Reliable Inconel-sheathed cartridge heaters rated at hundreds or thousands of watts have been in use for many years. Miniature heaters have been manufactured, having axial leads, which when treated with an emissive coating meet or exceed Globar emissivities in the ir. Free-air temperatures approximating 800°C have been obtained for power inputs of 10 W, while temperatures as high as 1150°C are possible for short lifetimes, e.g., 10 h, at power dissipations of 24 W or more. Sources can be designed to operate from dc or ac power with higher voltages and lower currents depending upon physical size limitations, resulting in simplified power supply problems. Although voltages from 12 V to 230 V are practical for larger units, the sources described below operated at 1060°C from a nominal 1.07 A at 18.7 V dc for a power consumption of 20 W variable continuously about this value. The small dimensions of these sources compare favorably to those of a 0.5-W electronic resistor. The present units are cylindrical, 1 cm long × 4 mm diam. The construction of smaller units is possible but difficult under present state-of-the-art limitations.
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