Sensors and Transducers Notes PDF Download

 Page 1


- 62 - 
 
 
CHAPTER (4) 
 
 
 
 
 
Sensors and Transducers 
 
Objectives:  
This chapter will consider different types of sensors. After you have read this 
chapter, you should be able to 
• Transform a temperature reading among different scales 
• Explain the operation of different temperature sensors 
• Describe the characteristics and applications of mechanical sensors 
• Describe advantages, disadvantages, and applications of limit switches, 
photoelectric sensors, inductive sensors, capacitive sensors, and ultrasonic sensors 
• Explain the operation of pressure, flow, and level transducers 
 
 
 
Page 2


- 62 - 
 
 
CHAPTER (4) 
 
 
 
 
 
Sensors and Transducers 
 
Objectives:  
This chapter will consider different types of sensors. After you have read this 
chapter, you should be able to 
• Transform a temperature reading among different scales 
• Explain the operation of different temperature sensors 
• Describe the characteristics and applications of mechanical sensors 
• Describe advantages, disadvantages, and applications of limit switches, 
photoelectric sensors, inductive sensors, capacitive sensors, and ultrasonic sensors 
• Explain the operation of pressure, flow, and level transducers 
 
 
 
Sensors and Transducers 
??????????????????????????????????? ?????????????? ?????????????? ???????????????? ????????????????? ????????????????? ????????????? ???????????? ?????????????? ???????????????????? ?????????????????? 
- 63 - 
4.1 What is a transducer? 
 
The sensor or the sensing element is the first element in a measuring system and takes 
information about the variable being measured and transforms it into a more suitable form to 
be measured. The following figure illustrates the difference between sensor and transducer. 
 
 
Figure 4.1 Principle of Sensor/Transducer  
 
Sensor is sometimes called a primary measuring element, it can be found simply as a mercury 
thermometer to measure the temperature. It may be embedded in the transducer to perform its 
function. That means the transducer consists of a primary element (sensor) plus a secondary 
element (signal conditioning circuit) that transforms the passive change or small voltage 
signal into active signal range that can be easily used in other chains of the control loop. 
 
Example: with a resistance thermometer, the resistance depends on the temperature value 
(sensor). It can be inserted into a bridge circuit (secondary element) in order to 
transform the change in the resistance value to a change in the voltage output. 
Finally, the output voltage from the bridge circuit express about the temperature 
change value. In general, we can say that:  
 
 Transducer = Sensor + Signal conditioning circuit  
 
In the following sections, we will present the main features of different transducers to 
measure: 
• Temperature 
- Using mechanical properties to operate 
- Using electrical properties to operate 
• Pressure 
- Static 
- Dynamic 
• Level 
• Flow 
- Direct measurement 
- Indirect measurement 
• Mechanical displacement 
 
 
 
 
Sensor 
Physical 
variable 
change 
Passive element 
Change as ?R or 
?L or ?C 
 
Transducer 
Physical 
variable 
change 
Active signal 
Change as ?V 
or ?I 
Page 3


- 62 - 
 
 
CHAPTER (4) 
 
 
 
 
 
Sensors and Transducers 
 
Objectives:  
This chapter will consider different types of sensors. After you have read this 
chapter, you should be able to 
• Transform a temperature reading among different scales 
• Explain the operation of different temperature sensors 
• Describe the characteristics and applications of mechanical sensors 
• Describe advantages, disadvantages, and applications of limit switches, 
photoelectric sensors, inductive sensors, capacitive sensors, and ultrasonic sensors 
• Explain the operation of pressure, flow, and level transducers 
 
 
 
Sensors and Transducers 
??????????????????????????????????? ?????????????? ?????????????? ???????????????? ????????????????? ????????????????? ????????????? ???????????? ?????????????? ???????????????????? ?????????????????? 
- 63 - 
4.1 What is a transducer? 
 
The sensor or the sensing element is the first element in a measuring system and takes 
information about the variable being measured and transforms it into a more suitable form to 
be measured. The following figure illustrates the difference between sensor and transducer. 
 
 
Figure 4.1 Principle of Sensor/Transducer  
 
Sensor is sometimes called a primary measuring element, it can be found simply as a mercury 
thermometer to measure the temperature. It may be embedded in the transducer to perform its 
function. That means the transducer consists of a primary element (sensor) plus a secondary 
element (signal conditioning circuit) that transforms the passive change or small voltage 
signal into active signal range that can be easily used in other chains of the control loop. 
 
