Identify the Start, Stop and
Seal-In Conditions
IEC Versions of Switches
Start:_______________________
Stop:_______________________
Seal-In:_____________________
Draw the ANSI and IEC versions of a
switch that is closed because a
mannequin has a foot
Draw the ANSI and IEC versions of a
switch that is open because an
employee placed a mannequin on a
pressure mate
Draw a switch that is open because
an ice-block is in place
CHAPTER 17
Temperature-sensing Devices
1
Begin with the End in Mind
• Review Chapter 17 questions
• Chapter 17 and Chapter 18 will be
combined for one homework assignment.
Expansion of Metal (1 of 4)
• The expansion of metal is a very common
and reliable method for sensing
temperature.
• Metal expands when heated.
• The amount of expansion depends on
Objectives
• Describe different methods for sensing
temperature.
• Discuss different devices intended to be
operated by a change of temperature.
• List several applications for temperaturesensing devices.
• Read and draw symbols for temperature
switches.
Expansion of Metal (2 of 4)
• Metal expands when heated.
– The type of metal used
– The amount of temperature
Expansion of Metal (3 of 4)
• Expanding metal operates a set of
contacts:
Expansion of Metal (4 of 4)
• Devices that use the expanding metal
principal
– Hot-wire starting relay
– The mercury thermometer
– The bimetal strip
2
Hot-wire Starting Relay (1 of 2)
Hot-wire Starting Relay (2 of 2)
• Uses a length of resistive wire connected
in series with the load so as to sense load
current
The Mercury Thermometer
• Mercury is a metal that is
in a liquid state at Room
Temperature.
• When confined to a glass
tube, it rises to the top as
temperature increases.
The Bimetal Strip (@ ambient Temp.)
The Bimetal Strip (1 of 5)
• The most common heat-sensing device
used in room thermostats and
thermometers
• 2 dissimilar metals bonded
The Bimetal Strip (Heated)
• A bimetal strip warps with a change of
heat.
3
The Bimetal Strip (Coiled)
• A bimetal strip used
as a thermometer
Thermocouples (1 of 4)
The Bimetal Strip (Snap-Action)
• A bimetal strip
used to operate a
set of contacts
• The Magnet
snaps the
contacts opened
and closed and
provides a
differential.
Thermocouples (2 of 4)
• Accomplished by joining two dissimilar
metals together at one end.
– When this junction is heated, a voltage is
produced also known as the Seebeck
effect .
• Amount of voltage determined by
– Type of materials joined together
– The temperature difference of the two
junctions
Types of Thermocouples
Thermopile (Voltage Adds)
4
Thermocouples (4 of 4)
Resistance Temperature
Detectors (RTDs) (1 of 2)
• They are made of platinum wire.
• The resistance platinum wire changes
greatly with temperature.
• When heated, resistance increases at
a very predictable rate.
• RTDs are ideal for measuring
temperature accurately.
• A thermocouple
provides power to
the safety cut-off
valve:
Resistance Temperature
Detectors (RTDs) (2 of 2)
RTD Temperature Response
Note:
Low
Sensitivity,
but high
“Linearity”
RTDs
Thermistors (1 of 2)
• From “Thermal Resistor”
• Thermally sensitive semiconductor devices
• Two types
– Negative temperature coefficient (NTC)
– Positive temperature coefficient (PTC)
5
Thermistors (2 of 2)
• A thermistor that has a negative
temperature coefficient (NTC) will
decrease its resistance as the temperature
increases.
• A thermistor that has a positive
temperature coefficient (PTC) will increase
its resistance as the temperature
increases.
Replacement for Hot-Wire
Thermistor Curve
Note Log axis and sensitivity of Thermistor. Is it a PTC or NTC?
The PN Junction (DIODE)
(1 of 2)
• Constant current generator w/ Sensing
Diode
As the
Temperature
rises, the voltage
across the diode
falls.
The PN Junction (DIODE)
(2 of 2)
Full Electronic Circuit
• Field effect
transistor
producing a
constant current
source.
6
Pressure and Temperature
Pressure and Temperature
(2 of 2)
(1 of 2)
• Senses change of temperature by the
increase of pressure of certain chemicals
(Refrigerant).
• Bellows contract
and expand with
a change of
refrigerant
pressure.
A Bellows Thermostat
Thermostat Symbols
Temperature Sensors
Resistive Temperature Sensors
• Which 2 temperature sensors change
resistance with Temperature?
• Which is more linear?
7
Resistive Temperature Sensors
• Which is more sensitive?
What Temperature sensing device has
the advantage of not needing an external
voltage source to use it?
What electronic device can be
used as a temperature sensor?
Summary
• In this chapter, we discussed:
– Different methods for sensing temperature
– Different devices intended to be operated
by a change of temperature
– Several applications for temperaturesensing devices
– Symbols for temperature switches
8