Transcript Lect2_temperature

Temperature Measurement
In microscopic world, temperature measures the average kinetic energy
of its atoms as they move. In macroscopic world, it a physical property
of a system that underlies the common notions of hot and cold.
Temperature scales
mixture of
water and ice
boiling water
Celsius (Centigrade)
100o C
0o C
Fahrenheit
212 o F
32o F
T o F  aT o C  b
T o F  95 T o C  32 o F
Absolute temperature (Kelvin)
Tk  T o C  273.15
Thermometer Calibrations
Three reference points
T  ax  b
Ice
Kelvin 273.15
Celsius 0.00
Triple
Point
273.16
0.01
Steam
373.15
100.00
Types of Thermometers
Primary thermometers: temperature can be calculated without any
unknown quantities.
Secondary thermometers: knowledge of the measured property is
not sufficient to allow direct calculation of temperature.
Need to be calibrated against a primary thermometer.
1. Gas thermometer
Vconst
T
P
mR
m: air mass;
R: the gas constant.
Environment
to be
measured
2. Liquid in glass thermometer
V
V
Volume expansion of glass:1.2-2.7x10-5 per 1.00oC,
Volume expansion of Hg:18x10-5 per 1.00oC
3. Maximum thermometer
4. Minimum thermometer
5. Six's thermometer
6. Bi-metallic strip
7. Electrical thermometers
Thermocouples
Seebeck effect: any conductor that is subjected to a thermal
gradient will generate a voltage.
Emf) output ~ temperature difference
Absolute temperature
Resistance
Electrical resistance of some materials changes with changing
temperature. Resistance thermal sensor can be made with small
wires to give fast response
film
Wire-wound
Thermistors
A thermistor is a type of resistor whose resistance varies with
temperature. Thermistors differ from resistance temperature
sensors in that the material used in a thermistor is generally
semi-conductor, such as ceramic or polymer. The temperature
response is also different; the resistance temperature sensors are
used for larger temperature ranges, while thermistors typically
achieve a higher precision within a limited temperature range
Radiation thermometers
Based on the principle that the radiation emitted from an object
depends on its temperature. By knowing the amount of infrared
energy emitted by the object and its emissivity, the object's
temperature can be determined. They are, sometimes, called laser
thermometers if a laser is used to help aim the thermometer to
enhance the device’s ability to measure temperature from a
distance.
Thermometer Response Time
The time a system or sensor takes to react to a given input ,
sometimes it is also called sensor’s time constant

Factors that control the time constant
C s  c s V
AH conv
inversely proportional to heat transfer rate.
proportional to the heat capacity
csV
c s d


AH conv H conv
  aU b , b  0.5
U: ventilation rate
Making temperature measurements in the air
1. Air is a poor conductor, thus, a good flow over the sensor
should be maintained.
2. Sensor to be thermally insulated from the mounting.
3. To prevent radiation, sensors can be polished or coated to
reflect solar radiation and to reduce the absorption of infrared
radiation. A shield can also be used to shelter the sensor, but it
needs to be aspirated to ensure proper ventilation.
4. Heating by adiabatic compression may occur when a sensor is
exposed to air moving at very high rates, e.g., aircraft
measurements. Adiabatic heating needs to be corrected.
5. Wetting of a temperature sensor will lower the measured
temperature due to evaporative cooling. Upper air measurements
can be affected as a sensor goes though a cloud. A special device is
needed to prevent sensor wetting. For surface measurements,
the radiation shield should keep the sensor dry.