Steel cylinders can hold the same amount of air as aluminum cylinders, but with a smaller size and/or lower pressure?

[RXTdiver]

Is working pressure = rated pressure = service pressure?

Or is working pressure = actual fill pressure?

 

[MR12111]

From the PADI Online course there are these parts:

1. "Steel cylinders usually hold the same amount of air as aluminum cylinders, but with a smaller size and/or lower pressure"

If a steel cylinder is smaller and is holding the same amount of air (capacity right?), wouldn't the pressure have to be higher instead of lower?​

2. How much air your cylinder holds depends on its internal capacity and its rated working (maximum) pressure. One cylinder can have a
higher working pressure than another, but hold less air because the internal capacity is lower.

This implies that the amount of air and the capacity are two different things. I thought that the amount of air a cylinder could hold is
the capacity.


3. Some websites say the amount of air is calculated by pressure x capacity. Others say pressure x size = capacity. The chart below
implies that size and capacity are two different things. These terms seem to be used interchangeably. This has me confused.
a. How is size defined?
b. How is capacity defined?
c. What is the overall equation used to calculate the amount of air in your cylinder? Is it pressure x size or pressure x capacity?

View attachment 794699


There is a lot going on here. Too many things to keep track of, so if you can answer point by point, that would be very helpful.

Great "question" great thread ...Thanks .. I just cannot see how the LP 95 can have that much volume at that low pressure ..The steel H.P. 100's I am used to are almost the same size but need 3500 to get the Volume ..Also I have never (or forgot) how negatively buoyant they are ... Just getting back into this and I'm gonna look for some L.P. 95's ..seems like the best choice for me ..Shops will more likely be able to give them good fills .. Thanks For the Help ..

 

[Angelo Farina]

Is working pressure = rated pressure = service pressure?

Or is working pressure = actual fill pressure?

Working pressure can be larger than rated pressure if, during hydro test, the tank receives a "plus"marking.
This is typical of steel LP tanks, in US, from what I understand..
Here in Europe there is no "working pressure", we have only the rated pressure and there are no "plus" markings.
We do not have burst discs, they are considered dangerous and are forbidden here.
We also do not have "total capacity" here: tanks are marked with liters (internal volume) and rated pressure.

 

[MR12111]

Working pressure can be larger than rated pressure if, during hydro test, the tank receives a "plus"marking.
This is typical of steel LP tanks, in US, from what I understand..
Here in Europe there is no "working pressure", we have only the rated pressure and there are no "plus" markings.
We do not have burst discs, they are considered dangerous and are forbidden here.
We also do not have "total capacity" here: tanks are marked with liters (internal volume) and rated pressure.

No burst discs ? Why do they think their dangerous ? Guess if your careful filling tanks and don't leave them in the heat/sun you'd be OK ?

 

[lexvil]

Great "question" great thread ...Thanks .. I just cannot see how the LP 95 can have that much volume at that low pressure ..The steel H.P. 100's I am used to are almost the same size but need 3500 to get the Volume ..Also I have never (or forgot) how negatively buoyant they are ... Just getting back into this and I'm gonna look for some L.P. 95's ..seems like the best choice for me ..Shops will more likely be able to give them good fills .. Thanks For the Help ..

85 not 95 the 95 is a big fat tank.

 

[lowwall]

Great "question" great thread ...Thanks .. I just cannot see how the LP 95 can have that much volume at that low pressure ..The steel H.P. 100's I am used to are almost the same size but need 3500 to get the Volume ..Also I have never (or forgot) how negatively buoyant they are ... Just getting back into this and I'm gonna look for some L.P. 95's ..seems like the best choice for me ..Shops will more likely be able to give them good fills .. Thanks For the Help ..

LP95 tanks have a larger diameter than an HP100. They are essentially the same size as an HP117. For steel tanks, get the HP version of the size you want. Even if the shop won't do a full HP fill, they can at least do a 3000 or 3200psi fill, so you get 10-20% more gas than the LP version of the same size tank. This may not apply to Central FL.

FWIW, the entire world's naming schemes for tanks is stupid. You need 2 numbers to understand a tank's potential and actual gas capacity: internal volume and maximum working pressure. And yet we only get 1 number. Europeans get the internal volume and Americans get a nominal gas capacity.

 

[MR12111]

85 not 95 the 95 is a big fat tank.

That chart shows it 3/4 inch bigger .. does it really seem that much bigger ?? I gotta look for a rental one to try ..

 

[lexvil]

That chart shows it 3/4 inch bigger .. does it really seem that much bigger ?? I gotta look for a rental one to try ..

The 85 is about the same as the HP 100 in the pic, it’s not easy to see here but yes they are fatter and have pretty bad diving characteristics

 

[Eric Sedletzky]

Close to 100, without doing all of the math, once they mistook an 85 for another HP 100 and I got that one back with 3800

I used to love it when they had a new air fill person working at Aquarius 2 at the Breakwater.
They’d fill my steel 72’s to 3000 (thinking they were AL 80’s) and I have an 86

 

[Eric Sedletzky]

Am I the only one to think this but is the + rating and the ten percent overfill to get the advertised cubic feet totally stupid and border on false advertisement?