Tube furnace configuration (without isothermal block): Base-metal thermocouples are often sheathed in formable materials such as braided fiberglass or PTFE. During calibration, they are bundled around a reference thermometer, held together with fiberglass cord or tape, and inserted into a tube furnace.
Isothermal block configuration: Metal- or ceramic-sheathed thermocouples are generally constructed with noble-metal thermoelements and therefore have higher calibration accuracy requirements. The isothermal block, which accommodates up to four 6.35 millimeter probes, improves heat transfer and temperature stability. This better equalizes the temperature between the reference probe and the UUT, which lowers measurement uncertainty compared to calibration without a block.