Example: with a resistance thermometer, the resistance depends on the temperature value 
(sensor). It can be inserted into a bridge circuit (secondary element) in order to 
transform the change in the resistance value to a change in the voltage output. 
Finally, the output voltage from the bridge circuit express about the temperature 
change value. In general, we can say that:  
 
 Transducer = Sensor + Signal conditioning circuit  
 
In the following sections, we will present the main features of different transducers to 
measure: 
• Temperature 
- Using mechanical properties to operate 
- Using electrical properties to operate 
• Pressure 
- Static 
- Dynamic 
• Level 
• Flow 
- Direct measurement 
- Indirect measurement 
• Mechanical displacement 
 
 
 
 
Sensor 
Physical 
variable 
change 
Passive element 
Change as ?R or 
?L or ?C 
 
Transducer 
Physical 
variable 
change 
Active signal 
Change as ?V 
or ?I 
Sensors and Transducers 
??????????????????????????????????? ?????????????? ?????????????? ???????????????? ????????????????? ????????????????? ????????????? ???????????? ?????????????? ???????????????????? ?????????????????? 
- 64 - 
4.2 Temperature sensors and transducers 
What is Temperature? 
In a qualitative manner, we can describe the temperature of an object as that which determines 
the sensation of warmth or coldness felt from contact with it. It is easy to demonstrate that 
when two objects of the same material are placed together (physicists say when they are put in 
thermal contact), the object with the higher temperature cools while the cooler object becomes 
warmer until a point is reached after which no more change occurs, and to our senses, they 
feel the same. When the thermal changes have stopped, we say that the two objects are in 
thermal equilibrium. We can then define the temperature of the system by saying that the 
temperature is that quantity which is the same for both systems when they are in thermal 
equilibrium. Temperature may be defined as "the condition of a body which determines the 
transfer of heat to or from other bodies" or the degree of hotness or coldness as referenced to a 
specific scale of temperature measurement. 
What is a Thermometer (sensor)?  
A thermometer is an instrument that measures the temperature of a system in a quantitative 
way (sensing element). The easiest way to do this is to find a substance having a property that 
varies regularly with its temperature. The most direct 'regular' way is a linear one:  
 
T(x) = ax + b, 
(4.1) 
where T is the temperature of the substance and changes as the property x of the substance 
changes. The constants a and b depend on the substance used and may be evaluated by 
specifying two temperature points on the scale, such as 32° for the freezing point of water and 
212° for its boiling point.  
4.2.1 Temperature scales 
It was in 1724 that Daniel Gabriel Fahrenheit, an instrument maker of Däanzig and 
Amsterdam, used mercury as the thermometric liquid. Fahrenheit measured the boiling point 
of water to be 212. He adjusted the freezing point of water to 32 so that the interval between 
the boiling and freezing points of water could be represented by the more rational number 
180. Temperatures measured on this scale are designated as degrees Fahrenheit (°F). In 
1745, Carolus Linnaeus described a scale in which the freezing point of water was zero, and 
the boiling point 100, making it a centigrade (one hundred steps) scale. Anders Celsius (1701-
1744) used the reverse scale in which 100 represented the freezing point and zero the boiling 
point of water, still, of course, with 100 degrees between the two defining points. In 1948 use 
of the Centigrade scale was dropped in favor of a new scale using degrees Celsius (° C). To 
convert from Celsius to Fahrenheit: multiply by 1.8 and add 32.  
° F = 1.8° C + 32 
(4.2)  
 Lord Kelvin (1824-1907) has proposed another scale its symbol is K. To convert from 
Celsius to Kelvin, add 273.  
o 
K = ° C + 273 
(4.3) 
Page 4


- 62 - 
 
 
CHAPTER (4) 
 
 
 
 
 
Sensors and Transducers 
 
Objectives:  
This chapter will consider different types of sensors. After you have read this 
chapter, you should be able to 
• Transform a temperature reading among different scales 
• Explain the operation of different temperature sensors 
• Describe the characteristics and applications of mechanical sensors 
• Describe advantages, disadvantages, and applications of limit switches, 
photoelectric sensors, inductive sensors, capacitive sensors, and ultrasonic sensors 
• Explain the operation of pressure, flow, and level transducers 
 
 
 
Sensors and Transducers 
??????????????????????????????????? ?????????????? ?????????????? ???????????????? ????????????????? ????????????????? ????????????? ???????????? ?????????????? ???????????????????? ?????????????????? 
- 63 - 
4.1 What is a transducer? 
 
The sensor or the sensing element is the first element in a measuring system and takes 
information about the variable being measured and transforms it into a more suitable form to 
be measured. The following figure illustrates the difference between sensor and transducer. 
 
