24-11-2012, 12:56 PM
Sensors and Transducers
[attachment=40954]
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
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.
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.
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.
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.
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.
Thermistors
The name thermistor is derived from thermally sensitive resistor, sense the resistance of a
thermistor varies as a function of temperature. It is an electrical device made of a solid
semiconductor with a high temperature sensitivity.
Signal conditioning:
The output from a thermocouple is in mV. This means that considerable amplification will be
necessary for a practical application. In addition, the small signal levels make the devices
susceptible to electrical noise. In most cases the thermocouple is used with a high gain
differential amplifier.
Reference junction compensation:
The output signal from the thermocouple depends on the difference between the two junctions
temperature. We have fixed one of them as a reference and the second one is considered to be
measured. If the temperature of the reference junction is not constant, that means, there is an
error will be produced.
Remarks
• In some applications, a group of thermocouples are connected in series so that there are
perhaps ten or more hot junctions sensing the temperature. The e.m.f. produced by each
is added together. Such arrangement is known as a thermopile.
• Temperature sensors can be immersed into a moving fluid to measure its temperature.
Therefore, a protective shield for a temperature measuring device, which penetrates into
the flow of the fluid being measured (thermowell)
• There are standard thermocouple tables that simply give the voltage for a particular type
of thermocouple when the reference junctions are at a particular reference temperature,
and the measurement junction is at a temperature of interest (Appendix A).
[attachment=40954]
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
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.
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.
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.
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.
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.
Thermistors
The name thermistor is derived from thermally sensitive resistor, sense the resistance of a
thermistor varies as a function of temperature. It is an electrical device made of a solid
semiconductor with a high temperature sensitivity.
Signal conditioning:
The output from a thermocouple is in mV. This means that considerable amplification will be
necessary for a practical application. In addition, the small signal levels make the devices
susceptible to electrical noise. In most cases the thermocouple is used with a high gain
differential amplifier.
Reference junction compensation:
The output signal from the thermocouple depends on the difference between the two junctions
temperature. We have fixed one of them as a reference and the second one is considered to be
measured. If the temperature of the reference junction is not constant, that means, there is an
error will be produced.
Remarks
• In some applications, a group of thermocouples are connected in series so that there are
perhaps ten or more hot junctions sensing the temperature. The e.m.f. produced by each
is added together. Such arrangement is known as a thermopile.
• Temperature sensors can be immersed into a moving fluid to measure its temperature.
Therefore, a protective shield for a temperature measuring device, which penetrates into
the flow of the fluid being measured (thermowell)
• There are standard thermocouple tables that simply give the voltage for a particular type
of thermocouple when the reference junctions are at a particular reference temperature,
and the measurement junction is at a temperature of interest (Appendix A).