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Application Guide

The Art of TemperatureSensing

Type J The Type J may be used, exposed orunexposed, where there is a deficien-cy of free oxygen. For cleanliness and longer life, a protecting tubeis recom-mended. Since JP (iron) wire willoxidize rapidly at temperatures over540°C (1000°F), it is recommended thatlarger gauge wires be used to compen- sate. Maximum recommended operat-ing temperature is 760°C (1400°F).

Thermocouples

Type K Continued Due to its reliability and accuracy,Type K is used extensively at tem- peratures up to 1260°C (2300°F). It’s good practice to protect this type of thermocouple with a suitable metal or ceramic protecting tube, especially in reducing atmospheres. In oxidizingatmospheres, such as electric furnaces,tube protection is not always necessarywhen other conditions are suitable; however, it is recommended for cleanli- ness and general mechanical protection. Type K will generally outlast Type J because the JP (iron) wire rapidly oxi- dizes, especially at higher temperatures. Type N This nickel-based thermocouplealloy isused primarily at high temperatures upto 1260°C (2300°F). While not a direct replacement for Type K, Type Nprovides better resistance to oxidationat high temperatures and longer life in applications where sulfur is present. Type T This thermocouple can be used ineither oxidizing or reducing atmos-pheres, though for longer life a protec-ting tube is recommended. Becauseof its stability at lower temperatures,this is a superior thermocouple for awide variety of applications in lowandcryogenic temperatures. It’s recommen-ded operating range is—-200°to350°C (-330°to 660°F), but it can beused to -269°C (-452°F) (boiling helium). W-5 Percent Re/W-26 Percent Re (Type C*) Types S, R and B This refractory metal thermocouplemay be used at temperatures up to Maximum recommended operatingtemperature for Type S or R is 1450°C (2640°F); Type B is recommended foruse at as high as 1700°C (3100°F). These thermocouples are easily con-taminated. Reducing atmospheres areparticularlydamaging to the cali-bration. Noble metal thermocouplesshould always be protected with a gas-tight ceramic tube, a secondarytube of alumina and a silicon carbide or metal outer tube as conditions require. 2315°C (4200°F). Because it has noresistanceto oxidation, its use isrestricted to vacuum, hydrogen orinert atmospheres.
*Not an ANSI symbol Turn to page46 for recommended uppertemperature limits forvarious thermocoupleconductor gauges. Thermocouple Conductor Gauge Thermocouple conductors come in avariety of sizes. Depending on yourapplication, the gauge selected will affect the thermocouple’sperformance.The larger the gauge size, the morethermal mass the thermocouple willhave with a corresponding decreasein response. The larger the gauge size the greater the stability and oper-ating life. Conversely, a smaller gaugesize will have a quicker response, but may not deliver the stability or oper-ating life required. 38