I seem to fail explaining the buoyancy concept to some of you. Let me try another way in terms of something that is more general in nature, i.e., density.
Do you all agree that if an object is neutrally buoyant in the water, that means, its bulk density is the same as that of water = 1.0 g/ml or 1.0 kg/l or 62.4 lb/cft? Let's use Sw = 1 for water and the object bulk density = Sb. If Sb < 1, it floats, i.e., it has positive buoyancy. If Sb > 1, it sinks, i.e., it has negative buoyancy. How much negative buoyancy it will be, depends on the difference between its density and the water density multiply by the bulk volume of the object.
Negative buoyancy = submerged weight = (Sb - Sw) x Vb = (Sb -1) x Vb
Where Sb = density of the object in kg/l & Vb = Bulk volume of the object in kg.
Here is where the confusion starts, that is, it is not easy to estimate the bulk density when you have an object such as DPV, with internals composing of battery, electronic, air space, etc., with each of the components has different density. If we know each component weight and volume, we can estimate Sb by:
Sb = (W1 + W2 + W3 + ....+ Wn) x Vb
Where W1 is the weight of component 1 in kg,..., subscript n = component n
Air density is so much smaller than the rest of the component and we can practically ignore, unless we are dealing with compressed air as in the SCUBA tank.
Let's now do a simple example of the dreaded flooded AL80. If its empty weight is say 14.2 kg as
@Storker estimated in post #28, page 3
W1 = 14.2 kg
We ignore the weight of atmospheric air in the tank.
W2 = 0 kg
Vb is about 16.3 liters as
@Storker estimated previously.
Therefore, empty AL80 bulk density becomes
Sb = (14.2 + 0) / 16.3 = 0.87 < 1 the AL80 will float if the nozzle is capped.
Now let's see what happen if it is completely flooded. If the cavity is 11 liters, then
W2 = 11 kg.
Therefore, the flooded AL80 density becomes
Sb = (14.2 + 11) / 16.3 = 1.55
Negative buoyancy = Submerged weight = (1.55 - 1) x 16.3 = 8.9 kg
That is not equaled to the weight of the water inside the AL80, which is 11 kg.
In
conclusion
Don't use the weight of water occupying the original cavity of an object (scuba tank, DPV, etc.) for estimating the negative buoyancy.
When the DPV imploded, Vb changes, i.e., it gets smaller, but Sb also changes, i.e, it gets bigger, accordingly. Whether it is flooded or imploded or exploded, it doesn't matter as now the Vb is the volume of the solid remains once the air is out of the cavity.
For
imploded,
exploded or
flooded AL80, Sb = density of aluminum = 2.7 kg/l and Vb = 5.26 liters, estimated by
@Stroker previously.
Negative buoyancy = Submerged weight = (2.7 - 1) x 5.26 = 8.9 kg.
and not water
