Steel 72 questions

[SteveC]

I think we should pick the phone back up. Spectrum was correct that the tank material has nothing to do with weight shift. 80 cubic feet of air weighs about 5 lb. 65 cubic feet would weigh less, but the shift would not be insignificant.

 

[texdiveguy]

I own one older steel 72 and really like it....mine is used for slinging as a stage bottle for bottom gas or as a deco bottle for 02.

 

[Jimmer]

I love them, I have 3 right now, one painted single, and one pair of galvanized doubles. They are so small on your back that you hardly notice them. Very little drag, and IMO the amount of air in them (assuming you got the 10% overfill) makes them equal for all intents and purposes to an al80, which are actually 77cf. I agree with everyone here that if they are clean on the inside, they sound like a good deal, particularly if they are galvanized.

Jim

 

[SparticleBrane]

Anyone ever overfill 72s? My girlfriend and I have two we're about to double up.
I know down in cave country LP tanks are frequently overfilled to 3600, although I know they're "today's" LP tanks rather than yesteryear's, so they're stronger and have thicker sidewalls.

I've never taken these two 72s over ~2600-2700, and that's only when they're getting a faster than normal fill. Otherwise I like to keep it to 2500 max. We want these tanks to last forever, so overfilling and hot filling these aren't something we plan on doing.

 

[fweber]

Anyone ever overfill 72s? My girlfriend and I have two we're about to double up.
I know down in cave country LP tanks are frequently overfilled to 3600, although I know they're "today's" LP tanks rather than yesteryear's, so they're stronger and have thicker sidewalls.

I've never taken these two 72s over ~2600-2700, and that's only when they're getting a faster than normal fill. Otherwise I like to keep it to 2500 max. We want these tanks to last forever, so overfilling and hot filling these aren't something we plan on doing.

This subject has been covered many times before, both here and on TDS. The official line is of course, to never overfill a tank, service pressure corrected for temp only. However, most foks that are routinely using old 72's have seen them filled repeatedly to 3000psi without adverse effect. But, since you have no intention or plan to ever overfill, this subject is of little consequence to you. So, why ask the question?

 

[TSandM]

We own two sets of doubled 72s and a couple of single tanks. The doubles make a good initial set, because they're fairly light, but my husband and I both found that other tanks balanced much better for us. (So we now own FOUR sets of doubles - yikes!) As single tanks, I found the 72s were adequate for me for a shallow dive, but were marginal for anything deeper. As it fell out, ours don't get used much.

 

[CBulla]

I've got 5 72s, 4 are doubled soo yeah.. I like them. With a single 72 and speedo diving, I have no lead on me at all.. in fact, I don't really start bringing that into play until Mil 3 with a wetsuit. Doubles.. lead, what lead? Paired 72's tend to balance out well for me as well.

Overfill of roughly 10% is not uncommon for me.

 

[SparticleBrane]

But, since you have no intention or plan to ever overfill, this subject is of little consequence to you. So, why ask the question?

Just curiosity. I hope that's allowed. :14:

 

[Luis H]

Anyone ever overfill 72s? My girlfriend and I have two we're about to double up.
I know down in cave country LP tanks are frequently overfilled to 3600, although I know they're "today's" LP tanks rather than yesteryear's, so they're stronger and have thicker sidewalls.

I've never taken these two 72s over ~2600-2700, and that's only when they're getting a faster than normal fill. Otherwise I like to keep it to 2500 max. We want these tanks to last forever, so overfilling and hot filling these aren't something we plan on doing.

The material on “today’s” 3AA tanks is not any stronger than those “old” tanks. Most 3AA tanks are made out of ASTM 4130 and the allowed stress of all 3AA cylinders during hydro testing is 70,000 psi (per CFR49, section 178.37).

Some of the newer 3AA cylinders are design to a working pressure of 2400 psi versus 2250 psi for the traditional 72 cu ft cylinder, but that is just a geometry change affected by the cylinder diameter and the wall thickness.

In general a hot fill will not deteriorate any steel cylinder. As a mater of fact you are allow to correct for temperature (overfill) as long as the pressure returns to the working pressure at 70º F (the maximum allowed temperature is 130ºF). If you do the math using ideal gas law you will see that at 130ºF the tank can be at 11% higher pressure than its rated pressure at 70ºF.

Therefore, since most of my tanks have passed the requirements and have been stamped with the “+”, their working pressure is 2475 psi. Under a hot fill they can go as high as 2747 psi (or 11% above working pressure). This is technically not a true overfill since it will return back to 2475 psi at room temperature.

I do not recommend over fills, but I am aware of many steel 72’s that have been used for years in the Florida cave region at pressures of 3000 psi. Statistically those thanks seem to pass hydro for a long time with out any signs of degradation.

Under highly controlled conditions overfilling a steel 72’s to 3000 psi is technically not detrimental to the integrity of the cylinder, since the test pressure is 3750 psi, but under real life conditions it is not leaving much of a safety margin. Material yield (in a structurally sound cylinder) should not start below the test pressure, but I do not recommend pushing it, especially not on a regular basis.

If on occasion a dive shop operator fills a steel 72 to 3000 psi by accident, I don’t worry about it. I would just make sure it doesn’t get any hotter before I dive it.

 

[DA Aquamaster]

I also dive double 72's and they work well on dives to about 150 feet for bottom times of 25minutes, but that is the limit with a 1/3 rd reserve. Still, I will keep them for NC coast wreck dives where Iam told 130-150 ft max is reasonable on most wrecks and where run times over an hour are not always a good idea anyway.

But I am also in the market for larger 100-120 cu ft doubles to allow deeper depths and longer run times.

As singles, they are basically as good as an 80 (assuming your air consumption os slightly better than your 80 equipped buddy) but are a few pounds lighter, a little shorter and allow you to take off about 4 lbs of lead weight compared to an 80.

The generic steel 72 does vary a bit from company to company but in general they hold 71.2 cf at 2475 psi and about 65 cf at 2250 psi.