# Why 34 lbs?

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Posted by Chris on October 23, 2004 at 20:47:57:

In Reply to: Question about require BC lift and weight. posted by Nitrox36 on October 22, 2004 at 21:54:14:

35 lbs. seems a bit excessive to me. Richard uses 19 lbs with a HP-120 which is neutral at 500 psi. If he used an Al-80 which is +5 lbs at 500 psi he would need 24 lbs. If he added a BC he would need to add another 2-3 lbs to compensate for the residual air in the BC hose, thus 27 lbs. I am curious why you use so much more?
As a rule you should use no more weight than you need to maintain your safety stop when your tank is near empty (500 psi) and your BC is completely deflated. Any more causes you to over inflate your BC during dives to compensate, thus increasing drag and air consumption.
If you change tanks measure the buoyancy at 500 psi on both tanks to determine your needed weight change. To measure tank buoyancy fill a trash can with water and weigh the tank (with a digital fish scale) while it is submerged. For buoyant tanks such as an Al-80 you will need to add some weight. A piece of sting or a wire tie over the valve with a dive weight works well.
With 500 si in the tank and submerged but not touching the bottom of the trash can the buoyancy is:

It is best to measure you own tanks because I have found the actual numbers are often different from the published ones. You can also measure the tank when it is full to see how the buoyancy changes during your dive. You can estimate the bouancy change as follows

Air weighs about 0.08071311 lbs. / ft3 at standard temperature and pressure.

Let us use a HP-120 3500 psi tank for an example:

120 ft3 · 0.08071311 lbs./ft3 = 9.69 lbs.

at 500 psi you will have

500 / 3500 · 120 ft3 = 17.24 ft3

17.24 ft3 · 0.08071311 lbs./ft3 = 1.38 lbs.

Thus the buoyancy change from 3500 psi to 500 psi for an HP120 is

9.69 lbs. - 1.38 lbs. = 8.58 lbs.

Thus a HP120 will be 8.6 lbs. heavier at the beginning of the dive than the end

For an AL-80 (76 ft3, 300o psi)

76 ft3 · 0.08071311 lbs./ft3 = 6.13 lbs.

at 500 psi

500 / 3000 · 76 ft3 = 12.67 ft3

23.67.24 ft3 · 0.08071311 lbs./ft3 = 1.02 lbs.

6.13 lbs. - 1.02 lbs. = 5.11 lbs.

Thus a AL-80 will be 5.1 lbs. heavier at the beginning of the dive than the end

I hope I have not confused you.

Chris

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