Measurements & Instrumentation – Module 3 Temperature Sensors Measurements & Instrumentation – Module 3

Objectives Explain the role of sensors/transducers in a measurement system Define the terms sensitivity and linearity of a sensor/transducer Identify different sensors used in temperature measurements Determine the sensor characteristics through experiments Understand and perform calibration of temperature sensor/transducer.

Introduction to Instrumentation Systems We use the term ‘measurements’ everyday in our lives. We make measurements while performing the following everyday tasks: Noting the time on our watch/clock Recording the temperature Tracing the speed of a car Estimating electricity consumption, and so on

Introduction 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.

Introduction 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.

Introduction 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.

Measurement System Elements Any Measurement system consists of three basic parts: Transducer/sensor Signal processor Recorder/display

Signal Processor Recorder Sensor/ Transducer Block Diagram of a measurement system Electrical / Mechanical Signal Sensor/ Transducer Signal Processor Recorder Physical Variable 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.

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.

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

Class Activity: Skill 1 Characteristic Definition Linearity Sensitivity Accuracy Precision Range Some measuring instruments/devices output a signal that is proportional to the input physical quantity. These instruments are called linear devices Sensitivity = Change in Output / Change in Input Accuracy of an instrument is how close a measured value is to the true value. The ability of an instrument to give the similar reading when the same physical quantity is measured more than once is called precision. The range of an instrument defines the minimum and maximum values that the instrument can measure

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’.

Temperature Measurement 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

Temperature Measurement There are various temperature measuring devices such as meters, gauges, or transducers, that could be used based on the measurement method and application. Thermometer Thermistor ThermoCouple

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.

THERMOCOUPLE Thermocouple is a temperature transducer that consists of two different metals, joined together at one end. 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.

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.

Input Physical quantity Class Activity: Skill 2 Sl No Sensor/ transducer Input Physical quantity Sensor output Picture 1 Ultrasonic sensor Distance Voltage or current 2 Thermocouple ________ 3 Thermistor Temp Voltage Temp Resistance

Lab Activity 1 & 2

Class Activity: Module Excercise

H.W. Internet Search : History of thermometers and pictures of different types of thermometers. Submit as a report in your own words . Include the references.