Thermometers
Igcse Physics
Prof.S. Palumbo
Define temperature
Temperature is a measure of the
average kinetic energy possessed by
each molecule of the substance.
What are the physical properties that are used to measure temperatures?
● Expansion of materials
● Liquids: mercury, alcohol
● Solids: Thermocouple
● Electromotive force: Thermocouple
● Change in electrical resistance
Physical properties may vary with temparature
Some materials have properties that CHANGE with temperature:
Liquids can expand if the temperature
increases, or contract if it decreases;
Metals and alloys also expand andcontract if the temperature changes;
Some substances change their colours
when the temperature increases
Some substances change their electrical
conductivity when its temperature rises
Thermometers
List the types of temperature meters?
● Liquid-based thermometers
● Solid-based thermometers
List liquid in glass thermometers
● Mercury thermometer
● Alcohol thermometer
● Clinical thermometer
Describe the structure of liquid in glass
thermometers
1- Glass tube
2- Bulb
3- Liquid
4- Thread
5- Scale
How liquid- in- glass thermometers operate?
The bulb of thermometer contains liquid (e.g. mercury, alcohol) which expands
when temperature rises and pushes a thread of liquid along the scale where
temperature can be read
Properties of thermometers
1. Sensitivity
2. Range
3. Linearity
4. Accuracy
What is meant the following terms ?
Range Sensitivity
It is the property in a thermometer It is the property in a thermometer
defined as the lowest temperature defined as the change in property
measured to the highest (length, volume etc) per unit
temperature. degree.
What is meant the following terms
Linearity Accuracy
The change in temparature It gives correct values of
produces the same change in temparature
length
L= L0 + k L0DT
More sensitive :
1. increase volume of bulb
2. decrease diameter of tube
3. use the alcohol
4. clinical thermometers
Wider Range :
1. decrease volume of bulb
2. Increase diameter of tube
3. Use the mercury
Better linearity
1. Uniform tube diameter
2. Use mercury because it is more linearity than alcohol
How do we increase the sensitivity of a
thermometer?
1- increase in the volume of the bulb
2- decreases the diameter of the tube (bore)
How do we increase the range of a thermometer?
1- decrease in the volume of the bulb
2- increases the diameter of the tube (bore)
Exercise
The basic scales Celsius and Kelvin
List the basic scales for temperature measurements?
Celsius Scale
Kelvin Scale
Celsius scale
What are the two fixed points in Celsius scale?
1- The lower fixed point
2- The upper fixed point
What is meant be "lower fixed point" and "upper fixed point"?
The lower fixed point is the temperature of pure melting ice. This is called the 0°
C.
The upper fixed point is the temperature of steam above pure boiling water under
standard atmospheric pressure. It is call the 100°C.
1. Describe the Kelvin scale?
The Kelvin scale is the same as the Celsius scale except it starts from
absolute zero. The a one degree Kelvin is equal to one-degree Celsius.
2. What is meant by "absolute-zero" temperature?
Absolute-zero is the lowest temperature in the universe and at this
temperature the molecules of a substance have the minimum possible
energy. It occurs at -273°C.
3. What is the relation between Kelvin temperature and Celsius
temperature?
TK=TC+273 where TK is temperature in Kelvin and TC is temperature in
Celsius.
Mercury thermometers
Advantages Disantvantages
1. Does not stick to the sides of tube 1. Freezes at -39°C. Not suitable for
low temperatures
2. thread easy to use
2. Poisonous
3. conducts heat well
3. Expensive
4. responds quickly to changes in
temperature
Alcohol Thermometer
Describe alcohol thermometer.
Alcohol thermometers
Advantages Disantvantages
1. Freezes at -115°C. Suitable for low temperature measurements.
2. Has expansion more than mercury.
3. Remains liquid over required range
4. No toxic
5. Expands uniformly
6. Low freezing point , melting
7. Measures low temperature
Disantvantages
1. Has to be colored to be seen easily
2. Clings (sticks) to the sides of tube
3. Thread has tendency to break
Thermocouple thermometer
What is a Thermocouple?
The Thermocouple is a thermoelectric temperature sensor which consists of two
dissimilar metallic wires, e.g., one chromel and one constantan, coupled at the probe tip
(measurement junction) and extended to the reference (known temperature) junction.
The temperature difference between the probe tip and the reference junction is detected
by measuring the change in voltage (electromotive force, EMF) at the reference junction.
The absolute temperature reading can then be obtained by combining the information of
the known reference temperature and the difference of temperature between probe tip
and the reference.
Thermocouple
If two junction are at different temperatures (e.g. one junction is placed in a
hotter region than the other), a small voltage is produced. The greater the
difference in temperature, the greater the voltage produced across the ends
of the two junctions.If one junction is kept at a fixed temperature such as
0*C, then the other junction can be used as a tiny probe to measure the
other temperature.
e.m.f produced is inversely proportional to the temperature difference
between reference junction and the probe.
How does a thermocouple thermometer work?
Describe the operation of a thermocouple meter?
When there a difference in temperature between the hot and cold
junctions an electric current is generated. This current is used to deflect
a galvanometer which is calibrated to temperature degrees.
Advantages
- ability to measure a large temperature range
- very useful in a variety of circumstances in many different industries
- respond rapidly to changes in temperature and can be used at high
temperatures at which other sensors would be destroyed.
- very versatile,robust,fairly accurate
- small mass
- made rugged, simply constructed and are immune to shock and vibration
- can be made very small, fitting into applications that other temperature
sensors
- measure the temperature, as well as be coupled with certain circuits in
order to discontinue the heating process once the wanted temperature is
achieved.
Practical
List three precautions a person must take when measuring
temperature?
Mercury thread should be kept along the scale the parallax errors
Line of sight should be perpendicular to scale.
Reading should be at the top of mercury thread.
Procedures for thermometer
Measurement thermometer
● AVOID PARALLAX ERROR, so liquid appeal sto touch the scale
● Don’t touch the thermometer at base of beacher
● Star the liquid before reading temperature scuola be at the same deph in the
liquid
Measuring volume
● Avoid Parallax
● Read bottom of meniscus
What does “meniscus” mean?
Meniscus
Water present “concave meniscus” Mercury present “ convex meniscus”
Thermometer
Alcohol thermometer
Thermocouple thermometer
