This document provides an overview of industrial temperature measurement. It discusses different temperature scales and units used in engineering. Common temperature measurement devices are described, including liquid-in-glass thermometers, bimetallic thermometers, resistance temperature detectors (RTDs), and thermocouples. RTDs and thermocouples are electrical sensors that change resistance or voltage, respectively, with temperature. Each device type has advantages and limitations for different applications and temperature ranges. Proper setup and wiring is important to reduce measurement errors from reference junctions or lead wire resistances.

1. Industrial Temperature
Measurement

2. Temperature
• Controlling temperature is one of the most
common processes in industrial electronics
and manufacturing.
• Manufacturing processes that are affected by
temperature are referred to as thermal
systems.

3. Temperature
• Temperature is a measure of the average kinetic
energy of the particles that make up a body.
-The greater the kinetic energy of the particles is,
the higher the temperature of the body will be
• Temperature is the ability of one body to transfer
thermal energy to another body
- If two bodies are in thermal equilibrium and no
thermal energy is exchanged, the bodies are at
the same temperature

4. Temperature
• Molecular motion creates heat known as
thermal energy
• Thermal movement from hot to cold is called
thermodynamics
• Absolute zero (no molecular motion) means
no heat is produced

5. Temperature scales & Units
There are 4 scales that can be used to measure
temperature.
• Celsius/Fahrenheit units are used in the
common everyday scales
• Kelvin/Rankine are used when working with
the Absolute Temperature Scale ( these are
typically used in engineering and research
calculations)

6. Temperature Measurement
Like other process measurements, temperature
measuring devices are divided into 3 general
categories:
indicators
sensors/transducers, switches
transmitters
The design and construction of these devices is
based on how different materials react when
subjected to heat and what type of measurement
is required i.e. indication only, point
measurement, analog output, etc.

7. Temperature devices are specified according to
their construction and are classified as follows:
• Liquid Expansion devices
• Bimetallic Elements
• Electrical Sensors (thermistors, RTD,
thermocouples, semiconductor)
• Optical pyrometers
• Change-of-state Devices

8. Liquid in Glass Thermometer

9. Advantages
• Easy portability
• Independence of auxiliary equipment
• Low cost
• Compatibility with most environments
• Moderate ruggedness
• Wide range (it has been use to measure
temperatures as low as 70K and as high as
1000°C, but it most frequent use is within -40°C
to 250 °C)

10. Disadvantages
• a large sensing element
• Impossibility for continuous automatic
readout,
• Long time constant
• Awkward dimensions, and hysteresis (except
for special types),
• breakage(mercury contamination)

11. Filled Thermal System
Filled thermometers, Gas Filled Thermometers
• Closed system, contains a gas or a volatile
liquid and relies on pressure measurements to
provide temperature indications.
• Various types are used but all have similar
components and share the same principle of
operation.

12. Closed System- no external energy
required

13. • When temperature changes , fluid either
expands or contracts, which caused Bourdon
tube to move, thereby moving the position of
the needle on the scale

14. Bimetallic Thermometers
• Based on the principle that different metals
expand at different rates as they warm up.
• By bonding two different metals together, you
can make a simple electric controller that can
withstand fairly high temperatures.
• This sort of temperature element is found in
many mechanical temperature switches as
well as indicators.

16. • Strips of metals with different thermal
expansion coefficients are bonded
together at the same temperature
• When the temperature increase, the
assembly bends.
• When this happens, the metal strip
together with the larger temperature
coefficient of expansion expands
more than the other strip.
• The angular position-vs-temperature
relation is established by calibration
so you can use the device as a
thermometer

17. Bimetallic Thermometer
• The bimetallic strip can be wound as a helix and will
twist when heated. This twisting action can be used to
drive a pointer over a calibrated temperature scale.
These temperature sensors are low cost and have
accuracy ranges of between 2-5% and are mostly used
for local readings. They are not suitable for providing a
continuous output measurement

18. Change-of-State Temperature
Measurement Devices
• Change-of-state temperature sensors consist
of labels, pellets, crayons, lacquers or liquid
crystals whose appearance changes once a
certain temperature is reached. They are used,
for instance, with steam traps – when a trap
exceeds a certain temperature, a white dot on
a sensor label attached to the trap will turn
black

19. • Response time typically takes
minutes, so these devices
often do not respond to
transient temperature
changes.
• Accuracy is lower than with
other types of sensors.
• Change in state is irreversible,
except in the case of liquid-
crystal displays.

