The document discusses various types of temperature sensors and thermometers. It begins by describing the Wheatstone bridge and how it can be used to precisely measure resistance changes in a thermistor or other resistive temperature sensor. Next, it discusses common temperature sensing devices like thermocouples, RTDs, infrared sensors, and bimetallic devices. It then focuses on different types of liquid-in-glass thermometers like mercury, alcohol, and pressure spring thermometers, explaining their working principles, advantages, disadvantages, and applications.

1. Contents
• Wheatstone bridge
• Conventional types of temperature sensing,
• Response of thermometer
• Glass thermometer
• Bimetallic thermometer
• Filled system thermometer like pressure spring
thermometer

2. Wheatstone Bridge
• Originally developed by Charles Wheatstone to measure unknown
resistance values and as a means of calibrating measuring
instruments, voltmeters, ammeters, etc.
• Consists of four resistors that are connected in the shape of a
diamond with the supply source.
• Used to find the unknown resistance very precisely by comparing
with a known value of resistances, by balancing two legs of a bridge
circuit, one leg of which includes the unknown component. Its
operation is similar to the original potentiometer.
• Used in two ways:
o to measure the value of an unknown resistor by comparison to standard
resistors
o to detect small changes in a resistance transducer (e.g. thermistor)

3. How Does Wheatstone
Bridge works
• Firstly, determine the type of thermistor used in the
experiment. Measure the resistance of thermistor at room
temperature, then heat a little bit and observe the resistance
change as increasing or decreasing.
• Then, built a Wheatstone bridge including thermistor and pot.
*Rx is thermistor and R2 is pot.
• After that adjust the pot until the voltage between bridge is
zero at room temperature. This is done for calibration and
going on the procedure from a reference value.

4. • Later, heat the thermistor and get at least 6 voltage
values.
• Use thermocouple and record voltages according to
temperature.

5. Applications of
Wheatstone Bridge
• Used for measuring the very low resistance values
precisely.
• Wheatstone bridge along with operational amplifier is
used to measure the physical parameters like
temperature, strain, light, etc.
• Measure the quantities capacitance, inductance and
impedance using the variations on the Wheatstone
bridge.

6. Advantages and
Disadvantages
• Advantages
o It operates on null deflection
o measurement can be extremely precise for, or extremely sensitive to, a given
change in the thermistor's resistance.
• Disadvantages
o not applicable for high resistances

8. Thermocouples
• Voltage devices that indicate temperature by measuring
a change in voltage.
• In the thermocouple, the sensing junction - produces a
voltage that depends upon temperature.
• Where the thermocouple connects to instrumentation -
copper wires? - you have two more junctions and they
also produce a temperature dependent voltage.
• Applications include temperature measurement for gas
turbine exhaust, diesel engines, other industrial
processes and fog machines.

9. Thermocouples
• Working principle
- Thermocouples are based on the principle that two
wires made of dissimilar materials connected at either
end will generate a potential between the two ends that
is a function of the materials and temperature difference
between two ends.
Advantages
• Temperature range
• Self – Powered
Disadvantages
• Accuracy
• Stability

10. Resistant Temperature
Detector(RTD)
• Rather than using a voltage as the thermocouple
does, they take advantage of another
characteristic of matter which changes with
temperature - its resistance.
• Applications of RTD: Air conditioning and
refrigeration servicing, food processing, stoves
and grills

11. Resistant Temperature
Detector(RTD)
• Working Principle
- temperature sensor that operates on the measurement principle that a
material’s electrical resistance changes with temperature. The relationship
between an RTD’s resistance and the surrounding temperature is highly
predictable, allowing for accurate and consistent temperature measurement
Advantages
• Linear over wide operating range
• Wide temperature operating range
• High temperature operating range
Disadvantages
• Low sensitivity
• Higher cost than thermocouples
• No point sensing

12. Infrared Sensors
• Infrared sensors are non-contacting sensors.
• As an example, if you hold up a typical infrared sensor
to the front of your desk without contact, the sensor will
tell you the temperature of the desk by virtue of its
radiation - probably 68°F at normal room temperature.
• Application: Detecting clouds for remote telescope
operation, Checking mechanical or electrical equipment
for temperature and hot spots, Measuring the
temperature of patients in a hospital without touching
them

13. Infrared Sensors
Working Principle
- consists of a lens to focus the infrared thermal radiation on to a detector,
which converts the radiant power to an electrical signal that can be
displayed in units of temperature after being compensated for ambient
temperature.
Advantages
• It facilitates measurement of moving targets (conveyor processes).
• Measurements can be taken of hazardous or physically inaccessible objects
Disadvantages
• Infrared thermometers cannot take measurements of gas or liquids
• The environment needs to be clean, without dust, high humidity, or similar

