Anomalies in the normal force measurement when using a force rebalance transducer

Torque and normal force measurements in torsional experiments provide a powerful technique for the efficient measurement of material properties. Over the past several decades NIST has been active in pioneering the use of such measurements for the characterization of polymer material behaviors, particularly in transient experiments. In a recent series of tests using a force rebalance transducer, anomalies in normal force measurements were observed. It was determined that the active nature of the transducer, which leads to excellent zero stability, causes thermal heating and consequent expansion of the system. The transducer length changes are shown to be the cause of the normal force anomalies and, in collaboration with the equipment manufacturer, suggestions for working around the problem and potential transducer redesign are discussed.