What is geographical calibration of a scale?
We assume that the mass of the unknown is stable (certainly true of calibration weights) and that we are teaching the scale the local value of the earth’s gravitational field, which is quite different from Los Angeles to Denver, CO. This technique, where we force the scale to read the weight of the standard mass correctly, and memorize the local value of gravity for future use, provides it with a second point, usually the full scale value of weight for the scale, at which the scale reads correctly. The first point at which the scale reads correctly is zero weight (empty weigh pan) where we had, just before putting the standard mass on the scale, forced the scale to read zero (called zeroing the scale and usually a push button operation). The scale now has two points at which it reads correctly, and two points define a straight line. If the scales readings follow that straight line as we weigh unknowns we have an accurate scale. However, if they don’t follow that straight line we will take erroneous readings. Scales that are used in projects requiring highly accurate readings should be checked at several points within their working range using a good set of standard weights to prove that the scale is linear. This is the true measure of quality.
OK, now let’s talk about geographical calibration. Let’s do the same LA to Denver transposition. What did we do with our full scale mass standard? We taught the scale the local value of gravity. Suppose we teach the scale the local value of gravity in a different way. If we had a program stored that would allow us to put in our geographical coordinates, and based upon these calculate the value of the local force of gravity and store it in its memory for future weight calculations, we would have calibrated the scale. This technique works well for Industrial scales with strain gauge type accuracies (0.01%), but is marginal for precision scales (0.001% and better) where calibration takes care of other secondary factors as well (temperature and magnetic drift effects). Industrial Scales are frequently very high capacity scales and calibration weights become hard to handle (requiring portable cranes in many cases) and this is where geographical calibration comes into its own. Be aware, however, that if the strain gauge load cell is overloaded to the point where its output constant (millivolts per unit weight) is changed only the use of a standard mass will detect the change.