Aluminum 80s versus larger steel tanks
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There is no need to go back and look at dive profiles--most computers and simple bottom timers can tell you what your average depth was on a dive. That's all you need to know to make the calculations.
WARNING! Math to follow!
Some facts:
1. For most adult male divers, a SAC rate of 0.5 cubic feet per minute is considered expert status. Very serious divers, like DMs, can do better, but 0.5 is considered a very good SAC rate.
2. A SAC rate of 1.0 is considered mediocre to poor. Both GUE and UTD divers use 1.0 to estimate SAC rates for each diver sharing air on an ascent when calculating emergency reserves, because they assume there will be some heavier breathing under those circumstances.
3. An AL 80 actually holds only 77.4 cubic feet of air when filled to 3,000 PSI.
Some calculations:
1. A diver with a 1.0 SAC rate will go through the tank in 77.4 minutes if swimming at the surface. That the diver's SAC rate. That diver should go through the tank in half that time, or 38.7 minutes, if the average depth of the dive was 33 feet (2 ATA). That diver would go through the tank in one third that time, or 25.8 minutes, if the average depth were 66 feet (3 ATA).
2. A diver with a 0.5 SAC rate (expert adult male) will take 154.8 minutes to go through the tank at the surface. That diver will go through the tank in 77.4 minutes at an average depth of 33 feet (2 ATA). That diver will go through the tank in 51.6 minutes at an average depth of 66 feet (3 ATA).
In all of those cases mentioned above, I am talking about draining the tank, with nothing left. Let's say you want to surface with 500 PSI, a typical requirement. That's roughly 13 cubic feet, so you actually have 64 cubic feet with which to dive.
3. In that case, your 0.5 cubic foot SAC rate diver will use that gas in 128 minutes at the surface, 64 minutes at an average depth of 33 feet, and 43 minutes at an average depth of 66 feet.
So what are typical average depths on dives? I haven't dived Cozumel in a couple of years, but here are my averages for the last set of single tank dives I did in Florida, each one using an 85 cubic foot tank: 56, 45, 71, 47, 85, 57, 59, 55, 65, 62, 54, 56, 76, 49, 80. That's an average of 62 feet. A person with a SAC rate of 0.5 should expect to get maybe 40 minutes per dive at those average depths. Those were not really multi-level dives, though. If that diver wanted to get 80-90 minutes out of an AL 80, the dive would have to be multi-level, and the average depths would have to be closer to 20 feet.
---------- Post added May 12th, 2014 at 05:08 PM ----------
I decided to check back in my log book to see about my average depths in Cozumel. Unfortunately, the last two times I was there I was either doing very deep decompression dives, or I did not record my average depths in my log book. The last time I did record average depths was in March of 2009. Those dives were with Aldora, and they did use steel 120s. Here are the averages for those dives. You will note the typical deep dive first sequence: 53, 43, 54, 42, 59, 42, 63, 39, 55, 46, 52, 40, 61, 49.
That's a 50 foot average per dive. A diver with a 0.5 SAC rate should expect to average about 53 minutes of bottom time on those dives, leaving a 500 PSI reserve.
I did not record how much gas I had left at the ends of these dives. I can only tell you my times: 59, 68, 61, 59, 46, 71, 52, 85, 50, 47, 64, 80, 51, 71. That's an average of 62 minutes per dive. As I said, I have no idea how much gas I had left after those dives, so I don't know how much I actually used.
d_aviddiver
Contributor
If one exception disproves the theory, then consider this:
Female diver, petit (5', 105lbs), experienced (approx. 1100 dives, half in Czm), lives at altitude (7100 ft), in good condition and close to qualifying for social security.
Last dives in CZM were a couple weeks ago. She wore a 5/7 full, with a hood, and no weights. Steel tanks would make her overweighted.
I believe one of our dives was in the profile provided by Christi early in the thread, but 75 minutes on a first dive, with max. depth of 120' or so, and working up to the shallows of Palancar is a typical dive. She will be back on the boat with 500 - 1000+ psi of gas. She does not need the extra gas in a big tank to do a long dive and is returning with plenty of gas for a safety margin.
As indicated, she is a small person and doesn't like the weight of the steel tanks on the boat or in the water (she dives with a Halcyon, so there should be no question about the fit of the gear).
She is typically running on the edge of deco requirements (or a few minutes in) on regular first dives that have no decompression stops in the dive plan. It is the NDL number that controls the dive, not the number on the spg - once again, what would she do with the extra gas?
Oh, one more thing, she dives both the first dive (on air) and the second dive (EAN 36) with special tanks that Christi provides for her - they are either 60 or 62 cf.
Of her dives about 97% have been with me & I am finally coming back with about the same gas pressure she has - but I start with an 80. One size does not fit all.
IMHO, it's not the gas you start out with, it's the gas left in the tank at the end of the dive that really matters.
Female diver, petit (5', 105lbs), experienced (approx. 1100 dives, half in Czm), lives at altitude (7100 ft), in good condition and close to qualifying for social security.
Last dives in CZM were a couple weeks ago. She wore a 5/7 full, with a hood, and no weights. Steel tanks would make her overweighted.
