Thermocouples What are Thermocouples? Thermocouples are temperature sensors. They operate under the principle that the junction of two dissimilar metals (forming a closed circuit) produces a measurable voltage (electromotive force) when the two ends of the thermocouple are at different temperatures (see Figure 1). Because thermocouples have simple construction and are superior in reliability, they have been used as industrial temperature sensors in a wide range of fields. Moreover, connecting a measuring instrument (recorders,DCS,PLC etc.) to one end of a circuit allows you to measure potential difference (electromagnetic force) (see Figure 2). There are many types of thermocouples to measure different range of temperature. Commonly-used types with superior characteristics have been standardized by JIS, IEC standards, and others. The following summarizes the typical thermocouple types (generally represented by symbols) and their features (advantages and disadvantages). Thermocouples Advantages & Disadvantages A thermocouple is comprised of at least two metals joined together to form two junctions. One is connected to the body whose temperature is to be measured; this is the hot or measuring junction. The other junction is connected to a body of known temperature; this is the cold or reference junction. Therefore the thermocouple measures unknown temperature of the body with reference to the known temperature of the other body.
Working PrincipleThe working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. They are as follows: 1) Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals. 2) Peltier effect: As per the Peltier effect, when two dissimilar metals are joined together to form two junctions, emf is generated within the circuit due to the different temperatures of the two junctions of the circuit.3) Thomson effect: As per the Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit.In most of the cases the emf suggested by the Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect plays a prominent role in the working principle of the thermocouple.
Resistance Temperature Detectors
What are Resistance Temperature Detectors?Resistance temperature detectors (RTD) are temperature sensors. They operate on the principle that the resistivity of a metal increases in proportion to its temperature. A Platinum RTD uses platinum (Pt) for a resistance temperature sensing element, which has good temperature characteristics and is linear and stable. Among the various types of temperature sensors, Platinum RTDs have been used widely with their high accuracy. In particular, Pt100 (resistance value at 0° is 100 ohm) has been popular worldwide. Nickel and copper are also used for RTDs. Thermistors are employed as resistors. Three types of wiring techniques are available: two-wire, three-wire, and four-wire.
b) Principle of the four-wire techniqueA constant current is passed through r1 and r4, and voltage is measured at the terminals of RTD, which is free from the effect of lead resistances in your measurement. Therefore, this system allows accurate temperature measurements. If a RTD of the four-wire technique is connected to a measuring instrument of the three-wire technique, disabling one of the RTD leads in the four-wire technique provides a simple configuration of temperature measurement. In this case, keeping the resistance of the three leads low and uniform is required in the same way as the three-wire technique. the unused lead must be terminated (insulated) to avoid the effects of noise and others. Original Source