How much air to surface with?

[stuartv]

Rock bottom is all about buddy diving. You can easily turn a 100 foot dive at 1000 psi even with an elevated 1.0 CF/min, but if you or your buddy have a total loss of gas at depth (100 ft) you better have 1500 psi in both tanks. Never know which buddy it will be. Both diving AL 80s..
Just my take on it.

We are still talking about Rec diving. So, a direct ascent is always an option. From 100' (30m), that means an average depth of 15m. That means average pressure of 2.5ATA. With a combined RMV of 2 CFM, that means you need 5 CFM (surface corrected) to get to the surface (for you and your buddy). From 30m, a direct ascent at 10m/min means 3 min. So, 15 cu-ft used, which is 584 psi in an AL80. Even if you assume you will breathe as if you're OOA when you have 200 psi left (which seems debatable from all the info presented in this thread), that still only means you need to start your ascent with (200psi in an AL80 plus 15 cu-ft =) 784 psi.

Yes, if I did the math correctly, you and a buddy can both breathe with an RMV of 1.0 CFM, off of one AL80, for a normal ascent from 100' depth (with no optional safety stop) to the surface, starting with 784 psi in the one AL80 and arriving at the surface with 200 psi still in the tank.

If you decide to do a 3 minute SS at 10m, again with your RMV of 1.0CFM, that's 4 CFM times 3 minutes or an additional 12 cu-ft needed, which is another 467 psi on an AL80. So, you can ascend from 100', with two people each breathing at 1.0 CFM, on one shared Al80, at a rate of 10m/min, doing a 3 minute safety stop at 10m, starting with 1250 psi, and STILL arrive at the surface with 200 psi left.

Where are you getting 1500 psi from?

 

[gfaith]

Rock bottom normally assumes 1 minute at depth to get every thing sorted out approximately 8 more CF/min. I was also using a normal SS 3 minutes @ 20 feet. Obviously you can skip the safety stop if you wish or need to, and I am keeping 200 psi at surface (nice to have air in your wing after you surface). Your calculation are good enough..

 

[BRT]

Right. It's my understanding that surface-supplied rigs use a different kind of regulator entirely.

I had no money. I used very old scuba 2nd stages.

---------- Post added September 9th, 2015 at 09:14 PM ----------

their first stages can't depth compensate due to not being at depth which is why they need special second stages, or at least specially tuned second stages. Brownies third lung is rated for 80FSW which is about 50psi of compensation from the surface. Regs have to be tuned to be able to compensate for that 50psi shift in IP.

Tuning in my case involved buying old 2nd stages and attaching them to a check valve on a harness on my back. Never went below 50 feet because I only had 2 25' air compressor hoses to hook together.

---------- Post added September 9th, 2015 at 09:21 PM ----------

I was speaking in terms of scuba. Since I've yet to see a 500 liter scuba tank, let's just assume that any single tank you purchase, 50psi is not a full breath.

-edit-
upon further contemplation... My lungs are 6 liters. So half a breath would be 3 liters. That my friend is volume.

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.

So when I turn off my valve it is impossible to get the last 140 psi out? I must have something wrong with all my first stages.

 

[dumpsterDiver]

I think that the 130 psi came from the IP of an unbalanced first, which you can suck down a little lower, but you would feel it all happening well before that. With a balanced first, you can breathe it down to the "balance pressure". Same deal with the second stage, but like you said, press the purge and you will get it all.

I, personally, would rather hit the surface with some air to play with, but I'm not a fanatic. More important to me is that I'm neutrally buoyant on the surface with an empty tank and an empty BC, so I'm not going back down without air which would be quite inconvenient.


Bob

My point was that there is air in the tank you can use in an emergency.. even when the pressure is low. You will need to be calm, breathing slowly and patiently. Not that common in an emergency, but I think people should experiment a little in a safe environment to find out what it feels like... rather than freaking out and shooting to the surface because they "ran out" of air at 22 feet - when they really had 250 psi or something in their tank.

A pair of divers needs to leave the bottom when each tank has enough air to get them (AND THEIR BUDDY) to the surface - safely.

 

[BDSC]

After hearing so many times how crap all the instructors I've had are, I'd like to take Edd's class and see for myself how crap all my previous instructors were.

What I've learned over the years on SB is that for the most part, all the instructors on SB are really really really great instructors who teach to a level not found in most other instructors and their students turn out better than all the other crappy students from crappy instructors. I know this is true because I read it here almost daily.

 

[archer1960]

...

Also, the idea of being on the boat with 500 psi is incredibly stupid.. uniformed divers will rush their ascent in order to make it to the boat with 500.. A much better rule would be to leave the bottom with XXX psi.. whatever that figure is, should make sense, but demanding 500 psi reserve is really treating people like imbeciles.

The op I was with in Cozumel last year always told us during the dive briefing to signal the DM and start up as a buddy pair when the first buddy got down to a given pressure (usually 500psi). The dive profiles were structured so that they ended shallow, and I rarely used more than 50 to 100 psi from the time I started up, until I was floating with an inflated BC, even after doing a safety stop.

