Rock Bottom Pressure and Turn-Around Pressure

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Wart, your statements regarding Rock Bottom and the Free Flow scenario indicates you do not really understand the concept of Rock Bottom. Your statement that you have 90 seconds to get to the surface is, quite simply, idiotic since it makes some major assumptions -- perhaps the most important being that one has a full(ish) tank when the free flow occurs. That, of course, is when Rock Bottom is really at its most irrelevant -- is it not?

Thus, the scenario where Rock Bottom becomes important is when one HAS a free flowing (or other misbehaving regulator) at the END of the dive, NOT at the beginning. And when does the dive end? When one reaches Rock Bottom (or before). BUT IF you are at 25 meters, you've been there so that you are AT your minimum air reserves (aka Rock Bottom) and your buddy's reg malfunctions (free flows for whatever reason), would it still be your position that "breathing from a free flowing reg" and ascending is the most appropriate response?

As an instructor, albeit one of those who does this for fun, rather than profit, I try to teach even my open water students that they need to be thinking divers. In addition, one of the things I try to convince them with the "free flowing reg" exercise in confined water is that IF they ever actually have a free flowing reg, to get it away from them! Almost all agree that they CAN breathe from the reg but that it makes much better sense to get that reg away from their bodies and breathe from their backup (Octo).

I would hope that IF any of them actually had this occur, they would have the good sense to go on their buddy's AAS, keep the free flowing reg away from their body and get out of Dodge. One can only do this IF one follows the rules of Rock Bottom.

Wart, do you really teach your students that breathing from a free flowing reg is the best possible solution?
 
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?

Hope you aren't going to accuse me of attacking you, but your characterization of SAC is not correct. When I learned to dive, I never remember hearing RMV, just SAC.

SAC is the air consumption rate (generally expressed in cu-ft/min) that would occur at a pressure of 14.7 psi (at the surface, depth = 0). There never was any assumption that SAC implied that the diver was resting at the surface (or on their couch) as you state.

I dusted off the NOAA dive manual which is "pretty technical" and they also describe calculating a divers SAC.

They seem to support exactly what I was taught in the past... They provide an example of a diver swimming at a certain depth, recording the volume of air consumed over a measured time period and then converting that consumption rate to an equivalent one which would occur at a depth of zero.... i.e., the surface. They describe using a SAC rate as an alternative to the RMV.

I've read several times in the past on SB where people seem to want to re-define SAC to be something that occurs on the couch and is therefore of marginal relevance. The term SAC never had that connotation and I do not think that it does now (at least according to NOAA).
 
Wart, your statements regarding Rock Bottom and the Free Flow scenario indicates you do not really understand the concept of Rock Bottom. Your statement that you have 90 seconds to get to the surface is, quite simply, idiotic since it makes some major assumptions -- perhaps the most important being that one has a full(ish) tank when the free flow occurs. That, of course, is when Rock Bottom is really at its most irrelevant -- is it not?

Thus, the scenario where Rock Bottom becomes important is when one HAS a free flowing (or other misbehaving regulator) at the END of the dive, NOT at the beginning. And when does the dive end? When one reaches Rock Bottom (or before). BUT IF you are at 25 meters, you've been there so that you are AT your minimum air reserves (aka Rock Bottom) and your buddy's reg malfunctions (free flows for whatever reason), would it still be your position that "breathing from a free flowing reg" and ascending is the most appropriate response?

As an instructor, albeit one of those who does this for fun, rather than profit, I try to teach even my open water students that they need to be thinking divers. In addition, one of the things I try to convince them with the "free flowing reg" exercise in confined water is that IF they ever actually have a free flowing reg, to get it away from them! Almost all agree that they CAN breathe from the reg but that it makes much better sense to get that reg away from their bodies and breathe from their backup (Octo).

I would hope that IF any of them actually had this occur, they would have the good sense to go on their buddy's AAS, keep the free flowing reg away from their body and get out of Dodge. One can only do this IF one follows the rules of Rock Bottom.

Wart, do you really teach your students that breathing from a free flowing reg is the best possible solution?

Are you teaching that the new OW diver should shut down their tank valve (or do it to their buddy) when this occurs to try to discontinue the freeflow?

