Resistance Coil Temperatures in Tubular Heating Elements

The life of any tubular electric heating element depends heavily on its coil operating temperature. Measurement of this temperature by conventional means is difficult because the coil is buried in magnesium oxide (MgO) insulation surrounded by a metal sheath. Coil temperatures can, however, be measured by making the coil from an alloy that changes resistance sharply with increasing temperature. Sample elements with Alumel resistors were first calibrated in a muffle furnace to determine resistance change with temperature, and then operated in free air. Coil temperature was studied as a function of watts per square inch of sheath and watts per square inch of wire, using coils with various stretch ratios, coil diameters, and wire diameters.