Thermocouples sense temperature as a result of a voltage generated between the joined end of two dissimilar metal wires and the open end when the temperature at the joined end, the "hot" junction, is different from the temperature at the open end, the "cold" junction. The voltage, known as the Seeback voltage after Thomas Seeback the discoverer of this effect, is proportional to the difference in temperature. Seeback voltage is given by the product of the change in temperature times the Seeback coefficient which represents the rate of change of thermal EMF (electromotive force) with respect to temperature. This is usually stated as microvolts per degree C, and is dependent on the temperature difference and the metals or alloys used in the thermocouple wire.
The combination of metals used in each thermocouple gives each type its name. For example, the wires used in a "Type J" thermocouple are iron (a relatively pure metal) and constantan (copper-nickel alloy).
Thermocouples are generally more rugged and less expensive than other types of sensors. Where thermocouple leads are connected to a voltmeter and the metals in each pair of wires are dissimilar, additional thermocouple-like junctions are created. For example, if the voltmeter connections are copper, connecting a Type T (copper/constantan) thermocouple creates two additional junctions: copper/copper and constantan/copper. No thermal EMF is created at the copper/copper junction. At the constantan/copper junction, however, an EMF is generated which opposes the EMF generated at the hot junction of the thermocouple.
If the temperature at the additional dissimilar junction is known, it is a simple matter to calculate the voltage at the hot junction, which yields the correct temperature of the process being measured. One method of compensating for this effect, is to create a "reference" junction through the use of "isothermal blocks'. Because the exact temperature of these blocks is known, usually a precise correction can be made, which yields an accurate value of the temperature at the "hot" or measured junction. These calculations are usually made by the microprocessor that is present in most modern digital controls.
Compensation can also be made with hardware. A battery may be used to offset the voltage generated at the reference junction. In addition, electronic circuits are commercially available which provide the electrical equivalent of 0 C at the reference junction.
Both the computer and hardware methods have their advantages and disadvantages, but both can be used to provide accurate temperature measurements.
Thermocouples can also be classified by the type of junction. Exposed junction thermocouples do not have their junctions protected by a sheath. Response time is fastest with this type of thermocouple but the maximum allowable temperature is less than that of a sheathed thermocouple.
A grounded thermocouple has its measuring junction in contact with a metal surface. This is the most common type. Where electrical isolation is necessary, an ungrounded junction is used. In this type, the junction is electrically isolated from its protective sheath. To provide a path for noise, the sheath can be grounded.
In selecting thermocouples, it is necessary to know the temperature range to be measured, the accuracy required, the materials to which the thermocouple will be exposed (e.g., acids), and the environment in which the thermocouple will be used (e.g., abrasive, vibratory, etc.).
Love manufactures a wide variety of thermocouples. All are manufactured to industry standards and meet stringent ANSI standards. This assures interchangeability with other standard thermocouples without requiring additional instrument recalibration.
Love thermocouples can be used in all types of applications, can measure wide temperature ranges, and are offered in a large variety of standard configurations.
Service temperature ranges are limited by the materials used in construction. See the application chart for wire temperature limits and service temperatures for the various thermocouples.
See Applications Guidefor additional information