice grouper, grabbed it by the gills and headed up immediately with no safety stop. Didn't check my watch or computer, but got on the boat with 3,000 psi. Took about a 25 minute surface interval.

Dropped down on the same wreck for another look around, max depth around 180 ft; average was maybe 165 ft, didn't check my bottom time, but it was probably around 9-10 minutes. Didn't shoot any fish and computer spanked me much harder than I expected, I never do deep bounces like that. Wound up 500 feet of line on the float reel on ascent and by the time I got to 20 feet still had like 27 minutes of deco. Spent a little time breathing harder than normal on the hang trying to shoot a pelagic fish.

Finished the deco and ascended immediately. Had around 1000 psi left,

A few hours later did a quick drop to around 74 feet with remaining air. Did about 15-18 minutes on bottom I guess, had a lot more no deco time and did a slow ascent and 3 minutes safety stop. Got on boat with around 300 psi.

 

[StreetDoctor]

I haven't tried to fathom out that last post! But please remember one thing - your SAC is not constant or anywhere near. When you're working hard, or stressed, it can easily rise by a factor of 7 or more, something that has caught out too many would-be rescuers and untrained public safety divers. SAC as being discussed in this thread is a guide only, subject to several major constraints.

:shocked2::shocked2::shocked2:

Lamont at some point you just have to give up... :shocked2:

 

[Scott L]

Rule of Thumb?

Today I wore a 149 cu-ft tank pumped to it's rated pressure of around 3,600 psi. First dive: quick drop to around 160 feet, shot a nice grouper, grabbed it by the gills and headed up immediately with no safety stop. Didn't check my watch or computer, but got on the boat with 3,000 psi. Took about a 25 minute surface interval.

Dropped down on the same wreck for another look around, max depth around 180 ft; average was maybe 165 ft, didn't check my bottom time, but it was probably around 9-10 minutes. Didn't shoot any fish and computer spanked me much harder than I expected, I never do deep bounces like that. Wound up 500 feet of line on the float reel on ascent and by the time I got to 20 feet still had like 27 minutes of deco. Spent a little time breathing harder than normal on the hang trying to shoot a pelagic fish.

Finished the deco and ascended immediately. Had around 1000 psi left,

A few hours later did a quick drop to around 74 feet with remaining air. Did about 15-18 minutes on bottom I guess, had a lot more no deco time and did a slow ascent and 3 minutes safety stop. Got on boat with around 300 psi.

If I did not live here and witness such dives taking place I would guess you were making all this up. But seriously DD, is this something you really feel should be posted???

 

[StreetDoctor]

If I did not live here and witness such dives taking place I would guess you were making all this up. But seriously DD, is this something you really feel should be posted???

The scariest thing is that it says instructor under his name. I was kinda hoping he was joking but after seeing your post and his other post...

Kinda sad really.

 

[TSandM]

Good luck getting her to pack doubles

Oh yeah?

It IS a very good point that the diver with the highest gas consumption should be the one who sets the gas reserves. But the safety factors we use are sufficiently generous that it really doesn't come into play in recreational diving. When using dissimilar tanks, the gas reserve needs to be calculated for each tank separately.

 

[Akimbo]

Oh yeah?

...

I stand corrected. Perhaps I should have said "good luck getting her to do anything that wasn't her idea"? Good thing my wife isn’t on this forum.

 

[elmer fudd]

Okay Check it out, lets say you have a buddy team diving AL80s to 120' with a SAC of .7.

-Now through some we can calculate under ideal conditions it will take us 4 min to go from 120' to the surface at a 30'/min ascent rate.

-Next we can calculate some basic stuff breaking our trip into ATA so from 120'-99' we'll call it 4 ATA so 5ATA x .7SAC=3.5cft/min, next 99'-66' 4ATA x .7SAC=2.8cft/min, next 66'-33' 3ATA x .7SAC=2.1cft/min, last 33'-surface 2ATA x .7SAC=1.4cft/min. So all that together is 9.8cft needed to surface (yes that's rough math but I could do it on a rocking dive boat).

-Okay so my AL80 has 3000psi so 3000psi/80cft=37.5psi/cft

-So the gas I need to get me from 120' to the surface is 37.5psi/cft x 9.8 cft = ~368psi in my tank + 175 so my reg will give me gas so lets call it 550psi I need to be no crap headed to the surface.

-So now my buddies reg free flows all his gas before he gets my attention to tell me to turn the dive so I need an additional ~368psi for him so now we are at ~900psi turn pressure.

Not to hard right? For sure don't want to be doing that math when I have 1500psi remaining and I'm at 120'.

Actually, although I can work the numbers myself, I don't usually bother because different people have different SAC rates and carry different sized tanks. What my buddies and I will usually do is agree on a maximum depth and a turn pressure before the dive. Since we've probably done over 100 dives together we have a pretty good idea of how much PSI we use up ascending from a given depth. Double it to account for an OOA buddy and then throw in a fudge factor too. For a dive to 120' using a 119, I would likely turn at 1500 PSI. Probably not a problem though since NDL's are pretty short at that depth and my computer would likely be going into the red before that.