Why can't compensation to the wire connected to the thermocouple temperature over 100 ℃ _ instrument2

Thermocouple for temperature measurement of the electric potential, is to use temperature difference but, because of the cold end temperature difference between temperature and display and control instrument, the use of thermoelectric potential and general thermocouple potential close to cheap instead of hot metal electrode extension of signal, which is compensation conductor. This kind of compensation conductor should be 1, copper - Compensation K dividing nickel chrome - constantan KC Nickel silicon thermocouple 2, copper - Copper and nickel SC compensation S dividing platinum rhodium - 10 Platinum thermocouple both compensation conductor should be belong to type of compensation, the main problem is to compensate the material after the thermoelectric potential of more than 100 degrees, error increases, at work, thermocouple cold junction temperature shall not be greater than 100 degrees, even using ptfe or glass fiber insulating sheath, the compensation temperature also shall not be greater than 100 degrees, if you are using PVC insulating sheath, its working environment temperature Thermocouple cold end. Shall not be greater than 70 degrees. Some inexpensive metal thermocouple and thermal electrode materials consistent conductor do compensation conductor, this kind of compensation conductor should be called expanded compensation conductor. They have 1, nickel chromium - Nickel silicon KX extension compensation degree nickel chrome - K Nickel silicon thermocouple 2, nickel chromium - Constantan EX extended compensation degree nickel chrome - E Constantan thermocouple 3, - of iron J dividing iron - constantan JX extension compensation Copper constantan thermocouple, 4 - Copper nickel T indexing copper - TX extension compensation Copper and nickel thermocouple such compensation conductor thermoelectric potential and thermocouple completely consistent, so, in the temperature of the thermocouple cold junction compensation is not restricted, if using ptfe jacket, you can use an ambient temperature of 250 degrees, the use of glass fiber sheath environment temperature is 500 degrees, if use PVC sheathed with environment temperature shall not be greater than 70 degrees. In addition, B dividing thermocouple due to potential very weak at low temperatures, below 400 degrees without precision, so B dividing platinum rhodium - 30 Platinum rhodium thermocouple 6 without compensation wire, double core is commonly used in copper sheath line.