Safe tank pressure at which to begin accent.

[stuartv]

@boulderjohn

Just checking my math. Will this rule of thumb,

D * 10 + 300

Get two divers back to the surface with one of them OOA? Will it permit a 3min stop at 15 ft? Assume AL80 at 1cf/min SAC for both.

Thanks

I did the algebra/pre-calc. Short answer, given the stated parameters, this formula works down to a depth of approximately 190 feet. Below that, the formula doesn't give you enough reserve. But, if you're going that deep, you don't need me to tell you that. And if you're going that deep on an AL80, you're probably going to die anyway.

Longer answer:

The formula given is linear. The formula for gas required to make an ascent from depth D is exponential. That is because the gas required per minute is a function of depth, and the time to ascend is also a function of depth. You multiply them together and the result is that the total gas required depends on the the square of D (i.e. D^2 is part of the equation).

Assuming the stated parameters of:

2 divers
RMV of 1.0 cu-ft/min for each diver
AL80 (so, 77 cu-ft of air @ 3000psi)
3 min stop at 15 ft (static coefficient of approx 9 cu-ft required)

Solving for the intersection of the two equations results in the deep side being approximately 190 feet. Shallower depths mean the rule of thumb for psi reserve gives you more gas than is required to make the specified ascent.

Note that this does not include a time allowance for "sorting things out" before starting the ascent. It also is a solution for arriving at the surface with 0 psi left in the tank. OTOH, at depths shallower than 190 feet, the formula for the reserve does result in more than 0 psi left when arriving at the surface. The actual amount left varies with the initial depth.

Caveat: This is IF I did all my Post-it Note algebra correctly.

Lastly, it's a handy rule of thumb to use as a starting point for Recreational sport diving. But, I think you should do your own math, for your own parameters and target depth, each time you dive, just to make sure you're not making a mistake, and to keep yourself up to date on being ABLE to calculate your gas management.

 

[scubadada]

The simplest questions on SB... My RMV is 0.35, I need less than 5 cf of gas to ascend to the surface at 30 ft/min, with a 3 min SS. That is about 200 psi, so yes, starting the ascent at 700 psi would be fine. Most of my diving is drift, I have full access to the surface whenever I want, this is actually pretty conservative for me, ending with 500 psi is not necessary. So maybe the OP has a RMV of twice mine, he can start to ascend at 900 psi. There is nothing here about needing to swim around to find an ascent line or anything else that would delay the ascent. This stuff about covering the entire gas requirement for 2 divers is simply not needed. Leave it to SB

 

[tbone1004]

so you don't believe that you need to reserve any gas for your buddy to make a safe ascent without a CESA? or leave any gas for time to handle a situation?

 

[boulderjohn]

The simplest questions on SB... My RMV is 0.35, I need less than 5 cf of gas to ascend to the surface at 30 ft/min, with a 3 min SS. That is about 200 psi, so yes, starting the ascent at 700 psi would be fine. Most of my diving is drift, I have full access to the surface whenever I want, this is actually pretty conservative for me, ending with 500 psi is not necessary. So maybe the OP has a RMV of twice mine, he can start to ascend at 900 psi. There is nothing here about needing to swim around to find an ascent line or anything else that would delay the ascent. This stuff about covering the entire gas requirement for 2 divers is simply not needed. Leave it to SB

A lot of people will agree with you--probably the vast majority, in fact.

As I said earlier, there are different approaches to gas management. This discussion has been dominated by different versions of the "rock bottom" philosophy, and if you look at the entire world of recreational diving, that probably represents only a tiny fraction of the divers. For most divers, planning for a possible OOA ascent will not include a safety stop. For most divers, an emergency ascent like that can be done at 60 FPM rather than 30 FPM. For most divers, simply making sure they follow the common "be back on the boat with 500 PSI" gives them enough reserve to deal with such an emergency. (Yes, you do dip into the 500 PSI in that case--that's what it's for.)

It is up to each diver to think this through and decide what works best.

 

[scubadada]

so you don't believe that you need to reserve any gas for your buddy to make a safe ascent without a CESA? or leave any gas for time to handle a situation?

I generally dive solo, I have my own extra gas. If I had a buddy, I would have about 400 psi, 10 cf to lend to a buddy if I left the bottom at 700 psi. What situations am I going to encounter between the bottom and the top? This is recreational diving, free access to the surface without a SS. If push came to shove, ascent could be a little quicker. Some make this considerably too complicated

 

[Patoux01]

One word: "boat".

 

[tbone1004]

diving solo changes things. If I'm diving truly solo, i.e. no one else is in the water, or I'm diving with a trusted buddy with redundant gas supply, then yes I 100% agree with your numbers and it honestly about where I would leave the bottom. Unfortunately I have been mugged on more than one occasion and because of that, I choose to leave plenty of gas in the tank because I am not risking myself by making a rapid ascent and blowing a safety stop. If you look at my original numbers of leaving the bottom at 1800 psi, and cut that in half for solo diving, I'm about where you are. My SAC rate is a bit higher around .45, and I dive with Poseidon Jetstreams so I have to be very careful about my ending pressure because they are upstream regulators and will freeflow if the IP drops much below 100psi, so my padding has a pressure minimum regardless of tank size that I choose not to violate, which is 400psi instead of 200psi, so we're actually in the same boat.

The major issue I have is the lack of time that you guys are factoring to remedy the situation and calm the OOG diver down enough to not make a super rapid ascent. That to me is very important and I plan for at least 2 minutes at depth to take of that just in case... Better to plan for the worst and hope for the best.

Oh, of note since "boat" was mentioned. If there are safety bottles on the hang line, then the gas planning changes since one of you can hop on that. In that case, that gives you an extra 300psi in an AL80

 

[scubadada]

A lot of people will agree with you--probably the vast majority, in fact.

As I said earlier, there are different approaches to gas management. This discussion has been dominated by different versions of the "rock bottom" philosophy, and if you look at the entire world of recreational diving, that probably represents only a tiny fraction of the divers. For most divers, planning for a possible OOA ascent will not include a safety stop. For most divers, an emergency ascent like that can be done at 60 FPM rather than 30 FPM. For most divers, simply making sure they follow the common "be back on the boat with 500 PSI" gives them enough reserve to deal with such an emergency. (Yes, you do dip into the 500 PSI in that case--that's what it's for.)

It is up to each diver to think this through and decide what works best.

Thank you, a voice of reason

 

[halocline]

Condoning reckless gas planning is not ok....

Consider the fact that every day, all around the world, for decades, literally millions of OW recreational dives are conducted by simply ascending when the diver reaches a pre-set reserve, (like 500 or 750 PSI) and these divers are not running out of air or dying. Calling this practice "reckless" is absurd and you should be smart enough to know it.

You need to realize that there is a difference between recreational, OW diving, and technical diving.

 

[tbone1004]

and how many of those are happening while sharing air? that is a perfect example of planning for the best and hoping for the best, if and when the worst happens, it may not end as well as it could have.