Steel vs Al

[FredT]

Originally posted by SpyderTek
How is it mathematically possible for me to get more gas into a smaller container at less PSI? And if it isn't possible, then why would I want a steel tank? Aren't they heavier than standard Aluminum?
ST

Three basic facts concerning steel and aluminum tanks, and the metals they are made of.

1. steel is abuout three times the density of aluminum. (0.2833 vs. 0.098 lbs.in^3)

2. Steel is about 1/3 the cost per unit weight of aluminum, thus a one cubic inch block of steel and aluminum is about a wash cost wise. Generally one base metal is ±10% of the other, with relative position being depending on energy pricing and metal stockpiles.

3. The useable strength of Al under the fatigue loading seen in tanks is LESS than 1/3 that of a good medium alloy steel, thus Al tanks often weigh MORE on deck a than comparably rated steel.

These facts lead to a few other relationships of interest.

1. The amount of air a tank holds is related to it's INTERNAL volume, not the external size of the tank.

2. In order for the steel and Al tank to cary the same volume of expanded gas at the same or similar pressures the internal volumes must be roughly equal.

3. Going up to point 3 above, an Al tank HAS to have a wall over 3 times thicker than a steel tank of similar pressure rating.

4. Buoyancy is a function of water displacement volume verses mass of the item displacing the water. Since the steel and Al tanks will have about the same internal volume and the weight of the metal is about the same in either tank due to the strength and density issues involved the Al tank MUST displace more water, thus the Al tank will be more buoyant. (The new Luxfer neutral tanks add Al to make buoyancy, not to increase strength. The metal is added at the tank base, where it is needed to counteract the "ass light empty" characteristics of the earlier Al tanks for trim, not at the top shoulder where neck cracking is still a possibility and a bit of extra metal would be handy as insurance.) A more buoyant tank may well mean more lead on the belt, thus your total gear load gets heavier if you dive with a dry suit, or if you naturally float a bit.

FT

 

[sharpenu]

Steel tanks are heavier than aluminum. That is actually a benefit. I will get to that in a minute. There are a few advantages to steel. They are:

1 My steel 120 is rated for 120cuFt at 3500 psi. Even at 3000psi, it holds 100cuFt of air. An Aluminum 80 holds 77.4cuFt at 3000psi.

2 Steel tanks as likely to develop stress cracks in the neck the way Aluminum tanks do. That means they last longer.

3 My steel 120 has a bouyancy of -1 when empty. An aluminum 80 is +4 when empty. That means less weight on the weight belt and better trim.

The disadvantages to steel:

1 They are heavier when not in the water.

2 The high pressure steel tanks require a din fitting on your reg, instead of the yoke fitting commonly found on tanks. If you want to rent or borrow a tank from somewhere else, you need an adapter to screw on to your reg.

All in all, I LOVE my steel tanks. I still use Aluminum tanks, but I do prefer the steel.

 

[devjr]

Rix, tanks "pressed"(deep drawn)from billets are better. These are seamless, relatively strong and lightweight. Common brands are PST(duh) and Faber. Adequate, but relatively heavy, are the seamless tubes which have been spun. Typical brands are Asahi and Heiser. Physically, the pressed type has a flatter bottom. The spun tank has a rounded, thicker bottom sealed with a plug. The spun tank is typically longer than a pressed tank, other things being equal.

Steel tanks have some advantages as discussed. That is not to say that the cheaper aluminum tank is to be dismissed. Your question seems vague but I think a set of double steel 80's might be in your future.

An interesting fact: A HP steel 100 weighs the same as an aluminum 80, 31 lbs. A Luxfer 80 has 77.4 cf actual.

http://www.scubadiving.com/gear/27tanktips/stats.shtml

 

[meekal]

quote:
--------------------------------------------------------------------------------
Originally posted by SpyderTek
How is it mathematically possible for me to get more gas into a smaller container at less PSI? And if it isn't possible, then why would I want a steel tank? Aren't they heavier than standard Aluminum?
ST
--------------------------------------------------------------------------------

from a very un-scientific point of view, what happens on the LP steel is that 'most' places will over fill them. that's what that +10 is for; well from what i'm hearing, 'most' places will actually +20 or even better at times! at my LDS they're quoting that

al80 = 77CF

FABER 95 = 122CF
FABER 108 = 140CF
FABER 121 = 156CF

 

[badgers]

from a very un-scientific point of view, what happens on the LP steel is that 'most' places will over fill them. that's what that +10 is for; well from what i'm hearing, 'most' places will actually +20 or even better at times! at my LDS they're quoting that

***************
what is a LP steel tank hydo tested to?

 

[FredT]

Originally posted by badgers
what is a LP steel tank hydo tested to?

With a few exceptions, Hydro is at 5/3 working pressure.

As an example a tank with a working presure of 3000 psi is hydrotested at 5000psi.

