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View Full Version : Stupid Question: Steel vs. Al



PaulThomas
10-13-2008, 20:39
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
10-13-2008, 21:06
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
10-13-2008, 21:10
So when we're talking about a tank volume, we're talking about the outside volume???

I'm completely confused...

Splitlip
10-13-2008, 21:35
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 (http://www.diversdirect.com/scuba-diving/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
10-13-2008, 22:19
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
10-13-2008, 22:46
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:


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 (http://www.scriptkiddie.org/diving/rockbottom.html); 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
10-14-2008, 00:45
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
10-14-2008, 04:22
"There are no stupid questions, only stupid people"
(Mr. Garrison, South Park Elementary School)

Jack Hammer
10-14-2008, 10:47
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
10-14-2008, 10:50
"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:smiley2:

navyhmc
10-14-2008, 10:55
"There are no stupid questions, only stupid people"
(Mr. Garrison, South Park Elementary School)

I have to disagree. The OP had a good question that we were all kind enough to help him out with. That's what divers do-help each other.

My quote is: There are no stupid questions, escept the one you didn't ask. Mind you, I have plenty of stupid answers, but your queston isn't stupid.

I hope we all gave you good answers Paul. :smiley20:

PaulThomas
10-14-2008, 11:33
[quote=scubastud;235857]I hope we all gave you good answers Paul. :smiley20:
You all did :smiley20:. I haven't read the online book yet, but it looks to be full of good information so I'll definitely read it. My OW class didn't spend much time, if any, on gas planning so I need to learn it myself.

Thanks everyone.

BTW: I already have an exposure skin, I'm still in the market for a reg, BCD, computer (?) and tanks but as I stated, I'm not in any hurry to buy. I'd rather wait and make good decisions and only buy once.

Jack Hammer
10-14-2008, 13:20
[quote=scubastud;235857]I hope we all gave you good answers Paul. :smiley20:
...Thanks everyone.

BTW: I already have an exposure skin, I'm still in the market for a reg, BCD, computer (?) and tanks but as I stated, I'm not in any hurry to buy. I'd rather wait and make good decisions and only buy once.
Not being in a hurry is probably the best thing you can do when it comes to buying gear. Try to borrow or rent different types/brands of gear to see what you like best, what you're most comfortable with, and what works best for you and the types of diving you do and are likely to do...

...then buy once and enjoy it.

Jack

CompuDude
10-14-2008, 17:04
"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:smiley2:

I don't disagree with your sentiment, but considering the OP titled the thread "Stupid Question:" I don't think the response was in any way out of line. :smiley2:

Catt99
10-14-2008, 22:32
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.

I agree with all of this wholeheartedly. I'd save my scuba dollars for many purchases before I'd look to buying tanks, and I think CompuDude's advice on how to prioritize various potential equipment purchases while at the same time concentrating on BCs, regs, and exposure suits, is bang on. I also happen to believe that the HP100 is the best choice if one were forced to choose a single tank best suited to general recreational diving -- but, and not to beat a dead horse, I'd place buying tanks at the end of my scuba purchase list.


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 agree; although many rentals are for more than a single day or a similarly narrow window of time (for example, in the SF Bay Area I can rent tanks for 5-6 days for the same cost as a day -- but of course I'd have to pay for fills after I use the initial fill that comes with a rental). I very much agree and would highlight that the principal attraction to owning tanks, IMHO, is the convenience that Jack Hammer describes.

scubastud
10-15-2008, 05:56
Quote:
Originally Posted by scubastud http://forum.scubatoys.com/images/buttons/viewpost.gif (http://forum.scubatoys.com/tanks/18076-stupid-question-steel-vs-al.html#post235857)
"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:smiley2:

Oh heavens no! I referenced the quote from the source to show humor... sorry if it was read wrong! It was a great question, actually I don't own a tank, and haven't decided al or steel, so I benifited from the post and responses as well.
(And... real men do not post smileys...hehe)

beperkins
10-17-2008, 13:28
I have a hard time getting Safety Sam fillers to give me even a good 3000 fill in my Al 80's so I know that getting 3442 psi fills is a laughing matter. I really am interested in LP steel tanks though. Might solve my short fill problems.

elijahb
10-17-2008, 20:18
The steel takes do better "cave fills"!

ianr33
05-24-2009, 21:34
The steel takes do better "cave fills"!

Nobody on their right mind would Cave Fill an Al tank.

When does a Cave Diver turn his dive?
When his tanks full! :smiley20:

Darthwader
07-04-2009, 10:05
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.

I love the search button! and, Cdude, you've come through once again. I picked up a couple of st72s in a trade, (with current hydro) and was wondering about life span compared to Aluminum and you guys spelled everything out very well.:smiley20:
I have no basis for this, but I don't trust aluminum, it just doesn't seem as durable as steel to me.

WD8CDH
07-06-2009, 09:07
The main reason for a fairly new diver to own a tank is when he wants a tank of a different size or type or valve than what is commonly rented. It's usually not a wise choice for a new diver to OWN an aluminum 80 if that is what is commonly rented.

Aluminum (assuming 6061 alloy that is used in current aluminum tanks) is just as long life as steel and handles corrosion better than steel. Actually, I would trust a 6061 alloy aluminum tank from an unknown origin more than I would trust an unknown steel tank.

But steel tanks have better buoyancy characteristics than aluminum tanks. Most steel tanks are smaller than aluminum tanks of equivalent capacity, especially HP steel tanks. For most divers an HP steel tank sized so you get enough air even if under-filled will be the most versatile choice.

Crimediver
07-06-2009, 09:48
Get steelies

navyhmc
07-06-2009, 13:23
The main reason for a fairly new diver to own a tank is when he wants a tank of a different size or type or valve than what is commonly rented. It's usually not a wise choice for a new diver to OWN an aluminum 80 if that is what is commonly rented.

Aluminum (assuming 6061 alloy that is used in current aluminum tanks) is just as long life as steel and handles corrosion better than steel. Actually, I would trust a 6061 alloy aluminum tank from an unknown origin more than I would trust an unknown steel tank.

But steel tanks have better buoyancy characteristics than aluminum tanks. Most steel tanks are smaller than aluminum tanks of equivalent capacity, especially HP steel tanks. For most divers an HP steel tank sized so you get enough air even if under-filled will be the most versatile choice.

Totally agree with the size issue on the steel tanks-especially the HP's I saw a steel 80 this weekend and it's smally than a AL50! With a true 80 cu ft. Thought my LP121 is a monster!!!!!