Temperature Sensor : Thermocouple Basics

Temperature Sensor : Thermocouple Basics

Let's start with T. J. Seebeck, who in 1821 discovered what is now termed the thermoelectric effect. He noted that when two lengths of dissimilar metal wires (such as iron and Constantan) are connected at both ends to form a complete electric circuit, an emf is developed when one junction of the two wires is at a different temperature than the other junction. 

  Basically, the developed emf (actually a small millivoltage) is dependent upon two conditions: (1) the difference in temperature between the hot junction and the cold junction. Note that any change in either junction temperature can affect the emf value and (2) the metallurgical composition of the two wires. 

  Although a "thermocouple" is often pictured as two wires joined at one end, with the other ends not connected, it is important to remember that it is not a true thermocouple unless the other end is also connected! It is well for the user to remember this axiom: "Where there is a hot junction there is always a cold or reference junction" (even though it may seem hidden inside an instrument 1,000 feet away from the hot junction). 

  Still in Seebeck's century, two other scientists delved deeper into how the emf is developed in a thermoelectric circuit. Attached to their names are two phenomena they observed--the Peltier effect (for Jean Peltier in 1834) and the Thompson effect (for Sir William Thompson a.k.a Lord Kelvin in 1851). Without getting into the theories involved, we can state that the Peltier effect is the emf resulting solely from the contact of the two dissimilar wires. Its magnitude varies with the temperature at the juncture. Similarly, the Thompson effect can be summarized as having to do with emf's produced by a temperature gradient along a metal conductor. Since there are two points of contact and two different metals or alloys in any thermocouple, there are two Peltier and two Thompson emfs. The net emf acting in the circuit is the result of all the above named effects. 

  Polarity of the net emf is determined by (a) the particular metal or alloy pair that is used (such as iron-constantan) and (b) the relationship of the temperatures at the two junctions. The value of the emf can be measured by a potentiometer, connected into the circuit at any point. 

  In summary, the net emf is a function primarily of the temperature difference between the two junctions and the kinds of materials used. If the temperature of the cold junction is maintained constant, or variations in that temperature are compensated for, then the net emf is a function of the hot junction temperature. 

  In most installations, it is not practical to maintain the cold junction at a constant temperature. The usual standard temperature for the junction (referred to as the "reference junction") is 32°F (0°C). This is the basis for published tables of emf versus temperature for the various types of thermocouples. 

  The Law of Intermediate Temperatures provides a means of relating the emf generated under ordinary conditions to what it should be for the standardized constant temperature (e.g., 32°F). Referring to Figure 4-1, which shows thermocouples 1 and 2 made of the same two dissimilar metals; this diagram will provide an example of how the law works. Thermocouple 1 has its cold junction at the standard reference temperature of 32°F and its hot junction at some arbitrary intermediate reference temperature (in this case, 300°F). It generates 2.68 mv. Thermocouple 2 has its cold junction at the intermediate reference point of 300°F and its hot junction at the temperature being measured (700°F). It generates 4.00 mv. The Law of Intermediate Temperatures states the sum of the emfs generated by thermocouples 1 and 2 will equal the emf that would be generated by a single thermocouple (3, shown dotted) with its cold junction at 32°F and its hot junction at 700°F, the measured temperature. That is, it would hypothetically read 6.68 mv and represent the "true" emf according to the thermocouple's emf vs. temperature calibration curve. 

  Based upon this law, the manufacturer of an infrared thermocouple need only provide some means of substituting for the function of thermocouple 1 to provide readings referenced to the standard 32°F cold junction. Many instruments accomplish this with a temperature-sensitive resistor which measures the variations in temperature at the cold junction (usually caused by ambient conditions) and automatically develops the proper voltage correction.