FT

 

[Darian Dunn]

No one has mentioned Steel + salt water = rust Fe3O2????
I thought this was a BIG problem?

It is time for me to buy a set of doubles, and I had almost decided on a set of AL80s before reading this thread, now I am just confused.

 

[FredT]

Originally posted by Darian Dunn
No one has mentioned Steel + salt water = rust Fe3O2????
I thought this was a BIG problem?

It is time for me to buy a set of doubles, and I had almost decided on a set of AL80s before reading this thread, now I am just confused.

This has been beaten to death several times, on many boards. However if you're just coming into it it still has value to go over it again. A lot of information can be had off Diverlink. See: http://diverlink.com/gear/gearguide.htm#tanks for a general treatise. There are several other links on that board to tank construction, buoyancy characteristics, maintenance techniques, etc.

Steel tanks do corrode if you allow them to get water or moisture inside. So will aluminum, but with aluminum you just don't call the corrosion products rust. The big difference is that once the corrosion starts in an aluminum tank it is scrap! Maybe not this year, but there is no way to get to the bottom of the corrosion cell and stop the pits from getting worse. It will eventually fail visual due to pitting depth, although it may still pass hydro. Rarely will an aluminum tank with pitting cells active survive in useable condition more than 5 years. A steel tank with light to medium internal corrosion can usually be salvaged with a phosphoric acid rinse combined with a light tumble. I have several tanks that have had this treatment up to 15 years ago that continue to pass visual and hydro. The phosphoric acid chemically kills the local corrosion cell on a steel tank, and provides a light protective film that is especially resistant to repeat corrosion a the same location.

External corrosion on a steel tank is an indication that the galvanizing is failing due to water being trapped next to the tank during storage. Wire brushing and a fast phosphoric acid rinse followed by cold galvanizing will rehab most external corrosion on a steek tank, then change your storage method!

I have many steel 72s, some of them I've been diving regularly since '68. A couple of the first ones I bought (already well used) had been originally manufactured in the early 50's. They still pass visual and hydro. Those 1/2"NGT tanks have been converted to emergency oxygen bottles due to the fact that SCUBA yoke valves and valve internals are no longer available to fit them. The tanks are fine, the valves just wore out from heavy use.

FT

 

[large_diver]

Jim,

Just to clarify a point above -- HP steels weigh LESS than AL tanks of the same capacity (dry land weight). HPs also weigh less than LPs (from Roakey's Diverlink site posted above -- PST HP 120 = 38 lbs; PST LP 120 = 52 lbs -- not sure if these stats are with tank full or empty, but you get the picture).

So, to go back to your original question -- one thing to do is ask yourself what kind of diving you do now and do you expect to do in the future? This may help you answer the question yourself.

A few random thoughts:

- Do you dive locally (New England) with drysuit or heavy wetsuit? Steels in general have negative buoyancy, helping you get weight off your belt.

- Do you dive shore sites where you may have long treks from vehicle to the water? Perhaps HPs make sense (lightest dry land tank per volume of the the choices above).

- Some folks say that their local shop has trouble providing complete HP fills -- something to explore.

- Some folks say HP tanks put additional stress on gear...but I've heard many others say that this is not a big concern given the high quality of today's gear

- Will you be diving doubles at some point (tech or deeper diving)? If you will be diving with a wetsuit, steel may not be the right choice, since with double steels you will now be wearing a significant amount of "non-ditchable " negative buoyancy (your tanks), making it difficult to swim up fom depth in the event of a BC failure....and with your wetsuit compressed at depth.....

- Most HP steels require DIN valves -- while this doesn't have to be a big issue...something to consider. Some divers prefer DIN due to a more secure connection. If you travel, you likely won't find DIN tanks, but converters are available for DIN regs to be used on yoke tanks (although not vice versa).

- Steels are more expensive than AL for tanks of similar capacity

Do some more reading in the "TANK" section of this board.....many threads on Steel vs. AL....while you may not get a definitive answer, you'll quickly become aware of the issues to be explored.

Also -- read the Diverlink tank link in FredT's post above -- this lays out the issues quite nicely.

One other suggestion -- explore this tank FAQ from scubadiving.com

http://www.scubadiving.com/members/gearreviews.php?s=178

Good luck!

Chris

 

[large_diver]

Jim,

Sorry -- just went back and read your original post -- you dive dry.

Then I would say steel.

Mfr.: Pressed Steel, since most folks tend to agree that the exterior finish on the Pressed Steel tanks is the most durable.

Where to buy? LDS, since you may be able to negotiate some free fills as part of the deal. Not sure where you live in CT, but I've always been impressed with the prices at Rex Dive Center in Norwalk. No idea if they sell Pressed Steel, but it's worth a shot if you are close to Norwalk.

http://www.rexdivecenter.com/rexdc/

-C