teknitroxdiver
Contributor
LOL. I guess I did all right, seeing as I figured the volume of a 72 out to six decimal places using the data of a different tank...
Detmining cubic footage
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It's probably a "72."
But a "72" doesn't really hold 72CF, even at 10% over, where it holds 71.2CF.
At 2250 it holds only about 65CF.
E.
But a "72" doesn't really hold 72CF, even at 10% over, where it holds 71.2CF.
At 2250 it holds only about 65CF.
E.
teknitroxdiver
Contributor
Hmm. So that must not be a 72 then, because I would've come out with 65 or so, right?
No, it probably is a "72".teknitroxdiver:
The steel used in "72's" isn't as strong as the steel (chrome moly) used in Faber 85's, so the wall thickness of the "72" (and the bottom and the neck) is greater than the wall thickness of the Faber (OMS), and the interior volume of the 72 is a bit less than the Faber 85.
E.
One thing about old steel tanks, the older 72s that had the markings and measurments as you described, some of them, especially those sold by USD in the late 60s and early 70s may have an epoxy inner liner. This is an nearly impervious coating. One of my tanks has this and it has been rejected for a VIP unless I remove the coating which can be costly if some one could even be found to do it via tumbling. Here is the thing---supposedly rust can form under the coating and go unnoticed---I say baloney. The rust gets under the coating when the coating is damaged by diveshop idiots poking at it with steel rods and pipes and whatever they can manage to stick into the cylinder thus damaging the integrity of the coating and rust resulting. If an undamaged liner were to have rust the coating would bubble away and flake. Nonetheless this logic will fall upon deaf ears.
A steel 72 is one of the best tanks you can have, it has near neutral bouyancy and is fairly compact and holds plenty of air for recreational diving. The capacity as I remember is closer to 68 cf at 2475 and less at 2250. I like diving them and have several and plan to get more. The diameter being 6.9 inches make sure your BC/BP can adjust to accept the smaller diameter. Alum 80s are typically 7.25 inches diameter and many tanks now are 8.0 inches etc. N
A steel 72 is one of the best tanks you can have, it has near neutral bouyancy and is fairly compact and holds plenty of air for recreational diving. The capacity as I remember is closer to 68 cf at 2475 and less at 2250. I like diving them and have several and plan to get more. The diameter being 6.9 inches make sure your BC/BP can adjust to accept the smaller diameter. Alum 80s are typically 7.25 inches diameter and many tanks now are 8.0 inches etc. N
It is most likely a steel 72 as not many other tanks with those general dimensions used a 2250 psi service pressure. They are normally approx 25 1/2 to 26" high and 6.9" in diameter. A steel 72 will hold 64.7 cu ft at 2250 psi and 71.2 cu ft at 2475 psi with the 10% overfill. My guess is the 27" measurement is with a fairly tall tank boot on the tank.rushrhees:
Do you really want to by an old used tank from someone who is not a diver, can tell you nothing about it, and that you can't even see? Take a trip to your LDS and buy a new one.
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Square half the diameter (radius) of the tank, multiply the results by pi and then mulitply that result by the length of the tank. Now divide that number by 144 and you'll have your approximate volume in cubic feet.
Doc Thomas
Registered
rushrhees:
Hey... Tom's got the right idea... pass it up. A few years ago a dive shop in Joplin Missouri tried to fill a tank that had been heated with a hot air gun. The owner of the tank didn't pass that bit of important information along to the dive shop operator. Apparently the owner of the tank had decided to change the color of the tank and used a hot air gun to put the finishing touches on the new paint job. The hot air weakened the tank's structural integrity, and when the shop owner got the tank pressurized, it blew. The owner survived, but became largely deaf in the explosion. The tank went through the wall of the shop, destroying the room, and landed in the street almost 50 yards away. In other words, if you don't know the history of the tank from production to your purchase, you could be buying a fatality. Don't do it. (And, by the way, again Tom seems to be right, I think you're dealing with an old steel 71.2 cf tank -- if that's the case, it's an antique to boot.) Doc
I appreciate people's conservatism, but most of the fears being expressed are based on mis-information.
Using a heat gun to strip an aluminum tank is a definite no-no as is using a power sander or wire wheel or baking the tank in a paint booth to cure the new paint as all four actions can potentially generate enough heat to ruin the heat treating of the aluminum. Many dive shops consequently will require a new hydro test of any used aluminum tank with a new paint job. (so if you repaint one, wait until the hydro test is due anyway.)
But none of that has anything to do with a steel tank as the same stripping and painting processes are not going to affect the temper of the steel. Plus, with an old tank that has been out of service, it will need a new hydro and VIP before you can use it anyway. And even if the hydro is current, if the tank is suspect, you can have the hydro repeated anyway.
