How much air to surface with?

[BRT]

You are correct about the doubles diver having more gas available in the tank compared with the single AL80 but that gas will not be available due to the fact that first stage regulators work off of pressure differences between the tank and ambient pressure, not on flow or capacity. For the sake of argument assume both divers have the same regulators but are diving on the tanks in your example. Once both divers reach the bottom rated working pressure of the regulator they will both start breathing harder (or not at all?) regardless of available gas. This assumption is based on what Superlyte27 wrote above and what others have said in this thread.I always assumed that the reg would stop delivering gas once it hit the minimum designed pressure. However, it appears other divers are getting gas below this minimum pressure albeit at a lower flow rate. Based on this I think the 140 psi is the minimum pressure that will deliver *rated flow*. Below this pressure gas is available but at reduced flow rates hence the comments that you have to breath slower. I could not, for the life of me, find a WOB (work of breathing) graph for my Aqualung Titan reg or any other reg for that matter. If the WOB is graphed against pressure then we will know just how far one can breathe down a tank. If anyone has this graph please post it.I'm going to breath down one of my AL80's tonight. I'll report back to this thread on my findings.

You don't have to breathe the tank down. Just shut off the valve and breathe. You will know right away how low you can breathe the 1st stage. I'm betting you can breathe it till you can remove the 1st stage from the tank.

 

[redacted]

I'm a technician for almost a dozen different regulators. On every single one of them, there is a seat and a cone. That seat is held down against the cone by a very thick spring. The only thing that can move that seat away from that cone is 140psi or more. So, no tank pressure or low tank pressure, seat is fast against teflon seat, no gas can go in or out.

You might want to take another look. All of my 1st stages are piston regulators. That heavy spring you refer to holds the valve (seat/cone) open until pressure reaches IP and the valve closes. And diaphragm regs work the same way.

Are you really a professional regulator technician?

 

[decompression]

This is why rock bottom is not going to catch on with rec divers. Most would be very surprised to know they have to begin their ascent after using 1/2 of their tank and reaching the surface with 1000 psi or more, still remaining. This means, for nitrox divers with good consumption, not even getting close to their NDL time underwater and giving up a big chunk of their dive, every time. They will sacrifice many, many minutes and eventually many hours of time underwater by anticipating the very worst situation that can happen once in 100,000 times, if even that often, based on exaggerated gas use assumptions that are not necessarily needed in rec NDL diving. I don't think rec divers are going to do this when it means sacrificing 30% of their normal dive time.

Put simply, if I have a buddy with a low/out of air situation underwater, he is getting my primary and we are heading for the surface, pronto. We are not going to worry about ascent rates, and are certainly not going to do a safety stop. A real OOA emergency calls for the surface, as fast as we can short of a full CESA. Formal "rock bottom" planning would be far more gas than needed to accomplish this. In fact, the "surface with 500 psi" guide means leaving the bottom with 700-800 at 80' (in my experience). If it takes me 300 to do a slow ascent and safety stop, even doubling that still leaves 200 left (close, but this is emergency planning). In reality, we would not use that much gas with a faster ascent and no stop. Remember, we are NDL diving so stops are optional and we have the safety margin of the tables or computer algorithm to help offset the fast ascent. I choose to plan this way, considering the very remote likelihood of the event even happening and the remote risk of injury with my plan, rather than sacrifice what, by now in my life, would have been days of underwater time as well as lots of unused gas.

Rock bottom is crucial if you have mandatory deco. At that point, you do have to calculate based on slow/proper ascent rates and the mandatory stops. But otherwise, I just don't see the crusade becoming widespread.

I didn't see in your post anything about recalculating rock bottom......most dives aren't squares but multi level, you just recalculate your rock bottom.

The real benefit to rock bottom is that if you are on your ascent with your couple of hundred PSI for you and suddenly someone signals out of air you don't have to make a "his life/my life" decision....


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[tbone1004]

On that discussion. I tend to break RB into two segments. Bottom assessment and ascent, and safety stop+reserve. If you do this then you can easily calculate the assessment + ascent time if you know your tank factors. Then again, you can also just put it in wetnotes in intervals. 15ft should be sufficient for the interval.

 

[guyharrisonphoto]

Well, I don't re-calculate it because I don't use rock bottom in the first place, for NDL diving.

Sure, I get the concept of multi-level allowances, but, on many of my local dives, 80' (which was just used because it was the example offered for rock bottom) is actually often the highest point of the dive and when the ascent begins. Also, I don't want to do math during the dive. I have my PSI to begin my ascent planned (and programmed into my computer), and follow that. I want to enjoy the sights, not try to do mental math fumbling with my slate or wetnotes when moving from 120 to 80', while imperceptibly narc'd, probably dealing with a current, and (hopefully) distracted by the joy of 80 goliath grouper or a dozen sharks.

