Thermocouple wire can be fabricated into accurate and dependable thermocouples by joining the thermoelements together at the sensing end. Thermocouple wire must be used to extend the thermocouple to instruments where temperature limitations exceed that for use of thermocouple extension wire. The conditions of measurement determine the type of thermocouple wire and insulation to be used. Temperature range, environment, protection, insulation requirements, response and service life should be considered. The following parameters serve as a guide to the selection of wire.
Temperature limits for standard thermocouples that are protected with a closed end protecting tube are shown. These limits are suggested for continuous temperature sensing where insulation is not a factor. For unprotected thermocouples where fast response is required these limits should be reduced for equivalent service life.
Thermocouple extension wire has approx- imately the same thermoelectric properties as thermocouple wire but is only guaranteed accurate within a limited temperature range. Thermocouple extension wire can be used to connect thermocouples to instruments at a lower cost and/or to provide improvement in the thermoelectric circuit. Thermocouple extension wire used with base metal thermocouples are substantially the same alloys. However, due to the high cost of noble metal thermocouple wire such as Platinum Rhodium vs. Platinum the extension wire for these applications is a different alloy like Copper vs. Copper Alloy. The “X” in the code denotes extension grade wire.