Question

1. What is air buoyancy and when might it be important in mass measurement?

Answer 1

Air Buoyancy:

Gravity acts upon every particle on the earth including air particles. But the pressure developed in the air creates an upward force which opposes the earth's gravity pull. As we approach the surface of the earth the air pressure is maximum. Air density builds to whatever level it can balance the force of gravity and after this point gravity isn't strong enough to pull down a greater number of particles. This difference in air pressure causes an upward buoyant force in the air all around us called Air Buoyancy.

Essentially, the air pressure is greater below the objects than it is above the objects, so air pushes up the object more than it pushes down due to pressure difference at the top and bottom. This buoyant force is often neglected as it is weak or negligible compared to the gravitational force. It is only equivalent to the weight of the air displaced by an object.

Importance of air buoyancy in mass measurements:

Air buoyancy is negligible when mass measurement is of high values but it is necessarily important if a measurement is carried out with high resolution( measurements of order micrograms and below). In the calibration of high sensitive mass measuring instruments, this air buoyancy must be corrected to reduce the error for precise measurements. The buoyancy force depends on the density of air which in turn depends on the pressure, temperature, and humidity.

Buoyancy Force = .g.V

Where, = density of air displaced

g = acceleration due to gravity

V = volume of displaced air

In mass measurements, the buoyancy force deviates the value from original mass value. The actual mass is the measured mass plus the Air buoyancy force(correction Force). Change in the density of air (variable buoyancy force) during mass measurement processes may cause errors of indication. The size of the error in measurement is also proportional to the density of weighed sample and its mass. In case of mass measurement of samples with low density ranging between(500 – 4000 kg/m3) the error due to the changes in air density may be significant. The size of the error also depends on the sample mass.