3 or 5 minute Safety Stop?

[chrpai]

At first I couldn't figure out your maths, but then I figured you were only accounting for you. By definition, "rock bottom" is enough gas for two divers to ascend at the normal rate and do standard safety stops. Maybe there needs to be another term for minimum gas needed for a solo diver?

Not at all. My math is for two divers. I don't always solo dive.

Rock Bottom and Gas Management for Recreational Divers | Spherical Chicken

 

[danvolker]

I guess you'd have to define a "90 foot recreational dive". On AIR my Hollis DG03 only gives me 19 minutes at depth. My Suunto Mosquito gave me 22 minutes. My SAC rate is .65 (not exceptional) so that's about 46CF or 1800 PSI for the bottom time portion of the dive. That's 1200 PSI remaining. 90 * 10 + 300 = 1200 so that's not a bad PSI to be ascending with. When I pop up to 45' now my rock bottom is 750. I've done the math and it should be fine.

Now if you throw in 36% and a longer bottom time... sure, bigger tanks can be needed.

No one on an advanced boat in Palm Beach uses air....Nitrox

 

[chrpai]

No one on an advanced boat in Palm Beach uses air....Nitrox

Then perhaps you should have said that instead of leaving it vague as some typical recreational dive.

 

[uncfnp]

Rock bottom, as I was taught it (by the inestimable JoeT himself), always includes all stops.

One definition of rock bottom:

RB is the minimum amount of gas needed for 2 divers to ascend safely to the surface while sharing gas from a single tank, expressed as psi (or bar for metric users) specific to the tank being used.

 

[tracydr]

No one on an advanced boat in Palm Beach uses air....Nitrox

Nitrox is 10-12 dollars a tank around here. Also, in Catalina, where we love to take short weekend trips, its hard to get nitrox, especially in winter months. The fills at the dive park are air only. When humping dive gear back and forth through town with our dive bags and/or a cart, we tend to only want to bring one tank and get it filled at the park after every dive.
The Channel Islands boat that we love to take for 3-4 day trips, air only.
So, in some crazy places they only have air or nitrox is impractical.

---------- Post added July 8th, 2013 at 05:32 PM ----------

The national 55mph speed limit, that was worse....

Geez, I hated driving 55mph!
Especially back when I was young and was driving a muscle car! 1966 mustang. 55mph, yuck!

---------- Post added July 8th, 2013 at 05:33 PM ----------

Then perhaps you should have said that instead of leaving it vague as some typical recreational dive.

As if everyone dives in Palm Beach,on boats, no less.

 

[lowviz]

Rock bottom is an absolute minimum, and thus it is intolerant of any under-estimated burdens like either one of the two divers hyperventilating, the ascent taking longer than planned due to buoyancy, coordination issues that add stop time, etc, etc...

Being at rock bottom at the turn point of a difficult dive is not at all the same thing as being at rock bottom on the anchor line at the end of a relaxing dive...

 

[danvolker]

Then perhaps you should have said that instead of leaving it vague as some typical recreational dive.

The issue is so 100% of the way people dive here....I really could not have imagined ANYONE not using nitrox that was doing charter boat dives. The only people we see using air, are rank beginners on very shallow sites...or on air at the BHB Marine Park with a depth range between 6 feet and 19 feet deep....

Sorry, I should not have made that assumption....
However, the one time I did dive Catalina, on the Great Escape, George and I used Nitrox, and I think pretty much all the others did too

 

[RJP]

No one on an advanced boat in Palm Beach uses air....Nitrox

If they were so advanced, they would know that air IS nitrox, no?

:d

 

[lamont]

This is controversial as when you do your "several stops" you are typically on-gassing as well as off-gassing.

Here's a DAN discussion about it.

