Use a spring balance to note the buoyancy effect on the metal as you suspend it in the water.

This gives you the weight taking into account the buoyancy effect (aka Archimedes' principle).

If you put the sample+acid+beaker on a top loading balance, you will lose the buoyancy effect.

Note that this buoyancy effect depends on what the liquid is.

When , the flow does not experience any buoyancy effect.

If there are also buoyancy effects, for example hot air rising, then natural convection can occur.

The difference would be caused by the buoyancy effect.

This effect is predominantly seen in shallow diving birds as buoyancy effects are strongest.

Care must be taken to keep the volume of the sample and the counterpart as close as possible to reduce the buoyancy effect.

Your second question has more to do with what air and the buoyancy effect in air are.