Sac Rating help.
- Thread starter HybridDiver
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Surface Air Consumption
rahuck
Contributor
You take your SAC and adjust for depth -- all part of gas management and pre-planning a dive. PM NWGratefulDiver -- he has a great packet of information on gas management he will probably email you.
Many OW students and new divers are told/taught to be back at the surface with 500 psi left in the tank. However, they are not taught how to determine that tank pressure at which point they need to start the ascent, at slow, controlled rate plus a safety stop (and deep stopGarrobo:
Here is a quick, concise primer for SAC and why you'd want to care about it. The bottom of that page gives additional links for info/calcs on SAC.
Rainer
Contributor
Garrobo:
Is this how you PLAN your dives? Golly, I'll just dive till I can't breathe anymore.
Probably so, but just in case...and should anyone else not know (I'm sure there are plenty that weren't taught this in OW class).
On the surface at sea level we are at 1 ATA pressure. 60feet's link suggests we breathe about 1 cf of air per minute here (a little much, that would mean an AL80 would only last us 77 minutes on the surface). When we descend to 33' (aka 2 ATA pressure) we now need twice as much air to expand our lungs the same amount. So at the same breathing rate, we will need 2 cf of air per minute or 38.5 minutes on an AL80 at that depth (no reserve left). At 66 feet, we would need 3 cf per minute or 25.66 minutes on an AL80.
Are you following this? This explains why we breathe through our tanks so much faster at depth than in shallower waters.
Some of us actually plan our dives. We don't just splash an come up when we hit a certain pressure. Some of us dive sites where getting to certain locations (such as in a cave) requires planning how much gas we need to bring with us. We plan this using our SAC/RMV rates. The 500psi rule is a bad one. As a new diver if you're not planning your dives, to include your air consumption, you're not diving very safely.
On the surface at sea level we are at 1 ATA pressure. 60feet's link suggests we breathe about 1 cf of air per minute here (a little much, that would mean an AL80 would only last us 77 minutes on the surface). When we descend to 33' (aka 2 ATA pressure) we now need twice as much air to expand our lungs the same amount. So at the same breathing rate, we will need 2 cf of air per minute or 38.5 minutes on an AL80 at that depth (no reserve left). At 66 feet, we would need 3 cf per minute or 25.66 minutes on an AL80.
Are you following this? This explains why we breathe through our tanks so much faster at depth than in shallower waters.
Some of us actually plan our dives. We don't just splash an come up when we hit a certain pressure. Some of us dive sites where getting to certain locations (such as in a cave) requires planning how much gas we need to bring with us. We plan this using our SAC/RMV rates. The 500psi rule is a bad one. As a new diver if you're not planning your dives, to include your air consumption, you're not diving very safely.
:shakeheadGarrobo:
... so does having less gas than needed for the dive
Yes, it's the one of those little data points we anal planners use to have a pretty good idea of how long we can safely plan to stay down on any particular dive profile with any particular gas supply.Garrobo:
If you don't care to know in advance, don't bother with it.
But as Brother Dave Gardner used to say, "don't knock it 'til you've tried it."
E.
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