20. Radiation Temperature Sensors
Optical Pyrometers
• In general terms these devices measure the
amount of radiation emitted by a surface.
Electromagnetic energy radiates from all
matter regardless of its temperature. In many
process situations, the energy is in the
infrared region. The intensity of an object’s
emitted IR energy increases in proportion to
its temperature and measured as the target’s
emissivity, that indicates an object’s
temperature

21. Non Contact Measurement

22. Used in High Temperature Applications
• Radiation Temperature sensors can be used as
hand-held local temperature devices or can be
installed to provide a continuous signal. They
are used in many industries where extremely
high temperature and/or non-contact
measurements are required. (Extrusion
presses, rolling mills, strip annealing, Tank
refractory’s, mold temperature, bottle
machines, Kiln shell, and many more)

23. Electrical Temperature Sensors
• Resistance
-Thermistors
- Resistance Temperature Detectors (RTDs)
• Thermo-electric
-Thermocouples

24. Thermistors
• Thermally sensitive resistors that change
resistance with changes in temperature (in a
predictable manner)
• They are highly sensitive and have very
reproducible resistance vs temperature
properties
• Typically used over a small temperature range,
(compared to other temperature sensors)
because of their non-linear characterstics

25. • Manufactured from oxides of nickel,
manganese, iron, cobalt, magnesium, titanium
and other metals
• They are epoxy or glass encapsulated, or bare
bead, many of the standardized types are
color coded.

26. Temperature vs Resistance
Characteristics
• Most thermistors
exhibit a negative
temperature
coefficient (NTC)
• Non-linear T vs R

27. Positive Temperature Coefficients
• Thermistors can
also be made to
have a PTC
response
• Used more for
current overload
than temperature
measurement

28. General Specifications (NTCs)
• usually specified by their resistance at room
temperature
-For example an NTC Thermistor T25 could
have a resistance of 3.0kΩ, 5.0kΩ, 10.0kΩ at
25°C
• Accuracy is very good to average
• Response time is fast to moderate
• Typically used over small temperature ranges

30. Resistance Temperature Detector RTDs
• Change resistance in a linear relationship to
the applied heat
• Very accurate temperature vs resistance
characteristics and reproducible
• Excellent interchangeability and stability
• Can be used ass a temperature standard

31. RTD Materials
• Platinum
• Nickel
• Copper

32. RTD Construction
• Wire wound
• Coil
• Hollow Annulus
• Film

33. Wire wound
• The wire wound
sensing element is
built by winding a
small diameter
platinum sensing
wire around a non-
conducting mandrel
Classical construction, excellent interchangeability

34. Coil
• The coiled element
sensor, made by
inserting the
helical sensing
wires into a packed
powder-filled
insulating mandrel,
provides a stain-
free sensing
element
Good Stability, rugged

35. Hollow Annulus
• The hollow annulus-type
element is made by
winding platinum sensing
wire around a hollow
corrosion-resistant metal
mandrel. The entire unit
is coated with an
insulating material
Fastest response, most espensive

36. Film
• Film type sensing
element is made by
depositing a thin layer of
platinum in a resistance
pattern on a ceramic
substrate. A layer of
glass applied for
protection
Newer design, easier to make,
interchangeability is not the best

37. Temperature- Resistance
Characteristics
• RTD’s are specified by their resistance at zero
deg C and the material they are made of
Platinum (Pt)
-Pt100 (100Ω@0°C) 0.4ohms/C
-Pt1000 (1000Ω@0°C) 4ohms/C
-Nickel Ni120 (120Ω@0°C)
-Copper Cu10 (10Ω@0°C)

