Has to do with the way the tanks are measured. In North America tank volume is dependant on being filled to the rated pressure (an AL80 is 77 cuft only IF filled to 3000 psi). Whereas, if you use metric your tank volume is the internal volume of the tank (if you fill it with water) and internal volume x bar = total volume. So basically your SAC is the same as your RMV (I calculate SAC as litres/minute - same with RMV) but SAC has problems with the cuft tanks because the volume (cuft) depends on the rated pressure fill (xxxx psi) which is not standard, ie. you don't know how much the total volume of your tank is unless you know its rated pressure. An 80cuft tank is 10.3 lt so if you use that instead and use your SAC as lt/min (28.3 lt/cuft) you cut out a conversion step.
Rock Bottom Pressure and Turn-Around Pressure
- Thread starter FPDocMatt
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No you don't.
You are taught to breathe from a free-flow at OW. There is easily enough gas to perform an ascent from 80ft, 1 minute 20secs is all that is required from the tank, with an ever reducing pressure. Unless you were incredibly low on air when the freeflow happens (unlikely) then the ascent is completable under their own power.
Matt:
If you are undertaking emergency ascents; Safety Stops are NOT required. That is why they are called safety stops. If your buddy is OOA then you both ascend directly at 60ft 18 pm.
Recreational diving is No-Stop diving. At any point during a dive within the No Decompression limits you are free to slowly ascend to the surface without penalty. In practice we always do safety stops for an added margin.
In no PADI course do they recommend you complete a Safety Stop in an OOA situation.
DevonDiver
N/A
Rock Bottom = Calculating sufficient gas reserve to allow a safe direct ascent whilst sharing air.
Turn Point = Calculating the point at which you need to begin travel towards the end-point of your dive. Nominally called the 'turn-point' because most dives involve a return to the starting location (thus you are turning around and going back). Doesn't matter if the start point is a boat or the shore - it's where you desire to surface/end the dive. Of particular value for dives in an overhead environment, where a return to the start-point is normally mandatory. In these cases, the turn-point is also a measure of 'rock-bottom' as you must plan for the contingency of sharing air during the return trip (as overhead precludes immediate ascent).
Turn Point = Calculating the point at which you need to begin travel towards the end-point of your dive. Nominally called the 'turn-point' because most dives involve a return to the starting location (thus you are turning around and going back). Doesn't matter if the start point is a boat or the shore - it's where you desire to surface/end the dive. Of particular value for dives in an overhead environment, where a return to the start-point is normally mandatory. In these cases, the turn-point is also a measure of 'rock-bottom' as you must plan for the contingency of sharing air during the return trip (as overhead precludes immediate ascent).
Wart, I respectfully differ, having been there. The MAXIMUM time you have is a minute and a half . . . by the time you have recognized the problem, banged on the regulator a couple of times to see if you can get the freeflow to stop, and signaled your buddy, you've probably spent more than half of that. Yes, you can breathe off a freeflow . . . but committing yourself to an ascent on that tank, when you have a buddy in front of you with all the gas in the world, is adding more urgency and risk to the situation than needs to be there.
DevonDiver
N/A
I see 'freeflow' breathing as an option that is taught - but there's no emphasis on it being the effected diver's primary or only response to the situation.
You can breath from a free-flow. Every diver must know that fact. Every diver must be able to accomplish the skill.
You must breath from a free-flow. Divers aren't taught that as a fact. Divers should be encouraged to ascertain the best available option open to them.
I had a catastrophic 'freeze' free-flow at 36m once. It occurred immediately on reaching bottom depth. It emptied a virtually full 15L cylinder before I had ascended to 15m (accounting for some time spent gaining situational awareness before ascent). I finished the ascent on my pony, with my buddy on-hand and ready to donate if necessary. The other issue I encountered was that free-flow breathing in very cold, fresh water (approx 3 celcius) actually coated my teeth with ice. That was far from ideal....and far from painless.
A diver should also account/plan for the increased stress caused by breathing from the free-flow whilst managing an ascent and maintaining buddy cohesion. In PADI classes, free-flow breathing is taught in a static position, with no added stress factors. In reality, especially with a novice diver who might otherwise be struggling with the basics - such as buoyancy control on ascent - the added stress loading is likely to cause a domino effect of resulting issues...such as rapid ascent and/or buddy separation.
Given those possibilities - I feel that air-sharing is actually a positive step towards diver safety in the first instance. The diver should also remove the free-flowing air as a distraction/stressor. That can mean holding the free-flowing second stage away at arms length, or if practicable, even shutting down the cylinder (with buddy assistance).
Also, as a general rule, I think it is better to preempt an out-of-air situation with air-sharing... rather than actually waiting until you are sucking on a dead tank before attempting to secure an alternate source of air. That applies to v.low on air scenarios as well as freeflows... especially where the depth indicates that there is no way the diver can achieve the surface before air supply is exhausted.
It seems far safer and more achievable to me that a diver should maintain control over a donated AAS on ascent, than try to gain control over a donated AAS on ascent. I've read more than a few accident reports that illustrate diver separation on ascent preceding or during an OOA emergency. Gaining physical buddy contact and access to an AAS should be an immediate priority to divers at any time they have cause for concern over their own air supply.
