Stupid Question: Steel vs. Al

[PaulThomas]

I've only used AL tanks but I find the current scuba deals on the steel thanks interesting. That being said, I need to be educated.

An 80 steel has as much air as an 80 AL, but their pressure if different. How do you compare with you buddy how much air each has left if you're using different tanks? I'm a new diver and during my training we used 500 psi as a pressure to get back into the boat with, so we typically came back up from 60ft when we broke 1000 psi. Would you use a lower pressure with steel tanks, 850 psi to come back up?

I understand that this would also depend on this size of the thank you'd be using. Any suggested readings on the subject, rule of thumbs?

Thanks,

p.s.: What does everyone think about the current deal on the steel thanks and how smart is it to buy a tank before a BC or reg setup? I'm not in a hurry and will buy good deals. I just don't have much time to dive during the winter and would rather rent and get what I want at a good price. I just like getting good deals

[Splitlip]

By reg first.

A steel 80 holds more gas than an aluminum 80 and has better buoyancy characteristics.

You can buy steel 80's tanks Divers direct for $209. If you have a coupon from their calander, this month you can get them for $167. ( don't have a coupon, just ask them when you get there.

Calculating gas based on remaining preassure is pretty much a simple ratio.

77.4 is to 3000 cf as "X" is to 1000 psi Aluminum solve for "X"

80cf is to 3442psi as x is to y psi for the steel then solve for "Y"


Or if you know your turn volume just sub in the equations for each cylinder and solve for psi

[PaulThomas]

So when we're talking about a tank volume, we're talking about the outside volume???

I'm completely confused...

[Splitlip]

Volume for breathing gas in the cylinder at 1 atmosphere.

I guess I complicated it. Too much information. If you are going to turn the dive at 1/3, the aluminum tank (the smaller), that would be 1000 psi. for the steel tank 1/3 is 1147. at 1000 psi the aluminum has 25.8 CF at 1 atmosphere. The steel 100's 33.3 at 1147. 30% more gas. The steel 80's 26.7 cf or 3% more.

to answer your original question. 1000 # in an aluminum 80 is equal to 888 psi in the 3442 Blue Steel faber 100s. 1110 PSI for the Blue Steel Fabers 80's below.

FABER Blue Steel FX80 Steel Cylinder 3442 PSI

EDIT Hold on! I was using a 100 CF tank. Let me redo it . sorry

Edit again. Calculator crapped out so I ended up doing it fast by hand. I think they are both right now.

Th 100's can be had for $247.

[navyhmc]

To give you the quick and easy answer, if your buddy has an AL80 @3000psi and you have a Steel 80 @3442, if you both have 850 psi, you will have 19.8 cu ft remaining and he will have 22 cu ft remaining. So your turn pressure is actually higher than his. you will both have very close to the same volume remaining if he has 850 and you have 950.

I cheated and made up a simple spreadsheet on excel to get the answer quickly.

Also, an AL80 is actually 77.8 cu ft @3000 psi and a steel 80 is a true 80 cu ft @3442 psi.

ETA: the steel 80 has 43.025 psi per cu ft and the AL80 has 38.56 psi per cu ft.

The advantage to an AL80 is the cost and the lower pressure can all but garauntee a full fill anywhere in the world. The disadvantage is the bouancy characteristics which have an AL80 about 1.5 lbs bouyant when at 500 psi.

A steel tank has much better bouancy characteristics as it is 3 lbs heavy when @ 500 psi which means less lead needed on your hips. the 3442 psi can get you less than a full fill, but the HP craze is reducing the chances of that.

If you're wanting an opinion on getting one of the tanks that ST has on sale, I would recommend a HP 100 as the weight and size is similar to the AL80 but you have more gas.

[Catt99]

Tank facts are screwy and not intuitive at all; there's no surprise you're struggling to figure this out and it's not a stupid question.

A few comments:

Originally Posted by PaulThomas

An 80 steel has as much air as an 80 AL . . .

Not necessarily. The extremely common AL80 aluminum tanks (the kind rented all over the place) actually only hold 77.4 cubic feet of air; why they're called "AL80" is something someone else will have to answer. Most steel 80 cubic feet tanks actually hold ~80 cubic feet of air.

. . . but their pressure if different.

