A thermocouple that is used as a temperature standard in the temperature range 631 c to 1064 c is made up of
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Temperature FUNDAMENTALS Temperature is a basic thermodynamic property of a substance, is determined by the amount of kinetic or heat energy in the substance. Increasing the kinetic energy in a substance will increase its temperature, while decreasing the kinetic energy will lower the substance's temperature. When two bodies at different temperatures are put into thermal contact, energy will be transferred from the warmer body to the colder body until the two bodies are at the same temperature. If two bodies are in thermal equilibrium with each other, they are at the same temperature. If two bodies have equality of temperature with a third body, they in turn have equality of temperature with each other. This is the zeroth law of thermodynamics. In order to be able to repeatedly and accurately report temperatures, standard temperature scales have been devised. The most common are the Fahrenheit and the Celcius scales. The relationship between these two scales is: F = 32 + (9/5) C In addition to the two common scales mentioned above, there are two absolute scales, the Rankine and Kelvin scales. These scales define temperatures independent of any particular substance and should be used in any calculation that requires reference to absolute temperatures, such as the ideal gas law. The English system for absolute temperature is the Rankine scale and is related to degrees F as follows: R = F The international system for absolute temperature is the Kelvin scale and is related to degrees C as follows: K = C TEMPERATURE SENSORS Temperature is the most commonly measured variable in process control. As a result, there are proliferation of sensors and physical construction options. However, while the options are numerous, the optimum choice for a particular application is not always clear. To make these decisions easier, the following information provides some theoretical background on temperature sensors and details on the standard sensors offered. Some of the background topics include the operation of thermocouple and RTDs, choosing between them, and application considerations. THERMOCOUPLE OPERATION The industrial thermocouple is based on the principle that when two dissimilar metals are joined together, an electromotive force (EMF) will exist. If the circuit is closed, a current will flow as a result of the EMF. This principle of operation has made it possible to construct thermocouples capable of measuring a wide range of temperatures using different metal combinations. As shown in Figure 1, a thermocouple circuit has two sides that are called junctions. The process side of the circuit is called the hot junction, while the opposite side is called the cold junction. The voltage (V) across this circuit is proportional to the temperature difference between the hot and cold junctions (T 1 and T 2, respectively). Figure 2 shows how this is implememted with a temperature transmitter. The hot junction is at the thermocouple, and the cold junction is at the transmitter s terminal strip. The voltage produced at the cold junction is proportional to the temperature difference between the thermocouple s hot junction and a transmitter's terminal strip. A reference junction sensor is typically used to measure the temperature of the terminal strip. By measuring the reference junction (T 2 ) and the voltage (V), the temperature at the hot junction (T 1 ) can be determined. T 1 FIGURE 1 Basic T/C Circuit T2 V = k(t - T ) 1 2 T 1 V T 2 V FIGURE 2 Temperature Transmitter T/C Circuit Americas ext Asia Pacific Europe