Volume and "+" rating

[masterof0]

I am looking at getting a second set of tanks and have a couple of basic questions

1) If a tank is rated at 85cf with a 2400+ rating, does this mean that the tank is rated at 85cf at 2400psi or 2640 psi? (have asked this to a couple of people and got different answers)

2) With all this discussion about overfills, something is no clicking with me. For the sake of this question, if a LP85 is roughly the same volume of a HP100, then wouldn't the reciprocal hold true is well? Why buy an LP85 and overfill it when an HP100 is designed to be filled at higher pressures which ultimately gives you the greater safety margin? In a situation where you get a short fill, then the worse you would get would be 85cf, correct?

 

[DA Aquamaster]

1. 2640
2. Yes, exactly - well almost. LP tanks do not make much sense outside of N Fl where an LP 108 filled to 3600 psi will hold 147 cu ft, but you get some minor volume advantages with the LP tanks.

For example, a worthington LP 85 holds 76.3 cu ft at 2400 psi, 82.9 cu ft at 2640 psi, and 113 cu ft at 3600 psi, weighs 34 lbs, is 7.25" in dia and is 24.7" tall.

In comparision a worthington HP 100 holds 99.5 cu ft at 3442 psi, holds 104 cu ft at 3600 psi and will hold 76.3 cu ft at 2640 psi. It weighs 33 pounds, is also 7.25" in diameter and is 24" tall.

So the LP 85 is slightly longer and a pound heavier but if overfilled to 3600 psi will hold 9 more cu ft than an HP 100 slightly overfilled to the same 3600 psi. Conversely the HP 100 holds slightly less (6.6 cu ft) than an LP 85 when both are filled to the same 2640 psi.

Unless you live in North Florida and can get overfills to 3600 psi, an LP 85 still does not make much sense. Even if underfilled to only 3000 psi, an HP 100 will hold 86.7 cu ft, 3.8 cu ft more than an LP 85 at 2640 psi and 11.3 cu ft more than an LP 85 filled to 2400 psi. Simply put the odds are a lot better that you will get a better fill in an HP 100 anywhere outside of N FL than you will with an LP 85 that most shops will not fill over the 2640 10% overfill pressure and many shops will not go over 2400 psi.

To get less air in an HP 100 than an LP 85 at 2640 psi, you'd have to get less than 2870 psi in an HP 100 and most shops can flog their compressors to at least 2870 psi.

 

[rjack321]

2) With all this discussion about overfills, something is no clicking with me. For the sake of this question, if a LP85 is roughly the same volume of a HP100, then wouldn't the reciprocal hold true is well? Why buy an LP85 and overfill it when an HP100 is designed to be filled at higher pressures which ultimately gives you the greater safety margin? In a situation where you get a short fill, then the worse you would get would be 85cf, correct?

There is no "safety margin". Neither tank will rupture in your grandchildren's lifetimes. Get the hp100 if your shop is a stickler for rated pressure fills. Get the lp85 if you can get >3000psi fills easily.

The 85 is actually more likely to last 8, 10 or more hydro cycles since its only tested to 4000psi. The hp100 is tested to 5250psi and since 1) they haven't been made that long and 2) in the early years there was a learning curve on the prestretching protocols, it hard to say how long they will last. They might plausibly fail hydro relatively early if the facility makes a mistake testing them. These mistakes are alot more common on this tank type than on the 3AA lp types.

 

[DA Aquamaster]

rjack is not incorrect as some shops initially had issues understanding the round out procedure needed with special permit steel tanks.

They are in effect not exactly round due to the finishing process used and they become round as they approach the test pressure. That often registers as an increase in volume that is mistaken as excess expansion.

The same lack of understanding can occur with a new hydro facility or tester, so it is cheap insurance to attach a copy of the procedure to the tank when you send it in.

That is not an issue with 3AA tanks and it is true that 3A and 3AA tanks will last for probably a century under normal use. However the practice of 3600 psi fills in 2400 psi tanks (with a test pressure of 4000 psi) is comparatively recent and it is not yet clear that tanks filled in that manner on a regular basis will last more than a few hydro test cycles anyway.

 

[masterof0]

I will probably stick with the HP tanks. I am not in cave country so the likelihood that I can take advantage of the overfills is little to none. This also eliminates the need to maintain a plus rating. (I am not saying that there is any difficulty with maintaining a plus rating, just one less thing I need to worry about) More importantly, to your last point, I am still a little nervous about such an extreme overfill, so the effects are better understand, I don't want to be the one to prove the theory wrong.

Thanks for the feedback

 

[DA Aquamaster]

I have 2 sets of double X7-100s that I use for offshore diving as they are a bit lighter than Faber LP 95s and hold more gas than I can get outside of cave country (a 95 filled to only 2400 psi only holds 86.4 cu ft).

But I also have 2 sets of double Faber 95s for use in cave country as with a 3600 psi fill, they hold 259.4 cu ft and are a couple pounds lighter and a couple inches shorter than high pressure X8-130's. That said, if I could have found X8-130's for the rpice I paid for both sets of used LP 95's I'd have went that route.

 

[Endler's]

For example, a worthington LP 85 holds 76.3 cu ft at 2400 psi, 82.9 cu ft at 2640 psi, and 113 cu ft at 3600 psi, weighs 34 lbs, is 7.25" in dia and is 24.7" tall.

Some Worthington LP85's, are available in my neighbourhood.

Is 34 lbs true weight on land with gas and valve?

Charts say, that with valve in salt water, a Worthy LP85 is 36.8 Lbs empty.

Verses Faber LP85 weighs in at 31.2 Lbs. without valve.

Anyone dive both Worthy' & Faber LP85 cylinders, and are able to weigh in, on the differences in BOTH land weight and buoyancy differences (with equivalent weight valves)?

Thanks in advance.