 

[tbone1004]

We are still talking about Rec diving. So, a direct ascent is always an option. From 100' (30m), that means an average depth of 15m. That means average pressure of 2.5ATA. With a combined RMV of 2 CFM, that means you need 5 CFM (surface corrected) to get to the surface (for you and your buddy). From 30m, a direct ascent at 10m/min means 3 min. So, 15 cu-ft used, which is 584 psi in an AL80. Even if you assume you will breathe as if you're OOA when you have 200 psi left (which seems debatable from all the info presented in this thread), that still only means you need to start your ascent with (200psi in an AL80 plus 15 cu-ft =) 784 psi.

Yes, if I did the math correctly, you and a buddy can both breathe with an RMV of 1.0 CFM, off of one AL80, for a normal ascent from 100' depth (with no optional safety stop) to the surface, starting with 784 psi in the one AL80 and arriving at the surface with 200 psi still in the tank.

If you decide to do a 3 minute SS at 10m, again with your RMV of 1.0CFM, that's 4 CFM times 3 minutes or an additional 12 cu-ft needed, which is another 467 psi on an AL80. So, you can ascend from 100', with two people each breathing at 1.0 CFM, on one shared Al80, at a rate of 10m/min, doing a 3 minute safety stop at 10m, starting with 1250 psi, and STILL arrive at the surface with 200 psi left.

Where are you getting 1500 psi from?

what gfaith said. You can never assume a direct ascent, you have to have time at depth to get everyone calmed back down so you don't bolt up to the surface. You bolt for the surface like you're suggesting and you're at risk of getting an embolism, infinitely worse than a mild case of DCS....

 

[RJP]

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

Yup... and you may be just as guilty.



Like most SB posters, our friend Stuart loves to overthink things. Especially things that require no thought whatsover, much less calculations.

Here's the original question:

"Is it pretty universal and standard to expect everyone to "get back on the boat with 500psi" even if they are diving bigger tanks or doubles?"

Phrased that way, the answer is "NO."

[/thread]

READ ON AT YOUR OWN RISK

Of course, phrased that way, the question defies answering. Phrasing the question that way is problematic because A.) it's not one question... it's two. and B.) It's worded in such a convoluted fashion with "pretty" used to modify "universal" ("universal" is binary, there is no "pretty universal" just as there is no "somewhat pregnant") then a redundant "AND standard" (if something is universal... it's standard). Then the whole thing is made conditional by the use of the phrase "...even if..." which is bad enough, but then the conditional situation proffered is one that essentially precludes the original question itself. (diving bigger tanks and doubles is neither universal nor standard.)

Let's break it down into the two questions that Stuart is really asking:

1. "Is 'be back on the boat with 500psi' common?"
2. "Even when diving bigger tanks or doubles?"

Then the answers are "YES" to the first question, and "NO" to the second question.

See? No calculations. No gas planning. No PSI. No unbalanced regs to worry about.

But Stuart (and most of SB) doesn't like simple answers... because then there's no argument to be had. There's no chance for each of us to go on and on to demonstrate how many of our brain cells can dance on the head of a pin.

But as long as I've taken the bait, here's some practical information to address Stuart's concern:

1. Practically speaking, the only time you'll ever hear an op say "be back on the boat with 500psi" is when you're doing recreational diving with a recreational op and you are using THEIR tanks. This is because one big part of the "be back with 500" mantra is they don't want idiots running their tanks down to zero. Beyond that fact that you might die... that might also cause water to enter the tank. And it might also cause water to enter the regs... and remember that many people on the boat are also diving the op REGS too. The fact that a diver having 500psi in the tank also means that they didn't drown is simply a pleasant side effect.
2. Most ops where the divers bring their own tanks/gear only care that you make it back on the boat and live long enough to get back to the dock. If you need to VIP your tanks and service your regs after every charter because you breath your tank down to zero... that's your problem.
3. Tech charters and other big-boy ops won't tell you "be back with 500" -- they just won't.
4. Points 1-3 notwithstanding, if you do happen to find yourself diving with an op who DOES ask you to "be back with 500psi" the correct response is "OK." Their pool, their rules. You don't have to like it, and you don't have to dive with them again, but you sure don't have to start doing dissimilar tank calculations and explaining rock-bottom dive planning to prove that you are smarter than they are.

Sixty-seven replies to a question where the answer is "NO" seems a bit excessive, even by SB standards.

 

[EFX]

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.

I was doing a training dive where we had a person acting as a dive safety officer. His briefing included telling everyone to arrive back on the surface with at least 500psi in their tank. I asked him: For people who are diving bigger tanks, do they still need to be back with 500 psi? For example, another diver had double 120s. If he got back to the surface with 250psi, he would still have more air in reserve than the guy with an AL80 at 500psi. The DSO said "no, everyone needs to be back on top with 500."

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.

 

[archer1960]

I don't know specifically how SCUBA regulators work, but pressure regulators generally open wider as the upstream pressure approaches the set pressure, in an attempt to maintain the set pressure. Most of the ones I've worked with (steam and industrial air) will go fully open if the supply pressure falls below the set pressure. If SCUBA works the same way, then you will be able to suck a tank down as low as you your lung power will allow.