max depth with an 80 cuft. tank

[MXGratefulDiver]

I already commented about SAC rates in an earlier post when I considered 1 cfm pessimistic, but based on the even more pessimistic scenario you just presented with not just one but 2 divers somehow managing to consume 35psi normally and 105psi under stress (!!!!), this buddy pair should not even be thinking about planning a dive to 130' on an AL80 until they get a lot fitter and a lot more comfortable in the water.
In fact they really need to find another more relaxing hobby.

Fitness is only one factor in air consumption. Stress is the real concern. As someone who built gas management exercises into my AOW curriculum, I've seen divers ... especially newer ones who are still working out their technique ... quadruple their "norrmal" consumption rates once they were stressed.

Furthermore, to "assume" that someone who's stressed out and relatively unskilled is going to make a routine ascent, following recommended ascent rates, isn't realistic ... that only ever really happens on the Internet. In real life, it may take them a while just to get squared away enough to begin an ascent ... particularly if the problem involves something confusing, like a free-flow that is going to have them attempting a valve shutdown just so they can manage to see each other through the air bubbles.

A direct ascent from 130 feet ... if it's done safely ... will take several minutes (exact minutes will depend on which agency standard you are TRYING to follow) ... and if it's not something you're used to doing, expect a bit of acceleration, deceleration, possibly even sinking a bit as you try to slow yourself down ... ESPECIALLY if you're attempting it while clutching your dive buddy's BCD strap in your right hand.

It's not the "simple" process that most people imagine ... particularly if you've never tried it before. Add in some additional factors like cold water, poor vis, unfamiliar equipment ... things that typical recreational divers the world over have to face from time to time ... and you've got a much more complicated scenario than the textbook claims you should have.

Which is why you want to leave adequate reserves to deal with these problems ... because until you're actually faced with them you have no way of knowing how you're going to respond ... and for damn sure you don't want to find out halfway to the surface that you got it wrong ...

... Bob (Grateful Diver)

 

[miketsp]

Fitness is only one factor in air consumption. Stress is the real concern. As someone who built gas management exercises into my AOW curriculum, I've seen divers ... especially newer ones who are still working out their technique ... quadruple their "norrmal" consumption rates once they were stressed.

Furthermore, to "assume" that someone who's stressed out and relatively unskilled is going to make a routine ascent, following recommended ascent rates, isn't realistic ... that only ever really happens on the Internet. In real life, it may take them a while just to get squared away enough to begin an ascent ... particularly if the problem involves something confusing, like a free-flow that is going to have them attempting a valve shutdown just so they can manage to see each other through the air bubbles.

A direct ascent from 130 feet ... if it's done safely ... will take several minutes (exact minutes will depend on which agency standard you are TRYING to follow) ... and if it's not something you're used to doing, expect a bit of acceleration, deceleration, possibly even sinking a bit as you try to slow yourself down ... ESPECIALLY if you're attempting it while clutching your dive buddy's BCD strap in your right hand.

It's not the "simple" process that most people imagine ... particularly if you've never tried it before. Add in some additional factors like cold water, poor vis, unfamiliar equipment ... things that typical recreational divers the world over have to face from time to time ... and you've got a much more complicated scenario than the textbook claims you should have.

Which is why you want to leave adequate reserves to deal with these problems ... because until you're actually faced with them you have no way of knowing how you're going to respond ... and for damn sure you don't want to find out halfway to the surface that you got it wrong ...

... Bob (Grateful Diver)

Maybe quadruple from a normal level, surely not quadruple from an already abnormal level.
I'm aware that SACs go up when people get stressed which is why I put as much stress on being comfortable in the water as being fit.
What I find hard to believe are the figures being quoted here. Did you do the conversion into normal SAC units?
105psi/min on an AL80 is over 80 liters/min or 2,85cfm!
I've been in situations where I've had a blown cylinder/1st stage o-ring at depth, I've also been in confusing situations like where my buddy's reg came unscrewed from the lp hose also creating an enormous cloud of bubbles and I've had to assist OOA divers. In none of these situations did we come anywhere near these levels of consumption and I doubt if the adrenaline peak lasted more than 30 seconds.
Once the diver with a problem had access to a new air supply things calmed down very quickly.

