Faber HP100 Bouyancy - way off??

[WetSEAL]

I recently just purchased my first tank, a Faber HP100 @ 3442 PSI.

I purchased it from LeisurePro:
Faber High Pressure Steel Tank

Bouyancy characteristics are listed here:
https://www.leisurepro.com/api/getSizeChart?productId=FBRHP

In summary:
Weight: 34.3 lbs
Bouyancy full: -8.41 lbs
Bouyancy empty: -0.59 lbs

The tank is currently empty and weighs in at 38.2 lbs = 17.327 kg. The valve is not installed, but the rubber boot is. I don't think the rubber boot can explain more than 1 lb of that at best...so this is really surprising to me.

To get an estimate of the total volume, I measured from the midpoint of the curve on the bottom to the midpoint of the curve on the top (I know that isn't 100% accurate, but using the rectangular approximation to a pill-shape should give me a fairly close estimate without having to do complex integration). This gives me a length of 56 cm. The diameter is known at 7.24 inches, so radius is 0.091948 m.

Using the cylindrical approximation of the main tank area, this gives:
Volume = Pi * (0.091948)^2 * 0.56m = 0.01425 m^3

Therefore, density = 17.327 kg / 0.01425 m^3 = 1215.92 kg/m^3

Density of saltwater = 1027 kg/m^3

So, I get empty bouyancy in saltwater=
( 1027 kg/m^3 - 1215.92 kg/m^3 ) * 0.01425 m^3 = -2.692 kg = -5.9 lbs

Now, I know these numbers are not perfectly exact, but this tank is supposed to have basically neutral bouyancy when empty, and I'm getting close to -6 lbs which seems like a pretty significant difference.

Am I missing something? Aren't these supposed to be neutral when empty?

Note: it's advertised as "12.9 L". I think that is interior capacity, which would be an under-estimate of the total displaced volume, but if I use that in the volume calculation instead of my own measurement, that gives volume 12.9 L = 0.0129 m^3, density = 1343.178 kg/m^3, and bouyancy = -9 lbs, which is even more off.

 

[BRT]

I think you do not have an hp100 but something around 120cf

 

[Got2Go]

Can you post some pics of the stamps on the cylinder?

 

[BRT]

Remember that 38lb of steel displaces 4.4 lb of water.

 

[sea_ledford]

Those buoyancy charts are notoriously incorrect. I put together a chart anc calculated teh air weight based on the specificiations given and it ranged from 0.024 to 0.084 lbs/cuft.. The weight of air doesn't change depending on the tank it is in...

Now the dry weights should be close to accurate though.

 

[WetSEAL]

I measured 7.26" diameter and just under 26" length without the valve, so it's definitely sized right for the HP100 model. The HP117 has a 8.02" diameter so it can't be that, and the HP120 has a 29" length, so it can't be that.

The stamps:
BSE 100S FABER MADE IN ITALY M8303 18/0216/025 04^18 TC-SU 7694-237
DOT-SP 13488-3442 DO NOT OVERPRESSURIZE REE 86 TP 5250

Most of those marking are explained here:
Cylinder Markings

Remember that 38lb of steel displaces 4.4 lb of water.

Not sure what you mean by that...it will displace water according to it's volume, which I measured at about 0.01425 m^3...seawater has a density of 1027 kg/m^3 so I estimate it displaces 14.635 kg of water = 32.26 lbs, I measured 38.2 lbs dry weight so 38.2 - 32.26 = 6 lbs, which is another way to get the -6 lbs bouyancy I calculated above.

 

[WetSEAL]

I removed the boot and re-weighed it, several times this time, to get a more accurate estimate of the dry weight.
I'm getting 34.9 lbs.

I also added in an approximation for the 1.5" neck using a second cylindrical approximation, which now gives me an estimate of the total displaced volume of 0.01497 m^3.

With these updates my revised estimate of the empty bouyancy of this tank in saltwater is -1 lb, which is close enough to the -0.59 lb advertised bouyancy that I feel the remainder could easily be due to approximation error.

I feel better now

 

[runsongas]

if that boot isn't self draining with the holes in the bottom, ditch it

 

[BRT]

I measured 7.26" diameter and just under 26" length without the valve, so it's definitely sized right for the HP100 model. The HP117 has a 8.02" diameter so it can't be that, and the HP120 has a 29" length, so it can't be that.

The stamps:
BSE 100S FABER MADE IN ITALY M8303 18/0216/025 04^18 TC-SU 7694-237
DOT-SP 13488-3442 DO NOT OVERPRESSURIZE REE 86 TP 5250

Most of those marking are explained here:
Cylinder Markings



Not sure what you mean by that...it will displace water according to it's volume, which I measured at about 0.01425 m^3...seawater has a density of 1027 kg/m^3 so I estimate it displaces 14.635 kg of water = 32.26 lbs, I measured 38.2 lbs dry weight so 38.2 - 32.26 = 6 lbs, which is another way to get the -6 lbs bouyancy I calculated above.

If it is a 12.9 l tank you know the water displacement of that volume and if you know the weight you can calculate the volume of steel in the tank. Adding the two together gives you total water displacement which gives you the ability to calculate how much flotation the tank has. Subtracting that from the weight tells you how much it will float or sink.

 

[BRT]

I removed the boot and re-weighed it, several times this time, to get a more accurate estimate of the dry weight.
I'm getting 34.9 lbs.

I also added in an approximation for the 1.5" neck using a second cylindrical approximation, which now gives me an estimate of the total displaced volume of 0.01497 m^3.

With these updates my revised estimate of the empty bouyancy of this tank in saltwater is -1 lb, which is close enough to the -0.59 lb advertised bouyancy that I feel the remainder could easily be due to approximation error.

I feel better now

That is realistic for a 3442 100.