University of Engineering and Technology TaxilaMechanical Engineering Department

Lecture No: 01

Course Title: Measurement and Instrumentation (ME-314)

Tutor: Khalid Masood Khan

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Recommended Books

1. Thomas G. Beckwith, John H. Lienhard V and Roy D. Marangoni: Mechanical Measurements, 6th Edition, © 2007 Pearson Education Inc.

2. Richard S. Figliola and Donald E. Beasley: Theory and Design for Mechanical Measurements, 3rd Edition, © 2004 John Wiley & Sons Inc.

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Variable

• Variable is the quantity targeted for measurement

• It can be independent or dependent, continuous or discrete

• Engineering variables such as displacement, pressure, strain, or temperature are continuous

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• A variable is controlled if it can be held at a constant value or at some prescribed condition during a measurement

• Variables that are not or cannot be controlled during measurement but affect the value of the variable measured are called extraneous variables

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Parameter

It is a functional relationship between variables.

Example is a fan flow coefficient (C1).

C1 = Q / (nd3) = f (Q, n, d)

WhereQ = flow raten = fan rotational speedd = fan diameter

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Measurement

• The process of quantifying a parameter

• An act of assigning a specific value to a physical variable

• Objective is to establish the value or the tendency of some variable

• Determination of amount/extent of a physical variable

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Mechanical Measurement

• Every thing that exists is related to the practice of mechanical engineering

• The determination of the amount/extent constitutes the subject of mechanical measurement

• The means to achieve the measurement may or may not be mechanical

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Significance of Mechanical Measurement• An essential activity in every branch of science and technology

• Allows monitoring and control of processes and operations • Temperatures, flows, pressures, and vibrational amplitudes monitored by measurement to ensure proper system performance

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• Allows experimental engineering analysis

• Measurement is one of the important basic subjects for engineers and scientists

• Measurement and the correct interpretation thereof are necessary parts of any engineering and development program

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• All research, design and development is based on measurement

• The experimental element of a product/process development is based on measurement

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Experimental Test Plan

• Conceptualize the factors that influence the test

• Plan the experiment around these factors

• Define test objectives

• Identify process variables, parameters, and means for their control

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• Define tolerances for errors

• Select measurement technique, equipment, and test procedure

• Plan in advance on how to analyze, interpret, and present the expected data

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True Value and Measured Value

• The input to the measurement system is the true value of the variable called measurand

• Output is the measured value

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Act/Process of Measurement

• Obtaining a quantitative comparison between a predefined standard and a measurand

• The act of measurement produces a result

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System

A system is composed of components that work together to accomplish a specific task.

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Measurement System

• A measurement system is an information system which presents an observer with a numerical value corresponding to the variable being measured.

• Measurement system is meant to include all components in a chain of hardware and software that leads from the measured variable to processed data.

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Measurement System Elements

There are four types and, in a system, one type of element may be missing or may occur more than once.

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1. Sensing Element (The Sensor)

The sensor is a physical element that uses some natural phenomenon to sense the variable being measured.

Examples:• Energy exchange between the liquid (the sensor) contained in the bulb of a thermometer and the surrounding air is the input signal

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• Thermocouple is the sensing element that measures temperature

• Load cell in a strain gauge is the sensing element that is used to measure mechanical strain

• Orifice plate is the sensing element that is used to measure the flow rate

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2. Signal Conditioning Element (The Transducer)

• The transducer takes output from the sensing element and converts it into a form more suitable for further processing. The transduced signal form might be electrical, mechanical, optical, etc.

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• Transducer may include both sensing and signal processing units. Each element in the measurement system may itself be a system made up of simpler components.

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Examples:

• Thermal expansion forces the liquid into the narrow capillary (transducer) of the bulb thermometer. It transforms thermal information into a mechanical displacement.

• Deflection Bridge converts an impedance change into a voltage change

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• Amplifier amplifies millivolts to volts

• Oscillator converts an impedance change into a variable frequency voltage

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3. Signal Processing Element

Takes the output of the conditioning element/ transducer and converts it into a form more suitable for presentation.

Example:• Analogue-to-Digital-Converter takes analogue signal as input and converts it to digital form for input to a computer.

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4. Data Presentation Element

Presents the measured value in a form which can be easily recognized by the observer.

Examples:• A simple pointer-scale indicator• Chart recorder• Alphanumeric display• Visual display unit