Doing the math

[DukeAMO]

I am curious - When you took your Nitrox class and I am assuming you took AOW - but I don't know this to be true. Did they review any of the calculations to determine your Gas requirements?

Yes, we've had AOW and Rescue. I can do the calculations. AOW, recent dives, and experience with several dives to 100 feet was required for that trip. I was just getting a sanity check because heading back up the line with 1600 PSI in the tank is hard to do. "But we just got down here!" I got to the wreck with about 2300-2400 PSI left in the tank. DH's SMB dislodged on entry, and I re-wrapped it and re-attached it at 15 feet, so it took us a few minutes to get down there, but I don't think using that much gas to descend is that unusual. So yeah, those dives do make you want a bigger tank or even doubles.

 

[fire_diver]

Errol -
Your numbers check out fine - of course.
My question for me - why am I carrying redundant air for 2? If I am on a charter in NJ - everyone is required to carry their own pony or redundant air source.
Diving in general - I have never seen or heard of two divers having catastrophic failure at the same time. So if each diver cares for themselves - I would cut your numbers in half. Everyone has their own approach - I am not saying yours is right or wrong - just my approach is different. But since I am recreational and dont cut razor thin margins - it seems to work for me.
If I have a true emergency - I am not doing deep stops with or without a buddy - I may chose to do a safety stop as normal but that will be decided by me based on the failure point(s) and how close I think I came to the NDLs.

If everyone on your boat is carrying their own seperate gas reserve, then you can adjust the math to what YOU need in case of catastrophic loss of back-gas. Just keep in mind that diving with ponies is unheard of in the vast majority of places in the world.

 

[DukeAMO]

In the dives you described your limiting factor was your NDL not the amount of air you had but you also didn't say how much air you actually had left when you got back on the boat. If your limiting factor is your NDL not your air why go to a larger heavier cylinder or cylinders when you will not get anymore bottom time out of it without getting trained for deco.

The dive to 100 fsw on air definitely had NDL as the limiting factor. The DM was checking computers frequently. On that dive, the anchor line was at about 80 fsw, so we got quite a few more minutes by moving up to the high point on the wreck after the guy with the most conservative computer ran out of time. I don't know why they would only offer air in the tanks, but that's what they did. I wouldn't choose that. They probably don't have Nitrox capability. Either that or they like to take their clients on short dives and get home in time for lunch.

The dive to 100 fsw on Nitrox was limited by PSI. I still had about 9 minutes left before reaching the NDL when we headed up to the upper deck at 90 fsw, and then the number went up even more. I forget to how much, maybe to 15 minutes remaining? It sure would have been nice to stay a few more minutes, but our pressure gauges said it was time to go.

Adding a pony bottle for emergencies is actually an intriguing idea. It's less to carry than a full second tank, and if you don't need it, you can use it again on the next dive, and the next dive...

 

[packrat12]

As this keeps popping up, I will also continue to remind people of the changes in diving through the decades.
Safety Stops became ‘fashionable’ in the 1990’s as a result of data, which was proved wrong, that micro bubbles may contribute to DCS. Since this has been proven wrong, Safety Stops have stayed fashionable as a method of adding a safety margin to the NDLs and giving a controlled break in the ascent, e.g. check for boats and things at the surface. They are not mandatory in any algorithm I know of nor are the penalized by most if not all computers, except maybe the Suunto algorithms :depressed:
Ascent rate used for much of recreational diving history was 60ft/min, not the 30ft/min. The reasons chosen for the 60ft/min rate are simplistic math and not a basis in DCS. It has valid basis to use the 30ft/min rate to reduce risk to DCS.
We, the pre 1990 divers, have completed many dives past 100’ using No Safety Stop and an ascent rate of 60ft/min. This was a normal dive profile for me for much of my diving life. One difference was that we had a conservation built into us by using square profiles to describe our multi-level dive when using dive tables. I have had numerous square profile dives to NDL, though.
As we have better scientific and empirical data, we have learned to use the 30ft/min rate and built a safety margin into our plan by using a safety stop. In an EMERGENCY, I am very willing to throw out the 30ft/min rate and safety stop as needed. In an emergency, I will if needed, surface as fast as necessary. If I get the bends, I will deal with that on the surface. I also suspect it will not be a major hit as I was not over NDLs, remember this is a Recreational profile plan. I have NEVER run out of air unexpected by the way and have very good situational awareness.
Also, with regard to air supply, every foot ascent on my gas will get me a foot closer to the surface. A free ascent from 100’ is a big deal. A free ascent from 30’ should not be to any proficient diver. Yes it is best to have gas all the way to the surface with a safety stop, but in an EMERGENCY, I will take what I can get and deal with it. I suspect that his is the reason we do not hear of deaths or DCS very often from emergency ascents, they are much closer to the surface then the dive depth when they lose gas supply, when in a known emergency! I, as any experienced diver can attest to, have seen many divers breach the surface out of air without further issues. While this is less common now, it used to be quite common on the boat.