 
Figure 4.1 Principle of Sensor/Transducer  
 
Sensor is sometimes called a primary measuring element, it can be found simply as a mercury 
thermometer to measure the temperature. It may be embedded in the transducer to perform its 
function. That means the transducer consists of a primary element (sensor) plus a secondary 
element (signal conditioning circuit) that transforms the passive change or small voltage 
signal into active signal range that can be easily used in other chains of the control loop. 
 
Example: with a resistance thermometer, the resistance depends on the temperature value 
(sensor). It can be inserted into a bridge circuit (secondary element) in order to 
transform the change in the resistance value to a change in the voltage output. 
Finally, the output voltage from the bridge circuit express about the temperature 
change value. In general, we can say that:  
 
 Transducer = Sensor + Signal conditioning circuit  
 
In the following sections, we will present the main features of different transducers to 
measure: 
• Temperature 
- Using mechanical properties to operate 
- Using electrical properties to operate 
• Pressure 
- Static 
- Dynamic 
• Level 
• Flow 
- Direct measurement 
- Indirect measurement 
• Mechanical displacement 
 
 
 
 
Sensor 
Physical 
variable 
change 
Passive element 
Change as ?R or 
?L or ?C 
 
Transducer 
Physical 
variable 
change 
Active signal 
Change as ?V 
or ?I 
Sensors and Transducers 
??????????????????????????????????? ?????????????? ?????????????? ???????????????? ????????????????? ????????????????? ????????????? ???????????? ?????????????? ???????????????????? ?????????????????? 
- 64 - 
4.2 Temperature sensors and transducers 
What is Temperature? 
In a qualitative manner, we can describe the temperature of an object as that which determines 
the sensation of warmth or coldness felt from contact with it. It is easy to demonstrate that 
when two objects of the same material are placed together (physicists say when they are put in 
thermal contact), the object with the higher temperature cools while the cooler object becomes 
warmer until a point is reached after which no more change occurs, and to our senses, they 
feel the same. When the thermal changes have stopped, we say that the two objects are in 
thermal equilibrium. We can then define the temperature of the system by saying that the 
temperature is that quantity which is the same for both systems when they are in thermal 
equilibrium. Temperature may be defined as "the condition of a body which determines the 
transfer of heat to or from other bodies" or the degree of hotness or coldness as referenced to a 
specific scale of temperature measurement. 
What is a Thermometer (sensor)?  
A thermometer is an instrument that measures the temperature of a system in a quantitative 
way (sensing element). The easiest way to do this is to find a substance having a property that 
varies regularly with its temperature. The most direct 'regular' way is a linear one:  
 
T(x) = ax + b, 
(4.1) 
where T is the temperature of the substance and changes as the property x of the substance 
changes. The constants a and b depend on the substance used and may be evaluated by 
specifying two temperature points on the scale, such as 32° for the freezing point of water and 
212° for its boiling point.  
4.2.1 Temperature scales 
It was in 1724 that Daniel Gabriel Fahrenheit, an instrument maker of Däanzig and 
Amsterdam, used mercury as the thermometric liquid. Fahrenheit measured the boiling point 
of water to be 212. He adjusted the freezing point of water to 32 so that the interval between 
the boiling and freezing points of water could be represented by the more rational number 
180. Temperatures measured on this scale are designated as degrees Fahrenheit (°F). In 
1745, Carolus Linnaeus described a scale in which the freezing point of water was zero, and 
the boiling point 100, making it a centigrade (one hundred steps) scale. Anders Celsius (1701-
1744) used the reverse scale in which 100 represented the freezing point and zero the boiling 
point of water, still, of course, with 100 degrees between the two defining points. In 1948 use 
of the Centigrade scale was dropped in favor of a new scale using degrees Celsius (° C). To 
convert from Celsius to Fahrenheit: multiply by 1.8 and add 32.  
° F = 1.8° C + 32 
(4.2)  
 Lord Kelvin (1824-1907) has proposed another scale its symbol is K. To convert from 
Celsius to Kelvin, add 273.  
o 
K = ° C + 273 
(4.3) 
Sensors and Transducers 
??????????????????????????????????? ?????????????? ?????????????? ???????????????? ????????????????? ????????????????? ????????????? ???????????? ?????????????? ???????????????????? ?????????????????? 
- 65 - 
We conclude that: 
• Fahrenheit scale is common used in U.S. and many other English-speaking countries. 
• Celsius scale is used in scientific measurements and industrial applications. 
• Kelvin scale is based on an extremely low temperature. 
4.2.2 Temperature measurement 
There are two basic categories either to use mechanical properties (expansion of a substance) 
or to use electrical properties to operate. 
Using mechanical properties 
 
1) Capillary tube thermometer 
The element mercury is a liquid in the temperature range of -38.9° C to 356.7° C As a liquid, 
mercury expands (moves) as it gets warmer, its expansion rate is linear and can be accurately 
calibrated.  
 
