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

P V n R T

Density Change

Density is a function of temperature --- for most materials, as temperature increases, density decreases

massρ=

volume

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 fragileCan be thin, so rapid responseLeads can give substantial errors

Thermistors

Semiconductor material

Resistance decreases with increasing temperature

Very sensitive --- large coefficient of thermal resistivity

ΔRlarge

ΔT

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 - ConstantanType J --- Iron - ConstantanType K --- Chromel - AlumelType E --- Chromel - ConstantanType 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 temperatureCompensate 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

4E T

Emissive power also a function of wavelength

5

1B

T

CE

e

Radiation Effects

Need to know emissivity

Measure emissive power via a photodetector sensitive to a given wavelength