Weight (W) is a force created by the acceleration of gravity (g) acting on the mass of an object. The simple formula becomes W = m x g. The acceleration of gravity, which is nominally 32.2 feet per second per second, is directed towards the center of gravity of the earth. This is what makes the leveling of a scale so very important, in order that the full effect of the gravitational acceleration acts on the object being weighed. In addition, the acceleration of gravity is not constant over the surface of the earth for a variety of reasons. Just think of the variation in altitude from place to place and how that varies the distance from the center of gravity. This variation in gravity must be compensated for in the calibration of any precision weighing device if weight is to be measured accurately from location to location.

The weight we use for calibrating a scale has been carefully adjusted to match a weight which has been defined to be a primary or secondary standard weight. These standard weights are kept under carefully controlled conditions at very specific locations and are very rigidly defined and controlled. Calibration weight should be traceable to these tightly controlled standard weights. The need for cleanliness and careful handling of calibration weights should be quite obvious by now.

When we become involved in weighing very small objects and the weighing resolution becomes micrograms we face a new set of problems. The amount and density of the air being displaced by the object being weighed must be compensated for accurate measurements. The density of the air varies with the barometric pressure and the humidity. Once again the location gets into weighing.

Add to these factors the reality that modern electronic scales drift with both time and temperature (for a variety of reasons), and the need for frequent re-calibration of the scale when doing fine weighing becomes apparent. This need has led to the introduction of scales which are self calibrating based upon automatic internal calibration mechanisms and weights. These scales must calibrate based upon temperature changes and the passage of defined time periods.