2.1 Check Bridge Circuitry and Zero Balance. (Numbers apply to standard 350 ohm bridges.)
Typical Connections Instrument required: Ohmmeter with 0.1 ohms resolution in the range of 250-400 ohms. Bridge Input Resistance: RAD should be 350 ±3.5 ohms (unless the cell has standardized output, in which case the resistance should be less than 390 ohms) Bridge Output Resistance: RBC should be 350 ±3.5 ohms Bridge Leg Resistances: Comparing the leg resistances at no load permits evaluation of the cause of any permanent damage in the load cell flexure. The computed unbalance of the bridge shows the general condition of the cell. The computed unbalance, in units of mV/V, is determined as follows: Unbalance = 1.4 (RAC - RAB + RBD - RCD) The Zero Offset, in units of % of Rated Output, is determined as follows: Zero Offset = 100 Unbalance ÷ Rated Output. If the ohmmeter resolution is 0.1 ohm or better, then a computed Zero Offset of greater than 20 percent is a clear indication of overload. A computed zero balance of 10-20% is an indication of probable overload. If the load cell has been overloaded, mechanical damage has been done that is not repairable, because overloading results in permanent deformation within the flexural element and gages, destroying the carefully balanced processing that results in performance to Interface specifications. While it is possible to electrically re-zero a load cell following overload, it is not recommended because this does nothing to restore the affected performance parameters or the degradation to structural integrity. If the degree of overload is not severe the cell may in some cases be used at the users discretion, although some performance parameters may violate specifications and the cyclic life of the load cell may be reduced.