Steel tanks also have a lifespan of at least a century as long as they are properly cared for and inspected. We routinely have steel welding tanks come through for hydro testing from the 1910-1920 era and they still not only pass hydro but still qualify for a plus rating. I personally own several steel 72's from the 60's and 70's that see extensive service every year and they still pass hydro with flying colors. In my experience, they are actually less prone to problems with rust internally than our newer faber steel tanks which seem to require tumbling more often.
So unless the tank has been stored empty with the valve off or been stored with standing water in the tank, it is unlikely to have extensive rust or pitting inside. It may require tumbling to remove a build up of flash rust, but this is a pretty normal procedure for any steel tank every 10 years or so.
Some tanks were internally coated with an epoxy finish and this coating can take about a week on a tumbler to remove, but is something that most shops can be talked into doing on the off season when the tumbler is idle anyway. A plastic coating on the outside is also a possibility. But contrary to popular opinion if the coating is fully intact it is much like any other paint and need not be removed for a visual inspection. If however it is chipped or peeling complete removal will be needed to ensure no rust is forming under the coating. Unlike the internal coating, it is also relatively easy to remove with a normal chemical stripper. Some tanks are galvanized under this coating and end up looking virtually new. Other tanks are not galvanized and will need to be painted. In my experience though, externally plastic coated tanks are relatively rare as steel 72's go - maybe 15% of the toal number out there. The number of internally coated tanks is even lower as if they were in active use, most had the coating removed in the mid to late 80's when the problems with the coating became a concern.
I have experiemented with several types of paint including spendy professional paints like Imron and I have yet to find anything short of powder coating that will stay on a tank long term. So if you have an ungalvanized steel tank, spray paint it with a rust resistant paint and just plan on touching it up or repainting it every year or two. I personally use a stainless steel colored paint made by Seymour with a high percentage of zinc in it designed for painting steel tire rims. It looks very nice, minimizes rust in any scratches and is about as durable as anything else I have found.
One of the larger issues with a steel 72 is the potential for it to have an older style 1/2 valve. This was the standard way to do it in the 50's and the change to 3/4" o-ring sealed valves was made in the early 60's. These tanks are still often in excellent condition, but the valves are very hard to find if they need replacement. Many are also a straight stem design and do not offer a covenient carrying handle.
So in summmary the issues to be concerned with when buying a used steel 72:
1) 1/2 valve? (it will be sealed with white thread tape visible at the base of the valve)
2) Hydro test dates? (first date will tell when it was made, last date will give last test date and spacing of the dates in between give agood idea of how consistently it was used in service. If used constantly, it will have a test date every 5 years - that would be a good thing)
3) does it have air in it? (a good thing if it does as it reduces the potential for water to have entered the tank. A potentially bad thing if it does not have some pressure still in it, especially if it is stored in a moist environment.)
4) plastic coating? (can be differentiated from paint as it nearly obscures the letters and numbers stamped on the shoulder of tank. I have only seen coated tanks in black or yellow)
5) J valve or K valve? (J's valves with their reserve levers are not used much any more so K valves tend to be preferred.)
6) Internal coating? (Impossible to tell unless the tank is devalved. But recent VIP's and Hydros are often an indicator that it may not be coated inside as most shops will not pass coated tanks.)
Five out of the six can be determined with a few questions and a bit of cooperation from a non diver over the phone. So if you get good news on on the phone, I'd buy the tank but price it on the assumption that you will need to have it hydro tested and tumbled. A used galvanized steel 72 with a new hydro test and a K valve is worth about $100 to $125. So start with that and subtract what it will cost you to get the tank to that condition as well as shipping costs to determine what the cash value of the tank actually is.
Locally, hydros are $15 including the shop's VIP and an air fill and tumbling is $10.00. Some shops really gouge for hydro tests and I have seen prices as high as $50.00 so the local cost will make a difference. If the seller can get it hydro'd and VIP's locally for less money that often makes more sense and it ensures you the tank will be serviceable. With a K-valve, a cheap $15.00 hydro and a $10.00 tumble and business rate shipping of about $15.00 if they box it themselves, the max the tank will be worth would be about $60.00.
The worth obviously drops below that as your costs go up so don't pay too much, but also don't just pass on the deal because someone on SB told you about someone who heard about someone who bought a used aluminum tank that blew up. Personally, I'd rather buy a used steel 72 than a new AL 80 as the steel 72 will probably still be in service long after the AL 80 has been turned into pop cans.
Using a heat gun to strip an aluminum tank is a definite no-no as is using a power sander or wire wheel or baking the tank in a paint booth to cure the new paint as all four actions can potentially generate enough heat to ruin the heat treating of the aluminum. Many dive shops consequently will require a new hydro test of any used aluminum tank with a new paint job. (so if you repaint one, wait until the hydro test is due anyway.)
