How much air to surface with?

[redacted]

500 psi is a good plan for those who do not want to put any real effort into planning. And I can live with that as a planning tool.

When it actually comes time to get out of the water, my standard is it is a fail if I can't still breath from that tank. It is rare for my tank to be less than 500psi but I also have little problem with 300psi. I avoid diving with ops that would even notice that small difference.

Regulators will behave differently at low tank pressures. Balanced 1sts and 2nds will generally be less effected than their unbalanced counterparts. My Mk7 (honker) is different in that it basically shuts down breathable gas to you primary around IP. A diver must switch to the alternate. It is good to take your rig down to "empty" just so you know how it will perform.

IP is not OOA.

 

[stuartv]

I dove a bunch with a surface supplied rig I build that only produced 50 psi. At 50 feet you had to breathe slow and steady but the regs worked fine.

50 psi is 3.4 ATM. 50' is ~2.5 ATA ambient pressure. So, what you're saying is just what I would have expected. And maybe even continue to work down to over 70' (i.e. close to 3.4 ATA ambient pressure). You were using a balanced 2nd stage regulator?

 

[scubadada]

It is good to take your rig down to "empty" just so you know how it will perform.

I've always thought that would be a good idea, I want to know what to expect, I will do it this trip to FL and report back. I will do it at my SS

I dive with both an AI (Oceanic VT 3) and an SPG. Interestingly, the SPG registers a slightly higher pressure than my computer though the reading is somewhat inexact. It will be interesting to see how the two compare at close to zero.

 

[Superlyte27]

Yikes, there's too much thought going into this.

First, the idea of surfacing with 500psi is antiquated and a poor practice (in my opinion) Call it the tech diver in me. Call it the alarmist in me. Call it.... who knows, there's just better ways of doing things. If you need reasons why, I'll expound later.

Secondly, the guy who wrote that SPG's are not accurate at low pressure is correct. So the guy who said he thought he finished a dive with 130 -150psi, I doubt those numbers were accurate. There's really only about 1 breath left at 140psi on most regs.

Now, for deep thoughts by Peter...

Surfacing with 500psi doesn't allow for much error. As stated twice above, SPG's are not very accurate. So 500psi shown on the gauge might only be 200psi. I've seen this several times recently. BUT, let's say the SPG is perfectly accurate... (I'm going to give you an event that happened while I was a captain in Key Largo) you've got 12 people surfacing from a dive in 100 feet of water. When we arrived the seas were 1-2' at 14 seconds interval. But an hour later, they've kicked up to 4-6'. The 3rd diver up the ladderer is trying to remove his fins, instead of holding the jon line, he's holding onto the ladder, the ladder catches him right in the face, knocking out his teeth. Blood is everywhere. He drops his fins, we hurry to try to resolve the situation while people are bobbing in the water overcoming 6' seas. In a heightened situation, with blood in the water, does your SAC rate go up? Yes, how long does 500psi last? Would you rather sit and wait for us to resolve the accident on snorkel power or tank power? I don't think I even own a snorkel.

Scenario #2
I came up from a dive and had some weird joint pain in my knee. I never felt it before and assumed it might be DCS. I mentioned to my buddy that I was going to go back down to 20' for 30 minutes and see if the joint pain subsided. As soon as I got to 20' the pain resolved itself (sure, it could have been mental, but why chance it). Could I have done that with only 500psi in my tank.

Scenario #3
Four people are diving in 40' of water... spearfishing. Two teams of two. 3 people surface, and the 4th guy does not. We wait a couple minutes, then decide we should probably go find him. Which is faster? Climb back in the boat, swap out tank, jump back in the water... or just go back down to find his speargun shaft is stuck under a piece of coral still attached to the grouper. I wouldn't have done that with 500psi in my tank.

And sure, none of these are perfect situations, but real situations none the less. 500psi works in a perfect scenario. But this is nature we're talking about. And she's a fickle biotch

 

[redacted]

I've always thought that would be a good idea, I want to know what to expect, I will do it this trip to FL and report back. I will do itat my SS

I do my testing in a pool or local FW facility in chest deep water. Make sure your op knows what you are doing.

 

[Superlyte27]

50 psi is 3.4 ATM. 50' is ~2.5 ATA ambient pressure. So, what you're saying is just what I would have expected. And maybe even continue to work down to over 70' (i.e. close to 3.4 ATA ambient pressure). You were using a balanced 2nd stage regulator?

Your reg is not going to work at any depth with 50psi in the tank.

