The magnetic characteristics of ferromagnetic steels, such as hysteresis loops, total permeability and differential permeability, are dependent on mechanical stress and temperature. This dependency can be the basis for a sensitive and non invasive sensing method for measuring the active stress in steel tendons and cables. This paper describes the current status of a stress sensor which can reliably monitor stress in tendons and cables. A transient magnetic field generated by a solenoid is utilized to bring the material to technical saturation. The induced voltage, which is affected by the presence of the ferromagnetic material, is measured and related to material characteristics. In order to obtain a stable and linear calibration curve between permeability and monitoring stress for a given material, an optimal applied field H0 must be determined. Initially, this field is estimated from the measured relationship between permeability and input voltage. The optimal working point is determined by searching for a linear relationship between permeability and applied stress at different temperatures. Temperatures from -20 C to 40 C were used. Experimental results for two common tendons, at two sizes (0.5' and 0.6'), were investigated using this technique. The results were demonstrated to match the accuracy and repeatability of a reference load cell for loading up to 70% of the yield stress. Additional tests were conducted on multi-strand cable assemblies. The experimental results indicate that the calibration procedure can be extended without significant error to accommodate such assemblies.