Lecture 1

Slide 1

University of Engineering and Technology TaxilaMechanical Engineering Department

Lecture No: 01

Course Title: Measurement and Instrumentation (ME-314)

Tutor: Khalid Masood Khan

Recommended Books

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. 2

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 variables4

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 diameter5

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 variable6

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 measurement10

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 data12

True Value and Measured Value

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

Output is the measured value13

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. 15

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.16

Measurement System Elements

There are four types and, in a system, one type of element may be missing or may occur more than once.17

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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 rate20

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 voltage24

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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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 unit26