North American Thermocouple Selection
Sheet 8745-1
Cover
Terminal Block
Gasketed Terminal Head
Elements and Insulators
Protection Tube
Exploded view of typical thermocouple assembly and protecting tube.
CONNECTION HEADS The purpose of the thermocouple connection or terminal head is to make positive electrical connections between thermocouple and extension wires to provide attachment for a protecting tube and extension wire conduit. The head contains a ceramic terminal block for electrical connections. Typical heads include a general-purpose head for most installations; a screw-cover head, for applications which must be weatherproof; and other special- ized connections. EXTENSION WIRE Extension wire connects the thermocouple to the reference junction in the instrument. This wire is generally a matched pair of conductors having insulation designed to meet the service needs of a particular application, and conductor type to match the thermocouple. mechanical damage; (5) to support the thermocouple. A wide variety of tube and well materials is available. Various ceramic insulators are available to support and protect the thermocouple wire within a tube or well. Protecting tubes and wells tend to slow the speed of response of thermocouples. For applications where faster response is desirable, "small mass," and "exposed tip" thermocouples are available.
A thermocouple is composed of two dissimilar metal wires butt- welded together in a circuit, as in Figure 1. The circuit develops a small dc voltage proportional to the temperature difference between the measuring junction (T 1 ) and the reference junction (T 2 ). This emf is the simple means whereby temperature can be measured by pyrometers.
Metal A
T 1 Measuring Junction
T 2 Reference Junction (30-T-91)
Metal B
Figure 1. Single Thermocouple Circuit.
THERMOCOUPLES Thermocouples (usually with a protecting tube or well) are installed directly in the process medium. A variety of thermocouple wire types is available to cover the temperature range –184 to +2330 C (–300 to +4000 F). Thermocouple wire is selected to provide as linear as possible a temperature-emf relation. Protecting tubes or wells are used for one or more of the following reasons: (1) to protect against deleterious gases; (2) to protect against corrosive fluids; (3) to provide suitable protection for a thermocouple in a pressurized vessel; (4) to protect against
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