Self weighing of tanks....

[danw2002]

I think i am going to try to get some weight figures for my tanks in the water myself. any one done this?
I would think a fish scale( the kind you hold and hang a fish from) should do it, and then attach a line to the tank, submerge it and weigh it at the air fill levels you need, say full, before dive and after at 500psi, and maybe even at 100psi, just to make sure....any other ideas on how to go about this?
One of the reasons, is that I have a nice twin 50 set I use and know one knows any figures for that set up, so it would be nice to know, then do my other tanks and have real world figures for my tanks and even print them on the tank, so it would be easy to change tanks and know what my weight would need to do. I dive with an Al 80, Twin Al 50's, LP104 and a LP120.

 

[Diver0001]

danw2002 once bubbled...
I think i am going to try to get some weight figures for my tanks in the water myself. any one done this?
I would think a fish scale( the kind you hold and hang a fish from) should do it, and then attach a line to the tank, submerge it and weigh it at the air fill levels you need, say full, before dive and after at 500psi, and maybe even at 100psi, just to make sure....any other ideas on how to go about this?
One of the reasons, is that I have a nice twin 50 set I use and know one knows any figures for that set up, so it would be nice to know, then do my other tanks and have real world figures for my tanks and even print them on the tank, so it would be easy to change tanks and know what my weight would need to do. I dive with an Al 80, Twin Al 50's, LP104 and a LP120.

The math involved in calculating it is straight-forward. You don't need to go to all this effort. Unfortunately, knowing the bouyancy of the tank is only part of the answer. It's the mass and volume of the *whole* diver that matters. Unless you know mass and volume of the diver without the tank on then you can't get anywhere with this.

If you're interested in the calculations, send me a personal message and I'll give you a spreadsheet you can play with.


R..

 

[Rick Murchison]

Using a scale won't give you quite accurate numbers directly, because lead displaces water too. To be accurate with a scale, you'd have to see how much lead "weighs" the same as the tank when submerged. What I use is a balance pole suspended under the diving board in a pool - hang the tank on one side and the weights to balance it on the other.
One thing I've found is that there is an awful lot of bad "information" from some seemingly reliable sources in print and on the internet about tank buoyancy - some off by as much as 10 pounds!
By all means measure them yourself.
Rick

 

[MikeS]

Another method is to attach a milk crate to a lift bag. Place the object being tested (i.e. tanks) in the milk crate and add enough air to the lift bag to make the combination neutrally buoyant. Then remove the tank and add enough weights to make it neutrally buoyant again. The amount of weight needed is equal to the negative buoyancy of the object being tested.

Mike

 

[nickjb]

I think your fish scale idea is the best option. I intend to do this with my tanks. I have a grocers scale with a large needle gauge which should be perfect.

 

[pescador775]

The fishscale is quick and dirty as engineers say, but good enough for govt work. Worrying about a few ounces or error either way is not productive. Make sure that the test is conducted in the same water as you dive, eg salt, fresh or whatever. The only time that I give a hoot whether I'm one or two pounds off is when freediving/spearfishing. Deep free diving, I weight for neutral at about 15 feet, shallow diving, I weight for neutral at the surface. You see, if the diver must kick to submerge or to stay submerged, fish will spook, especially in shallow water.

For tank divers, to avoid having to calculate your volume, proceed, fully suited, to do an 'Archimedes' test. Weight yourself with everything except tank to neutral buoyancy at the preferred depth. Measure the tank buoyancy with scale and add or subtract the weight from your person to achieve the desired result. If you're worried about the change in displacement when adding or removing one or two lead weights you are overthinking the problem, IMO.

 

[The Chairman]

Weight is not the same as mass, but it is dependent on it. Weight takes into account mass, gravitational pull, and buoyancy.

Mass is constant... it is the same on land, in the water or on the moon.

Weight will vary from the land to the water due to buoyancy.

Weight will vary from the land to the moon due to gravitational pull.

A balance beam is used to determine mass.

Springs are used to determine weight.

Buoyancy for most solid objects in air is negligible.

If two objects are measured against each other in the water using a balance beam, then you are comparing mass and buoyancy (densities), but not weight.

The only way to actually measure "weight" in the water is to use a spring style guage. Or... you can do the math. :tease:

Rick's method, however goes even further... because it takes into account that lead has a tiny bit of buoyancy. That is, it does not weigh the same in air as it does in water. It has the same mass, but it is buoyed up by the mass of the water it displaces.

Say you have a tank that has 5 lbs of buoyancy at the end of a dive... will five pounds of lead make it neutral???

Lead has a specific gravity of 11.34, which means it weighs 11.34 times as much as PURE water. Fresh water weighs 63.4 lbs/cf so
63.4lbs/cf X 11.34= 719.0 lbs/cf or 1lb of lead is equal to 1.39 x 10-3 cf/lb. In salt water this means that lead displaces 8.90 X 10-2 pounds of sea water... in other words, a pound of lead weighs 0.910 pounds in the water. So 5/0.910= 5.49! You would need at least 6 pounds of lead weights to not be buoyant.

No, this is not the most useful post I have ever made, but I hope that it was fun to follow.

 

[donacheson]

NetDoc once bubbled...

Lead has a specific gravity of 11.34, which means it weighs 11.34 times as much as PURE water. Fresh water weighs 63.4 lbs/cf so
63.4lbs/cf X 11.34= 719.0 lbs/cf or 1lb of lead is equal to 1.39 x 10-3 cf/lb. In salt water this means that lead displaces 8.90 X 10-2 pounds of sea water... in other words, a pound of lead weighs 0.910 pounds in the water.

It's much easier to note that the specific gravity of pure water is 1.00, so any weight of lead in air is reduced by a factor (11.34-1.00)/11.34=0.912 in water. For salt water, the factor is about (11.34-1.03)/11.34=0.909.

 

[The Chairman]

I almost put those calculations in too... felt it was too long as it was and the long way helps people to grasp the concept.

Pure water by definition has a specific gravity of 1.00.

 

[Epinephelus]

As much as I love working with numbers, I note that the balance method mentioned by Rick is a direct measurement and therefore has no math and no errors. It is even better because if you use the weights you dive with to do the balancing, the method automatically compensates for any inaccuracies in the actual weight of the weights themselves.
I like it.
E. itajara