vintage lp50s

[runsongas]

is there any reason to dive the older 1800 psi 3aa tanks over more modern tanks other than nostalgia?

they seem to be heavier, fatter, and more positively buoyant than the more modern 2400 psi faber lp50. i was expecting them to be lighter considering the lower service pressure

 

[Bob DBF]

Nostalgia is a good reason and most likely the best reason, unless you have a set laying around your dive locker. Waste not want not.

The reason the new tanks are better is that there are better stronger alloys available, and have been designed for SCUBA . The 1800# tanks may well have been designed for another use and repurposed into SCUBA tanks back in the day.


Bob

 

[tbone1004]

@Bob DBF the 3aa tanks were designed to a 3aa engineering standard, definitely not designed for scuba. Just happen to have the right neck threads that we use. definitely no reason to not use them if you want to, but there are a lot of reasons to use modern higher pressure tanks if you have them available.

 

[2airishuman]

is there any reason to dive the older 1800 psi 3aa tanks over more modern tanks other than nostalgia?

None that I've been able to figure out. I have one, and only use it for regulator service and shop air.

they seem to be heavier, fatter, and more positively buoyant than the more modern 2400 psi faber lp50. i was expecting them to be lighter considering the lower service pressure

The 1800 PSI 3AA cylinders are typically 53 cf and weight 23 pounds. The Faber LP50 cylinders are 50 cf and weigh 19 pounds. They are also 3AA cylinders, but with a 2400 PSI rating. I believe the difference in weight is mainly due to tighter manufacturing tolerances.

Typically 3AA cylinders all have about the same ratio of weight to usable capacity regardless of pressure rating. Except for some antiques (twin 38s, for example) that used the 3A standard, all LP SCUBA cylinders are made to the 3AA standard. The higher the pressure rating, the more negative the buoyancy becomes, and vice versa.

3AA2400 cylinders are approximately neutrally buoyant when empty, which is generally a good thing. I have a 3AA3000 cylinder which is 9 pounds negative when empty, which makes it useful for diving a single tank in 7mm.

The reason the new tanks are better is that there are better stronger alloys available, and have been designed for SCUBA . The 1800# tanks may well have been designed for another use and repurposed into SCUBA tanks back in the day.

In general any cylinder with 3/4" NPSM threads was probably designed and intended for SCUBA service, because no other application uses this thread.

Substantially all 3AA cylinders are made from 4130X alloy, although a few others are permitted by the CFR. 4130X is a fairly broad specification. The newer HP cylinders are made from highly specified alloys that fit within the 4130X specification, and are more carefully heat treated, to provide a higher tensile strength steel and therefore a somewhat thinner wall.

@Bob DBF the 3aa tanks were designed to a 3aa engineering standard, definitely not designed for scuba. Just happen to have the right neck threads that we use. definitely no reason to not use them if you want to, but there are a lot of reasons to use modern higher pressure tanks if you have them available.

All steel scuba cylinders are designed to 3AA except for the newer HP cylinders starting with the PST cylinders made under E-9791. LP50, LP72, LP85, LP95, LP108, the older LP80 and older HP95, HP40, HP13, are all 3AA. The pressure ratings vary from 1800 to 3130 PSI.

The PST, Worthington, and Faber HP cylinders are/were made under different special permits, but the permit terms are nearly identical. These are all 3500 PSI cylinders by design although most are derated to 3442 PSI for regulatory compliance. The wall thickness is about 70% of the wall thickness a 3AA3130 cylinder of the same size would have.

 

[dead dog]

You can carry less weight around your waist with these tanks.

 

[runsongas]

from what i could google up these become positively buoyant when empty whereas the fabers are about neutral. so you would need more weight compared to the fabers and they are heavier also.

 

[Fish&beer]

I have a twin set of US Divers 1800 psi 3AA 53's (52.8 cf) dated 1971 that I dive with often. Here are the specs that I have found on my twin 53's-
1800+10%= 1980 psi (twin 48's at 1800 psi)
Twin dry wt full- 57 lbs.
Aprox. 2.75 inches shorter than a 72 cf tank and 6.9" in diameter.
-2.5 lbs full in sea water
4.5 lbs buoyant at 260 psi (13 cf)
When I use them I just pump them up to 2000 psi and throw on an extra 4lb wt and have no problems. Like Bob DBF said, I love the nostalgia using them w/ my DH regulator. Plus the low pressure is easy on my old Bauer compressor. IMO use them if you got em.

 

[John C. Ratliff]

I have a set of those USD twin 53s, and use them a lot. I also throw on some weight to make them neutral (I dive mostly freshwater). These replaced my twin AL 50s, which I have decommissioned due to the potential stress cracks on their threads. I like them, and also use them with my double hose regulators. You can see them in use in the YouTube video below.

U.S. Divers Company added them as an alternative specifically for women divers, as they shorter than a single 72. Here's their description in their 1972 catalog:

SINGLE 53--YELLOW
A tough Molycrome-Steel tank with yellow Tuff-Koaté exterior and 0525 "J" valve or 0530 "K" valve. 52.8 cu. ft. air capacity.
A. 0670 with "J" valve reserve..........$117.95
A-1 0672 with "K" valve/no reserve...$102.95

SeaRat

 

[Bob DBF]

In general any cylinder with 3/4" NPSM threads was probably designed and intended for SCUBA service, because no other application uses this thread

I must have missed the thread size in the OP. Over the years I have dealt with a number of tanks that were adapted from, or used, pipe thread to be used for SCUBA.


Bob

 

[2airishuman]

I must have missed the thread size in the OP.

Let's call it a rebuttable presumption I made.

The rest of the story is that there are very few 3AA1800 cylinders that were repurposed for SCUBA use. The only 1800 PSI cylinders close to the shape and size of a SCUBA cylinder that have been made are CO2 fire extinguisher cylinders. Those were almost all 3A, not 3AA, for cost reasons, and because of the thicker wall they are negatively buoyant.

But the presence of 3/4" NPSM threads is a pretty sure sign the cylinder was manufactured specifically for diving.

Over the years I have dealt with a number of tanks that were adapted from, or used, pipe thread to be used for SCUBA.

The earliest LP72s had 1/2" NGT (pipe thread) necks. Steel medical gas cylinders do, also, and are sometimes repurposed as stage cylinders. Most of these are spun and are stamped "SPUN." Aviation oxygen cylinders, back when they were steel, had NGT threads, either 3/8" or 1/2" depending on the cylinder size. All kinds of oddball stuff out there too, and some of it manufactured with SCUBA in mind.