General purpose thermocouples can be grouped by their base metal element type and by their construction type. There are 11 industry standard base metal combinations developed by the Instrument Society of America and adopted by International Electromechanical Commission. These have set calibration specifications approved by the American National Standards Institute. The most popular base metal element types are Type J, Type K, Type T and Type E with Type J being the most commonly used of the four. Type J is constructed with Iron wire as the positive lead and Constantan as the negative wire. This base metal combination is well suited for general purpose thermocouple (GPT) applications because it provides a wide operating temperature range of 0° to +750°C (32° to 1380°F), depending on the wire size, while offering a relatively low cost to build. Once the base element is selected, the construction type, essentially, the sheath enclosing the element, determines which general purpose category.

The second category for identifying general purpose thermocouples is their construction type which is based on the structure and material structure forming the protective enclosure (“sheath”) around the element. It is commonly referred to as the probe or protection tube. The most common construction type is the stainless steel tube. It is available in several standard OD sizes beginning at .188 and incrementally scaling up to .500 inches. The OD can be as small as .010 inch all the way up to 2 inches. The primary tubing material is stainless steel which is available in 304, 310 & 316 grades. Although Inconel is available as an alternative tubing material, it is rare to see any applications requesting Inconel. The second most common construction type for general purpose thermocouples is the thermowell. There are several standard mechanical configurations for thermowells such as threaded well, heavy duty threaded well, socket-weld, flanged, and Van Stone. Some other general purpose thermocouples construction types worth mentioning are closed-end protection tube types which come in a variety of forms for both ceramic and metal material.

General purpose thermocouple assemblies combine the thermocouple element and the protection tube with a transition. Transitions are an intermediate “coupling” and often serve as a reducer or adapter making it easier to mechanically connect to existing tubing or plumbing. An example of an industry standard transition would be a 3/8 OD sheath compression fitting to ½ NPT pipe. This transition allows any ½ NPT attachments to be threaded on to the probe. One of the most common attachments to the transition is the threaded aluminum cover head. The cover head is used as a water proof enclosure to house internally mount terminal blocks or transmitters. Cover heads provided access through a replaceable cap. Terminal blocks are generally used to provide wiring convenience for extension wires or cabling. Internally mounted transmitters, used with RTDs, provide analog signal for HVAC building applications. The cover head is also available in a UL94 rated plastic. The transition can also serve as the mechanical mounting mechanism. A common practice is to use a flange style transition, making a convenient mounting method for a general purpose thermocouple assembly. Being a universal connecting joint, the transition is often used with extension wire to provide connection to a remote a termination point. The complete general purpose thermocouple assembly will be the element, probe, transition, housing, and extension wire which may or may not have terminations. At C-Temp International, we are eager to provide your general purpose thermocouple assembly.