14. Bimetallic Devices
• Bimetallic devices take advantage of the expansion of
metals when they are heated.
• In these devices, two metals are bonded together and
mechanically linked to a pointer.
• Application: For various household appliances such as
oven, Thermostat switches, wall thermometers

15. Bimetallic Devices
Working Principle
- When heated, one side of the bimetallic strip will expand more than the other.
And when geared properly to a pointer, the temperature is indicated.
Advantages
• Easy Construction
• Fully mechanical devices, thus no need of power source.
Disadvantages
• Not accurate
• Not suitable for measuring lower temperature

16. Thermometer
• A thermometer is a device that measures
temperature or a temperature gradient.
• A thermometer has two important elements: a
temperature sensor mercury-in-glass
thermometer) in which some physical change
occurs with temperature,
• Some means of converting this physical change
into a numerical value

17. Silicon Diode
• The silicon diode sensor is a device that has been
developed specifically for the cryogenic temperature
range.
• Essentially, they are linear devices where the
conductivity of the diode increases linearly in the low
cryogenic regions.
• Application : a heat engine which converts a heat
difference directly into electric power, a heat engine
working backwards as a refrigerator, such as a Stirling
engine

18. Working Principle
- The forward voltage of a silicon diode, which may be the base-emitter
junction of a bipolar junction transistor (BJT), is temperature-dependent
Advantages
• Low cost
• Accurate and reliable
• Highly consistent measurements
Disadvantages
• Self-heating
• No resistance standards
• Requirement of additional circuitry to control application loads
Silicon Diode

19. DYNAMIC RESPONSE
• First order system
Input – Output = Rate of Accumulation
hA(x-y) – 0 = mC
𝑑𝑌
𝑑𝑡
Where
h = Convection coefficient of heat transfer
(J/m2.h.0C)
A = Surface Area of the sensor(m2)
m = mass of thermocouple(g)
C = heat capacity (J/g.oC)
t = time(h)

20. Deviation Variables
hA [X-Y] = mC(
𝑑𝑌
𝑑𝑡
)
• 𝑚𝐶/ℎ𝐴= 𝜏
Taking Laplace Transform
X(s) – Y(s) = τsY(s) – Y(0) ;Y(0)=0
𝑌(𝑠)
𝑋(𝑠)
=
1
τ𝑠 + 1
=
𝑜𝑢𝑡𝑝𝑢𝑡
𝑖𝑛𝑝𝑢𝑡

21. GLASS THERMOMETER

22. Definition
 Glass capillary tube with a
liquid-filled bulb at one end
 Use the variation in volume of a
liquid in temperature
 The fluid is contained in sealed
glass bulb and its expansion is
measure by using scale attached
to the stem of the thermometer

23. As the temperature
increases, the fluid
expands and rises up the
capillary
As the temperature
decreases the fluid
contracts and goes down
in the capillary

24. Mercury
Thermometer
• Uses mercury as the liquid
to measure the
temperature
• It remains liquid in range
from –38°C to 356°C
• The mercury is in glass
tube and when the heat is
applied the mercury rises
• Mercury is toxic and it
should be handled with
care
Alcohol
Thermometer
•Uses alcohol (ethanol )as
the liquid to measure
temperature
•Alcohol expands when
absorbs temperature and
contract in colder
temperatures
•Alcohol is non toxic and are
much safer to use
•Alcohol thermometers can
be used to measure very
low temperatures.(-115°C –
70 ° C )
Two major types of liquid-in – glass
thermometer

25. Construction
• ThethinglassBulb: Thin glass reservoir that holds
the liquid. Allows conduction of heat quickly
through glass the liquid
• TheStem: The glass tube having a tiny capillary
along which the liquid expand or contract with
changes in temperature
• TheScale: A narrow temperature scale for reading a
reference temperature
• InertGas is used for mercury intended to high
temperature. The thermometer is filled with an
inert gas such as argon or nitrogen above the
mercury to reduce its volatilization.

26.  Contraction Chamber: The purpose of this chamber is to
shorten the total stem length needed to reach the main
scale.
 Expansion Chamber : To prevent the build-up of pressure
if the temperature of the liquid rises past the top of the
scale.

