When 80 does not equal 80

[Drew Sailbum]

It's the not-exactly-secret fact of life. An S80 doesn't always hold 80cuft at rated service pressure. An S80 from Luxfer for example only holds 77.4cuft. Other sizes vary as well so that a Luxfer S72 holds 69.6cuft, but their S63 holds 63cuft!?!

But an HP80 from PST does hold 80cuft. Of course the PST is a steel tank.

In fact Catalina Cylinders makes all of its tanks to the exact volume except for the S80, which they make to the "industry standard" of 77.4cuft.

So is it just Luxfer & Catalina jerking us around? Why the heck is 77.4cuft for an S80 the "industry standard"? Any real reason for this?

I've got a theory, and wonder if anyone knows why there are sometimes such variations but not always.

 

[scubafanatic]

...so, what's the 'theory' , Drew?


Karl

P.S.....steel tanks are subject to this 'rounding' error as well.....just take a look a Faber LP tanks and compare their capacities to those listed by OMS, and realize OMS tanks are all manufactured by Faber.

 

[nickjb]

The European system is much easier ut:

 

[OneBrightGator]

scubafanatic once bubbled...
P.S.....steel tanks are subject to this 'rounding' error as well.....just take a look a Faber LP tanks and compare their capacities to those listed by OMS, and realize OMS tanks are all manufactured by Faber.

Faber and OMS use different temperatures in the volume calcs of thier tanks, it's not a rounding error.

Ben

 

[HarleyDiver]

For Nick jb

The European system is much easier

Why do you feel it's important to know how much water a compressed air tank will hold? I rarely fill mine with water as it makes it too heavy & doesn't breathe well.

Try air or Nitrox, it's a lot easier. lol

 

[CincyBengalsFan]

Drew Sailbum once bubbled...
It's the not-exactly-secret fact of life. An S80 doesn't always hold 80cuft at rated service pressure. An S80 from Luxfer for example only holds 77.4cuft. Other sizes vary as well so that a Luxfer S72 holds 69.6cuft, but their S63 holds 63cuft!?!

But an HP80 from PST does hold 80cuft. Of course the PST is a steel tank.

In fact Catalina Cylinders makes all of its tanks to the exact volume except for the S80, which they make to the "industry standard" of 77.4cuft.

So is it just Luxfer & Catalina jerking us around? Why the heck is 77.4cuft for an S80 the "industry standard"? Any real reason for this?

This has been debated for years with never a good answer. Catalina's actually hold 77.7 and the Luxfers do hold 77.4. But I don't really know what you want to hear.

I've got a theory, and wonder if anyone knows why there are sometimes such variations but not always.

This has been debated for years with never a good answer. Catalina's actually hold 77.7 and the Luxfers do hold 77.4. But I don't really know what you want to hear.

 

[Drew Sailbum]

I haven't tried to run the math to see if it works out, but here's my guess.

Possibly some manufacturer made a steel 80 that actually holds 80cuft at rated pressure. For whatever reason, the same exterior dimensions were used in constructing an aluminum cylinder. Of course the walls of the aluminum cylinder would have to be thicker to reach the same pressure, thus resulting in less gas being held at the same pressure.


Maybe? Anyone know?

 

[RichLockyer]

Drew Sailbum once bubbled...
Of course the walls of the aluminum cylinder would have to be thicker to reach the same pressure, thus resulting in less gas being held at the same pressure.

Nope. The steel 80 is a relatively new cylinder. They used to be 72cf, and were shorter than the AL80. The cylinders begin life as a disc, and are drawn over a form that ensures a consistent inside dimension. Literally, take a pizza dough disc and pull it over the bottom of a bottle, slip the bottle out, then pinch the open end closed.

Of course, they ALSO have to meet some kind of standard outside dimension, while maintaining a wall thickness suitable to contain the rated pressure.

What it boils down to is the LENGTH is what is adjusted to increase/decrease the capacity of a standard diameter cylinder.

Calling it an 80 when it's a 77.x is merely rounding for the purposes of marketing... and less than 3cf is really of no consequence... a short or slight overfill will make more of a difference than that.

How do we KNOW that the S100 is actually 100cf and the S40 is really 40cf? Start fillin' balloons!

 

[No Fish]

Now I won't be able to sleep. There HAS to be a reason...... even if it is a screwed up reason.

Was there an alloy specification change after the bottle size was established?

Maybe ......... oh never mind....

 

[tivey]

PST-Scuba

Cylinder Internal Volumes:
All rated volumes for PST-Scuba products are minimums. For example an E7-80 (replacement for the HP-80) holds a minimum of 80 cubic foot at 3442 psi at 70 degrees.

Best regards,

Thomas Ivey
PST-Scuba Product Mgr.
tivey@pressedsteel.com