Views
8 months ago

tidu271

Theory of Operation

Theory of Operation www.ti.com The most misunderstood important point about a thermocouple is that virtually no voltage is developed at the junction. The junction is basically an electrical bond between two metals which complete the circuit, so that current flow can take place. Voltage is developed across each wire as temperature changes, as shown in Figure 5. The voltage difference is observed at the measuring end, because two dissimilar metals have different Seebeck coefficients. Figure 5. Thermocouple and Developed Voltage Depending on the required temperature range, vibration resistance, chemical resistance, response time, and installation and equipment requirements, the designer can choose the appropriate thermocouple. Thermocouples are available in different combinations of metals and calibrations as given in Table 3. Table 3. Types of Thermocouple TYPE TYPE E TYPE K TYPE J TYPE R TYPE S TYPE T Nickel –10% Nickel –10% Chromium Platinum –13%, Platinum –10%, Junction Chromium Iron versus Copper versus versus Nickel Rhodium versus Rhodium verus Material versus Constantan Constantan –5% Aluminum Platinum (–) Platinum (–) Constantan Silicon Seebeck Coefficient 62 µV/°C 40 µV/°C 62 µV/°C 7 µV/°C 7 µV/°C 40 µV/°C at 20°C Temperature –100°C to Range 1000°C 0°C to 1370°C 0°C to 760°C 0°C to 1450°C 0°C to 1750°C –160°C to 400°C High Resistance to Standard for Cryogenic oxidation and Often used in General General Higher calibration for the use; non- corrosion, differential Application purpose Sensitivity melting point of magnetic use calibration measurements gold purposes 8 Temperature Sensor Interface Module for Programmable Logic Controllers TIDU271–May 2014 (PLC) Submit Documentation Feedback Copyright © 2014, Texas Instruments Incorporated

www.ti.com Theory of Operation Each thermocouple sensor has its own sensitivity (µV/°C), a temperature range, and a nonlinear voltage curve over that temperature range, depending on its metals and calibration type. Thermocouple voltage curves are nonlinear over their operating temperature ranges, as can be seen in Figure 6. Figure 6. Thermocouple Output as a Function of Temperature The Seebeck coefficient is a nonlinear function of temperature and causes the output voltages of thermocouples to be nonlinear over their operating temperature ranges. Thermocouples are not perfectly linear across temperature. As a best practice, always choose a thermocouple having less variation in its Seebeck coefficient. Figure 7. Temperature versus Seebeck Coefficient TIDU271–May 2014 Submit Documentation Feedback Temperature Sensor Interface Module for Programmable Logic Controllers (PLC) Copyright © 2014, Texas Instruments Incorporated 9