 

[tbone1004]

there is a difference between "it breathes", and "it breathes like it is supposed to". The first stage is supplying whatever your IP is set to above ambient, so I try to keep tank pressure above 200psi when I surface so the first stage isn't fighting. Made worse with an unbalanced reg as the IP is already dropping off. Do whatever you guys want, but regardless of what the cf is, I don't like to surface with less than 300psi, whether it's in a LP104 representing 12cf, or a HP80 representing 7cf.

As far as rock bottom, whoever told you that Rock Bottom is for technical diving only vs. recreational diving is an idiot. It's far more important for recreational diving. That said, how do you know when to start your ascent if you aren't calculating Rock Bottom? You can choose whether or not to pad the SAC rates up to 1cfm, and whether or not to factor it in for 2 divers, but padding SAC rates and calculating for 2 divers is far more important on rec dives than tec dives. Think about it. Recreational divers are generally less experienced, generally have higher SAC rates, generally take longer to get things corrected before being able to make a safe ascent, are more likely to need to borrow your gas due to not paying attention to their SPG, and may be more prone to having equipment failures due to rental gear *this is debatable, but I've seen more hose leaks and o-rings burst on rental gear than personal gear*. All that factored in means planning for a 2cfm gas requirement for ascent is much higher on my priority list in a recreational dive than it is on a technical dive. 80ft dive on an AL80 means start your ascent with 1400psi, 1100 on your 120. That gets you to the surface with 200psi which is a reasonable buffer.

Pete gave a bunch of good reasons on why you should always pad your ascent pressures. True Rock Bottom planning allows for all of those situations to happen assuming you aren't sharing air.

 

[scubadada]

Secondly, the guy who wrote that SPG's are not accurate at low pressure is correct. So the guy who said he thought he finished a dive with 130 -150psi, I doubt those numbers were accurate. There's really only about 1 breath left at 140psi on most regs.

I'm the guy. Those were the pressure readings off my AI, downloaded. If there was one breath left at 140, how did I get down to 130? Whatever, I'll go ahead and do my experiment, N=1, on my next trip. I don't think the risk at 15 feet is very great. I have a good regulator, Scubapro MK25/S600, we'll see

 

[stuartv]

there is a difference between "it breathes", and "it breathes like it is supposed to". The first stage is supplying whatever your IP is set to above ambient

So, with balanced regs, if ambient is 30, IP is (supposed to be) 140, and the tank is at 170, I should get one last breath more or less normal. But, are you saying that, once the tank drops to 100, I'm going to get nothing? Or I'll have roughly 100 psi of IP and so I'll still be able to breathe off the 2nd stage, just not breathing "like it's supposed to"?

As far as rock bottom, whoever told you that Rock Bottom is for technical diving only vs. recreational diving is an idiot.

I never said anyone told me that, because nobody ever did tell me that. But, nobody ever mentioned rock bottom planning in OW training, either. It only was ever taught (to me) after I started tech training.

 

[redacted]

there is a difference between "it breathes", and "it breathes like it is supposed to". The first stage is supplying whatever your IP is set to above ambient, so I try to keep tank pressure above 200psi when I surface so the first stage isn't fighting. Made worse with an unbalanced reg as the IP is already dropping off. Do whatever you guys want, but regardless of what the cf is, I don't like to surface with less than 300psi, whether it's in a LP104 representing 12cf, or a HP80 representing 7cf.

As far as rock bottom, whoever told you that Rock Bottom is for technical diving only vs. recreational diving is an idiot. It's far more important for recreational diving. That said, how do you know when to start your ascent if you aren't calculating Rock Bottom? You can choose whether or not to pad the SAC rates up to 1cfm, and whether or not to factor it in for 2 divers, but padding SAC rates and calculating for 2 divers is far more important on rec dives than tec dives. Think about it. Recreational divers are generally less experienced, generally have higher SAC rates, generally take longer to get things corrected before being able to make a safe ascent, are more likely to need to borrow your gas due to not paying attention to their SPG, and may be more prone to having equipment failures due to rental gear *this is debatable, but I've seen more hose leaks and o-rings burst on rental gear than personal gear*. All that factored in means planning for a 2cfm gas requirement for ascent is much higher on my priority list in a recreational dive than it is on a technical dive. 80ft dive on an AL80 means start your ascent with 1400psi, 1100 on your 120. That gets you to the surface with 200psi which is a reasonable buffer.

When tank pressure falls below IP, the 1st stage valve just stops closing so it is pretty much out of the equation. With an unbalanced 1st, the IP is actually changing as tank pressure falls. The difference is most noticeable at low tank pressures where IP could drop 30+ psi.

A balanced 2nd uses spring pressure and air pressure to close the valve. Air pressure is balanced so all you really need to overcome is the lighter spring pressure plus the reduced air pressure as tank pressure falls below IP. With a balanced 2nd you always have to deal with the full effect of the stronger spring pressure so it is more noticeable.