This technology break through saw the traditional laboratory and analytical equal arm balance based upon knife edges and hanging pans replaced by a Roberval equal arm suspension where three trusses with taut bands wrapped around them are tied together by two beams.  This parallelogram arrangement allows the user to place the unknown weight and the balancing weights on top of the mechanism (top loading), and also allowed for using weight loading to increase the sensitivity of the skeleton making the bands appear to be softer than they actually were.  Torbal continues to manufacture these mechanical scales today for use in the Pharmacy where they have set the standard for years.  


The Roberval suspension is the key to today’s digital scales as well.  However, the taut bands of  earlier balances are now replaced by flexures (look like a bow tie where the knot has been hammered thin to increase their flexibility).  In addition the substitution weights have been replaced by a magnetic force restoration motor working in conjunction with a digital computer.  The computer is necessary to correct for the non-linearity of the force generated and the temperature characteristics of the magnetic motor.  The Torbal force restoration magnetic motor load cell is very accurate and durable because of careful attention to flexure design (usually the cause of balance failure in overloading).


The Torbal line of industrial scales is based upon modern computer enhanced strain gauge technology.  The strain gauge load cell is based upon bonding resistance wires, foils, or semiconductors, which vary their resistance depending upon the stress or strain they are subjected to, in a Wheatstone Bridge arrangement.  The wires are bonded to a metal structure which bears the unknown weight and puts half of the resistive elements under stress and the other half under strain.  There are a variety of configurations that can be used, the simplest of which is a simple cantilever beam with half the wires on top of the beam and half bonded to the bottom of the beam.   Most of Torbal’s Industrial Scales use an S beam configuration where the center leg has the wires bonded to it.  


Just as the magnetic force motor has its inherent limitations, the strain gauge load cell has problems due to the nature of the metals used carry the load that make it less accurate and stable than the magnetic force motor load cell.  Some of the problems are non-linearity, hysteresis, creep, non-repeatability, temperature coefficients, and a few lesser problems.  The computer can compensate for those that are mathematically predictable, however, things like non repeatability and creep cannot be predicted.  Rest assured that we compensate as best we can, and, most of all , we stay on top of the technology.