Questions about LP72 tanks

[Luis H]

I read right over this when you posted it and it didn't "click". Just noticed it. Please elaborate...

The pre-test procedure is very important when testing any galvanized cylinder. Both, PST and Worthington have published bulletins specifying the pre-test procedure. The difference in test results is very significant if the procedure is not performed right before the actual test. I would make certain the hydro test facility follows the proper procedure before letting them test any of my cylinder.


You can download a better copy from VintageDoubleHose.com, but this is what the bulletin looks like:

---------- Post added August 28th, 2015 at 11:53 PM ----------

Now understood. Thanks.


On another 72 topic:

I'm making progress on alkaline electrogalvanizing. Room temp. The two best references I've found are given below.

Zinc coatings in general: http://www.galvanizeit.org/images/uploads/publicationPDFs/Zinc_Coatings.pdf
Alkaline zinc electrogalvanizing: http://chemistry.unt.edu/~tgolden/group/2643_0_art_0_lc718m.pdf

-added this info as this is becoming a definitive thread on 72's and I'm still waiting for something better than crappy zinc paint...

I have had excellent results with ZRC and other similar galvanizing paint. The zinc does form a chemical bond with the steel that is stronger than the iron-oxide. Therefore, the paint can still be scratch, but rust will not migrate under the paint.

Rust migrating under the paint is the week-link for all other paints applied to steel cylinders.

Zink rich galvanizing paint is essentially what Faber is doing now-a-days. They do have what seems to be a very good process, but it seems to rely on the iron-zinc chemical bond.



Over the decades (starting in the 1970’s at my first LDS job) I have seen several attempts at aftermarket galvanizing scuba cylinders. The outside finish on some of them looked excellent. But, on every single one of those attempts, some of the acids used in the process (cleaning or otherwise) leaked inside the cylinder and ruined it.

It is very possible that someone has been successful with aftermarket galvanizing process, but the ones that I have seen had to be condemned due to excessive rust an pitting on the inside after the process.

I have also seen a few cylinders (in different dive shops) that were chrome plated for display. I never asked how did the inside looked, but they were only used for display so it probably didn’t matter.

Just be aware that you may risk ruining the cylinder.



Note: galvanized steel 72 with 3/4" valve neck are not that hard to find. I dive mostly in salt water so I do not own any steel 72 that is not galvanized (and have 3/4" neck).

 

[lowviz]

...//... Over the decades (starting in the 1970’s at my first LDS job) I have seen several attempts at aftermarket galvanizing scuba cylinders. The outside finish on some of them looked excellent. But, on every single one of those attempts, some of the acids used in the process (cleaning or otherwise) leaked inside the cylinder and ruined it.

I started with acid baths due to "everybody does it that way". I can get a dull coating that works fine on iron/steel samples but it is just too much trouble. Looking for something easy. I have a bit of experience in electroplating from one of my past jobs.

Note: galvanized steel 72 with 3/4" valve neck are not that hard to find. I dive mostly in salt water so I do not own any steel 72 that is not galvanized (and have 3/4" neck).

I have quite a few 72's some of them have "history" and I want to continue using them, thus the reason for galvanizing.

Zinc is amazing stuff. I have a tank from a dive legend that has patches of bare steel on it that are still being protected by the traces of remaining zinc. Exactly the same effect as the sacrificial magnesium anodes in all of our mild steel home water heaters that would rust through in a couple of months. The bare steel won't corrode as long as the magnesium rod is contacting it somewhere. Take one apart someday, remove the anode, watch how fast the iron rusts, and then put a torch to the magnesium rod just for grins.

I remain convinced that a fast, safe, environmentally reasonable plating solution that will plate out a bit of dense and highly adherent zinc onto a cylinder is worth working for. Not worried about anything leaking in. Sacrificial valve and 500 psi...

---------- Post added August 29th, 2015 at 12:25 AM ----------
... double post due to edit...

 

[Eric Sedletzky]

One thing I can say, I certainly have never seen this kind of fanatical fussing over any other tank.
That says something about 72's.
People probably didn't pay any mind to them back when they were the standard and a million of them around.
Now they're like gold to a dedicated following.
If they ever quit making Standard AL80's do you think they would ever have this kind of following?

 

[Bob DBF]

If they ever quit making Standard AL80's do you think they would ever have this kind of following?

I doubt it, their main claim to fame is their price. The size helped their popularity some back in the day and they are ok if you don't use a lot of thermal protection, but there are a lot more choices today. Divers will go on about their favorite tank, but I can't remember anyone arguing for an AL80.


