module3 temperature sensors

Measurements & Instrumentation

Module 3: Temperature Sensors

PREPARED BY

Academic Services Unit

August 2013

© Institute of Applied Technology, 2013

ATE 314– Measurements & Instrumentation

Module 3: Temperature Sensors 2

Module 3: 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.

3.1 Introduction to Instrumentation Systems 3

3.2 Measurement System Elements 4

3.3 Temperature Measurement 7

3.4 Lab Activity 1 11

3.5 Review Exercise 15



3.1 Introduction to Instrumentation Systems

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 3.1: 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 3.3) or an air-conditioner could be used

to maintain the room temperature.

Figure 3.2: Fixing a flat tire Figure 3.3: Room heater



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.

3.2 Measurement System Elements

Any Measurement system consists of three basic parts:

Ø transducer/sensor

Ø signal processor and

Ø recorder/display

The basic block diagram of a measurement system is shown in Figure 3.4:

Figure 3.4: 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



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 3.5: 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 3.6: Thermocouple Figure 3.7: Microphone

Sensors are mostly an integral, built-in part of a transducer.



Skill 1: Sensor/Transducer Characteristics

As indicated previously, sensors/transducers are the primary elements of

any measurement system. In Module 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



3.3 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 3.8

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,

RTD and the thermocouple.

Figure 3.9

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 3.10



2. Thermocouple

Thermocouple is a temperature transducer that consists of two different

metals, joined together at one end (Figure 3.11). 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 3.11: Thermocouple Construction Figure 3.12: 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 3.13: Thermistor



4. RTD

A Resistance Temperature

Detector (RTD) operates on the

principle of changes in electrical

resistance of pure metals. The

RTD is characterized by a linear

positive change in resistance with

temperature. Typical elements

used for RTDs include nickel (Ni)

and copper (Cu), but platinum

(Pt) is by far the most common

because of its wide temperature

range, accuracy, and stability.

Figure 3.14: RTD



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 Thermocouple ___________ ___________

2 Thermistor ___________ ___________

3 RTD ___________ ___________



3.4 Lab Activity

You will be given three unknown temperature sensors: thermocouple,

thermistor and RTD sensors. Use the following equipment to design and

perform an experimental setup to do the following tasks:

1) Identify the name of the given sensors, their inputs and outputs

2) Plot the temperature curve of each sensor

3) Identify the relationship between the input and output of each

sensor.

4) Determine the linearity of each sensor

5) Obtain the calibration equation of linear sensors

List of Equipment:

• Heat Bar System

• Calibration Tank

• Thermometer

• Set of various temperature sensors

• Multimeter

Procedure:

Step 1: Based on the knowledge acquired in Module 1, draw the setup of

your experiment



Step 2: Describe the main steps of the experiment

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________

Step 3: Label Each Sensor as Sensor 1, Sensor 2 and Sensor 3

Step 4: Perform the experiment

Step 5: Fill the following Tables:

Sensor No. Input Output

Sensor 1

Sensor 2

Sensor 3

Sensor 1 Sensor 2 Sensor 3

Input

Reading

Output

Reading

Input

Reading

Output

Reading

Input

Reading

Output

Reading



Step 6: Use Microsoft Excel to plot the temperature curve of each sensor.

Sensor 1

Temperature Curve

Sensor 2

Temperature Curve

Sensor 3

Temperature Curve

Step 7: use the curves drawn from step 6 to fill the following table:

Sensor Number Linearity Input/Output Relationship

Sensor 1

Sensor 2

Sensor 3

mm

Step 8: for linear sensors only, obtain the calibration equation of that

sensor. Then, check your answers by using Microsoft Excel.

1. Hint: to obtain the calibration equation in Excel, right click on the

graph à Add trendline à display equation on chart

_______________________________________________________

_______________________________________________________

_______________________________________________________

_______________________________________________________

_______________________________________________________



Step 9: Identify the name of each sensor based on the above results.

Justify your answer:

Sensor No. Sensor Name Justification

Sensor 1

Sensor 2

Sensor 3

Question:

• List one application for each sensor

Sensor No. Application

Sensor 1

Sensor 2

Sensor 3



3.5 Review 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

RTD Temperature ___________



4. List the differences between a RTD and a thermistor.

RTD Thermistor

Linearity

Sensitivity

5. Compare both readings obtained by completing the table below:

Sensor 1 Sensor 2

Temp.

(0C)

Voltage

(mV)

25 0.1

30 0.2

35 0.3

40 0.4

45 0.5

50 0.6

Temp.

(0C)

Resistance

(ohm)

25 100

30 200

35 300

40 400

45 500

50 600

Sensor

Name

Linearity

Input

Output

Sensitivity

Calibration

Equation



Notes

module3 temperature sensors

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