Think of it this way -- you could hold 80 cubic feet of air in a breadbox or a phone booth. In each case, you have the same amount of air; there's just a difference in the physical size of the container which means there is a difference in the pressure of the air in each container (the bread-box-sized container will be at a much higher pressure).

How do you compare with you buddy how much air each has left if you're using different tanks?

To do so accurately, you need to do the math on the particular tanks you're diving (so you know that 800 psi on tank 1 happens to mean the same cubic feet of air held at 1000 psi on tank 2 at the same depth). For most divers, understanding the relationship between pressure and available air won't require more than a few memorizatons (because they'll focus on the tanks they own or those tanks commonly available for rental in their area).

I'm a new diver and during my training we used 500 psi as a pressure to get back into the boat with, so we typically came back up from 60ft when we broke 1000 psi. Would you use a lower pressure with steel tanks, 850 psi to come back up?

Any short-hand rules such as "back on the boat with 500 psi" almost certainly assume AL80 tanks. More importantly, such rules can be misleading or counterproductive if not understood in context of the behavior they're trying to promote (my view of the "500 psi on the boat" rule is that it is promoting being on the surface with a decent reserve -- there are other ways to promote this behavior, but I have no great quarrel with the basic premise).

I understand that this would also depend on this size of the tank you'd be using. Any suggested readings on the subject, rule of thumbs?

I'd recommend Rock Bottom and Gas Management for Recreational Divers; if you can grasp the approach and understand the math, it will help you determine your own approach to using the air in your and your buddy's tanks -- understanding the concepts doesn't mean you must slavishly follow the approach promoted.

What does everyone think about the current deal on the steel thanks and how smart is it to buy a tank before a BC or reg setup?

No opinion on the tank sale; on what to buy first, I'd say that the tank should be just about the last thing to buy -- it's simply too cost effective to rent tanks (that come filled with air or nitrox!) than invest in purchasing tanks for most divers; those that are diving regularly and frequently will see cost savings by buying -- but they'll see more by investing in other gear that they'd need to rent to dive regularly and frequently before tanks. It's usually very good advice to invest in exposure suits, regulators, BCs, etc. well before tanks.

[CompuDude]

Good advice, Catt99, and well stated.

I second the advice to read the rock bottom link, above. It gets a little complicated, but all the answers are in there.

As to which tank... I feel pretty safe in recommending the HP100 as the best all-around tank for recreational diving in the vast majority of situations. It beats the Aluminum 80 in every respect, except for cost, of course, but even there, it can hold it's own over time since the lifespan of a moderately-well-cared-for steel tank is 30+ years (and theoretically no max age), while 20 year old aluminum tanks that are still in service are pretty rare.

That said, I agree that tanks should probably be a new diver's LAST purchase. Good deals are great but there will always be good deals. At this point in your diving, buoyancy is the single most important skill to master, and the single most important part of that is a BC. Buy your BC first, then your regs. Exposure protection is a wildcard... if you can fit rentals well enough and you receive adequate thermal protection from them, the BC and regs are by far the most important parts. Otherwise, your own wetsuit or drysuit becomes arguably the most important thing, both for reasons of safety and comfort, and because after the BC, the exposure protection is the next most important part of establishing proper buoyancy.

[scubastud]

"There are no stupid questions, only stupid people"
(Mr. Garrison, South Park Elementary School)

[Jack Hammer]

Owning tanks really doesn't save you money over renting unless you dive alot. What owning a tank gives you is convenience. The convenience of having a tank to dive with on short notice and the conveniece of not having to pick up and return your tank within a 24 hour period to avoid paying an extra days rental cost.

I'd buy a regulator and exposure protection first. For a bc, some people swear by a bp/w setup, others like a stab jacket. You should rent or borrow both to see which you prefer and buy one. (lets not start that debate here ) A tank should be one of the last things you buy.

If you're still set on getting a tank, the two I'd give the strongest consideration are either a steel LP85 or steel HP100. They are very similar in size physically and both have similar and excellent bouyancy characteristics.

Jack

[Jack Hammer]

Originally Posted by scubastud

"There are no stupid questions, only stupid people"
(Mr. Garrison, South Park Elementary School)

Whereas that quote is pretty funny and so is South Park, it looks like you're calling the OP stupid for asking a legitimate question. Perhaps you might consider using a smiley to avoid misinterpretation.

Jack