Figure 4.2 Thermometer 
The mercury-in-glass thermometer illustrated in the above figure contains a bulb filled with 
mercury that is allowed to expand into a capillary tube. Its rate of expansion is calibrated on 
the glass scale. Mercury can be replaced by alcohol for low temperature measurement. 
2) Filled thermal system 
 
The device consists of a bulb filled with expanding substance connected to a Bourdon tube 
mechanism via a capillary tube ( ˜30m long) as shown in figure. 
 
 
Figure 4.3 A filled system thermometer 
Temperature scale
Moving 
pointer 
Bourdon
tube
Bulb with 
expanding 
substance 
Capillary 
tube
Control 
Room 
Process 
Field 
Page 5


- 62 - 
 
 
CHAPTER (4) 
 
 
 
 
 
Sensors and Transducers 
 
Objectives:  
This chapter will consider different types of sensors. After you have read this 
chapter, you should be able to 
• Transform a temperature reading among different scales 
• Explain the operation of different temperature sensors 
• Describe the characteristics and applications of mechanical sensors 
• Describe advantages, disadvantages, and applications of limit switches, 
photoelectric sensors, inductive sensors, capacitive sensors, and ultrasonic sensors 
• Explain the operation of pressure, flow, and level transducers 
 
 
 
Sensors and Transducers 
??????????????????????????????????? ?????????????? ?????????????? ???????????????? ????????????????? ????????????????? ????????????? ???????????? ?????????????? ???????????????????? ?????????????????? 
- 63 - 
4.1 What is a transducer? 
 
The sensor or the sensing element is the first element in a measuring system and takes 
information about the variable being measured and transforms it into a more suitable form to 
be measured. The following figure illustrates the difference between sensor and transducer. 
 
 
Figure 4.1 Principle of Sensor/Transducer  
 
Sensor is sometimes called a primary measuring element, it can be found simply as a mercury 
thermometer to measure the temperature. It may be embedded in the transducer to perform its 
function. That means the transducer consists of a primary element (sensor) plus a secondary 
element (signal conditioning circuit) that transforms the passive change or small voltage 
signal into active signal range that can be easily used in other chains of the control loop. 
 
Example: with a resistance thermometer, the resistance depends on the temperature value 
(sensor). It can be inserted into a bridge circuit (secondary element) in order to 
transform the change in the resistance value to a change in the voltage output. 
Finally, the output voltage from the bridge circuit express about the temperature 
change value. In general, we can say that:  
 
 Transducer = Sensor + Signal conditioning circuit  
 
In the following sections, we will present the main features of different transducers to 
measure: 
• Temperature 
- Using mechanical properties to operate 
- Using electrical properties to operate 
• Pressure 
- Static 
- Dynamic 
• Level 
• Flow 
- Direct measurement 
- Indirect measurement 
• Mechanical displacement 
 
 
 
 
Sensor 
Physical 
variable 
change 
Passive element 
Change as ?R or 
?L or ?C 
 
Transducer 
Physical 
variable 
change 
Active signal 
Change as ?V 
or ?I 
Sensors and Transducers 
??????????????????????????????????? ?????????????? ?????????????? ???????????????? ????????????????? ????????????????? ????????????? ???????????? ?????????????? ???????????????????? ?????????????????? 
- 64 - 
4.2 Temperature sensors and transducers 
What is Temperature? 
In a qualitative manner, we can describe the temperature of an object as that which determines 
the sensation of warmth or coldness felt from contact with it. It is easy to demonstrate that 
when two objects of the same material are placed together (physicists say when they are put in 
thermal contact), the object with the higher temperature cools while the cooler object becomes 
warmer until a point is reached after which no more change occurs, and to our senses, they 
feel the same. When the thermal changes have stopped, we say that the two objects are in 
thermal equilibrium. We can then define the temperature of the system by saying that the 
temperature is that quantity which is the same for both systems when they are in thermal 
equilibrium. Temperature may be defined as "the condition of a body which determines the 
transfer of heat to or from other bodies" or the degree of hotness or coldness as referenced to a 
specific scale of temperature measurement. 
What is a Thermometer (sensor)?  
A thermometer is an instrument that measures the temperature of a system in a quantitative 
way (sensing element). The easiest way to do this is to find a substance having a property that 
varies regularly with its temperature. The most direct 'regular' way is a linear one:  
 
