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The Measurement of Temperature

The Measurement of Temperature. Chapter 17. Definitions……. Heating is the transfer of energy from an object with more random internal energy to an object with less Temperature is a measure of the average energy of the randomly moving molecules in a substance

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The Measurement of Temperature

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  1. The Measurement of Temperature Chapter 17

  2. Definitions…… • Heating is the transfer of energy from an object with more random internal energy to an object with less • Temperature is a measure of the average energy of the randomly moving molecules in a substance • Same temperature=thermal equilibrium

  3. History • Daniel Gabriel Fahrenheit • Chose two reference points • Coldest (mixture of salt, water, ice) 0F • Body temperature 96 (slightly in error = 98.6) • First to utilize mercury • Anders Celius • His two reference points: boiling point of water and the freezing point of water • Lord Kelvin • Zero point is absolute zero (-273.15oC) and 0oC is 273.15 K • Review Figure 17.1 and the conversion tables

  4. Fixed Reference Points and Thermometer Calibration • Systems whose temperatures are determined by some physical process that is universal and repeatable • Phase transitions are commonly used as fixed reference points

  5. Transducers (sensors) used to measure temperature • Liquid expansion devices • Bimetallic expansion devices • Change-of-state indicators • Metallic resistance devices • Thermistors • Thermocouplers

  6. Liquid Expansion Thermometers • Contain liquid that expands or contracts with temperature changes • Mercury or alcohol • Partial, total or complete immersion

  7. Bimetallic Expansion Thermometer • Constructed by fusing together two metal strips, typically one of brass and one of iron, which have different coefficients of expansion

  8. Change-of-State Indicators • Varied products, all of which change color or form when exposed to heat

  9. Resistance Thermometry • Metallic resistance thermometers and thermistors are two types of thermometers based on the principle that the electrical resistance of materials changes as their temperature changes • RTDs use metallic wires • Thermistors use semiconductor materials

  10. Thermocouples • Consists of two wires made of different metals that are joined together Metal wire 1 Current flows when the Two junctions are at Different temperatures Metal wire 2

  11. Verifying the Performance of Laboratory Thermometers • 95% of all possible malfunctions will affect the ice-point reading • Review Box 3: Verifying the Performance of a Thermometer • OR verify against a NIST-traceable thermometer

  12. Problems • Do 1, 3, 4, and 5

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