Doing the math

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

DukeAMO

Contributor
Messages
485
Reaction score
98
Location
North Carolina, United States
# of dives
50 - 99
Tatiana's thread got me thinking...

Instead of theory, I'd like to base this on my own data...
On a recent dive in the Atlantic ocean, we used AL 80s. The maximum depth was about 100 fsw.
We started our ascent from the line at about 90 fsw.
We're still pretty new divers, so our SAC rates are reasonable, but not great. I don't have exact numbers because our computers can't give us an average depth. :-( I may yet pay $80 for the stupid data cable so I can figure that out.
It took us each about 300 PSI to ascend at a leisurely rate (around .5 feet/second) to the hang bar at 15 fsw.
The 3-minute safety stop used another 100 PSI apiece. The boat was bucking, so we were not especially relaxed there. :)
Swimming to the tag line and getting back on the boat (hard exercise in current and 3 foot waves) used another 100 PSI apiece.
So that means each of us needed 500 PSI to ascend and get back on the boat safely.
If we were stressed and breathing hard, we might need double on the ascent portion, so that would be 300 + 300 + 100 apiece, or 700 PSI, to get to the surface.
A safety stop is mandatory if you've been at those depths.
You have to assume you can't use the last 200 PSI, but you don't have to use the regulator on the surface (although it sure does help).
700 + 700 + 200 = 1600
So... would 1600 PSI be a good point to start our ascent on the next deep dive of that type?

Our DM in Hawaii advised us to head up from a similar dive at 1500 PSI, but I think they were basing that on the pool-like conditions we had there.
Maybe we'll use AL 100s next time, so then we'd have to do some more fancy math to figure that out, but it's not too difficult to convert to cubic feet.

Also a good data point for learning: on the dive in Hawaii we had to use air, and we really had to watch our NDLs closely. One guy got down to 1 minute remaining mid-dive; I think I had 3 minutes remaining at that point. Then we moved 10 feet up to the upper deck. On the Atlantic dive we used Nitrox 30, and had plenty of time remaining before the NDL (closer to 9 minutes remaining at 100 fsw before we moved up to the deck). Hooray Nitrox.
 
I'm not feeling very mathy rihgt now, but just to clarify a point in your statement above. A safety stop is not mandatory. If it were mandatory it would be a deco stop. If in an emergency you have to miss the safety stop it should be ok as long as your ascent rate was good and you didn't exceed NDLs. Of course people still get bent even following the rules.

I use ScubaCalc on my phone to calculate Min ascent pressures. I have high pressure tanks so it tells me 1817psi as 100ft. (You start to see why people dive doubles.) I dive to 100 ft regularly so I dive doubles. Otherwise I wouldn't get to stay long. My doubles are sidemount.
 
Does your computer download to your laptop? If so, can you look at a graph of the dive? You can do a pretty good job of guesstimating your average depth from the graphic profile.

But to answer your question . . . Doing the math formally, I come up with 40 cf for two people to get from 100 feet to the surface, sharing gas. That's about 1600 psi in an Al80. Now, that is using a fairly conservative ascent strategy, and you can cut some of that by a more rapid ascent rate, and probably still be adequately safe.

But the real lesson here is how MUCH gas you need to reserve, and how little time at depth you have when you use small tanks to dive deep. Someone once suggested a rule of thumb of not diving deeper than the number of cubic feet in your tank, and it's a pretty good rule, and this is why.
 
It is astounding that anybody survives diving in Cozumel. 100+ feet on an AL80. And they do it over and over again, pushing NDL's if they are on tire gas, with total dive times in excess of an hour. Some of the calculations I hear about for required gas for rec diving are almost amusing when you think about the reality of tourist diving in Mexico.
 
Does your computer download to your laptop? If so, can you look at a graph of the dive? You can do a pretty good job of guesstimating your average depth from the graphic profile.
...

But the real lesson here is how MUCH gas you need to reserve, and how little time at depth you have when you use small tanks to dive deep. Someone once suggested a rule of thumb of not diving deeper than the number of cubic feet in your tank, and it's a pretty good rule, and this is why.

Unfortunately, neither of our computers (a Mares Puck and a Suunto Zoop) come with the data cable that lets you download the data to a computer, and both of their data cables are hideously expensive. Boo hiss.

Yes, it does make for some very short dives if you're on an AL 80, assuming you're being safe. I'm getting more and more motivated to buy HP 100s, because I'm not really tall enough for AL 100s to be comfortable. I really wish shops would rent the HP 100s. As for doubles... yes, I can totally see why people go that route. I'm just not ready for that much gear yet. Maybe someday.

---------- Post added August 22nd, 2014 at 01:24 AM ----------

I'm not feeling very mathy rihgt now, but just to clarify a point in your statement above. A safety stop is not mandatory. If it were mandatory it would be a deco stop. If in an emergency you have to miss the safety stop it should be ok as long as your ascent rate was good and you didn't exceed NDLs. Of course people still get bent even following the rules.

I guess you're not in deco, if that's what you mean by mandatory. It's not a "hard ceiling" where you're likely to die if you skip it, so in an emergency, you could. On the other hand, my PADI dive tables tell me a safety stop is required for all dives below 100 fsw and for some long dives at shallower depths, and my computer will go into an error mode and tell you not to dive again that day if you skip the safety stop from that depth.
 
I have done similar calculations for myself, and that led me to take a 30cf pony on deeper dives, deeper than 80ft. in cold water, or 100 ft. in warm water. The latter is difficult, as it doesn't seem easy to find shops that rent ponies in warm-water locations. Not so difficult here in Great Lakes territory. I also just bought a pair of HP119s for diving around here, that I get filled with an appropriate mix (usually nitrox 28%). This gets me around the limitations of the Al80s, and I can enjoy my NDL time to the fullest and have ample reserves and redundancy if things go wrong at depth.
 
It is astounding that anybody survives diving in Cozumel. 100+ feet on an AL80. And they do it over and over again, pushing NDL's if they are on tire gas, with total dive times in excess of an hour. Some of the calculations I hear about for required gas for rec diving are almost amusing when you think about the reality of tourist diving in Mexico.

One of my first ocean dives was in Cozumel, on AL 80 tanks filled with air. We went to the Palancar Caves (but not in any caves, just in little canyons). Anyway, the maximum depth was about 85 feet. We ended our dive in about 70 fsw, and the DM said to signal him when we got to 700 PSI, and he'd send us up to the boat in pairs as we got to that level of air remaining. The reserve gas plan? The DM said, "I don't use much air, so you can share air with me if there's any problem." Sound familiar?
 
Unfortunately, neither of our computers (a Mares Puck and a Suunto Zoop) come with the data cable that lets you download the data to a computer, and both of their data cables are hideously expensive.

You don't need a data cable to get your average depth with a Suunto Zoop. I have a Zoop but no data cable, and I can tell you the average depth of every single dive that I've done with the Zoop. The average depth is calculated and recorded automatically and readily available on the Zoop if you scroll through the on-computer logbook.
 
https://www.shearwater.com/products/swift/

Back
Top Bottom