Tank buoyancy numbers

[Luis H]

I understand using the weight of the compressed air to determine the capacity of the tanks but I'm not understanding how you are using the water capacity to determine air volume. What's the formula for doing that, and how do you convert ounces to cubic inches?

My main reason for asking is because I have two small steel tanks and I don't know the capacity. They are about 18.5" measuring to the bottom of the neck and a steel 72 measures about 25". That's roughly 74% of the size of a steel 72. They are 2250s and the same diameter so I'm coming up with approximately 53 cu/ft. Does that sound right? I cannot find any clue to the existence of either a "50" or a "55" or any such thing. They say "Voit" on them and were born February '62 (first hydro). I naturally found many references to "38s" but these seem to be a littler larger than that.

They are due for a hydro test so I'm planning on weighing them empty and once again filled with air and calculating the capacity that way but any clues you may have would be appreciated. I'm planning on selling them and would prefer to give an accurate description of their size.

Steel 50 and 55 were available in 2250 pressures (and a few other pressures like 2015 and 1800 psi). I have twin sets with all three pressures.

Assuming that those are about 53 cuft is not unreasonable..

You can try to measure the weight of the air, but you would need a very accurate scale to measure very precise small weight changes on something that is going to weigh over 60 pounds (the tanks and manifold) when it is assembled.


The more accurate way to do it (and the standard way to do it at a factory) is to measure the weight of the empty cylinder, then fill the cylinder with water and measure the weight of the cylinder and the water. The difference between the two weights is the weight of the water.

The approximate water density at room temperature is about 62.4 pounds per cuft.

If you measured how many pounds of water will it take to fill a cylinder (or any volume) then all you have to do is divide that weight (mass) by the density of the water (in this case it is in pounds / cuft). The result in this case will be in cuft, but it is very easy to convert to cubic inches or any other volume units that are preferred.

It is very important to keep the units consistent. My digital scale reads in pounds and ounces, which I hate, but it is not uncommon for a scale in the US. I have to convert the ounces to decimals of a pound and add them to the total pounds reading. That is just one of the issues with some of our systems of measurements, but as long as I can convert it to decimals I can live with it.

This measurement is of the "actual" volume inside a cylinder.

For example, the actual volume inside a steel 72 is about 0.42 cubic feet. To get the volume of standard air that will fit inside the cylinder after it is compressed, you need to multiply that actual volume with the fill pressure in atmospheres (or Bars is close enough).

A steel 72 is filled to 2475 psi for its final air volume. The 2475 psi is approximately 168.4 atmospheres or 170.7 Bars.

If you multiply 0.42 cuft with 168.4 Atm you get 70.7 cuft.

If you multiply 0.42 cuft with 170.7 Bars you get 71.7 cuft.

The 70.7 cuft is the accurate volume (if the 0.42 cuft was the actual), but the Bars used in the metric system is close enough for rough calculations.

For you purpose (since you are just going to sale it) I would say that is about 50 cuft, or you can measure the air weight, but that is not going to be very accurate either.


I hope this is clear and it makes sense. Thanks

 

[Luis H]

Expansion volume has nothing to do with the original tank rated total volume.

This not really correct.

The expansion volume will be a result of the pressure applied and the original actual volume.

This is a true statement, but the hydro tester does not need to know the actual volume in order to measure the expansion volume experienced during the cylinder pressurization. The hydro testes just zeroes the measuring burette when the tank pressure is zero and then takes a reading when the test pressure is reached. That is the total volume differential or expansion.

Then he/ she releases the pressure and then takes another volume reading. This reading would have returned to zero if all the expansion was purely elastic, but most of the time there is some residual volume.

The pass/ fail criteria is based on the percentage of the residual expansion volume to the total expansion volume.

The actual total cylinder volume is not directly used for any calculation during the test.


The expansion volumes are proportional to the size of the cylinder (the actual internal volume), but the hydro tester does not really need that information. At least not accurately. He does need a rough idea of how big is the cylinder in order to pick the best burette size for the test.


The hydro tester has several burette sizes and he should pick the most appropriate size for the predicted expansion volume. This is the only reason why the hydro technician need to judge the rough size of the cylinder, but they normally just look at it and pick a similar burette that work on a cylinder of similar size. They normally would not even think about the (numerical) actual volume of the cylinder.

 

[JamesBon92007]

Steel 50 and 55 were available in 2250 pressures (and a few other pressures like 2015 and 1800 psi). I have twin sets with all three pressures.

Assuming that those are about 53 cuft is not unreasonable..

You can try to measure the weight of the air, but you would need a very accurate scale to measure very precise small weight changes on something that is going to weigh over 60 pounds (the tanks and manifold) when it is assembled.


The more accurate way to do it (and the standard way to do it at a factory) is to measure the weight of the empty cylinder, then fill the cylinder with water and measure the weight of the cylinder and the water. The difference between the two weights is the weight of the water.