Part of the problem in this discussion about "deep stops/slow ascent" vs "on-gassing" is that you're actually in an edge case where a minute or two or 10 or 20 feet make a whole lot of difference when you're talking about recreational diving. On a 200 foot dive, if you start your deep stops at 160 or 150 or 140 its not going to make a huge difference overall to the whole profile. When you're talking about 100 foot recreational dive, the differences in the profiles between "deep stops" at 60 or 50 or 40, or between 1s from half depth to the surface are very different profiles.

On a 100 foot dive on 32% nitrox, your tissue saturation is only to 60 fsw (ppN2 of 2.72 ata). What that means is that you absolutely will not be pushing a gradient in any tissue until you ascend at least that far. No bubble creation, you only get some Boyle's law expansion of bubbles that already exist. If you've been doing a lot of up and down and have violated that ceiling previously in the dive (maybe a gnarly cave profile that pushes you shallow somewhere in the middle of it) then you might have a lot of bubble volume already at that point in the dive. Otherwise, when coming off the bottom you can go fairly fast. Mostly the things to be worried about in transitioning from the bottom to the point where oversaturation begins is avoiding lung barotrauma and loss of buoyancy control -- otherwise you can go as fast as you want and largely ignore DCS issues. I'll regularly scooter shallow and let my computer scream at me about ascent rate violation when coming off the bottom. As long as you can stop and don't hurt yourself a 60fpm ascent to 60 feet on a 100 foot dive is fine, in fact, its good because it decisively slows down the DCS and gas clocks.

Somewhere around the point that oversaturation starts, you clearly need to slow down, but at 60 feet coming off a 100 foot dive you're still not pushing any gradient that matters, and neither are you pushing much of a gradient at 50. In the link you posted Bennet argues for a 2.5 minute stop at 1/2 depth coming off a 100 foot dive, and I really can't see that doing much good. There's just not much of a gradient there to warrant sitting there that long. You're probably better off taking 2.5 minutes to go from 50 to 30 and gradually start to press a gradient. And I expect that overall deco time is going to be correlated more with less bubbles than deep stops, so if you want to hang for 2 more minutes I'd do it at 30 or above where you're driving more of a gradient and where you've got less gas pressure and less concerned about ongassing.

On the two different hands, I would definitely not recommend a 60fpm ascent from 100 directly to 20, neither would I recommend "deep stops" starting at 70 feet on a recreational 100-foot dive. "deep stop" recreationally at most means stops starting at 40 or 50 (and probably bump it 10 feet deeper for every 10 feet deeper on max depth that you're coming off of)

Also, because this is recreational diving, if you need to get to the surface, just go... A 60 fpm ascent directly to the surface from 100 feet will beat drowning every time. This is where if rec divers start reading about 'slow ascents' and 'deep stops' and start spending 10 minutes coming off the bottom to the surface, with a lot of that time spent below 50 feet it is just putting the divers at more risk. We're talking about reducing an already very negligible DCS risk, and if we increase the likelihood of OOA/drowning at all in the process then the result is probably not safer divers.

 

[John_B]

One definition of rock bottom:

RB is the minimum amount of gas needed for 2 divers to ascend safely to the surface while sharing gas from a single tank, expressed as psi (or bar for metric users) specific to the tank being used.

Rock bottom is an absolute minimum, and thus it is intolerant of any under-estimated burdens like either one of the two divers hyperventilating, the ascent taking longer than planned due to buoyancy, coordination issues that add stop time, etc, etc...

Being at rock bottom at the turn point of a difficult dive is not at all the same thing as being at rock bottom on the anchor line at the end of a relaxing dive...

IIRC, those that developed the concept and coined the phrase "rock bottom" say it does include calculations for elevated SAC rate, time to focus, all normal stops, etc. Rock bottom also changes with conditions, can't be calculated without knowing your buddy's SAC rate, is the same for the buddy team, and dictates an absolute turn time.

That's why I said it might be better to call your version "minimum gas" or "chrpai's/uncfnp's/lowviz's minimum gas" (or whatever) if you're inclined to come up with your own alternate definitions. Obviously you're free to calculate that in whatever way you'd like.