A Mineral insulated, metal sheathed Thermocouple or a Thermocouple element insulated with re-crystallized Alumina tube fitted into a terminal head & provided with or without head extension, a support tube and Re-crystallized Alumina protecting fitted tube would form a typical assembly for temperature measurements of furnaces, combustion chambers, recuperators & similar applications. The closed end Protecting tube normally employed is re-crystallized Alumina type 710 containing 99.7% Aluminum Oxide & conforming to Type 999 as per DIN VDE 0355 & can be used for temperatures upto 1700 deg C. These tubes are impervious & are fixed to metallic support tube by means of ceramic to metal cement. The low cost option is Pythagoras Tubes, Type 610 containing approximately 60% Aluminum Oxeide as per DIN VDE 0335 & can be used for temperatures upto 1400 deg C. The standard execution is with plated CS extension & Aluminum head with conduit entry of 1/2"NPT. The thermocouple wire size when used as a beaded Thermocouple is 24 AWG (0.51mm) for type R, S & 'B' &14 AWG for all base metal Thermocouples.
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