Steel tank ?..again

[Rick Murchison]

A few minor points..
1. LP tanks aren't necessarily heavier than a comparably sized AL tank. (In fact I thought both the PS 80 and AL 80 were 32, vice 32 and 31.6, but if the PS is 6 ounces heavier so be it) An OMS LP 85, for example, is 31 pounds and another LP80 (can't remember the brand) is 27.7 pounds.
Bottom line, LP steel tanks weigh about the same as comparably sized LP AL tanks.
2. The yoke limit of 3000 psi has to do with the yoke itself, and especially the o-ring arrangement (pressed, not captured) - not the first stage's ability to handle higher pressures.
3. High pressure seats and hoses and o-rings subjected to higher pressures will wear out sooner than those operated at lower pressures, all other things being equal. Like 140 mph rated tires, they'll last longer at 50 than at 100. The good news is that if you have your reg serviced regularly it shouldn't be a problem anyway.
4. Tanks overfilled routinely will last fewer cycles than tanks kept within their operating pressures.
Rick

 

[Warhammer]

Rodale’s Oct ’00 issue listed the PS LP80 @ 32lbs and the Catalina S80 AL80 @ 31.6, which is where my data came from. But the Diverlink Chart lists the PS LP80 @ 34lbs and the Catalina S80 @ 31.6lbs. So I guess it depends on where you look and who you ask. But like you said, a few ounces makes no difference.

 

[turnerjd]

For all the europeans here......

There is a draft Euro Normal just finished called EN144, which is to replace DIN and Yoke for cylinders filled with other than air (ie nitrox).

For PADI People in the UK, there is a small article about it in the member update that arrived with First quarter UJ.

Apparently, this is going to be the legal requirement within europe in about 10 years, although they are talking about removing the old style from the market in 5 years, giving then another 5 years to have removed all old style from circulation.

The valve itself is apparently similar to DIN, but a different size, and thread form.

Why did they have to get involved!

Jon T

 

[The Chairman]

Hey all,

Some great posts, with a few glaring errors in them.

1)Yes, Al weighs less than steel per volume (We are talking about the raw metals here, not the manufactured tanks). However, Steel is far stronger per unit weight. Consequently, Steel tanks have thinner walls than AL, and the total weight can be less.

2)HP Steel tanks are smaller than similer capacity Al tanks. Remember that first class in OW, where they tried to define neutral bouyancy??? Its the diplacement of the steel tank that REALLY makes the difference. My steel PS 3300psi, 120cf tank is just a tad larger and heavier than my Al Luxfer 3000psi, 80cf cylinder. Consequently, its bouyancy is far less in the water (where the unit density can be really felt).

3) Capacity is NOT the same thing as size (or displacement). Capacity is a calculated figure. Using simple gas laws (STP) and measuring the actual volume of a cylinder you can easily determine how many cf of gas at ambient pressure and temperature that cylinder can hold at X psi (don't forget that you have to use absolute pressure and temperature). The working pressure of a tank is not determined by how much air you want, but rather the strength of the tank walls.

Pete from Orlando...

 

[jobowker]

I currently dive with al80's, but they have a few drawbacks for me. 1) They are a little too long - a tank 3 inches shorter would make life easier.2)I always get short fills - if I start out with 2700psi it's a good day 3)any chance of removing even a little weight off the belt would be great. Cost aside, steel seems a better choice. I'm not going to be diving steel doubles way down deep in a wetsuit, so the worry of not having any ditchable weight is not an issue. That leaves me with LP vs. HP. It seems like HP would require me to buy either a din fitting or a din/yoke adapter, and short fills would be even more of an issue. If I went LP, short fills would hopefully be less of a problem, would require no modifications or additions to my regulator, may possibly be less wear and tear, and might even let me drop a pound or two of lead. This makes it seems like the way to go, but there's one rub. The LP 80 must have more internal volume than an al 80, since it holds around 80cf with less pressure. This means that if I surface with 500psi (which I almost always do), there's more cf left in an LP80 than in an al80. So even if I have a good fill, do I end up with less usuable air than if I stuck with my old aluminum tank?

 

[neil]

Rick Murchison wrote:
" High pressure seats and hoses and o-rings
subjected to higher pressures will wear out sooner than
those operated at lower pressures, all other things
being equal. Like 140 mph rated tires, they'll last longer
at 50 than at 100. The good news is that if you have
your reg serviced regularly it shouldn't be a problem
anyway. "

Rick, all true except that the only closing force on a modern first stage (and the seat on a 1st stage is only closed when the air is ON) is INTERMEDIATE pressure, which on modern regs is always the same regardless of tank pressure. Therefore, the seat is not subjected to higher forces when using a HP tank. The balance chamber o-ring on a diaphragm reg will be subjected to the higher pressure, but that shouldn't matter so much if, as you say, the reg is serviced. The only hose involved would be the HP hose to your comp or SPG, and that is rated to well over what it is subjected to. It's a popular misconception that higher pressure tanks wear out things faster, but I don't see it. Let me know if you have better info.
Neil

 

[Rick Murchison]

Though it is true that the IP side is what pushes the seat home and seals it, it must still overcome the higher pressure on the HP side of the seat. I would submit that the chief wear and tear on an HP seat is the rush of HP air across the seat as the seat unseats and reseats, and that the greater pressure differential between the HP and IP environments when using a high pressure tank (while it's at higher pressures) will cause more wear and tear than a lower pressure differential for the same number of unseat/reseat cycles.
Again, all this is academic as long as you replace the high pressure seat once a year as you should.
Rick

 

[devjr]

Warhammer, I used to be married to a lady diver the same size as your wife. When the Al 80 tanks first came out in the 70's I encouraged her to try one. We quickly found out that the Al80 IS TOO LONG AND TOO HEAVY. The 70 cu ft/2250 psi steel tank is the MAXIMUM that a woman of this size can safely handle. The 80 cf hp steel tank is probably worth a try if you absolutely must have the extra capacity. At least, they are short but they are still larger in diameter with the attendant roll factor, surface swimming issues, and water resistance.

 

[devjr]

Modern regulators are designed to operate at "high pressures". I see no reason or rationale to argue that equipment parts will wear out sooner. This idea probably originated some time ago when failures were reported with USD regulator first stages. Until finally corrected , there was a problem with the high pressure poppet used by US Divers. High pressure air would cause the soft seat to delaminate, leak or blow. USD replaced this with a new design and seat material claimed to operate reliably at high pressure.

 

[Bob]

Evidently Mares considers 3500psi to be high pressure. Mares is now producing a line of regulators designed to be used with the high pressure tanks now avaible. My Mares first stage comes with the caution to avoid using with pressure above 3500psi or the first stage could be damaged, resulting in a free flow of air. That's what Mares recommends and I tend to take their advise.
:tree:Bob