The effect of gravity acceleration on non-automatic weighing systems

Abstract

Weighing systems belong to the group of force transducers and give the mass value of a body by exploiting the gravity force, that is, the weight force acting on the body. Its value is obtained from the relation existing between the deformation of an elastic body and the force exerted on it, although other relations are also possible but seldom used. The gravity force depends on the acceleration due to gravity, which is a geophysical quantity varying at different sites. Gravity acceleration variations on the Earth (equal to about 0.53%), mainly due to the shape and the rotational motion of the globe, depend on latitude, altitude and the density of the terrestrial crust, which is the cause of the so-called Bouguer or gravity anomalies. Consequently, since g can be considered an influencing quantity, it can be taken into account in the use of weighing systems, when transported from one place to another, whenever its variations are of the same order of magnitude as that of the instrument accuracy. The most direct way of taking account of such variations is to correct the scale value of the system when the g value at the place of use is known and then to modify the scale of the system when it is transported elsewhere. Such an operation is conventionally avoided when the territory within which the system is operated is divided into zones of use or gravity zones (as established by the EEC Directive 90/384), where the minimum, maximum and mean values of g are known. The correction based on the average local value having been made, the weighing system can be moved and its accuracy remains within the limits of g variation in the site selected for use. In the present paper general criteria are examined that may be adopted for establishing the number of the zones of use, in particular as regards the Italian territory, for which a proposal has been advanced and is compared with the solutions applied by other countries.