Stress Sensitivity of Manganin Resistor in High-Current Precision Coaxial Shunt

The ring-magnet current of the 12.5 BeV Zero Gradient Synchrotron (ZGS) and the current of a magnet test fixture at Argonne National Laboratory (ANL) are monitored with precision, four-terminal resistors (shunts) of coaxial design. These shunts, designed and fabricated at ANL, have a frequency range from dc to over 100 kHz, and can continually dissipate up to 24 kW. They have been certified by the National Bureau of Standards (NBS) with direct current to be within 0.01 percent of their measured value. Their resistance changes by about 3 ppm/°C over a range of 29 to 42°C. Unique features of the design are that thermally induced stresses in the manganin resistance element are minimized and that end effects are reduced by means of slotted current electrodes. Thermoelastic effects are often not considered sufficiently in the design and installation of precision shunts. It will be shown that the compressive stress due to a rise of 1.7°C will, in a rigidly mounted piece of manganin, change its resistance by 0.01 percent. Considering the temperature coefficient of resistance alone, a rise of 1.7°C would change the resistance value by only 0.0007 percent in the range of 47 + 10°C. It is recommended that manufacturers of resistance materials include resistance-stress coefficients in their general information.