Temperature Measure of internal energy of substance Four common scales Celsius (C) Kelvin (k) Fahrenheit (F) Rankine (R)

Temperature Multiple ways to measure Ideal Gas Thermometer Density Change Electrical Effects Radiation Effects

Ideal Gas Thermometer Best suited for low to moderate temperatures where gas is ideal Essentially measure pressure of known volume of gas Use ideal gas equation to compute temperature

Density Change Density is a function of temperature --- for most materials, as temperature increases, density decreases Since mass fixed, then volume must change. To decrease density, volume must increase

Density Change Liquid thermometers Liquid expands upon heating Level rises in capillary Actual expansion is quite small

Liquid thermometers Can be calibrated to NIST standards Can be quite accurate Level of immersion is important Fairly slow response Fragile

Density Change BiMetallic Thermometers Two strips of different metals bonded together Temperature change causes differential expansion Differential expansion causes strip to bend

BiMetallic Thermometers To T1 Can be strips or coiled. Commonly used in household thermostats.

Electrical Effects RTD -- Resistance Temperature Detector Thermocouples --- Differential EMF Thermistors --- Semiconductors For most materials, resistance increases with temperature

RTD Wire resistance is known function of temperature Accurately measure resistance Use known R - T relationship to compute temperature Mechanically fragile Can be thin, so rapid response Leads can give substantial errors

Thermistors Semiconductor material Resistance decreases with increasing temperature Very sensitive --- large coefficient of thermal resistivity Need to carefully measure resistance

Thermocouples When different metals in contact, emf generated Emf is function of temperature

Thermocouples Are standard metal pairs Type T --- Copper - Constantan Type J --- Iron - Constantan Type K --- Chromel - Alumel Type E --- Chromel - Constantan Type S --- Platinum - (Platinum + 10% Rhodium Each metal pair has its own EMF vs T curve

Thermocouples Thermocouple must have at least 3 metal - metal junctions Where the wires are twisted together One wire terminates at emf measuring point Other wire terminates at emf measuring point

Thermocouples If terminal points same material and at same temperature, do not need to correct for termination Need to compare to reference temperature Add reference temperature Compensate in measuring device

Thermocouples Reference junction Process Measuring Device Ice Bath Material 1 Material 2 Material 1

Thermocouples Cheap Fast response time Robust Easily mounted to surfaces

Radiation Effects All objects emit radiation based on their temperature Emissive power also a function of wavelength

Radiation Effects Need to know emissivity Measure emissive power via a photodetector sensitive to a given wavelength