Bob

 

[halocline]

The nice thing about 72s is their weight and size, plus being roughly neutral when empty. It's easy to get caught up in the "more air is better" idea and end up lugging around a behemoth tank. But the truth is for many dives, you just don't need that much air and it's more fun to dive with a smaller, lighter tank on your back, especially when you are walking around with it. The neutral buoyancy makes them extremely versatile and basically takes the tank out of the ballast equation. Then the cherry-on-top for a cheapskate like me is that you can frequently buy them for $50 or less. What's not to like?

AL80s are bigger, heavier, require 4lbs of added ballast, (making their effective land weight that much higher) and only have a few more cuft of air, which is easily made up on a 72 with a slight overfill.

 

[Eric Sedletzky]

The nice thing about 72s is their weight and size, plus being roughly neutral when empty. It's easy to get caught up in the "more air is better" idea and end up lugging around a behemoth tank. But the truth is for many dives, you just don't need that much air and it's more fun to dive with a smaller, lighter tank on your back, especially when you are walking around with it. The neutral buoyancy makes them extremely versatile and basically takes the tank out of the ballast equation. Then the cherry-on-top for a cheapskate like me is that you can frequently buy them for $50 or less. What's not to like?

AL80s are bigger, heavier, require 4lbs of added ballast, (making their effective land weight that much higher) and only have a few more cuft of air, which is easily made up on a 72 with a slight overfill.

This is pretty much where I'm at.

Like Bob said, the only reason AL80's became popular was because they were cheap. Dive shops and operators could buy 20 or 30 of them at time and get the price down to below $100 ea.
Then I remember aluminums came out that were heavier thicker neutral versions of 80's and 100's making them even more awkward. I never saw the point of those.
Now those are gone and it seems the super huge water heater mega tanks are all the rage with people claiming "more 'gas' is better even if you don't use it". Yeah, but it still needs to be hauled around topside, and most of the time I can't use that much volume on one of my typical shore dives, so why haul it around?
My HP 120 is now pretty much just a boat diving tank.

The only other modern tank that comes close in feel to a 72 is an HP 100.
I think the HP 100 is currently the most popular tank overall.

 

[Luis H]

-

My favorite tank is still a rental AL80 down in the Caribbean (or in the Philippines).


When I dive with an AL80, the water is always warmer, the visibility is much better, and there are all this cool colorful critters to see.

 

[redacted]

This is pretty much where I'm at.

Like Bob said, the only reason AL80's became popular was because they were cheap. Dive shops and operators could buy 20 or 30 of them at time and get the price down to below $100 ea.
Then I remember aluminums came out that were heavier thicker neutral versions of 80's and 100's making them even more awkward. I never saw the point of those.
Now those are gone and it seems the super huge water heater mega tanks are all the rage with people claiming "more 'gas' is better even if you don't use it". Yeah, but it still needs to be hauled around topside, and most of the time I can't use that much volume on one of my typical shore dives, so why haul it around?
My HP 120 is now pretty much just a boat diving tank.

The only other modern tank that comes close in feel to a 72 is an HP 100.
I think the HP 100 is currently the most popular tank overall.

My HP100 is nice when I just need more gas. But I really can't feel any difference between my HP100 and my Al80 with 4 lb added to the cam bands. And come time for a VIS, nobody has ever found any rust in my Al80s.

 

[halocline]

The only other modern tank that comes close in feel to a 72 is an HP 100.

I think AL63s are more like 72s. They're small and light, and fairly close to the buoyancy of a 72.

 

[guruboy]

The test pressure fore 3AA cylinders is 5/3 of the stamped pressure.

For:
2250 psi , hydro test pressure is 3750 psi (the +10% working pressure 2475 psi)
2400 psi, hydro test pressure is 4000 psi (the +10% working pressure 2640 psi)
3000 psi, hydro test pressure is 5000 psi (the +10% working pressure 3300 psi)

The hydro test pressure takes the cylinder to the lower range of the material yield strength. The purpose of the hydro test is to actually verify the material lower yield strength limit. This checks the material condition (for work hardening or heat/ fire damage that could have affected the heat treatment).

All of my steel 72’s are either PST or Norris. I don’t own any Walter Kidde, but I have hydro tested two for my neighbor.

Do the WK steel tanks need the special pre-test procedure used for PST tanks?

I just had a WK LP72 steel tank from '68 fail hydro due to over expansion (14%).