38. RTD’s Temperature vs Resistance
Charactersitics

39. Temperature Coefficient
• Temperature coefficient or alpha α is used by
the manufacturers to standardize the RTD’s
slope of TR curve
• The alpha describes the average resistance
change per unit temperature from the ice
point to the boiling point of water
• A Pt100 and Pt1000 have an α=0.00385

41. Wiring configuration
• RTDs are typically use with a bridge circuit

42. Wiring Configuration
• These bridge circuits are built into the
transmitter, PLC, DCS, PID Controller etc

43. Effects of lead wire resistance
Assume the RTD is measuring 100°C
R(PT100)= 138.5Ω RLEAD=20Ω
Total resistance measured will be 158.5Ω which
is a temperature of 153°C

44. 3- wire RTDs

45. RTD Specifications
• Platinum RTD’s can measure temperatures
from -200°C to 650°C. (IEC says -200°C to
850°C)
• A “bare wire” RTD has a fast response time
however the protective “sheaths” and “wells”
slow down the response.

46. Example of Manufacturers Specs
• Time Constant of 2.2 sec or less
• Temperature Range: -200 to +600°C
• Pressure Range: Vacuum to +50,000 psia
• Accuracy: ± 1ohm@0°C(±0.1% for 100ohm)
• Reproducibility: ±0.1% of resistance for
100ohm element

48. Spring loaded RTD Thermowell

49. RTD color coding

50. RTDs Pros and Cons
Advantages
• Linear
• Stable output over a long period of time
• Ease of recalibration
• Accurate readings over narrow temperature
spans
Disadvantages (when compared to thermocouples)
• Smaller overall temperature range (-330 to 930°F)
• Higher initial cost
• Fragile in rugged industrial environments

52. Thermocouples (TCs)
• A thermocouple consists of two pieces of
dissimilar metals with their ends joined
together (by twisting, soldering or welding)
• Based on Seebeck Effect which simply states
that an electromotive force (emf) is created at
the junction of 2 dissimilar metals when
heated.

53. Seebeck Effect produces a mV
• When heat is applied
to the junction of 2
dissimilar metals, a
voltage , in the range
of millivolts (mV), is
generated at the open
leads.
The mV per degree will depend on the combinations of metals used.
Manufacturers produce a variety of combinations specified as “types”
examples – Type T, J, K, E

54. Thermocouple Types
• Manufacturers have perfected a variety of metal
combinations and specify them as “types”
Example -Type T, J, K, E
• Each produces its own specific mV per degree, these values
are published in the TC cables
• Each type has a different temperature range (Type T can only
measure up to 400°C, Type K 1300°C)
• The types are color coded to make it easy to identify them in
the field.

57. Thermocouple Table
Reference Junction is 0°C

58. Measuring Temperatures- Example
According to the type J table the
meter should read 5.269mV when
the TC is measuring a temperature
of 100°C
But it reads 4.25mV instead
Because the meter leads form
another junction which produces
another emf equal to room
temperature
The mV at the reference junction temperature must be added to the meter

59. Measuring & Reference Junction
Type J(Iron/Constantan)

60. Reference Junction Compensation
• Ice Bath
• Electronic Ice Point
• Thermocouple Transmitters and Controllers
are internally compensated
• Make sure to match TC types with the
equipment ( some instruments will allow
several types but must be configured.

61. Ice Bath Compensation

65. Thermocouples Pros and Cons
Advantages
• A wide temperature from -300 to 2300°F
• Fast response time (under a second in some cases)
• Low initial cost and durability
• Thermocouples are able to withstand rugged applications
Disadvantages
• wide accuracy range, especially at elevated temperature
• Difficult to recalibrate seeing though they are dependant
upon the environment, and
• Installation can be very expensive if long lengths of
thermocouple wire are needed

73. Limitations of electrical thermometers
• Sensor cable’s resistance and its temperature
dependency
• Junction resistances
• Thermal voltages
• Thermal noise in resistors
• Measurement current
• Non-linear temperature dependencies
• Electrical perturbations
• Inaccuracy at least ± 0.1 °C