How do you presume from my example that I begin my ascent from 80' with a "couple hundred" left in the tank, only for me? The example clearly shows that ascending at 700-800 gives 250-300 for me and an additional equal amount for my buddy, with still a 200 potential cushion. Guess what--we are going to make it.

The "bottom" line is that rock bottom cuts way too much into dive time for NDL diving, unnecessarily, in my view, as Tbone's example above shows. I do not plan for dallying on the bottom, for a leisurely ascent, or for any stops with my OOG buddy. But, it will never be "who dies" because I will have enough gas to get us up, as my alternate example above shows. I called rock bottom for rec diving a "crusade" precisely because of the attitude that if you don't do it that way and only that way, you are endangering lives. It is a "you're gonna die" cliche that is just not true.

 

[dmaziuk]

it will never be "who dies" because I will have enough gas to get us up

And besides, a CESA from 25 metres shouldn't kill you. It'll likely get you bent, but there's still some distance between that and "you're gonna die". So even if you're stupid enough to suck the tank really really dry while remaining at depth, chances are you ain't gonna meet Dr. Darwin just yet.

 

[Couv]

....I'm a technician for almost a dozen different regulators. On every single one of them, there is a seat and a cone. That seat is held down against the cone by a very thick spring. The only thing that can move that seat away from that cone is 140psi or more. So, no tank pressure or low tank pressure, seat is fast against teflon seat, no gas can go in or out.

Superlyte, you probably know this already and just had one of those moments; but to clarify for others.

Try this experiment. Remove the dust cap from a regulator-don't hook it up to a tank. Breathe in on the second stage. Yes, you can breathe in because even at ambient pressure you can get air through the first and second stage. Why? See AWAP's post explaining that the first stage spring holds the valve OPEN. The second stage spring force will be overcome as soon as the diaphragm pushes on the lever which opens the second stage demand valve.

 

[dumpsterDiver]

And besides, a CESA from 25 metres shouldn't kill you. It'll likely get you bent, but there's still some distance between that and "you're gonna die". So even if you're stupid enough to suck the tank really really dry while remaining at depth, chances are you ain't gonna meet Dr. Darwin just yet.

NOPE ... on a recreational dive it is EXTREMELY unlikely that a CESA from 80 ft will get you bent.

Superlyte, you probably know this already and just had one of those moments; but to clarify for others.

Try this experiment. Remove the dust cap from a regulator-don't hook it up to a tank. Breathe in on the second stage. Yes, you can breathe in because even at ambient pressure you can get air through the first and second stage. Why? See AWAP's post explaining that the first stage spring holds the valve OPEN. The second stage spring force will be overcome as soon as the diaphragm pushes on the lever which opens the second stage demand valve.

Finally there is some reality in the discussion..

 

[halocline]

I'm a technician for almost a dozen different regulators. On every single one of them, there is a seat and a cone. That seat is held down against the cone by a very thick spring. The only thing that can move that seat away from that cone is 140psi or more. So, no tank pressure or low tank pressure, seat is fast against teflon seat, no gas can go in or out.

Wow, are you sure you don't want to re-think this? Because it is absolutely wrong, and conveys that you don't know the first thing about how 1st stage regulators function. If you're a technician, and you don't understand that the 1st stage valve is open when unpressurized, and that IP is the pressure required to close the 1st stage valve, that's pretty scary. The very thick spring you are referring to keeps the seat OFF the orifice (cone), not on it.

But back to the entertaining discussion of how a regulator works when the tank pressure dips below IP, it's really simple. The 2nd stage will continue to allow air to pass as long as it exceeds ambient. You'll just have to work increasingly harder to pull air through the hose as the supply pressure goes down. If you want to get an idea about how much effort it takes to pull ambient pressure air through a regulator, just take the dust cap off a normal regulator (with no auto-close gizmo) and suck away. You could breathe off it all day. But it's a LOT of effort.

Edit: Didn't mean to pile on, it looks like my esteemed (or is it "steamed") colleagues awap and couv beat me to it.

 

[dumpsterDiver]

I think we should get this thread back on target.. And that is, how to deal with the boat Nazi's who may demand seeing 500 psi on the boat... The answer is very simple, as climbing up the ladder.....

Press the purge of the octopus, yell and swear a little, make sure the crew knows you find this extremely irritating and do you best to demonstrate that you are trying to stop the freeflow (as you continue to press the button) It will be more dramatic if you can do part of it with the reg submerged. Wave your arms around a bunch and they won't be able to focus on your finger on the purge.

The whole thing will probably distract them, but if they stay on point, just show them your gage and swear up and down that there was 700 psi when you got on the ladder and the remainder was lost during the freeflow. How can they prove you wrong?