7. Temperature

Methods of temperature measurement

1. pressure/volume(thermal expansion)

2. contact potential btw diff metal

3. resistance of metal/semiconductor

4. energy radiation

Working mechanism of a thermoelectric temperature sensor (thermocouple)

• 2 different metal joint at both end

• Seebeck effect: when junctions of the metal experience different temperature, electric current flows

• contact potential (difference in electric potential difference) increases with temperature

how does contact potential occur

junction of 2 different metal always produces contact potential

How to account for contact potential of wires connecting the voltmeter (2 ways)

1. ensure junction of wire and voltmeter wire is kept at same temperature, therefore the contact potential produced cancels out

2. connect in second configuration, such that V1=V2-V3

Factors to consider when choosing type of thermocouple

• sensitivity and temperature range

• application to corrosive environment

• withstand thermal and mechanical stress

• accuracy

• price

Difference between thermocouple probe ends:

• soldered bare wire

• metallic sheath

Metallic sheath:

• can touch conductive surface due to inner insulation

• longer response time as sheath and insulation has to be heated up before it reaches junction

Insulation material has high resistance and thermal conductivity

Working mechanism of basic metallic resistance thermometer + formula

resistance of certain metal changes with temp

α: temperature coeff of resistance

Reasons platinum is commonly used for metallic resistance thermometer + 1 disadvantage

platinum typically used for

• stability

• reproducibility(linear relationship)

• inert and resistant to contamination

disadvantage: less sensitive compared to other metals

3 types of metallic resistance thermometer construction

a. wire directly exposed to fluid measured

b. isolated by ceramic rod to prevent contamination that could change relationship to temeperature

c. metal deposited as thin film pattern on insulating substrate and attached to flat surface

2 Potential errors occur in metallic resistance thermometer

1. self heating effect-electric current heats up wire due to energy dissipation

2. Thermoelectric effect(seebeck effect): if resistance wire and lead wire is made of different material, additional current will flow

Relationship of resistance and temperature of a thermistor (semiconductor resistance thermometer)

• resistance decrease with temperature increase

• larger negative temperature coefficient of resistance

difference between thermistors and metallic resistance thermometer

• smaller

• responds to temperature variance more quickly

BUT

• self heating effect is greater→ should operate at lower current levels

How does resistive thermometer be converted into voltage

due to small current levels, thermistor x is large, and approximation of small x does not apply

How to determine temperature by varying R2

change R2 to ensure V0=0

therefore R2=RT

Working principle of radiation pyrometer

detect infared radiation given by object measured

temperature transducer used in detector (e.g. thermopile) is converted to radiation energy using Stefan-Boltzmann law - detect temperature from radiation NOT surface temperature)

energy converted to temperature of measured surface using black body energy plot

Definition of black body

black body emit radiation in all wavelengths

a ‘body’ at a certain temperature emits certain amounts of enerhy at certain spectrum

any object that temperature is not absolute zero will emit radiation

Advantages and disadvantages of pyrometers

GOOD:

short response time

distance independent measurement

BAD:
error for reflective or transparent/translucent surface

Equation of temperature response time for a constant temperature

speed of change is categorised by time constant (1/K) or time when it reached 0.63(Tf-T0)

Equation of temperature response time for a sinusoidally varying temperature

• the lag causes the measured temperature to not accurately measure the maximum temperature