Introducing a New Vacuum Furnace Control Thermocouple

Reviewed will be a short survey of past and present thermocouple practice and application problems to meet the AMS 2750E standard over a temperature range from ambient to 2800F. A new ceramic closed end tube and internal platinum thermocouple, type S or type R, with reduced mass and thermal conductivity has been developed for furnace control, to track the survey work thermocouples to meet an AMS class 2 standard, without operator control thermocouple offsets.

Included in the presentation will be thermocouple drawings of prior designs as well as drawings for the new operational low-mass thermocouple design. Also included will be a graph detailing the primary problem for the alumina ceramic tube and internal ceramic, clearly showing the added thermal conductivity for the ceramics from 1400F down to ambient. This is the primary problem and one that intuitively would not be suspected under normal circumstances. An additional drawing will be presented highlighting the addition of thermal heat shields to the back side of the hot zone ring (or box) to further reduce thermal conductivity from the thermocouple to the water-cooled outer chamber. Actual comparative charts and data will be presented detailing the thermocouple improvements over prior art.