The Isotech Saturn 877 Thermocouple Calibration Sphere comprises a number of concentric shells. The outer shell of spun metal is for containment and support. Inside this is a layer of ceramic fiber. Within the fiber is a ceramic spherical mantle containing the heater windings. In the middle of the furnace is a solid cast-ceramic sphere with 8 or 16 tubes, to be used for the thermocouples requiring calibration.

The spherical design ensures a central zone of constant temperature. Thermocouples are inserted around the circumference of the furnace. When fully inserted, the measuring junctions sit within a few millimeters of each other at the center of the sphere. Up to 15 thermocouples can be calibrated simultaneously. The accuracy is between ±0.25°C and ±0.1°C at 1000°C.