Measurements & Instrumentation. Module 3: Temperature Sensors

Transcription

1 Measurements & Instrumentation PREPARED BY Academic Services Unit August 2013 Institute of Applied Technology, 2013

2 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 Measurement System Elements Temperature Measurement Lab Activity Review Exercise 15 2

3 3.1 Introduction to Instrumentation Systems ATE 314 Measurements & Instrumentation 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 3

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. 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: Physical Variable Sensor/ Transducer Electrical/ Mechanical signal Signal Processor Recorder 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. 4

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 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. 5

6 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 6

7 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 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

8 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 8

9 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 9

10 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 10

11 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 11

12 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 Output Input Output Input Output Reading Reading Reading Reading Reading Reading 12

13 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 13

14 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 14

15 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 Thermocouple Sensor input (physical quantity) Sensor output (electrical quantity) Temperature Resistance RTD Temperature 15

16 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. ( 0 C) Voltage (mv) Temp. ( 0 C) Resistance (ohm) Sensor Name Linearity Input Output Sensitivity Calibration Equation 16

17 Notes 17