I know TSM and I went a few rounds on that quite a while ago, and my recommendation was to scramble for the surface, as fast as was safe.
 
They seem to support exactly what I was taught in the past... They provide an example of a diver swimming at a certain depth, recording the volume of air consumed over a measured time period and then converting that consumption rate to an equivalent one which would occur at a depth of zero.... i.e., the surface. They describe using a SAC rate as an alternative to the RMV.

This is also true for PADI...

PADI Tech Instructor Manual:
Gas consumption

1. In this course, you’ll learn to plan gas use based on how fast you consume it.

a. Your Surface Air Consumption (SAC) rate is the rate you use gas (in litres or cubic feet per minute) if swimming at a moderate speed in all your equipment at the surface.
b. SAC changes with equipment and anything else affecting drag.
c. SAC changes as you gain skill and fitness, and with physical variables such as temperature.
d. SAC can be expressed in bar/psi per minute, or gas volume per minute.
e. You’ll also hear references to Respiratory Minute Volume (RMV), and it is often the term used in decompression software. The medical community defines RMV as:
SAC.jpg
RMV = Vt -Vd x respiratory rate
where Vt = tidal volume, Vd = respiratory dead air space and respiratory rate = breaths per minute.


• Tec divers sometimes use RMV to mean SAC rate based on volume – this is not technically accurate, but close enough.
• You’ll hear SAC rate and RMV used interchangeably; this is fine, but do not confuse SAC rate based on bar/psi with SAC rate based on volume. They’re not the same thing.
 
dd, no I do not teach shutting down the offending tank. I hope I teach students to think. Today, during a rescue scenario I free flowed my reg and went OOA to the student. He couldnt see me through the bubbles and couldnt donate. It was eye opening to him. Bubbles are nasty and expecting a diver to breathe off a free flow reg for 90 seconds is nuts.
 
Speaking of free flowing regs - has anybody tried kinking a free flowing reg hose (like you would a garden hose)? They are low pressure so presumably it wouldn't be too tough to choke off at least partially and reduce the air escaping while breathing off your occy. I'd like to try this but I don't really feel like busting my reg hose.

The whole breathe off a free flowing reg surely must be a throw back to when PADI, etc. started and occys weren't standard? Can't say anything seems particularily logical to me other than to swim over to your buddy and ascend treating it as similar to an OOA incident.
 
Speaking of free flowing regs - has anybody tried kinking a free flowing reg hose (like you would a garden hose)? They are low pressure so presumably it wouldn't be too tough to choke off at least partially and reduce the air escaping while breathing off your occy. I'd like to try this but I don't really feel like busting my reg hose.

If the free-flow originates from the 1st stage (likely), then you'd only succeed in diverting the air egress into the AAS. That'd make your problem worse - as you'd then have two free-flowing second stages to contend with.

The whole breathe off a free flowing reg surely must be a throw back to when PADI, etc. started and occys weren't standard?

As you can now see.... it isn't :wink:
 
Yea I was thinking of a second stage reg failure - granted your gear has to be in a pretty bad state of disrepair/clogged with something for that to happen as second stages aren't exactly complicated
 
Wart, your statements regarding Rock Bottom and the Free Flow scenario indicates you do not really understand the concept of Rock Bottom. Your statement that you have 90 seconds to get to the surface is, quite simply, idiotic since it makes some major assumptions -- perhaps the most important being that one has a full(ish) tank when the free flow occurs. That, of course, is when Rock Bottom is really at its most irrelevant -- is it not?

I teach my students to plan dives easily.

Rock bottom is completely useless for the the newbie dive.

Why?

No OW courses teach SAC/RMV. Hence why in the opening salvos of this thread we had comments asking what SAC even was. Most people don't know it.

You can plan a dive without something as convoluted as Rock Bottom. It's so dirt simple. Take out the 500psi your supposed to come back with add a little for you safety stop (say 300psi) and then divide by 2; to get your turn around.. Easy done. And that will leave enough gas for you and your buddy for safety stop. My OW students can understand this.


The first time you have a dyslexic student, or a 10 year old student, you'll understand why this method is far more efficient
 
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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.

Ask PADI.

I'm not taking flak for the PADI curriculum. :no:
 
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