If you're properly weighted at the start of a dive; are you over-weighted?

[MNJoe]

If at the beginning of the dive you are neutrally bouyant plus added enough weight to offset the air you are going to use during the dive, then you are perfectly weighted.

 

[fnfalman]

"Properly weighted" does not equate to "neutral buoyancy" at 15-ft and 500-psi at the end of the dive.

 

[TMHeimer]

Yes, what they all say. You are properly weighted, but not "perfectly weighted". Other than perhaps one millisecond in time this can't happen because you are constantly emptying your tank.

 

[EFX]

You are properly weighted but let me correct you on one item. For whatever reason many people think there is a weight/bouyancy SHIFT difference between steel and aluminum tanks, there is not. The weight or more correctly bouyancy shift of a tank soley depends on it's volume and has nothing at all to do with the material it is made of. An 80 cf/12L tank made of any material will have the exact same shift, assuming you use the same amount of gas out of it. You will need more weight (lead) to offset your overall positive bouyancy with an aluminum tank as opposed to a steel one but shift in bouyancy, which is what you need to be concerned about, is no different as long as the tank volume is the same.

The buoyancy (shift) of a tank and anything else for that matter depends on both its volume and (dry) weight. Here is the relavent equation: B = VD - W, and here is the definition:

According to the equation the buoyant force B is the difference between the weight of the water displaced by the object (VD) and the dry weight (W) of the object. If B is zero the object is neutrally buoyant and will neither sink nor float. If B is negative (-) the object will sink and if B is positive (+) the object will float.

As you can see from the equation the weight has an effect. If an aluminum and steel tank both have the same volume the steel tank will be less buoyant than the aluminum because in the equation W for the steel tank is higher. The weight of the tank is determined by its mass which is dependent on the material used.

 

[oszillodrom]

The buoyancy is different, but not the buoyancy SHIFT, as herman pointed out. The buoyancy shift only depends on the mass of air being consumed during the dive.

 

[herman]

The buoyancy is different, but not the buoyancy SHIFT, as herman pointed out. The buoyancy shift only depends on the mass of air being consumed during the dive.

Exactly, many people, including EFX, confuse buoyance of the tank with buoyance shift of the tank, totally different things. Buoyancy of the tank is definately affected by the material of the tank but buoyancy shift, which is what a diver is really worried about once the proper amount of weight is established has nothing at all to do with the material of the tank. For any tank that has a varible content, your equasion is not complete, it must also include the weight of the contents. In our case that is air or some gas mix like Nitrox and that is the varible we are concerned about, which for an 80 cf tank reguardless of material is about 4 to 5lbs depending on how far you drain the tank. The only difference between steel and aluminum tanks of equal capacity is the amount of external ballast (lead on your weightbelt) that must be used.

 

[diver 85]

From what I can read you're saying/asking-------I'd say no(to your last statement---Q)............

EDIT:...check yourself out: --with the weight you think you need to be 'neutral' @ 15 feet or so with 500 or so lbs of pressure..........Then you'll know if you are high, low ,---or right on it.......

 

[cooperscuba]

Thank you for all your answers. Appreciated.

 

[EFX]

Exactly, many people, including EFX, confuse buoyance of the tank with buoyance shift of the tank, totally different things. Buoyancy of the tank is definately affected by the material of the tank but buoyancy shift, which is what a diver is really worried about once the proper amount of weight is established has nothing at all to do with the material of the tank. For any tank that has a varible content, your equasion is not complete, it must also include the weight of the contents. In our case that is air or some gas mix like Nitrox and that is the varible we are concerned about, which for an 80 cf tank reguardless of material is about 4 to 5lbs depending on how far you drain the tank. The only difference between steel and aluminum tanks of equal capacity is the amount of external ballast (lead on your weightbelt) that must be used.

OK, Sorry. I thought you meant a buoyancy shift between aluminum and steel tanks. What you meant was a buoyancy shift between the start and end points of a dive on the same tank. The W in the equation is total weight and includes tank and gas weight. So, the equation is complete but must be calculated for both points in the dive to get the buoyancy shift.

 

[cooperscuba]

From what I can read you're saying/asking-------I'd say no(to your last statement---Q)............

Based on my understanding of weighting, and the general consensus of the replies, I'm going to disagree with you If you're weighted to compensate for the buoyancy shift, then you must be overweighted to begin with (don't confuse my use of over-weighting to mean you have too much weight for the whole dive, I'm refering only to the start of the dive i.e. the cylinder is full). No?