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Chapter 6

Chapter 6. Electrical Thermometers. Thermocouples • Resistance Temperature Detectors (RTDs) • Thermistors • Semiconductor Thermometers. A thermocouple creates an electrical potential when the junction is at an elevated temperature.

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Chapter 6

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  1. Chapter 6 Electrical Thermometers Thermocouples • Resistance Temperature Detectors (RTDs) • Thermistors • Semiconductor Thermometers

  2. A thermocouple creates an electrical potential when the junction is at an elevated temperature.

  3. The Seebeck effect causes an electrical potential when two dissimilar wires are joined and the end is heated.

  4. The Peltier effect can be used to build thermo-electric coolers.

  5. The Thomson effect causes heating or cooling when there is current flow through a temperature gradient in a wire.

  6. The law of intermediate temperatures states that the temperature at the end of the wires determines the electrical potential regardless of the inter-mediate temperatures.

  7. The law of inter-mediate metals states that other metals may be used in a thermocouple circuit as long as the junctions are at the same temperature.

  8. A complete thermocouple circuit includes extra junctions from the copper wire in the leads to the voltmeter.

  9. A 32°F ice bath is the reference temperature for thermocouple tables.

  10. An isothermal block can be used to establish a reference temperature for the cold junction.

  11. A modern digital thermo-couple system includes a voltage to temperature conversion, cold junction compensation, and a digital readout of the temperature.

  12. Conventional thermocouple construction uses insulator beads to isolate the two thermocouple wires.

  13. Thermocouple designations require that the wires follow a particular voltage-temperature curve.

  14. Thermocouple color codes have been standardized in many countries.

  15. Sheathed thermo-couples can be wired in several ways for different applications.

  16. Difference thermo-couples are made of two thermocouples wired in series with reversed polarity.

  17. A thermopile consists of several thermocouples wired in series in order to amplify the signal.

  18. A swamping box uses resistors in each thermocouple circuit to eliminate errors when measuring an average reading of a set of thermocouples.

  19. A thermocouple pyrometer uses a variable potentio-meter to balance loop resistance.

  20. A resistance temper-ature detector (RTD) contains a resistor with a resistance that varies with temperature.

  21. RTDs consist of precision wires wrapped around an insulator and encapsulated in a protective sheath.

  22. Thermistors are available in a variety of shapes and sizes.

  23. The changing resistance of a thermistor can be used as a temper-ature switch.

  24. PTC thermistors have a resistance that increases with increases in temperature.

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