Temperature is the most frequently measured process variable, and any material with a temperature sensitive characteristic can be used as a Thermistor. Electrical temperature sensing devices can be used for remote temperature measuring and signaling. We will compare the thermocouple, thermistor and RTD methods.

Thermocouples

A thermocouple is a measuring device made by joining materials with different Seebeck coefficients. Two wires from dissimilar metals are joined at one end. The emf is generated when a temperature gradient exists between this wire junction and a reference junction. This measurable change of electric potential is the basis of the thermocouple method. The most common materials used in thermocouples are iron-constantan, copper-constantan, chromel-constantan, and two different platinum alloys. Table 1 shows various thermocouple types and materials.

Thermocouple junctions come in three basic forms: (1) exposed, (2) grounded and (3) ungrounded. Exposed junction was designed for faster response. Insulation is sealed beyond the exposed junction tip to prevent penetration of moisture or gas to the inner thermocouple. Grounded junction is used for high-pressure gas and liquid applications. The junction is electrically-joined to the protective sheath. This provides faster response. Ungrounded junction is used for measuring in corrosive conditions. The welded tip of the wire is physically insulated from the jacket/sheath. Thermocouples can be portable or permanent.