18 - Mass, force and torque measurement

Abstract

This chapter discusses the alternatives to load cells that are either mass-balance instruments or the spring balance. Mass describes the quantity of matter that a body contains. Load cells are the most common instrument used to measure mass, especially in industrial applications. In an electronic load cell, the gravitational force on the body being measured is applied to an elastic element. Pneumatic and hydraulic load cells translate mass measurement into a pressure measurement task. The output pressure measured in the cell is approximately proportional to the magnitude of the gravitational force on the applied mass. Mass-balance instruments are based on comparing the gravitational force on the measured mass with the gravitational force on another body of known mass. The electromagnetic balance uses the torque developed by a current-carrying coil suspended in a permanent magnetic field to balance the unknown mass against the known gravitational force produced on a standard mass. Spring balances provide a method of mass measurement that is both simple and cheap. The technique of applying a force to a known mass and measuring the acceleration produced can be carded out using any type of accelerometer. Measurement of applied torques is of fundamental importance in all rotating bodies to ensure that the design of the rotating element is adequate to prevent failure under shear stresses. Measuring the strain induced in a shaft due to an applied torque has been the most common method used for torque measurement in recent years.