The approximate water density at room temperature is about 62.4 pounds per cuft.

If you measured how many pounds of water will it take to fill a cylinder (or any volume) then all you have to do is divide that weight (mass) by the density of the water (in this case it is in pounds / cuft). The result in this case will be in cuft, but it is very easy to convert to cubic inches or any other volume units that are preferred.

It is very important to keep the units consistent. My digital scale reads in pounds and ounces, which I hate, but it is not uncommon for a scale in the US. I have to convert the ounces to decimals of a pound and add them to the total pounds reading. That is just one of the issues with some of our systems of measurements, but as long as I can convert it to decimals I can live with it.

This measurement is of the "actual" volume inside a cylinder.

For example, the actual volume inside a steel 72 is about 0.42 cubic feet. To get the volume of standard air that will fit inside the cylinder after it is compressed, you need to multiply that actual volume with the fill pressure in atmospheres (or Bars is close enough).

A steel 72 is filled to 2475 psi for its final air volume. The 2475 psi is approximately 168.4 atmospheres or 170.7 Bars.

If you multiply 0.42 cuft with 168.4 Atm you get 70.7 cuft.

If you multiply 0.42 cuft with 170.7 Bars you get 71.7 cuft.

The 70.7 cuft is the accurate volume (if the 0.42 cuft was the actual), but the Bars used in the metric system is close enough for rough calculations.

For you purpose (since you are just going to sale it) I would say that is about 50 cuft, or you can measure the air weight, but that is not going to be very accurate either.


I hope this is clear and it makes sense. Thanks

Thanks Luis!

This is great info and I'm printing it out and putting it with my other scuba info.

For my purposes it doesn't need to be precise so weighing the water with a bathroom scale should be OK. Or I supposed I could just measure the amount of water in cu/ft and determine the weight based up 62.4 pounds per cu/ft. It always seemed like the total was around or possibly over 100 cu/ft based upon how long the air lasted. If each tank was only 38 cu/ft it wouldn't be worth carrying the extra weight compared to a 72.

Also, on the chart you posted early-on in this thread you stated that the capacity of some aluminum tanks was 85.3 and 85.0 but you said the working pressure was 3442. Why did you use that particular pressure? Since most of them are filled to around 3000 I would have thought that would be the standard. According to Huron spec chart that results in the well-known 77.4.

 

[Luis H]

You can try measuring the water volume directly and not bothering with the water weight, but I find it easier to measure the weight. A bathroom scale that is halfway decent should be accurate enough. That is part of the beauty of using water weight. The water is heavy enough that a small error in measuring the weight produces a relatively small over all error in the volume calculation.


Those two cylinders are steel 80 made by PST. They are the high pressure type. I was surprised that they held more volume than advertised (at the rated pressure.

I do have two aluminum cylinders, one Luxfer and one Catalina. I have also measured their volume and they also came out a bit higher than the publish data.

I am not surprised about some variation, but the two steel HP80 do seem a bit too high, but I have triple checked all my numbers.

You probably saw some of my steel 72 volume data. There is some variation, but they do tend to run a bit below the advertised volume.

 

[JamesBon92007]

You can try measuring the water volume directly and not bothering with the water weight, but I find it easier to measure the weight. A bathroom scale that is halfway decent should be accurate enough. That is part of the beauty of using water weight. The water is heavy enough that a small error in measuring the weight produces a relatively small over all error in the volume calculation.


Those two cylinders are steel 80 made by PST. They are the high pressure type. I was surprised that they held more volume than advertised (at the rated pressure.

I do have two aluminum cylinders, one Luxfer and one Catalina. I have also measured their volume and they also came out a bit higher than the publish data.

I am not surprised about some variation, but the two steel HP80 do seem a bit too high, but I have triple checked all my numbers.

You probably saw some of my steel 72 volume data. There is some variation, but they do tend to run a bit below the advertised volume.

Ah yes, now I see that they are all steel tanks. By the time I enlarged my screen the top category was no longer visible

 

[NAM001]

I was crunching some numbers and trying to figure out the buoyancy of a Luxfer AL80 tank when empty.

It listed as +4.4 lbs when empty here
Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

But comes out to +3.4lbs if you do the math, such as here
Scuba tank size and buoyancy calculator

Not sure where the discrepancy is coming from.

Note - I am trying to figure out buoyancy of tank alone without valves or air.

This is a topic that comes up a lot. I tell everyone to tanke the tank with 50# in it add a 10# leqad weight on it and put it in a pool dangling from a scale. It the scale says 5.6# then the tank is 10# less ( 4.4 light) than that with the valve. You can weigh the valve on a postal scale.

You can do that same thing for your entire kit and get maybe 13# on a full tank. (13# neg)

Hmmmm same process for wet suits also.

