How much air to surface with?

[stuartv]

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

So, based on this:

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.

I gather that that is because of something to do with the 1st stage?

 

[tbone1004]

and that is a break down in your OW training. They should have taught you how to calculate that, whether the agencies teach it or not, it should be something any good instructor teaches...

WRT the pressure, it depends on the regs on how they're going to behave. On my Poseidons if the IP gets below 100, they will freeflow. Doesn't matter how it gets below that, but they're going to freeflow, it's the design. On the Cyklons, especially with the unbalanced first stages, the breathing goes to complete **** when the tanks get below about 300-400psi. Think how normal regulators breathe while you are laying on your back. Not fun. On a MK25/S600, you are unlikely to notice a significant difference because they are balanced very well. On a sealed diaphragm such as the Apeks/Hog/DiveRite/Hollis etc, remember that the diaphragm overbalances the first stage, so at depth the IP is actually higher than what you set it at at the surface. Not significantly higher, but it can help a little bit.

Point being, surfacing with less than 500psi is really not a great idea as it invites the potential for water ingress into the system if you blow it by accident. Could be you surface with 500psi, but you power inflate your BC and aren't really good at breathing around your mouthpiece and it gets down to 200-300 psi on a small tank, could be you have to hold your safety stop an extra 3-5 minutes because of surface conditions etc. and while I have no problem with a 200psi buffer if you're planning Rock Bottom, I would never recommend anyone plan to surface with less than 300psi.

Thanks for clarification AWAP. Don't use balanced second stages, so that might be the real world difference between them functioning properly and not.

 

[scubadada]

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.

I generally dive solo with a pony, much does not apply to me. Your assumptions regarding "recreational diver" seem condescending, may be generally applicable, but, again, do not apply to me. I don't disagree that divers need to be well educated regarding gas planning and practice what they know.

 

[stuartv]

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.

I assume you meant "an UNbalanced 2nd" there?

In which case, your whole post seems to be saying that, as long as you're using balanced 1st and 2nd stages and the tank pressure is above ambient, you could still pull a breath. For example, even if ambient pressure is 30psi and tank pressure is 50psi. Yes?

 

[redacted]

and that is a break down in your OW training. They should have taught you how to calculate that, whether the agencies teach it or not, it should be something any good instructor teaches...

.

Not really a breakdown in his training. I don't believe there are any agencies that teach that at the OW level.

 

[tbone1004]

Not really a breakdown in his training. I don't believe there are any agencies that teach that at the OW level.

agencies not that I'm aware of, outside of GUE/UTD which I assume do, but lots of instructors do....

 

[stuartv]

Point being, surfacing with less than 500psi is really not a great idea as it invites the potential for water ingress into the system if you blow it by accident. Could be you surface with 500psi, but you power inflate your BC and aren't really good at breathing around your mouthpiece and it gets down to 200-300 psi on a small tank, could be you have to hold your safety stop an extra 3-5 minutes because of surface conditions etc. and while I have no problem with a 200psi buffer if you're planning Rock Bottom, I would never recommend anyone plan to surface with less than 300psi.

I understand. My point was, do you says "500 psi" no matter what size tank? Or is there a calculation you can do, based on the tank size, where we assume 500 psi with an AL80 is the reference standard?

My second post took your earlier statement and translated that to "surface with 200 psi plus 8 cu-ft". If it's good enough with an AL80, to allow for the scenarios you just described, then isn't that good enough with an HP120, even though the HP120 would only have 433 psi, instead of 500?

And, yes, I understand what other folks have said about SPGs getting less accurate at low pressures, etc.. But, that just means that even 500 on an AL80 is just a rough estimate. Is 450 on an HP120 really even less safe?

 

[Superlyte27]

You're missing some things here. Ambient pressure has no bearing on a rigid tank with 50psi in it. Even on the surface, 50psi is about half a breath. What good does that do for you?

Secondly, while a surface supplied reg MIGHT function at 50' with 50psi supply gas... actually it should... you're still going to be starved for gas unless you're asleep. I exaggerate, but you get my point. The reason it won't work so great through a first stage is two fold... 1. It's still less than a solid breath. 2. 50psi doesn't really flow (and it's not being forced) through the .006 orifice size in the first stage (again, I exaggerate, but you get my point), to get to the second stage. This is the hole that reduces 3000 psi to 140psi that the second stage reg then reduces to a pressure we can breathe.

 

[stuartv]

agencies not that I'm aware of, outside of GUE/UTD which I assume do, but lots of instructors do....

Suffice it to say that SB has already advised me on numerous occasions that every single instructor I've had was rubbish. I think I'm something like 0 for 5 now.

It doesn't change the fact that it's probably not reasonable to expect Recreational divers, in general, to be doing rock bottom calculations. However, it might be reasonable to think that they could understand something like "your tank is a 120, so you can get back on the boat with 450psi, instead of 500" and leave them to whatever devices they have been using to ensure they get out with the "target" amount still in reserve.

 

[Superlyte27]

Come to Florida. I'll give you a cavern class. You'll learn what real gas planning is. And your life will be changed forever. And, you'll be a safer diver.