A thermocouple with a protecting tube is constructed with thermocouple bare wires, ceramic insulators, a terminal block and a terminal head as shown at Figure 2.1. Mineral insulated thermocouple is a thermocouple with such integrated construction that powdery inorganic insulator (for example, magnesium oxide; MgO) is filled up between metal sheath and thermocouple wires and sealed as shown at Figure 2.2. The MI thermocouple can be made 3(three) types of measuring junctions such as “Ungrounded”, “Grounded,” and “Exposed” junctions. The advantages of MI thermocouple are (1) thinness, (2)fast response, and (3)bendability---bending radius 10 shall be 5 times of the outer diameter of metal sheath as shown at Figure 2.3

The limits of normal operating temperature and elevated operating temperature are given in JIS C 1602-1995 “Thermocouples” and JIS C 1605-1995 “Metal Sheathed Thermocouples”.  

“Normal operating temperature limit” means the limit of temperature within which thermocouples can be continuously used in the air. “Elevated operating temperature limit” means at which thermocouples can be used for a short time in case of unavoidable needs. Regarding the normal operating temperature and the elevated operating temperature, refer to Table 2-1 for thermocouples, Table 2-2 for Metal Sheathed Thermocouples and Refer to Table 2-3 for the normal operating temperature of protecting tubes. Moreover refer to Table 2-4 for the normal operating temperature of lining and Teflon tube. 

Extension and/or Compensating cables Extension and/or Compensating cables Extension and/or compensating cable is identified with color according to the type of thermocouples and have nearly the same thermoelectromotive force characteristic as the thermocouple which is to be used in combination. Tolerance of compensating cable is determined as Table 4 according to the temperature of connecting junction with thermocouple. When wiring thermocouples, be sure to use the proper compensating cables. If the unproper cable such as the cable for a different type of thermocouple or copper lead wires are used, it will generate the unproper EMF and will not be able to expect the accurate measurement. Be sure to keep the temperature of compensating junction properly at connecting point with thermocouple and compensating cable. 

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