Figure 9.

Cross-section drawing of original NBS/NIST gas-thermometry thermal expansion furnace. The outermost shell was made of brass in three sections. PTFE gaskets were employed at the joints to permit evacuation of the furnace. Cooling coils on the outside surface of the outer shell could be connected to refrigerated fluid in order to cool the furnace to temperatures as low as −30 °C. Successively smaller radiation shields of copper, silver (three shields), and gold enclosed three Inconel 600 shields, I1, I2, and I3. Shell I1 was equipped with separate resistive heaters placed in grooves in its top, bottom, upper sidewall, middle sidewall, and lower sidewall. Shell I2 supported heater circuits located on its top plate, on its sidewall, and on its bottom plate. The innermost shell, I3, was prepared from a solid cylinder of Inconel of 7.5-cm diameter and 13-cm length; a central hole was bored to house the sample assembly, and an off-center hole accommodated a calibrated PRT. Three-element “Platinel” thermocouples were arrayed along the surfaces of shells I1, I2, and I3; these thermocouples were connected in differential circuits for temperature control. A quartz window, 1.9-cm diameter and 16-cm thick, was sealed in place above the sample. Sample loading was accomplished by removal of the lids of the shields and the slotted plugs in the Inconel shells. All shields and shells rested upon steel balls that were constrained by slots in the next lower plate. The furnace could be rotated on a ball-bearing assembly; it was leveled by means of three sets of screws that contacted the circular rotation unit as shown in the figure. (Reprinted from reference [19].)