The standard assembly consists of an 1/8″ O>D> MgO insulted type “J” thermocouple in a stainless steel sheath. The body is stainless steel with a 1/2″-20UNF-2A thread. A thermal barrier reduces heat transfer from body to thermocouple. Other parts include a packing seal, compression barrel nut, hex adjusting screw and polarized quick-disconnect male plug. Standard immersion is adjustable from flush to 1″. Standard body lengths are 3″, 4″, and 6″ measured from the beginning of wrench flats to the bolt tip. The adjustable plastic melt thermocouple has a movable probe activated by an inner screw mechanism. The probe can be immersed into or withdrawn from the plastic melt stream while the extruder is in full operation at maximum flow rates and pressures. This feature permits temperature measurement at various depths in the melt stream, eliminates thermocouple breakage at start-up, and prevents hang-up. The thermocouple is self-cleaning; as it is retracted into the body any film formed on it is scraped off.