Question

What is buoyancy and how does it cause errors in mass measurement with electronic balance?

Answer 1

Buoyancy refers to the upward force that an object suffers from the water and when compared to the weight force of the object, determines that the object float, sink, or remain neutrally buoyant in the water.

The value of buoyancy force is equal to the amount of liquid displaced by the object.

Then we can write the following formula

Where ρ is the density of a medium in which an object is placed (liquid or gas), g is the gravitational acceleration force and V the volume of displaced liquid (equal to the volume of the object inmersed)

For electronic balances we have to take in account that the weight force (gravitational force) is compensated by a signal from the balance (usually caled compensating force). The value of this last one, is linked with the mass to give the corresponding actual mass.

In these balances, if we take in account that the bouyancy is also a force acting from the bottom to the object, we can say over the object exists 3 forces acting at the same time: the weight force (acting vertically down), bouyancy (because the air is displaced by the object) and the compensating force from the balance.

As the weight force is a constant in a specific operation place and the compensation force is balance’s reaction to the weight, thus is also a constant. The only variable, that required continuous calculating in electronic balances is the buoyancy force.

The buoyancy force is dependent on the density of atmospheric air at work place and the density depends on pressure,temperature, humidity .

When the obtained measurement is corrected by the buoyancy force the obtained value of mass is equivalent to weight the object in vacuum.

The size of error related in the evaluation of the buoyacy force depends on the density of the object that is being weighted. when the object density is much lower than steel density, the error can be significant.

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