Watt-Flex can be manufactured with a groove along the outside of the sheath to accommodate a needle-type thermocouple for accurate sensing and control in temperature-critical processing applications such as molding heat-sensitive resins. Unlike those in internal-thermocouple heaters, the thermocouple in this split-sheath cartridge heater can be replaced independently of the heater. While the thermocouples in conventional heaters are fixed and measure the temperature at either the middle or tip of the cartridge, the thermocouple in the split-sheath cartridge heater is movable, allowing temperature measurement at any point along the heater sheath. Split-sheath construction allows each half of the heater to expand independently when energized to ensure secure, metal-to-metal contact with the surrounding bore. When the heater is de-energized, the split sheath contracts uniformly to ease removal, and the company heaters warranted its heaters to be removable from bores and sleeves.