Module1 Temperature Sensors
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Transcript of Module1 Temperature Sensors
Instrumentation
Module 1: Temperature Sensors
PREPARED BY
IAT Curriculum Unit December 2011
© Institute of Applied Technology, 2011
ATE 1124– Instrumentation
Module 1: Temperature Sensors 2
Module 1: Temperature Sensors
Module Objectives Upon successful completion of this module, students should be able to:
1. Explain the role of sensors/transducers in a measurement system
2. Define the terms sensitivity and linearity of a sensor/transducer
3. Identify different sensors used in temperature measurements
4. Determine the sensor characteristics through experiments
5. Understand and perform calibration of temperature
sensor/transducer.
Module Contents: Topic Page No.
1.1 Introduction to Instrumentation Systems 3
1.2 Measurement System Elements 4
1.3 Temperature Measurement 7
1.4 Lab Activity 1 10
1.5 Lab Activity 2 13
3.6 Module Exercise 17
ATE 1120– Measurements & Instrumentation
Module 1: Temperature Sensors 3
1.1 Introduction to Instrumentation Systems
As you learned in Measurements course, we use the term ‘measurements’
everyday in our lives. We make measurements while performing the
following everday tasks:
Noting the time on our watch/clock
Recording the temperature
Tracing the speed of a car
Estimating electricity consumption, and so on
An instrument is a device used to measure a physical
variable. A ruler, for example, provides
measurement of length; hence, it is an instrument.
Many different physical variables such as
temperature, distance, pressure, force, humidity etc.
can be measured by instruments or measuring
devices.
Figure 1.1: Ruler
One of the most important uses of instruments or measuring devices is in
the field of process control. A process is a sequence of operations carried
out to achieve a desired result. For example, fixing a flat tire is a process.
Similarly, maintaining the temperature of a room within the desired limit is
a process. A room heater (Figure 1.3) or an air-conditioner could be used
to maintain the room temperature.
Figure 1.2: Fixing a flat tire Figure 1.3: Room heater
ATE 1124– Instrumentation
Module 1: Temperature Sensors 4
Fixing a tire is a manual process, whereas, the process of maintaining the
room temperature is an automatic process. However, the fundamental
part of any process control system is a measurement system.
1.2 Measurement System Elements
Any Measurement system consists of three basic parts:
transducer/sensor
signal processor and
recorder/display
The basic block diagram of a measurement system is shown in Figure 1.4:
Figure 1.4: Elements of a Measurement System
The sensor/transducer is the primary element of a measurement system. It
senses the input physical variable and converts it into a measurable signal.
The signal processor converts the signal to a format that can be displayed
or recorded by the recorder.
Sensor/
Transducer
Signal Processor
Recorder
Electrical/ Mechanical signal
Physical Variable
ATE 1120– Measurements & Instrumentation
Module 1: Temperature Sensors 5
Sensor
A sensor is a device that senses a physical variable, such as temperature,
force, or pressure etc. Sensing the variable means detecting the presence
of the variable as well as to what degree it is present. For example, a
human finger is a sensor that can tell you whether an object is hot or cold,
but a thermometer can determine accurately to what degree the
temperature is present.
Figure 1.5: Thermometer
Transducer
A transducer is a device that converts a physical quantity into a measurable
quantity, usually an electrical signal. Examples of transducers include
microphones and thermocouples. A microphone converts sound energy
into electrical energy, whereas, a thermocouple converts heat energy into
electrical energy.
Figure 1.6: Thermocouple Figure 1.7: Microphone
Sensors are mostly an integral, built-in part of a transducer.
ATE 1124– Instrumentation
Module 1: Temperature Sensors 6
Skill 1: Sensor/Transducer Characteristics
As indicated previously, sensors/transducers are the primary elements of
any measurement system. In Term 1 you studied few of the characteristics
of these sensing instruments. Try to recall some of these characteristics by
completing the following table.
Characteristic Definition
Linearity
Sensitivity
Accuracy
Precision
Range
ATE 1120– Measurements & Instrumentation
Module 1: Temperature Sensors 7
1.3 Temperature Measurement
Temperature is the degree of hotness or coldness
measured on a definite scale. The various units for
measuring temperature are Celsius, Kelvin and
Fahrenheit. The SI unit of temperature measurement is
the ‘Kelvin’.
Figure 1.8
Temperature can be measured using any one of the following methods
depending on the application:
1. Immersion in a gas or liquid
2. Surface Contact with the solid
3. No contact
There are various temperature measuring devices such
as meters, gauges, or transducers, that could be used
based on the measurement method and application. In
this module, you will study the thermometer, thermistor
and the thermocouple.
Figure 1.9
1. Thermometer
Mercury thermometer is a temperature sensor that
converts the measured temperature into expansion
and contraction of a liquid which can be read on a
calibrated glass tube.
Figure 1.10
ATE 1124– Instrumentation
Module 1: Temperature Sensors 8
2. Thermocouple
Thermocouple is a temperature transducer that consists of two different
metals, joined together at one end (Figure 1.11). Examples of metals
used: Iron and Constantan. When the metal junction is heated or cooled,
the thermocouple produces a voltage proportional to the temperature. The
output voltage is very small and is in mV. Since thermocouples are
rugged, and can measure within wide temperature ranges, they are
especially useful in industries.