27. How does glass thermometer operate
Alcohol thermometers work by enclosing a narrow capillary
attached to a bulb of reserve fluid.
As the temperature goes down, the fluid contracts,
dropping down the capillary.
As the temperature heats up, the alcohol expands, rising
up the capillary.
Markers along the capillary indicate the temperature,
with people reading the temperature by finding the
marker which corresponds to the meniscus of the fluid
inside the capillary
This can be tricky to do with a narrow capillary, as the
thin thread of alcohol may appear almost invisible
even with dye.

28. Mercury thermometer
~It is easy to see, have a
shining colour (silver)
~ It has regular expansion
~ Does not stick to the side
of the glass capillary tube
of a thermometer
~ Measure the temperature
quickly and accurately
~Very expensive
~It is a poisonous substance
~Unsuitable for measuring a
very low temperature because it
has high freezing point

29. • Suitable for measuring high boiling point 357 oC
• Not recommended for cold climates and should be
brought indoors when the temperature starts dipping
below -30oC
• Applications :
 Cooking kettles
Pipe lines for fluid flow
Open tanks containing liquids
Meteorology

30. Alcohol Thermometer
~ It can measure at low
temperature because its
freezing point is low (-
115°C)
~ It has regular expansion
~ Can measure the
temperature accurately
because it has large
expansion coefficient
~Cannot measure at high
temperature because its
boiling point is low
~It is colourless so must add
with dyestuff in order to
visible

31. • Alcohol has a very low freezing point of about −115 oC
suitable in thermometers to record very low
temperatures.
• Used in cold atmospheres such as freezers.
• Not suitable in vey hot atmospheres due to the low
boiling point (78 ͦC).

32. Differences between Mercury and Alcohol
thermometer

33. FILLED
SYSTEM
THERMOMETER

34. • That work on pressure or volume of a gas or
changes in vapor pressure of a liquid
• Depends on 3 well known physical phenomena:
The thermal expansion of a
liquid
The temperature dependence of
the pressure of a gas
The temperature dependence of the
saturated vapor pressure of a liquid
Definition

35. • Measurements are listed as below:
1) Mercury in glass thermometer
2) Bimetallic thermometer
3) Pressure Spring thermometer ( √ )
Measurements

37. Bimetallic Thermometer
• Two metals whose coefficient of linear expansion
is different are welded and rolled together to the
desire thickness
• The actual movement of a bimetal is its flexivity
with one end fixed, a straight bimetal strip deflects
in proportion to its temperature, to the square of
its length and inversely with its thickens

39. Advantages Disadvantages
• They are cheap to buy
• Easy to install
• They are not extremely
accurate
• Are slow to react to a
temperature change.

40. Pressure Spring Thermometer
Liquid filled – Class 1
Vapor pressure – Class 2
Gas filled – Class 3
Mercury filled – Class 4

41. Types Class Working Principle Advantages Disadvantages
Liquid
filled
1 - thermal expansion
- indicating, recording
or controlling
mechanisms are
attached to pressure
spring
- Cheap and
widely
- slow response
time
- sometimes
inconvenient
dimensions.

42. Types Class Working Principle Advantages Disadvantages
Vapor
pressure
2 - Is a thermometer that
is used to measure
vapor pressure of the
liquid
- The pressure of the
gas is converted to
temperature by means
of a bourdon gauge
- free from
errors due
to bulb
expansion
- Small
capillaries
may lead to
heat
leakage

43. Types Class Working Principle Advantages Disadvantages
Gas
filled
3 - The system is filled
under high pressure
- As the temperature
rises, the pressure of
the gas inside the tube
increases
proportionally,
causing the spring to
unwind.
- This rotary movement
amplified by a
mechanism, action the
pointer of the
thermometer.
- It is very
accurate
- In fact its
accuracy allows
it to be utilized
to calibrate
other
thermometers
- Ideal for
distance
reading
- Expensive to
manufacture
and keep
- It is not easy
to handle
and read

44. Types Class Working Principle Advantages Disadvantages
Mercury
filled
4 - Works the same
as the liquid
filled type
- Mercury is a
naturally
opaque liquid
(Silver)
- This means
that it can be
directly
utilized in its
pure form
- Mercury
poses a
potential
toxic hazard
if the glass
container is
ruptured

45. Bimetallic
Thermometer
Thermostat
switches
Wall
thermometers
Grills
Circuit breakers
for electrical
heating devices
Applications

46. Pressure Spring
Thermometer
Application
Liquid Filled
Thermometer
Meteorological
and
Oceanographic
applications
Vapor Pressure
Thermometer
Piping, boilers,
pipeline
Gas Filled
Thermometer
Pharmaceutical,
Chemical,
Petrochemical
Mercury Filled
Thermometer
Common
household uses
Measure body
temperature