 

[MXGratefulDiver]

Maybe quadruple from a normal level, surely not quadruple from an already abnormal level.
I'm aware that SACs go up when people get stressed which is why I put as much stress on being comfortable in the water as being fit.
What I find hard to believe are the figures being quoted here. Did you do the conversion into normal SAC units?
105psi/min on an AL80 is over 80 liters/min or 2,85cfm!
I've been in situations where I've had a blown cylinder/1st stage o-ring at depth, I've also been in confusing situations like where my buddy's reg came unscrewed from the lp hose also creating an enormous cloud of bubbles and I've had to assist OOA divers. In none of these situations did we come anywhere near these levels of consumption and I doubt if the adrenaline peak lasted more than 30 seconds.
Once the diver with a problem had access to a new air supply things calmed down very quickly.

You're applying your own consumption rates to someone who's probably got much less experience than you do. Perhaps you should recall what your consumption rate was when you started diving ... I know that mine was very much higher than it is now.

105 psi/minute SAC for two stressed-out divers breathing off the same reg isn't unrealistic. Keep in mind that his measured, "calm" SAC was 35 psi/minute ... which is fairly high (about 0.9 CFM), but not abnormally so for a newish diver. I have measured several new divers in excess of 1.0 CFM for what would be considered a "normal" dive. For stressed out, or "working" SAC, you can typically double that ... which in this diver's case would give you 70 psi/minute (1.8 CFM) just for the one diver. Add in a buddy breathing off your reg who's equally stressed out and you're probably being optimistic at 105 ... it's likely going to be higher than that.

... Bob (Grateful Diver)

 

[UCFKnightDiver]

I think the SAC rate that I used a few posts back was at least reasonable miketsp.

 

[Cave Diver]

You have about 10 minutes to make it to the surface. HP hose does not flow much air since most regs use a pinhole oricice for the HP port

This is correct. But, the HP hose I lost recently was an o-ring failure. I lost 1k air in less than a minute.

Interesting. My DiveRite regs have pinhole orifices machined into the HP port of the first stage body. Even if I blew an o-ring on the HP hose it still can't flow much gas due to the orifice restriction. I always assumed other regs were similar.

 

[Cave Diver]

Oh, so we don't drown but now we're bent?

Bent is (usually) fixable. Dead (usually) aint.

 

[Gombessa]

Bent is (usually) fixable. Dead (usually) aint.

Unless you subscribe to the "drown and resuscitate" school of air sharing

 

[miketsp]

..snip..

105 psi/minute SAC for two stressed-out divers breathing off the same reg isn't unrealistic. Keep in mind that his measured, "calm" SAC was 35 psi/minute ... which is fairly high (about 0.9 CFM), but not abnormally so for a newish diver. I have measured several new divers in excess of 1.0 CFM for what would be considered a "normal" dive. For stressed out, or "working" SAC, you can typically double that ... which in this diver's case would give you 70 psi/minute (1.8 CFM) just for the one diver. Add in a buddy breathing off your reg who's equally stressed out and you're probably being optimistic at 105 ... it's likely going to be higher than that.

... Bob (Grateful Diver)

Reread his post. He's talking about 105 for EACH diver not for the pair:
"assumptions: are heart rate is up, our consumption is up. Worst case lets say our total consumption for both divers now is triple or 105 psi sac rate x 2= 210 psi sac rate."

 

[miketsp]

I think the SAC rate that I used a few posts back was at least reasonable miketsp.

As you already commented in post #73, the SAC of 1cfm that you used in post #46 may be a reasonable conservative value and is probably a much better value to use by inexperienced divers.
It's certainly much more reasonable than a value of 2,85cfm suggested by dktexas54.

But in answer to Bob's comment
"You're applying your own consumption rates to someone who's probably got much less experience than you do. Perhaps you should recall what your consumption rate was when you started diving ... I know that mine was very much higher than it is now."

By the time I started diving below 100ft my SAC was already down to much better levels than are being quoted on this thread.

 

[Superlyte27]

Interesting. My DiveRite regs have pinhole orifices machined into the HP port of the first stage body. Even if I blew an o-ring on the HP hose it still can't flow much gas due to the orifice restriction. I always assumed other regs were similar.

Just checked my oceanic regulators. Dive Rite is a little ahead at least in this area. The holes in my HP regulator is about the size of a bic pen tip. It will absolulely empty a tank quickly. I blew my wetsuit from across the garage just now.