Resistance Alloy
Nickel chromium based precision wire demonstrates exceptional resistance stability combined with a minimal temperature coefficient of resistance across wide temperature bands. In reference standard manufacture, evanohm provides stable resistance values required for precision current shunts and metrology grade bridge networks. The nominal composition includes approximately seventy-five percent nickel, twenty percent chromium, along with small additions of aluminum and copper.
Controlled heat treatment modifies the crystal structure to shift the temperature coefficient curve close to zero near room temperature. High volume resistivity allows compact winding configurations in ultra high precision resistors.
Thermal Stability
Linear temperature coefficients of resistance below one part per million per kelvin are achievable within typical laboratory ambient ranges. The curvature of the resistance temperature relationship follows a flat parabola around twenty-three degrees Celsius. Thermal electromotive force against copper remains exceptionally low, preventing parasitic DC voltages at terminal junctions.
Mechanical stress alters the internal lattice structure, shifting the temperature coefficient away from nominal calibration targets. Proper thermal cycling relieves drawing stresses to establish long term resistance stability.
Processing Constraints
Winding tension and heat treating protocols dictate the final drift rate of wirewound standard assemblies. Solder joints introduce thermal strain, requiring specialized crimping or spot welding techniques during lead attachment. Enclosure in hermetically sealed oil baths or gas filled cans isolates the alloy from atmospheric moisture and oxygen.
Strain relief annealing at elevated temperatures stabilizes the resistive element prior to initial calibration. Environmental temperature variations during processing cause permanent zero shifts if stress relief remains incomplete.
Metrological Sourcing
Receiving inspection verifies resistance drift rates against national measurement institute certified working standards over thousands of operational hours. Resistance standards wound from this material require calibration intervals based on documented historical drift rates, typically verified via automated DC comparator bridges. Low drift rates under one part per million per year allow extended recalibration cycles in primary metrology laboratories.
Failure to verify supplier annealing certs risks uncharacterized baseline resistance creep in production instruments. Traceable measurement data must confirm zero point stability across the operational span.
