Al100 Vs. Al80
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Soggy
Contributor
Depends on the depth and your SAC rate. It's ~20cft more gas. At 100 ft with a SAC of .5 cft/min, it'll give you 10 more minutes.
Here's a way to look at it.....
The common AL-80 really only holds 77.4 CF at 3000 (rated) PSI.
For lack of a gas management discussion let's say that 500 PSI or 1/6 of that gas is off limits except for emergencies. That's the minimum you want to get out with.
So 5/6 of 77.4 is: 64.5 cubic feet.
In the same manner lets look at the 100 which is either 100 CF actually or very close depending on the brand and model.
5/6 of 100 is 83.3 CF of gas to enjoy.
So let's visualize a dive profile. Make the same dive with each cylinder. The only difference will be staying on the bottom longer. The ascents and descents will be identical. Make it a nice little shore dive at 33 feet for easy math. Being a new diver let's say your SAC (a measurement of how much air you use, do a search) is .75.
At 33 feet you are breathing air at 2 atmospheres, the surface plus 33 feet. At 1 ATM you are using .75 cubic feet of per minute so at 33 feet (1atm below the surface) you will use 1.5 cubic feet of air.
The difference in usable air in the cylinders was 83.3 - 64.5 = 18.8 cubic feet.
Since you a re using 1.5 cubic feet per minute the answer is 18.8 / 1.5 = 12.5 minutes.
So what does that all mean?
Let's start with that AL80 again where you had 64.5 cubic feet to play with. Let's say that you write off 14.5 cubic feet for testing your regulator, pumping your BCD throughout the dive, ascent and descent. That leaves you with 50 cubic feet to explore at 33 feet. So take 50 cubic feet and divide by your usage of 1.5 cubic feet and you see that you get to roam around down there for just over 33 minutes.
Jump in with the 100 cubic feet and the extra 12 minutes of bottom time and you can stay down for 45 minutes, a 36 percent increase, not too shabby.
Now stay tuned because it gets better. If you dive frequently and develop a nice relaxed technique that SAC rate may easily drop to .5. That drops your consumption at 33 feet from 1.5 to 1 CFM. If you run the numbers and use 5/6 of the 100CF cylinder and allow the same 14.5 CF for other than bottom time then your bottom tiome soars to just over 72 minutes.
The big cylinder may let you save face as a novice and latter as a more seasoned diver it will extend you capabilities.
Hint... The AL80 is popular largely because it is a relatively cheap commodity. A 100 CF aluminum will be less of a bargain. Seriously consider a HP-100 if you wish to enjoy the extra capacity.
http://home.gwi.net/~spectrum/scuba_al80.html
Have fun.
Pete
The common AL-80 really only holds 77.4 CF at 3000 (rated) PSI.
For lack of a gas management discussion let's say that 500 PSI or 1/6 of that gas is off limits except for emergencies. That's the minimum you want to get out with.
So 5/6 of 77.4 is: 64.5 cubic feet.
In the same manner lets look at the 100 which is either 100 CF actually or very close depending on the brand and model.
5/6 of 100 is 83.3 CF of gas to enjoy.
So let's visualize a dive profile. Make the same dive with each cylinder. The only difference will be staying on the bottom longer. The ascents and descents will be identical. Make it a nice little shore dive at 33 feet for easy math. Being a new diver let's say your SAC (a measurement of how much air you use, do a search) is .75.
At 33 feet you are breathing air at 2 atmospheres, the surface plus 33 feet. At 1 ATM you are using .75 cubic feet of per minute so at 33 feet (1atm below the surface) you will use 1.5 cubic feet of air.
The difference in usable air in the cylinders was 83.3 - 64.5 = 18.8 cubic feet.
Since you a re using 1.5 cubic feet per minute the answer is 18.8 / 1.5 = 12.5 minutes.
So what does that all mean?
Let's start with that AL80 again where you had 64.5 cubic feet to play with. Let's say that you write off 14.5 cubic feet for testing your regulator, pumping your BCD throughout the dive, ascent and descent. That leaves you with 50 cubic feet to explore at 33 feet. So take 50 cubic feet and divide by your usage of 1.5 cubic feet and you see that you get to roam around down there for just over 33 minutes.
Jump in with the 100 cubic feet and the extra 12 minutes of bottom time and you can stay down for 45 minutes, a 36 percent increase, not too shabby.
Now stay tuned because it gets better. If you dive frequently and develop a nice relaxed technique that SAC rate may easily drop to .5. That drops your consumption at 33 feet from 1.5 to 1 CFM. If you run the numbers and use 5/6 of the 100CF cylinder and allow the same 14.5 CF for other than bottom time then your bottom tiome soars to just over 72 minutes.
The big cylinder may let you save face as a novice and latter as a more seasoned diver it will extend you capabilities.
Hint... The AL80 is popular largely because it is a relatively cheap commodity. A 100 CF aluminum will be less of a bargain. Seriously consider a HP-100 if you wish to enjoy the extra capacity.
http://home.gwi.net/~spectrum/scuba_al80.html
Have fun.
Pete
Also, the aluminum 100 is more "floaty" when nearly empty, so you need more lead to counteract that effect near the end of your dive. Steel tanks don't have this effect, being close to neutral when empty. Plus, less lead needed when diving steel.
Leadking
Contributor
25% More!!!!
Actually, that's not correct. For a Luxfer AL-80, the empty buoyancy is 4.4 pounds, but for a Luxfer AL-100, the empty buoyancy is only 3.1 pounds. The difference of a pound and a half at 500psi I've not found to really be significant enough to merit a weight change. Since the tank's centered, there's obviously no lateral trim issue, and I haven't found a need to adjust fore/aft trim when switching between AL-80/100 tanks.shoredivr:
There are only two "gotchas" about AL-100s as compared to AL-80s. First, they are noticeably heavier (10 pounds dry weight). It's a true 10 pounds heavier, too, as the added weight of the additional air is just about the same as the weight you can drop due to the reduced empty buoyancy; the tank itself adds 10 pounds.
The second "gotcha" is that AL-100s are 3300psi tanks, which some shops simply cannot or will not fill completely. If you have a shop that routinely fills HP steels, it'll be a cakewalk getting good fills, but if you have a shop that exclusively uses AL-80s, your AL-100 is likely to be a 3000psi AL-90 most of the time. Still more, but not as much as it should be.
Leadking
Contributor
The best choice for you would be a Worthington steel 100. Its 3442 psi vs alum 100 at 3300 psi and weighs 8 lbs less. It's negatively buoyant thru-out the dive, smaller than an alum 80 and price wise is only a few dollars more.
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