 

[JamesBon92007]

Steel 50 and 55 were available in 2250 pressures (and a few other pressures like 2015 and 1800 psi). I have twin sets with all three pressures.

Assuming that those are about 53 cuft is not unreasonable..

You can try to measure the weight of the air, but you would need a very accurate scale to measure very precise small weight changes on something that is going to weigh over 60 pounds (the tanks and manifold) when it is assembled.


The more accurate way to do it (and the standard way to do it at a factory) is to measure the weight of the empty cylinder, then fill the cylinder with water and measure the weight of the cylinder and the water. The difference between the two weights is the weight of the water.

The approximate water density at room temperature is about 62.4 pounds per cuft.

If you measured how many pounds of water will it take to fill a cylinder (or any volume) then all you have to do is divide that weight (mass) by the density of the water (in this case it is in pounds / cuft). The result in this case will be in cuft, but it is very easy to convert to cubic inches or any other volume units that are preferred.

It is very important to keep the units consistent. My digital scale reads in pounds and ounces, which I hate, but it is not uncommon for a scale in the US. I have to convert the ounces to decimals of a pound and add them to the total pounds reading. That is just one of the issues with some of our systems of measurements, but as long as I can convert it to decimals I can live with it.

This measurement is of the "actual" volume inside a cylinder.

For example, the actual volume inside a steel 72 is about 0.42 cubic feet. To get the volume of standard air that will fit inside the cylinder after it is compressed, you need to multiply that actual volume with the fill pressure in atmospheres (or Bars is close enough).

A steel 72 is filled to 2475 psi for its final air volume. The 2475 psi is approximately 168.4 atmospheres or 170.7 Bars.

If you multiply 0.42 cuft with 168.4 Atm you get 70.7 cuft.

If you multiply 0.42 cuft with 170.7 Bars you get 71.7 cuft.

The 70.7 cuft is the accurate volume (if the 0.42 cuft was the actual), but the Bars used in the metric system is close enough for rough calculations.

For you purpose (since you are just going to sale it) I would say that is about 50 cuft, or you can measure the air weight, but that is not going to be very accurate either.


I hope this is clear and it makes sense. Thanks

Well, I finally got my tanks back from the hydro tester--I found that you get them back sooner if you give them the correct phone number

I filled one of the small tanks with water and weighed the water. It came to about 17.5 lbs. From this I calculated that the capacity at 2475 psi is approximately 47.2 cu/ft, which is right about what you figured it would be.

The sad news is that one of my small tanks failed hydro It was clean and shiny inside with no sign of any pitting or even flash rust and the galvanized finish still looked good. They stamped "CONDEMNED" on it. At least I got to keep the plastic boot--they are becoming difficult to find in 6.9" round bottoms.

I stuck a K-Valve on the little tank so now it have a "kiddie" tank. The US Divers J-Valve double manifold is still in excellent working condition but I'm not sure if I could even stand up with double 72s. The backpack only has single bands so I haven't thought of another good use for it and I already have some double bands if I should ever decide to set up some double 72s but it had better be soon as I'm not getting any younger. I have a 10-year-old grandson who is interested in diving but somehow it doesn't seem like it would really work out to have him using a 50 and everyone else using a 72. Once-upon-a-time I had a AL50 that I ended up giving to my LDS so he could use it for his kid's class. I expected nothing in return but he gave me a freshly hydroed, galvanized steel 72! Maybe I'll give my little tank to a dive shop but won't expect anything in return.

In any case thanks for all the information which made it possible for me to determine the tank capacity. I am also wondering if this the beginning of the end--will we finally start to see many steel 72s fail hydro?

 

[guruboy]

It's possible it failed because they did not use the suggested pre-test procedure on the tank.

Maybe it was stressed somehow from overfills or heat.

No way of knowing, especially since they stamped it condemned.

 

[JamesBon92007]

It's possible it failed because they did not use the suggested pre-test procedure on the tank.

Maybe it was stressed somehow from overfills or heat.

No way of knowing, especially since they stamped it condemned.

They both passed the hydro in 2010 and I've only filled them a little over so they would be at about 2475 when cooled. According to the guy I picked them up from the hydro tester expected both tanks to fail, based upon his previous experiences. These two tanks are essentially the same as a steel 72, only shorter. I have a '62 Sportsways steel 72 with a 1/2" valve that initially failed the hydro test (same tester) but after he bled the system it passed. I'm beginning to wonder if I should get a new hydro tester.

 

[RayfromTX]

They both passed the hydro in 2010 and I've only filled them a little over so they would be at about 2475 when cooled. According to the guy I picked them up from the hydro tester expected both tanks to fail, based upon his previous experiences. These two tanks are essentially the same as a steel 72, only shorter. I have a '62 Sportsways steel 72 with a 1/2" valve that initially failed the hydro test (same tester) but after he bled the system it passed. I'm beginning to wonder if I should get a new hydro tester.

yes