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PowerPoint® Presentation Chapter 6 Electrical Thermometers Thermocouples • Resistance Temperature Detectors (RTDs) • Thermistors • Semiconductor Thermometers Chapter 6 — Electrical Thermometers A thermocouple creates an electrical potential when the junction is at an elevated temperature. Chapter 6 — Electrical Thermometers The Seebeck effect causes an electrical potential when two dissimilar wires are joined and the end is heated. Chapter 6 — Electrical Thermometers The Peltier effect can be used to build thermoelectric coolers. Chapter 6 — Electrical Thermometers The Thomson effect causes heating or cooling when there is current flow through a temperature gradient in a wire. Chapter 6 — Electrical Thermometers The law of intermediate temperatures states that the temperature at the end of the wires determines the electrical potential regardless of the intermediate temperatures. Chapter 6 — Electrical Thermometers The law of intermediate metals states that other metals may be used in a thermocouple circuit as long as the junctions are at the same temperature. Chapter 6 — Electrical Thermometers A complete thermocouple circuit includes extra junctions from the copper wire in the leads to the voltmeter. Chapter 6 — Electrical Thermometers A 32°F ice bath is the reference temperature for thermocouple tables. Chapter 6 — Electrical Thermometers An isothermal block can be used to establish a reference temperature for the cold junction. Chapter 6 — Electrical Thermometers A modern digital thermocouple system includes a voltage to temperature conversion, cold junction compensation, and a digital readout of the temperature. Chapter 6 — Electrical Thermometers Conventional thermocouple construction uses insulator beads to isolate the two thermocouple wires. Chapter 6 — Electrical Thermometers Thermocouple designations require that the wires follow a particular voltagetemperature curve. Chapter 6 — Electrical Thermometers Thermocouple color codes have been standardized in many countries. Chapter 6 — Electrical Thermometers Sheathed thermocouples can be wired in several ways for different applications. Chapter 6 — Electrical Thermometers Difference thermocouples are made of two thermocouples wired in series with reversed polarity. Chapter 6 — Electrical Thermometers A thermopile consists of several thermocouples wired in series in order to amplify the signal. Chapter 6 — Electrical Thermometers A swamping box uses resistors in each thermocouple circuit to eliminate errors when measuring an average reading of a set of thermocouples. Chapter 6 — Electrical Thermometers A thermocouple pyrometer uses a variable potentiometer to balance loop resistance. Chapter 6 — Electrical Thermometers A resistance temperature detector (RTD) contains a resistor with a resistance that varies with temperature. Chapter 6 — Electrical Thermometers RTDs consist of precision wires wrapped around an insulator and encapsulated in a protective sheath. Chapter 6 — Electrical Thermometers Thermistors are available in a variety of shapes and sizes. Chapter 6 — Electrical Thermometers The changing resistance of a thermistor can be used as a temperature switch. Chapter 6 — Electrical Thermometers PTC thermistors have a resistance that increases with increases in temperature.