Figure 1.11: Thermocouple Construction Figure 1.12: Thermocouple types
3. Thermistor
The thermistor is a thermally
sensitive resistor, the resistance
of which varies with temperature.
It is a non-linear device in that
the resistance of a thermistor is
not proportional to the input
temperature.
Figure 1.13: Thermistor
ATE 1120– Measurements & Instrumentation
Module 1: Temperature Sensors 9
Skill 2: Sensor Input/Output
Identify the input physical quantity and the output for the sensors given in
the table. Refer to the first example and complete the other two:
Sl No Sensor/
transducer
Input Physical
quantity Sensor output Picture
1 Ultrasonic
sensor Distance
Voltage or current
2 Thermocouple ___________ ___________
3 Thermistor ___________ ___________
ATE 1124– Instrumentation
Module 1: Temperature Sensors 01
1.4 Lab Activity 1
Objective:
a. To measure the temperature of the heat bar using a thermometer.
b. To plot the temperature curve.
c. To determine the sensitivity of the thermometer.
Background Information:
The heat bar assembly is shown in Figure 1.14. The bar conveys heat from
the heater to the heat sink by conduction. The heat sink conveys the heat
to the surrounding atmosphere by convection. Therefore, the heat sink end
of the bar is only a little above the room temperature.
Figure 1.14
Procedure A:
1. Measure the room temperature with the
mercury thermometer. Record the data.
2. Remove the cap of the calibration tank and fill
it with water upto approximately 16mm from
the top.
3. Mount the calibration tank on the heat bar at
notch N10.
4. Insert the thermometer into the larger hole of
the tank and make sure that its top surface is
at the 10C mark.
Figure 1.15
ATE 1120– Measurements & Instrumentation
Module 1: Temperature Sensors 00
5. Switch on the heat bar supply. Note the time, and record the first
reading.
6. Record the thermometer reading for every two minutes for the first
20 minutes and note down the measurements in table 1.1.
7. Take the thermometer reading for every 5 minutes until no further
increase is observed.
8. Record the readings in the table.
Time
(minutes)
Temperature
(0C)
0
2
4
6
8
10
12
14
16
18
20
25
30
Table 1.1
ATE 1124– Instrumentation
Module 1: Temperature Sensors 02
9. Plot a graph of temperature vs time.
10. Find the slope of the curve to determine the sensitivity of the
thermometer.
Sensitivity = change in temperature/change in time
= ______________________0C/min
ATE 1120– Measurements & Instrumentation
Module 1: Temperature Sensors 03
1.5 Lab Activity 2
Objective: To determine the linearity of a thermocouple and a thermistor.
Part A: Linearity
Procedure:
1. Connect the thermometer, calibration tank, the heat bar and the
multimeter to build the set up shown in Figure 1.16.
Figure 1.16
2. Set the multimeter on voltage function and mV range.
3. Connect the two leads of the thermocouple to the multimeter.
4. Note down the temperature on the glass thermometer. Record the
value in table 1.2
5. Note down the voltage on the multimeter. Record the value in table
1.2
ATE 1124– Instrumentation
Module 1: Temperature Sensors 04
Thermometer Reading
(C)
Multimeter Reading (mV)
Table 1.2
ATE 1120– Measurements & Instrumentation
Module 1: Temperature Sensors 05
6. Plot a graph of temperature versus voltage, with the temperature
values on the X-axis and the voltage values on the Y-axis.
7. Observe the graph. Is the output voltage proportional to the input
temperature? Do you think the thermocouple is a linear, or a non-
linear device?
_____________________________________________________
_____________________________________________________
8. Replace the thermocouple with the thermistor, and repeat the
experiment. Set the multimeter to read resistance instead of the
voltage. Record the values in table 1.3
ATE 1124– Instrumentation
Module 1: Temperature Sensors 06
Thermometer Reading
(C)
Multimeter Reading
(kΩ)
Table 1.3
9. Plot a graph of temperature versus voltage, with the temperature
values on the X-axis and the resistance values on the Y-axis.
10. Is the thermistor a linear, or a non-linear device?
_____________________________________________________
ATE 1120– Measurements & Instrumentation
Module 1: Temperature Sensors 07
1.6 Module Exercise
1. The sensitivity of an instrument is the
a. smallest increment in the input that can be detected by the
instrument.
b. largest input change which the instrument fails to detect.
c. Ratio of change in the output to the change in the input.
d. closeness of the ouptut values for repeated applications of a
constant input.
2. The output signal of the thermistor is:
a. Voltage
b. Current
c. Resistance
d. Power
3. Complete the table below:
Sensor/
transducer
Sensor input
(physical quantity)
Sensor output
(electrical quantity)
Thermocouple ___________ __________
__________ Temperature Resistance
_________ Distance ___________
ATE 1124– Instrumentation
Module 1: Temperature Sensors 08
4. List two differences between a thermocouple and a thermistor.
5. Determine the sensitivity of the thermometer by referring to the
values in the table below:
Time
(minutes)
Temperature
(0C)
0 23
3 27
6 31
9 35
Sensitivity = __________________________________________
ATE 1120– Measurements & Instrumentation
Module 1: Temperature Sensors 09
Notes