 

[fire_diver]

The dive to 100 fsw on air definitely had NDL as the limiting factor. The DM was checking computers frequently. On that dive, the anchor line was at about 80 fsw, so we got quite a few more minutes by moving up to the high point on the wreck after the guy with the most conservative computer ran out of time. I don't know why they would only offer air in the tanks, but that's what they did. I wouldn't choose that. They probably don't have Nitrox capability. Either that or they like to take their clients on short dives and get home in time for lunch.

The dive to 100 fsw on Nitrox was limited by PSI. I still had about 9 minutes left before reaching the NDL when we headed up to the upper deck at 90 fsw, and then the number went up even more. I forget to how much, maybe to 15 minutes remaining? It sure would have been nice to stay a few more minutes, but our pressure gauges said it was time to go.

Adding a pony bottle for emergencies is actually an intriguing idea. It's less to carry than a full second tank, and if you don't need it, you can use it again on the next dive, and the next dive...

Be VERY careful here! You become much, much more likely to have a DCS hit if you ride the computer's NDL line. While this isn't as big of a deal with a range from 80-100 ft I have heard from divers doing it on reef dives. Going to the max depth until they run up against their NDL, then go up 20 and hang around until NDL, then going up a little higher, etc, etc.

 

[Basking Ridge Diver]

Just keep in mind that diving with ponies is unheard of in the vast majority of places in the world.

When I found myself diving Solo to 70 feet and thinking about the risk and how my wife would kill me if hurt myself - I figured it was time to Pony up... My tolerance for risk has gone down since I have grown older... But my son is the one that is benefiting the most - I believe - when I was his age I took more risks with less sense of self preservation - now I try to point out the benefits of being self-reliant, respecting the depths and being prepared...

 

[Steve_C]

When diving locally in the ocean I sling a small pony. Assuming I have plenty of reserve, which I should, I can always pass the pony to my buddy if needed.

If you are doing a multilevel dive, then as you ascend the needed amount of reserve also decreases. The amount of gas you would need for your buddy at 50 ft is a lot less than what you need when you start at 100 ft.

 

[DukeAMO]

I like this cheat sheet too, which TSandM just posted on the other thread. http://www.tsandm.com/gm/gm.pdf

I think this sort of thing would work well for most recreational divers. Redoing a full calculation for every dive based on your buddy's SAC rate is a lot to ask of people.

 

[tursiops]

I am OW certified from YMCA - have been for a long time and last year finally took the SDI Solo Dive class (at my wife's request) - where it was hammered into us how to determine your Gas needs. I have not taken an AOW class - never needed it for the dives that I do

Interesting. AOW or equivalent is a prereq for Solo.

 

[dumpsterDiver]

I like this cheat sheet too, which TSandM just posted on the other thread. http://www.tsandm.com/gm/gm.pdf

I think this sort of thing would work well for most recreational divers. Redoing a full calculation for every dive based on your buddy's SAC rate is a lot to ask of people.

I remember our instructor teaching us the most simplistic air calculations when I was 13 yrs old. He said, if you plan for 1 cu-ft /min, it is hard (but not impossible) to get into trouble. Seems like that very reasonable approximation continues to be relevant 30 years later.

Although it makes you wonder how they took us on a training dive to 60 ft with no pressure gages and a reserve that was activated around 300 psi..