BTS steel tanks?

[pescador775]

Heh, funny, your cylinders are estimated -4 lbs ea. Calculation is by ratios. The benchmark 12 liter tank is the "72" (take my word for it). The LP72 weighs 26 lbs and is neutral buoyant. Your 12 liter tank weighs 4 pounds more than the 72. 26 (net)-30(net) = -4. Moreover, my 12 liter tanks come out to 26-35 = -9. (So, yes to -4 kilos but wrong tank). Tare weight of your tank is 14 kilo, net is 13.5 kilo, just for clarification.

 

[evil_xander]

Heh, funny, your cylinders are estimated -4 lbs ea. Calculation is by ratios. The benchmark 12 liter tank is the "72" (take my word for it). The LP72 weighs 26 lbs and is neutral buoyant. Your 12 liter tank weighs 4 pounds more than the 72. 26 (net)-30(net) = -4. Moreover, my 12 liter tanks come out to 26-35 = -9. (So, yes to -4 kilos but wrong tank). Tare weight of your tank is 14 kilo, net is 13.5 kilo, just for clarification.

I was just doing the same math yesterday, based on the following:

14kg /(7.8 kg/L) = 1.7948 L + 12 L = 13.7948 L volume in total

13.7948 L /(1.03 kg/L)= 13.39 kg (displacement or similar, not sure of the english term)

12 L x 0.0012 kg/L x 232 bar = 3.34 kg (weight of air per tank)

13.39 kg - 14 kg = -0.61 kg (empty)
13.39 kg - 14 kg - 3.34 kg = - 3.95 kg (full)

Per tank, of course.

Or I could be completly insane.

 

[pescador775]

The math looks OK with one small error. It affects things a little. The tare weight of 14 kg and supposed buoyancy of -.61 would indicate the bare tank is neutral buoyant although I believe now that it is a little bit positive.

I went out and weighed a 72 on the old bathroom scale and the result was 27.5 lbs, heavier than I thought (if the scale is accurate). Using ratios, that would put your tank buoyancy (bare tank) at -2.5 or about 1 kilo which is still heavier than your calc, so I started wondering if the 72 is neutral after all. Using ratios is still an estimate only but the result should be pretty close if "conventional wisdom" of weight and buoyancy of the 72 is right. Using that as fact may have been a mistake. The 72 internal volume is actually about 1% larger than 12 liter but that should not make a difference. It may be that I also heard wrong about the buoyancy of the "72" and that it is actually positively buoyant. It doesn't take a large discrepancy to change the result so as to be significant for diver's buoyancy. The 72 weighs 12.5 kilos = 1.6 liters steel. 12.1 + 1.6 = 13.7 L salt water = 14.1 kilo = 31 lbs. IOW, the 27.5 lb tank displaces 31 lbs suggesting the bare tank has a buoyancy of +3.5 lbs. A 72 with valve would have a buoyancy of about +2.5 pounds. I don't know for sure but the math seems to confirm.

13.7948 L /(1.03 kg/L)= 13.39 kg (displacement or similar, not sure of the english term)

12 L x 0.0012 kg/L x 232 bar = 3.34 kg (weight of air per tank)

13.39 kg - 14 kg = -0.61 kg (empty)
13.39 kg - 14 kg - 3.34 kg = - 3.95 kg (full)

Per tank, of course.

In order to obtain the displacement it is necessary to multiply the weight of water X the total volume of the tank. Thus, 13.7948 L X 1.03 = 14.21 kilos. So, your tank with valve is buoyant by .21 kilo. (subtract 14 from 14.21). You understand this, right? Salt water is heavier than unity and creates more buoyancy than fresh water.

It is more accurate to calculate this without the valve in which case your tank weighs 13.5 kilo. 13.5/7.8 = 1.73 L steel. 1.73 + 12 = 13.73 L. 13.73 X 1.03 = 14.14 kilos. Thus, using this method the bare tank has a pos buoyancy of 14.14 - 13.5 = +.64. Add weight of valve (.5 kilo) and the buoyancy is + .14 which is only slightly less than .21 calculated the other way. That is about as close to neutral as one can get.

 

[pescador775]

If the bathroom scale is right, the 72 is pos buoyant. Conventional wisdom from divers says that tank is neutral when empty. Well, what is empty? I guess that when equipped with valve, bands, pack and 500 psi air the tank does trim about neutral. Still waiting to hear if anyone has a good number for weight so I can check this.

By the same token, your double tanks would trim a bit negative but your basic calculations and assumptions are right on.

 

[pescador775]

Summary:
the BTS 12 ltr tank is neutral buoyant with valve.
the vintage 12.1 ltr tank known as a "72" is +2.5 lbs buoyant with valve.