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 ( o C) and 0 o C is 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 Current flows when the Two junctions are at Different temperatures Metal wire 1 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