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


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



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



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.



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



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



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



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 ___________ ___________



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



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



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



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



Thermometer Reading

(C)

Multimeter Reading (mV)

Table 1.2



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



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?

_____________________________________________________



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 ___________



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 = __________________________________________



Notes