71.2 cuft tanks

[oldmossback]

This question has no doubt been asked and answered before, however, I am asking again......

Why did the manufactures stop making the old style 71.2 cuft tanks? The ones that are rated at 2250psi plus 10% etc etc...........

If these are so popular for their balance and desireability, and can withstand additional pressure fills, ie: a nominal 25 or 2600 psi; why take them off the market?

I know people desire more air and other tanks meet this desire, but in general these were chrome moly as are the newer tanks, so why not up the pressure certification vs. redesigning and manufacturing heaver and large tanks........or is there some other criteria that limits this design?

Oldmossback

 

[captain]

At the time production stopped the big issue then as today was internal rust. Aluminum was thought to be the ultimate solution. The first aluminum tanks were 72's, same diameter as steel but longer but they were much more positively buoyant. The AL80 came a few years later with less positive buoyancy and 5 cu/ft more air. Divers no longer wanted steel and the rust problems. Economie of scale made the aluminum 80's cheaper than steel.
Tech diving and nitrox resulted in a new generation of larger steel tanks because of the need for more gas than AL80's could hold.

 

[oldmossback]

Thanks Captain,

I understand that........But again, there are several steel tanks out there now that it seems tried to solve the issue..but they never succeeded...PSD had that little HP 80.......Worthington has a a short 80 hp and 100 that is about the same length as the original 71.2, but both are larger dia. Seems a 71.2 at original dimensions would fit between these.
Faber has a 72, but it is heavy, and again not the same size.......yet no one reintroduced the original size 71.2......even with a pressure increase, this size would seem optimum.......ei: 24x6.75" at 2475psi plus 10% (2722cuft) would give 75/76 cuft.......or 1 or 2 cuft less than the Aluminum 80....and a much handier bottle.

Just wonder why the young engineers have to reinvent something when the answer is right in front of their nose................

 

[halocline]

The closest common tank might be the LP85, 7" in diameter and about 26" long, so it's almost the same width and a few inches longer. It's also neutral when empty, but weighs about (I think) 5lbs more than the LP72.

Certainly faber could do a slightly shorter 7" 3AA tank that would hold about 77-80 cft at 2640, and probably weigh in just under 30 lbs. That would be pretty close, and a great alternative to the AL80.

 

[captain]

Also the BC changed the way people dived, they no longer were concerned about heavy tanks, just blow some air in the BC.

 

[Eric Sedletzky]

The Al 80 took the market away for steel 72's, then when steel tanks came back into vogue 72's were too small for modern applications.
The 3442 E7 100 is overall probably the most popular tank out there right now. I'd say as far as any steel tank goes it probably comes the closest to a replacement for the 72 in size except you get 28 more CF.
I don't like HP 80's, too stumy for a guy 6'4"

I have eight 72's now ranging in dates from 1959 to 1980.
I'm about to acquire another 1960 72 and possibly a set of WW2 oxygen bomber bottles on a 1960 USD doubles manifold with a center post and a J lever on the side all for free.
I have some great friends!

 

[Luis H]

The 3AA code is very specific about the allowable stress (70 ksi max at hydro testing pressure) and the equations used to calculate the stress are also specified. You can’t just change the pressure without affecting the design.

The calculated stress is affected by the diameter and the wall thickness. If you want to increase the working pressure (and corresponding test pressure) without changing the diameter, you have to increase the wall thickness.


BTW, in this case the age of the engineer is irrelevant. The codes are very clear and a young engineer will come up with the same numbers as a vintage one.
Note: Actually , you don't really need an engineering degree to do the calculations. The calculations are relatively simple... and are well spelled out in the codes.

 

[halocline]

Okay then, Luis, could you make a slightly shorter version of the LP85 that would hold 75-80cft of air at 2640 and weigh less than 30 lbs? 7" diameter, 3AA spec, 24" tall, it could basically be like a LP72 only with 2400+10% service pressure and slightly more weight.

 

[elmer fudd]

I think HP80's are the modern equivalent of 72's. They hold slightly more air and have somewhat similar buoyancy characteristics.

The primary differences as I see them are 6" shorter, 2 lbs. heavier, 3 lbs. more negative, but in terms of total weight on your back and your belt they're within a pound of each other.

Assuming the 72's have a good fill, the air capacity is about the same. The total weight is about the same and the buoyancy swing from full to empty is also about the same.

As much as I like my 72's, the only real advantage that I can see over HP80's is their low pressure design. I can use them more safely with low pressure vintage regs and I can more easily top them off with a transfill whip. A 119 with 3300 psi in it will top off a couple of LP72's nicely, but it won't add much to an HP80.

Unless you're somebody who really likes old gear though, I don't think that would be much of a selling point. If you are chances are you'd still buy used. LP72's are still cheap and plentiful and what's cooler, a modern tank or something from 1959 with an ICC stamp, a 1/2" upright valve and the name of a defunct company on it?

 

[halocline]

I think HP80's are the modern equivalent of 72's. They hold slightly more air and have somewhat similar buoyancy characteristics.

The primary differences as I see them are 6" shorter, 2 lbs. heavier, 3 lbs. more negative,...

Those are big differences IMO. The neutral empty part is especially important. I also like the smaller diameter of the 72s; it makes for great small doubles. I assume you're talking about the worthingtons? The 28lb weight is pretty attractive to me; maybe I'll try to find a used one.