Rock bottom & SAC rate calculators

[spatman]

I am coming back on this interesting subject. The link to the metric calculator does not work. Can you send me a valuable link or the spredsheet by MP? Thanks

Hello

Yes, the url has changed for the metric calculator. It is now DiveNerd - SAC Rate / RMV Calculator, Metric Units

There are also tabs on each calculator page for their metric versions.

 

[D_B]

Perhaps these should only be used as a check for newer divers learning to do the calculations, because it seems like at least rock bottom (min gas) should be quickly calculable on the fly.

Cool tools for sure though; now if you can make them for the iphone it would be perfect.

I use Lamonts Rock Bottom rule of thumb ( ... Spherical Chicken » Rock Bottom and Gas Management for Recreational Divers ) for my mostly above 80 foot recreational diving and it ties in nicely to your Rock Bottom Calculator

for my HP100 it's depth x 10 (or add a 0 ) + 300 = Rock Bottom PSI ... something even I can recalculate on the fly

 

[scubadada]

One of our local computer folks has put these resources together, and I thought they were too good not to share with the world at large:

SAC rate calculator

Rock bottom calculator

He wanted me to be sure to say that these are works in progress, but they look pretty good so far to me!

Hi Lynne, thanks for the interesting post. Sorry, but rock bottom is not for all of us. Using the app, my buddy and I would start ascent from a 100 foot dive using AL80S at 1700 psi. I just don't get the opportunity to dive and reserve that kind of gas. This is the extreme of conservatism. To each their own. Perhaps I'm just not paranoid enough.

 

[TSandM]

Scubadada, if you are willing to use a different ascent strategy from the one posited in the calculator, you can cut that a bit. A direct, 60 fpm ascent without a safety stop will reduce it to 20 cubic feet, which is about 900 psi in an Al80. Personally, I don't want to be a position to have to rocket to the surface and potentially compound a bad situation, so I'm willing to reserve the gas not to have to do so. The lesson to me was that diving deep on small tanks is something to be regarded cautiously, if not avoided, but you may be less risk averse than I am.

 

[rongoodman]

Hi Lynne, thanks for the interesting post. Sorry, but rock bottom is not for all of us. Using the app, my buddy and I would start ascent from a 100 foot dive using AL80S at 1700 psi.

Interesting. What did you use for SACs? 1300 psi is what I usually see for 100' on AL80s, and it's about what I come out with if I do the math.

Whatever your comfortable with, you have to be able to get you both to the surface with it.

 

[TSandM]

The DIR ascent from 100 feet requires 40 cubic feet of gas. Since an Al80, full, has 77 cubic feet of gas in it, this is more than half of the tank. Using the tank factor of 2.5 cubic feet/100 psi, it's 1600 psi, which would leave him 1400 psi to use on his dive. It DOES make for a short dive, which is why small tanks aren't great for deep diving. But if you don't shape the ascent the way the DIR folks do, and use the maximum emergency ascent rate and omit stops, you can cut it down. It's still a big chunk of the tank, though. If you don't reserve that much gas, you really ought to be carrying a pony or something, because if one of you has a freeflow or other major gas loss, you will NOT have enough gas to get both of you to the surface.

 

[tattooedjeeper]

Hi Lynne, thanks for the interesting post. Sorry, but rock bottom is not for all of us. Using the app, my buddy and I would start ascent from a 100 foot dive using AL80S at 1700 psi. I just don't get the opportunity to dive and reserve that kind of gas. This is the extreme of conservatism. To each their own. Perhaps I'm just not paranoid enough.

When I first joined this site there was an excruciating thread about rock bottom that continued forever and basically turned into a giant arguement, but it was very informative for me. During that thread I asked TSandM about this rock bottom calculator and she mentioned to me then that the ascent strategy used in the calculations may not be exactly what I needed and the calculator may be a bit conservative for my diving. I later found an app "scubacalc" that I use now, I add a 200psi buffer to the pressure that they recommend and it has helped me a lot with planning. Maybe these numbers will be a little more inline with what you think are better for you and your buddies. Also, if anyone with lots more experience than me find these numbers a bit too liberal please chime in I'd like to hear that as well.

 

[scubadada]

Thanks for the responses to my post regarding rock bottom calculations. I have been calculating my gas requirements for years and am very comfortable doing so. I generally dive solo and carry a 19 cu ft pony. I always have at least enough gas in my primary cylinder to make my own ascent, I could always donate my pony to a buddy with a gas deficit.

Using a conservative calculation of a minute at depth, a 30 ft/min ascent, and a 3 min safety stop, at a little more than twice my average SRMV, I need just over 17 cu ft of gas to ascend from 130 feet and 13 cu ft from 100 ft. A direct ascent to the surface from 100 ft at 60 ft/min would require a little over 3 cu ft of gas.

I probably did not give enough information in my comment regarding rock bottom, sorry

Good diving, Craig

 

[Basking Ridge Diver]

Using a conservative calculation of a minute at depth, a 30 ft/min ascent, and a 3 min safety stop, at a little more than twice my average SMRV, I need just over 17 cu ft of gas to ascend from 130 feet and 13 cu ft from 100 ft. A direct ascent to the surface from 100 ft at 60 ft/min would require a little over 3 cu ft of gas.

I was curious -
I used my own spreadsheet and backed into 13 cu ft used for a direct ascent from 100 ft depth - came up with SMRV of about 1.1 (twice your average) - or .55 for average SMRV.
I am curious if this is what you used? I am really validating my spreadsheet.
What I have noticed for my diving is my rates change with depth - as I dive deeper my rates tend to go up. So I tend to use those higher rates when I calculate my pony needs.

 

[scubadada]

I was curious -
I used my own spreadsheet and backed into 13 cu ft used for a direct ascent from 100 ft depth - came up with SMRV of about 1.1 (twice your average) - or .55 for average SMRV.
I am curious if this is what you used? I am really validating my spreadsheet.
What I have noticed for my diving is my rates change with depth - as I dive deeper my rates tend to go up. So I tend to use those higher rates when I calculate my pony needs.

My average SRMV is 0.37 cu ft/min. I use 0.75 for my calculations, assuming stress/duress. I've tracked my last 430 dives for SRMV, it actually varies very little, the standard deviation is 0.04 (i.e. 95% of my dives are between 0.29 and 0.45). Depth does not affect my SRMV, it goes up modestly when I am cold and, obviously, with exertion. My highest SRMV was 0.62, diving a deep wreck in 47 degree water with a 7 mm full suit/hooded vest and reasonably vigorous exertion.

I make direct calculations regarding my gas requirements for dives or ascents