steel 72's

[M&P+4]

An uncle of my wifes gave me a couple steel 72's that I just got back from hydro - I had to replace the valves and they didn't get the + rating that they got the last time they were hydro'd in '88.

How do they decide whether they are OK for the + rating or not?

[theskull]

Quite often it comes down to whether the tester understands the + and whether he cares to bother with it.

Not getting the + this time doesn't mean you won't get it back again with the next tester.

theskull

[MgicTwnger]

What exactly is the + rating?

[swimjim 144]

A '72 without the + rating can be filled to 2250 psi. If it gets the plus rating it can be filled to 2475 psi.

Jim

[MgicTwnger]

A '72 without the + rating can be filled to 2250 psi. If it gets the plus rating it can be filled to 2475 psi.

Jim
That's what I thought. I understand that new tanks come with the + rating, but what are the criteria for older tanks?
Of course you can do what I do: put 300 bar valves on the suckers and stuff them good.:cool:

[M&P+4]

I'm told the 72 with 2450 psi has more gas than an al80 with 3000 psi - it would have been nice to get the +. As it is, the valves have a 2400 psi burst disks in them.

It would have been nice to get the + but truthfull they are for my wife and we don't often dive below the thermocline. 72 cf is plenty of gas for a 30' max dive and it will lighten up the lead load.

[M&P+4]

That's what I thought. I understand that new tanks come with the + rating, but what are the criteria for older tanks?
Of course you can do what I do: put 300 bar valves on the suckers and stuff them good.:cool:

One of them was manufactured in '66 and the other in '72

[MgicTwnger]

I'm told the 72 with 2450 psi has more gas than an al80 with 3000 psi
That's backwards.:p

[theskull]

That's backwards.:p

Righto. Cubic feet is how much gas you have to breathe; pressure is just how compressed it has to be to get to the tank's rated capacity.

The steel 72 holds app. 72 cu. ft. of gas at the rated plus pressure of 2450.
The Al 80 holds app. 80 cu. ft. of gas at the rated pressure of 3000--actually closer to 77, no truth in advertising.

theskull

[SLIM]

A AL80 is not a true 80 cft tank. A luxfer is 77.2 cft and Catilinas are about the same but I think like 76 cft. It is just easyet to call them and 80it comes from years back when they first started making AL tanks.

SLIM

[VuDuLou]

...I have 2 X's the steel 120's, Baby!!!!!

[Atlaua]

I'm told the 72 with 2450 psi has more gas than an al80 with 3000 psi - it would have been nice to get the +. As it is, the valves have a 2400 psi burst disks in them.

Be carefull with that.


Often, ok always, when a steel tank is sold as X cubic feet, they are selling it as X cubic feet with the plus rating.


So in 5 years, if your don't pay, or get, the plus rating, the tank is rally X-Y cubic feet.

People are leading right whey they say to pay attention to the volume, it never changes,

Buy a 95 and it is "legally" a 95 for 5 years, longer if you extend the plus.

Then there is the whole steel overfilling... Fill a 95 to 3200 and you get a lot of air.

The plus indicates hou you are "legally allowed" to fill the tank. How it is actually filled is anoteher matter.

Then there is HP steel and Carbon. That really messes with things. They are only what they are rated to if filled to rated pressure. More often than not they are less.

Confused? Welcome to tank world.

James

[scububa]

I'm told the 72 with 2450 psi has more gas than an al80 with 3000 psi - it would have been nice to get the +. As it is, the valves have a 2400 psi burst disks in them.

It would have been nice to get the + but truthfull they are for my wife and we don't often dive below the thermocline. 72 cf is plenty of gas for a 30' max dive and it will lighten up the lead load.

What you might have been told is that a 72 with a working pressure of 2450 and plus rating, when filled to the plus rated pressure (2695) holds more gas than an AL80. The 72 which holds 72 ft3 at 2450 will have 79.2 ft3 at the 10% higher pressure of 2695. As pointed out the 80AL is a rounded off short cut designator and really only holds around 77 ft3 when filled to its rated pressure.

Properly designed/fabricated steel can get a plus rating because of the properties of steel. It has a very different material fatigue characteristic than AL. So it can repeatedly expand and contract without failure. AL does not have the same fatigue properties and is much more limited in this regard. (I'd go into alloys and annealing, yields and strengths, but I hardly understand all of that myself ;-)

This link does a good job of describing a hydro test. http://www.sounddive.com/hydro%20test%20illustration.htm
To get a plus rating the tank must return to original size after being tested to 5/3's of it's plus rating.

[sobfrogg]

how about a al80 at 3300 ??

[scububa]

Well, the math would be:

3300/3000 * 77.4 = 85.14

if you are talking about increasing your pressure 10%, you are going to increase your ft3 of air 10%, so if you know you have 77 ft3, then 10% is 7.7 ft3 and you have 84.7 or about 85 ft3.

This assumes the tank is spec'd at 77.4 ft3 at 3000 and that you can get it filled to 3300. I don't know that AL tanks can have a + rating. I have never heard of it. I have heard AL100s with a rating of 3300. I assumed that they have different design parameters.

If you have a steel spec'd at 85 ft3 at the plus pressure and you fill it to the stamped pressure

2400/2640 * 85 = 77.3 ft3

or if you overfill it say to 2800

2800/2640 * 85 = 90.1 ft3

[M&P+4]

This link does a good job of describing a hydro test. http://www.sounddive.com/hydro%20test%20illustration.htm
To get a plus rating the tank must return to original size after being tested to 5/3's of it's plus rating.

Interesting read - I wondered how the procedure worked. My browser wouldn't open up all the pictures but I got the jist.

[scububa]

Interesting read - I wondered how the procedure worked. My browser wouldn't open up all the pictures but I got the jist.

Actually I think some of the images were dead links at that site. This one opens them all
http://www.deep-six.com/page37.htm