The force we call weight is directed to that center and follows the simple formula; force = mass x acceleration, where the acceleration is that of gravity which is approximately 32 feet per second per second. In order to measure weight accurately the weight sensor, usually a force motor type load cell in the case of Laboratory and Analytical Scales, must have its weigh axis aligned with the gravitational field. In most scales the weigh pan must be orthogonal (90 degrees) to the gravitational field in all directions. The base plate is made to be orthogonal to the weigh axis of the force motor by internal adjustment, and must not be changed. A bubble type level adjustment indicator is mounted carefully on the base plate and is used to trim the base plate to a level condition.
Scales are designed to be leveled in this way. The best and most common system for level adjustment involves a 3 point stance for the base plate (in order to eliminate rocking). Two legs are adjustable and one is fixed. Leveling bubbles are made with different angular sensitivities. Generally speaking the bubble in an analytical balance must be about ten times as sensitive as that in a laboratory scale because of the higher resolution of the analytical balance (1 part in 1,000,000 for the analytical and 1 part in 100,000 for the laboratory).
The reduction in weight as we deviate from the orthogonal axis to the base plate follows a cosine curve shape. If the angle of error is 0 degrees (perfect alignment) the cosine of 0 degrees = 1.000…) there is no weight error. If the angle of error is 0.0833 degrees (5 minutes of angle) then the cosine = 0.9999989, or 1.1 parts per million of force reduction, about the max we would tolerate for an analytical scale. If the angular error is o.25 degrees (15 minutes) the cosine = 0.9999904, or close to 1 part per 100,000 of force reduction, the max we would tolerate for a laboratory scale.
These are very small angles and care should be taken to center the bubbles when leveling the scale. After leveling do not move the scale on the surface of the bench. If it is moved the level indicator must be checked before using the scale. Most surfaces that people use precision scales upon are not of the quality of surface plates and moving the scale can introduce weighing errors in many instances. Be warned. Be wise.