Again... PADI doesn't state that you cannot act proactively to secure an alternate air source, if you felt that requirement. Nor, I believe, does it specifically state that freeflow breathing is the only or even preferred method of response to a freeflow.
You can breath from a free-flow. Every diver must know that fact. Every diver must be able to accomplish the skill.
You must breath from a free-flow. Divers aren't taught that as a fact. Divers should be encouraged to ascertain the best available option open to them.
I had a catastrophic 'freeze' free-flow at 36m once. It occurred immediately on reaching bottom depth. It emptied a virtually full 15L cylinder before I had ascended to 15m (accounting for some time spent gaining situational awareness before ascent). I finished the ascent on my pony, with my buddy on-hand and ready to donate if necessary. The other issue I encountered was that free-flow breathing in very cold, fresh water (approx 3 celcius) actually coated my teeth with ice. That was far from ideal....and far from painless.
A diver should also account/plan for the increased stress caused by breathing from the free-flow whilst managing an ascent and maintaining buddy cohesion. In PADI classes, free-flow breathing is taught in a static position, with no added stress factors. In reality, especially with a novice diver who might otherwise be struggling with the basics - such as buoyancy control on ascent - the added stress loading is likely to cause a domino effect of resulting issues...such as rapid ascent and/or buddy separation.
Given those possibilities - I feel that air-sharing is actually a positive step towards diver safety in the first instance. The diver should also remove the free-flowing air as a distraction/stressor. That can mean holding the free-flowing second stage away at arms length, or if practicable, even shutting down the cylinder (with buddy assistance).
Also, as a general rule, I think it is better to preempt an out-of-air situation with air-sharing... rather than actually waiting until you are sucking on a dead tank before attempting to secure an alternate source of air. That applies to v.low on air scenarios as well as freeflows... especially where the depth indicates that there is no way the diver can achieve the surface before air supply is exhausted.
It seems far safer and more achievable to me that a diver should maintain control over a donated AAS on ascent, than try to gain control over a donated AAS on ascent. I've read more than a few accident reports that illustrate diver separation on ascent preceding or during an OOA emergency. Gaining physical buddy contact and access to an AAS should be an immediate priority to divers at any time they have cause for concern over their own air supply.
Again... PADI doesn't state that you cannot act proactively to secure an alternate air source, if you felt that requirement. Nor, I believe, does it specifically state that freeflow breathing is the only or even preferred method of response to a freeflow.
Perhaps. However CW 2 & 3 emphasise sucking on a dead tank.
Knowledge Review 3, Quiz 3 and the Final exam have the 5 situations drilled into the OW diver.
1: Normal ascent/ Low on Air
2: OOA
3: CESA
4: Buddy Breathing
5: Buoyant
Freeflow by the way PADI words it, puts it in the 1st category.
They don't state it. However they do drill on the feeling of sucking on a dead tank before signalling.
Wart, honestly, would you rather have a basic diver try to start an ascent on a freeflowing reg, probably get separated from his buddy in the process, and draw the last breath out of the tank before he got to the surface, just because the only response to a freeflow that PADI teaches is the one of breathing the reg?
To me, ascents are one of the most difficult things for new divers to keep under control, and it's worse when they are stressed. We DO teach them, and make them practice, air-sharing ascents. Establishing an air-share, getting into solid touch contact, and ascending, seems a MUCH safer response to me than desperately trying to see and stay in control while trying to breathe off the free-flowing regulator -- and even if you throw that one away and go to your own octo, you still have VERY VERY little time before there's no gas coming out of that one. I just don't get it.
To me, ascents are one of the most difficult things for new divers to keep under control, and it's worse when they are stressed. We DO teach them, and make them practice, air-sharing ascents. Establishing an air-share, getting into solid touch contact, and ascending, seems a MUCH safer response to me than desperately trying to see and stay in control while trying to breathe off the free-flowing regulator -- and even if you throw that one away and go to your own octo, you still have VERY VERY little time before there's no gas coming out of that one. I just don't get it.
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Why not? Mine are. Any malfunction of your gas supply dictates a decision of an ascent in the failure mode (with possible repair on the way up) or obtaining gas from your team mate and then trying to sort it out. The latter makes a whole lot more sense, especially since, "the OW diver" has likely only done one ascent, and had the crap scarred out of him about it.
I don't think your question was answered here, so I will try to help.
In general, people mix up the two terms, so it is hard to tell which one people are really talking about. In the long run, it only matters that you know what you are talking about.
As I use the terms in te emost technical sense, SAC is the rate at which you would use air whilst sitting benignly on a bench on the surface, and RMV is what you will actually use on a dive in whatever conditions you face at whatever depth you are diving. By that definition, you need to know SAC first, and then you multiply that by the depth and the exertion factors to get the RMV. It is not that you are scrapping SAC--you are using it to calculate the RMV.
But that is not how almost everyone I know uses the terms. They only use SAC, and they vary it by depth and conditions, meaning they are actually using RMV. You will hear people say, "My SAC on the working portion of a dive with a dry suit is 8.0, and during deco it is 6.0." When they say that, they should be using RMV instead of SAC, but who cares as long as you understand what they mean?
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