I believe one of our dives was in the profile provided by Christi early in the thread, but 75 minutes on a first dive, with max. depth of 120' or so, and working up to the shallows of Palancar is a typical dive. She will be back on the boat with 500 - 1000+ psi of gas. She does not need the extra gas in a big tank to do a long dive and is returning with plenty of gas for a safety margin.
As indicated, she is a small person and doesn't like the weight of the steel tanks on the boat or in the water (she dives with a Halcyon, so there should be no question about the fit of the gear).
She is typically running on the edge of deco requirements (or a few minutes in) on regular first dives that have no decompression stops in the dive plan. It is the NDL number that controls the dive, not the number on the spg - once again, what would she do with the extra gas?
Oh, one more thing, she dives both the first dive (on air) and the second dive (EAN 36) with special tanks that Christi provides for her - they are either 60 or 62 cf.
Of her dives about 97% have been with me & I am finally coming back with about the same gas pressure she has - but I start with an 80. One size does not fit all.
IMHO, it's not the gas you start out with, it's the gas left in the tank at the end of the dive that really matters.
Mike
Contributor
Thank you John for taking the time to show the math that verifies my "opinion stated as fact" from 8 pages ago that has alarmed and annoyed so many.
drrich2
Contributor
d_AvidDiver:
I believe you prove the point going in the other direction; 80 cf tanks, like any other size, are not ideal for everyone, and just as some people are best served with a larger tank, some are best served with a smaller tank.
I suspect that the number better served with a 120 is great than the number served with a 60 or 62, but both situations exist.
Richard.
I believe you prove the point going in the other direction; 80 cf tanks, like any other size, are not ideal for everyone, and just as some people are best served with a larger tank, some are best served with a smaller tank.
I suspect that the number better served with a 120 is great than the number served with a 60 or 62, but both situations exist.
Richard.
Yes, you are correct John, and of course I am aware of that - but "chilly" did specifically ask:
but I'm not stupid, I know what that was about, as well as her subsequent comments - and they have nothing to do with this thread.chilly:
Secondly, as I stated, I pulled the trip logs from a random week as examples of our typical dives - I wasn't on the dives, so I don't have a computer to refer to - I was going based on the information I have from the DM's trip reports they complete each day - and each diver has a computer. The week I chose is a typical week for us and 85 - 90% of our divers. Obviously there are exceptions in both directions - I never claimed that EVERYONE gets these profiles ALL of the time on 80's.
Unlike some, I'm not worried about getting bogged down in the semantics and details on a ridiculous thread. Some people just like to nitpick and be "right fighters". To me, there are such bigger and more important things in life to worry about rather than what tank size a diver that I don't even know prefers. I don't have that much time on my hands nor do I care - live and let live!
Bottom line, it's up to each diver to decide what size tank he/she needs and wants. It's also up to each diver to be responsible for his/her own dive and to make sure that they plan their dive appropriately to have a safe level of reserve gas upon surfacing and/or in case of emergency at depth. It's also up to each diver to continue to work on and improve their skills to be able to maximize safe bottom time with enough reserve gas regardless of what size tank they use.
Some people like bigger tanks some people don't.
Some people need more gas, some don't.
The End
Hold the phone BJ, let me understand this.
So looking at my last dive March 4 of this year. My average depth was 35 feet. I was down 82 minutes.
The first dive that day, average was 51 ft and I was there 64 minutes.
Day before 52 average and 64 minutes.
So you are saying that I am on the bloody high end of expert? Is there a badge I can sew on my BC?
Seriously, while I am towards the end, I am not last out. And I dive with girls, like my missus, that I cannot beat as well as one guy in particular.
So really I am that good? And those others are fabulous? I never knew we were THAT good. Those numbers just seem low?
On this thread: http://www.scubaboard.com/forums/basic-scuba-discussions/65982-sac-rate-what-normal.html almost 45% report better than .5 SAC, so it makes me wonder. Are half the people in the basic scuba board discussion liars or expert divers? Seems unlikely.
Of course if I am wrong, where can I get my expert patch for the old BC?
---------- Post added May 12th, 2014 at 10:54 PM ----------
Well I don't fly till Wednesday, so I got time to argue. So I don't need the five minute argument, I can have the full half hour. Careful of room 12 if you know what I mean:
[youtubehq]kQFKtI6gn9Y[/youtubehq]
Were it only that easy....
This is all too stupid.
Is it ever more better to have less air than more air? We put 100cf tanks on small people and they weigh lass than Alum 80s yet carry 25% more air, and are never positively buoyant. For the extremely small outliers we put on HP steel 80s which look lie pony tanks compared to the aluminum 80s. Am I crazy to deal with the extra costs of Steel tanks? Maybe?
Dave Dillehay
Aldora Divers
Is it ever more better to have less air than more air? We put 100cf tanks on small people and they weigh lass than Alum 80s yet carry 25% more air, and are never positively buoyant. For the extremely small outliers we put on HP steel 80s which look lie pony tanks compared to the aluminum 80s. Am I crazy to deal with the extra costs of Steel tanks? Maybe?
Dave Dillehay
Aldora Divers
Why when you say it does it seem less insulting to everyone? And less argumentative too. Reasonable even.
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