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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 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 • Same temperature=thermal equilibrium
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
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
Transducers (sensors) used to measure temperature • Liquid expansion devices • Bimetallic expansion devices • Change-of-state indicators • Metallic resistance devices • Thermistors • Thermocouplers
Liquid Expansion Thermometers • Contain liquid that expands or contracts with temperature changes • Mercury or alcohol • Partial, total or complete immersion
Bimetallic Expansion Thermometer • Constructed by fusing together two metal strips, typically one of brass and one of iron, which have different coefficients of expansion
Change-of-State Indicators • Varied products, all of which change color or form when exposed to heat
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
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
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
Problems • Do 1, 3, 4, and 5