Stainless Steel Tanks?

[GDCB]

Wow you guys are coming in quickly. That's a lot to ponder. I'm NOT an engineer or a materials specialist. I just build the pictures they give me. This was a passing thought and knowing the wide breathe of the SB membership I knew I would get some intelligent food for thought. I was right. Thanks!

Carl

 

[ChrisA]

Are scuba cylinders made so much different or are they held to higher standard?

Steel is cheap. If you are building a welding cylinder that you want to last a long time all you need to do is make the wall thinckness "way thick" then (1) the tank will withstand abuse and (2) DOT will let you go 10 years between hydro tests.

But we want light weight tanks for scuba. We want tanks that are about 1 pound negative in water when empty so they have to make the walls as thin as possable and we pay for it with a five year hydro test and yearly inspection.

It's a trade off between durability and weight

 

[spectrum]

Thanks Pete. I didn't know that. I know 4130 is tough to work with and is usually resistant to the elements. I wonder if the same or similar tooling would work in 304/316 stainless?

Some of the welding cylinders I've used are very old pre WWII and used very hard. Banged around quite a bit. Still being regularly filled to 3k. In fact most industrial cylinders are charged the majority of the time. Fatigue isn't a big issue. Are scuba cylinders made so much different or are they held to higher standard?

Carl

As ChrisA mentions in post 9 scuba cylinders run a balancing act between capacity, weight and buoyancy. They can't just throw more metal at them. Also being a consumer item they need to be good enough and affordable.

Pressure cycles are where a lot of the aging occurs and a scuba cylinder can see multiple cylces in a day and they will see a significant temprature range as well.

Pete

 

[rjack321]

10 year hydros (called star service in the industry) are only allowed under specific circumstances and they don't have anything to do with wall thickness.

And of course there are ASTM cylinders which aren't hydroed at all. Of course they are built like M1 tanks to a completely different standard- more like boilers in a power plant than a "cylinder"

Neither type are allowed for scuba service

 

[GDCB]

Okay Let's say the PST 130 is 48 lbs (my guess) empty and the 72 is roughly 31 lbs .. How much of that differential can I take off in lead? I know it's off topic but the buoyancy comments have my brain wandering.
Carl

 

[rjack321]

Not that simple, you have to factor in internal volume which is (obviously) positive.

In the end you should be able to remove 3 lbs of lead.

 

[meesier42]

Okay Let's say the PST 130 is 48 lbs (my guess) empty and the 72 is roughly 31 lbs .. How much of that differential can I take off in lead? I know it's off topic but the buoyancy comments have my brain wandering.
Carl

Check out this site Scuba.com. They have all the actual wieghts, and full/empty bouyancy numbers for the tanks they carry

 

[rjack321]

Steel 72s can vary alot and there are many different types. E.g. MP 72s are still made. LP72s were made back in the 1970s and are are no longer made. Presumably you were talking about these older cylinders which are approximately neutral.

Your HP130 is about -2.5 empty

 

[GDCB]

rjack321 wrote

Steel 72s can vary alot and there are many different types. E.g. MP 72s are still made. LP72s were made back in the 1970s and are are no longer made. Presumably you were talking about these older cylinders which are approximately neutral.

Your HP130 is about -2.5 empty

The truth is I don't know. I went diving last weekend with a new back inflate. It was a first dive in that bc. I took AL 80 with me but rented a steel at the Breakwater. I used the steel but should have went with the AL 80. I knew my weight with the AL and my old BC. Re weighting would have been simple. But I used the steel of an unknow buoyancy. I thought it was an lp 72 like I used on a boat a while back. I dropped 4 lbs to start and was all over the place on the dive. I left the 4lbs off with the AL80 then dropped 2 more. I was able to exhale desend with a little effort with 700 lbs in the tank then hold 10' with an empty BC. All of this changing weight and adjusting trim got me thinking about owning my own tanks and what tanks would I like to own. Hence the SS tank question.
rjack321 lead me to Scuba.com which had good weight and buoyancy information. Thanks! rjack321 very helpful. The compact AL 80 @-1.8 lbs empty looks really good. BUT that's at 3300 lbs. will I have a hard time getting 3.3k fills? What's the capacity at 3000 lbs if I can't.

Carl

 

[Brewone0to]

rjack321 wrote
BUT that's at 3300 lbs. will I have a hard time getting 3.3k fills? What's the capacity at 3000 lbs if I can't.

Carl

Well, I don't have my calculator out but here goes.
Aprox. 9% less by volume.
If your 80 actually holds 80 cu ft.
Then that would be about 7.2 cu ft short of a full fill.
Or, in other words 72.7 cu ft.

Get used to that number because you'll be
lucky to find anyone that will overfill Al tanks.

Hell, sometimes I can't get a full fill on my HP100's
Some places don't have a compressor to do much over 3000 psi fills,
and other places do such hot fills that you lose 10% or better
by the time you get it in the water.