But none of that has anything to do with a steel tank as the same stripping and painting processes are not going to affect the temper of the steel. Plus, with an old tank that has been out of service, it will need a new hydro and VIP before you can use it anyway. And even if the hydro is current, if the tank is suspect, you can have the hydro repeated anyway.
Steel tanks also have a lifespan of at least a century as long as they are properly cared for and inspected. We routinely have steel welding tanks come through for hydro testing from the 1910-1920 era and they still not only pass hydro but still qualify for a plus rating. I personally own several steel 72's from the 60's and 70's that see extensive service every year and they still pass hydro with flying colors. In my experience, they are actually less prone to problems with rust internally than our newer faber steel tanks which seem to require tumbling more often.
So unless the tank has been stored empty with the valve off or been stored with standing water in the tank, it is unlikely to have extensive rust or pitting inside. It may require tumbling to remove a build up of flash rust, but this is a pretty normal procedure for any steel tank every 10 years or so.
Some tanks were internally coated with an epoxy finish and this coating can take about a week on a tumbler to remove, but is something that most shops can be talked into doing on the off season when the tumbler is idle anyway. A plastic coating on the outside is also a possibility. But contrary to popular opinion if the coating is fully intact it is much like any other paint and need not be removed for a visual inspection. If however it is chipped or peeling complete removal will be needed to ensure no rust is forming under the coating. Unlike the internal coating, it is also relatively easy to remove with a normal chemical stripper. Some tanks are galvanized under this coating and end up looking virtually new. Other tanks are not galvanized and will need to be painted. In my experience though, externally plastic coated tanks are relatively rare as steel 72's go - maybe 15% of the toal number out there. The number of internally coated tanks is even lower as if they were in active use, most had the coating removed in the mid to late 80's when the problems with the coating became a concern.
I have experiemented with several types of paint including spendy professional paints like Imron and I have yet to find anything short of powder coating that will stay on a tank long term. So if you have an ungalvanized steel tank, spray paint it with a rust resistant paint and just plan on touching it up or repainting it every year or two. I personally use a stainless steel colored paint made by Seymour with a high percentage of zinc in it designed for painting steel tire rims. It looks very nice, minimizes rust in any scratches and is about as durable as anything else I have found.
One of the larger issues with a steel 72 is the potential for it to have an older style 1/2 valve. This was the standard way to do it in the 50's and the change to 3/4" o-ring sealed valves was made in the early 60's. These tanks are still often in excellent condition, but the valves are very hard to find if they need replacement. Many are also a straight stem design and do not offer a covenient carrying handle.
So in summmary the issues to be concerned with when buying a used steel 72:
1) 1/2 valve? (it will be sealed with white thread tape visible at the base of the valve)
2) Hydro test dates? (first date will tell when it was made, last date will give last test date and spacing of the dates in between give agood idea of how consistently it was used in service. If used constantly, it will have a test date every 5 years - that would be a good thing)
3) does it have air in it? (a good thing if it does as it reduces the potential for water to have entered the tank. A potentially bad thing if it does not have some pressure still in it, especially if it is stored in a moist environment.)
4) plastic coating? (can be differentiated from paint as it nearly obscures the letters and numbers stamped on the shoulder of tank. I have only seen coated tanks in black or yellow)
5) J valve or K valve? (J's valves with their reserve levers are not used much any more so K valves tend to be preferred.)
6) Internal coating? (Impossible to tell unless the tank is devalved. But recent VIP's and Hydros are often an indicator that it may not be coated inside as most shops will not pass coated tanks.)
Five out of the six can be determined with a few questions and a bit of cooperation from a non diver over the phone. So if you get good news on on the phone, I'd buy the tank but price it on the assumption that you will need to have it hydro tested and tumbled. A used galvanized steel 72 with a new hydro test and a K valve is worth about $100 to $125. So start with that and subtract what it will cost you to get the tank to that condition as well as shipping costs to determine what the cash value of the tank actually is.
Locally, hydros are $15 including the shop's VIP and an air fill and tumbling is $10.00. Some shops really gouge for hydro tests and I have seen prices as high as $50.00 so the local cost will make a difference. If the seller can get it hydro'd and VIP's locally for less money that often makes more sense and it ensures you the tank will be serviceable. With a K-valve, a cheap $15.00 hydro and a $10.00 tumble and business rate shipping of about $15.00 if they box it themselves, the max the tank will be worth would be about $60.00.
The worth obviously drops below that as your costs go up so don't pay too much, but also don't just pass on the deal because someone on SB told you about someone who heard about someone who bought a used aluminum tank that blew up. Personally, I'd rather buy a used steel 72 than a new AL 80 as the steel 72 will probably still be in service long after the AL 80 has been turned into pop cans.
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