Model 426 is for Aluminium, Silver, Gold or Copper slim fixed point cells as well as for comparison calibration. Heat pipes provide the ideal conditions for the creation and maintenance of slim ITS-90 cells. The furnace core is a specially-designed stress-free isothermal heat pipe, which provides a very low thermal gradient along the core working length. The heat pipe is designed so that the inner wall is not subject to thermal expansion stresses from the outer wall before the heat pipe reaches conduction temperature. The working fluid is permanently and safely sealed within the plasma-arc welded enclosure. The Oberon can be used with Blackbody Fixed Point Cells.