T(x) = ax + b, 
(4.1) 
where T is the temperature of the substance and changes as the property x of the substance 
changes. The constants a and b depend on the substance used and may be evaluated by 
specifying two temperature points on the scale, such as 32° for the freezing point of water and 
212° for its boiling point.  
4.2.1 Temperature scales 
It was in 1724 that Daniel Gabriel Fahrenheit, an instrument maker of Däanzig and 
Amsterdam, used mercury as the thermometric liquid. Fahrenheit measured the boiling point 
of water to be 212. He adjusted the freezing point of water to 32 so that the interval between 
the boiling and freezing points of water could be represented by the more rational number 
180. Temperatures measured on this scale are designated as degrees Fahrenheit (°F). In 
1745, Carolus Linnaeus described a scale in which the freezing point of water was zero, and 
the boiling point 100, making it a centigrade (one hundred steps) scale. Anders Celsius (1701-
1744) used the reverse scale in which 100 represented the freezing point and zero the boiling 
point of water, still, of course, with 100 degrees between the two defining points. In 1948 use 
of the Centigrade scale was dropped in favor of a new scale using degrees Celsius (° C). To 
convert from Celsius to Fahrenheit: multiply by 1.8 and add 32.  
° F = 1.8° C + 32 
(4.2)  
 Lord Kelvin (1824-1907) has proposed another scale its symbol is K. To convert from 
Celsius to Kelvin, add 273.  
o 
K = ° C + 273 
(4.3) 
Sensors and Transducers 
??????????????????????????????????? ?????????????? ?????????????? ???????????????? ????????????????? ????????????????? ????????????? ???????????? ?????????????? ???????????????????? ?????????????????? 
- 65 - 
We conclude that: 
• Fahrenheit scale is common used in U.S. and many other English-speaking countries. 
• Celsius scale is used in scientific measurements and industrial applications. 
• Kelvin scale is based on an extremely low temperature. 
4.2.2 Temperature measurement 
There are two basic categories either to use mechanical properties (expansion of a substance) 
or to use electrical properties to operate. 
Using mechanical properties 
 
1) Capillary tube thermometer 
The element mercury is a liquid in the temperature range of -38.9° C to 356.7° C As a liquid, 
mercury expands (moves) as it gets warmer, its expansion rate is linear and can be accurately 
calibrated.  
 
Figure 4.2 Thermometer 
The mercury-in-glass thermometer illustrated in the above figure contains a bulb filled with 
mercury that is allowed to expand into a capillary tube. Its rate of expansion is calibrated on 
the glass scale. Mercury can be replaced by alcohol for low temperature measurement. 
2) Filled thermal system 
 
The device consists of a bulb filled with expanding substance connected to a Bourdon tube 
mechanism via a capillary tube ( ˜30m long) as shown in figure. 
 
 
Figure 4.3 A filled system thermometer 
Temperature scale
Moving 
pointer 
Bourdon
tube
Bulb with 
expanding 
substance 
Capillary 
tube
Control 
Room 
Process 
Field 
Sensors and Transducers 
??????????????????????????????????? ?????????????? ?????????????? ???????????????? ????????????????? ????????????????? ????????????? ???????????? ?????????????? ???????????????????? ?????????????????? 
- 66 - 
The pressure inside the bulb changes as the temperature changes. Consequently, the pressure 
moves the pointer at the moving end of Bourdon tube. This movement is marked using the 
temperature scale. 
 
3) Bimetallic thermometers 
 
This type of temperature sensor has the characteristics of being relatively inaccurate, having 
hysteresis, having relatively slow time of response. This sensor consists of two materials 
(metals) with gross different expansion coefficients and bonded together. Therefore, the 
temperature will make each metal to expand with a different length. Consequently, this effect 
can be used to close switch contacts or to actuate an on/off mechanism when temperature 
increases to some operating set point.  
 
 
   
 
Figure 4.4 Bimetallic on/off switch  
 
The effect also is used for temperature indicators, by means of assemblages, to convert the 
curvature into dial rotation.   
 
 
Figure 4.5 Bimetallic thermometer 
 
Using electrical properties 
 
1) Resistance Thermal Detectors (RTDs) 
 
It is a temperature sensor that is based on a metal resistance increasing with temperature. 
Metals used in these devices vary from platinum, which is very repeatable, quite sensitive, 
and very expensive, to nickel, which is not quite as repeatable, more sensitive, and less 
expensive. For pure metals, the characteristic relationship that governs resistance is given by: 
Temperature scale
Moving 
pointer 
Bimetallic 
strip