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Instrumentation & Measurement _____________________________________________________ _ AAiT _____________________________________________________________________________________ Compiled by Yidnekachew M. Page 1 of 16 Chapter 1 Basic Concepts of Measurement and Instrumentation 1.1 Introduction Measurement techniques have been of immense importance ever since the start of human civilization, when measurements were first needed to regulate the transfer of goods in barter trade to ensure that exchanges was fair. The industrial revolution during the nineteenth century brought about a rapid development of new instruments and measurement techniques to satisfy the needs of industrialized production techniques. Since that time, there has been a large and rapid growth in new industrial technology. This has been particularly evident during the last part of the twentieth century, encouraged by developments in electronics in general and computers in particular. This, in turn, has required a parallel growth in new instruments and measurement techniques. Measurement systems have important vital applications in our everyday lives, whether at home, in our vehicles, offices or factories. We use measuring devices in buying our fruits and vegetables. We assume that the measuring devices are accurate, and we assume that we are all referring to the same units (e.g., kilogram, meter, liter…). The consequence of inaccurate measuring devices in this case leads to financial losses on our part. We check the temperature of our homes and assume that the thermostats reading the temperature are accurate. If not, then the temperature will be either too high or too low, leading to inconvenience and discomfort. We pay for our electricity in units of kWh and we assume that the electricity meter is accurate and faithfully records the correct number of electricity units that we have used. We pay for the water we consume in liters, and we also assume that the water meter is correctly measuring the flow of water in liters. In this case as well, the error will lead to financial loss.
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Transcript of Chapter 1 - Yidnekachew · PDF fileInstrumentation & Measurement _____ _ AAiT

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    Chapter 1 Basic Concepts of Measurement and Instrumentation

    1.1 Introduction

    Measurement techniques have been of immense importance ever since the start of human

    civilization, when measurements were first needed to regulate the transfer of goods in barter

    trade to ensure that exchanges was fair. The industrial revolution during the nineteenth century

    brought about a rapid development of new instruments and measurement techniques to satisfy

    the needs of industrialized production techniques. Since that time, there has been a large and

    rapid growth in new industrial technology. This has been particularly evident during the last part

    of the twentieth century, encouraged by developments in electronics in general and computers in

    particular. This, in turn, has required a parallel growth in new instruments and measurement

    techniques.

    Measurement systems have important vital applications in our everyday lives, whether at home,

    in our vehicles, offices or factories.

    We use measuring devices in buying our fruits and vegetables. We assume that the measuring

    devices are accurate, and we assume that we are all referring to the same units (e.g., kilogram,

    meter, liter). The consequence of inaccurate measuring devices in this case leads to financial

    losses on our part.

    We check the temperature of our homes and assume that the thermostats reading the temperature

    are accurate. If not, then the temperature will be either too high or too low, leading to

    inconvenience and discomfort.

    We pay for our electricity in units of kWh and we assume that the electricity meter is accurate

    and faithfully records the correct number of electricity units that we have used. We pay for the

    water we consume in liters, and we also assume that the water meter is correctly measuring the

    flow of water in liters. In this case as well, the error will lead to financial loss.

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    The accuracy of the measurement systems mentioned above is very important, but is more

    critical in some applications than others. For example, a pharmacist preparing a medication is

    reliant on the accuracy of his/her scales to make sure he/she includes the correct amounts of

    ingredients in the medication. Another example is the manufacturing of present-day integrated

    circuits and photo-masks that requires a high degree of accuracy. Certain chemical reactions

    require high accuracy in the measurement and control of temperature.

    The massive growth in the application of computers to industrial process control and monitoring

    tasks has spawned a parallel growth in the requirement for instruments to measure, record and

    control process variables. As modern production techniques dictate working to tighter and tighter

    accuracy limits, and as economic forces limiting production costs become more severe, so the

    requirement for instruments to be both accurate and cheap becomes ever harder to satisfy. This

    latter problem is at the focal point of the research and development efforts of all instrument

    manufacturers. In the past few years, the most cost-effective means of improving instrument

    accuracy has been found in many cases to be the inclusion of digital computing power within

    instruments themselves. These intelligent instruments therefore feature prominently in current

    instrument manufacturers catalogues.

    1.2 The evolution of measurement

    We can look at the evolution of measurement by focusing on invented instruments or by using

    the instruments themselves. We will list the steps of progress in measurement, which we define

    somewhat arbitrarily, according to human needs as these emerged throughout history:

    the need to master the environment (dimensional and geographical aspects);

    the need to master means of production (mechanical and thermal aspects);

    the need to create an economy (money and trade);

    the need to master and control energy (electrical, thermal, mechanical, and hydraulic

    aspects);

    the need to master information (electronic and optoelectronic aspects).

    In addition to these is the mastery of knowledge which has existed throughout history and is

    intimately connected:

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    measurement of time;

    measurement of physical phenomena;

    measurement of chemical and biological phenomena.

    1.3 Functions of Measurement systems

    Measurements are made or measurement systems are set up for one or more of the following

    functions:

    To monitor processes and operations

    To control processes and operations

    To carry out some analysis

    1.3.1 Monitoring

    Thermometers, barometers, anemometers, water, gas and electricity meters only indicate

    certain quantities. Their readings do not perform any control function in the normal sense.

    These measurements are made for monitoring purposes only.

    1.3.2 Control

    The thermostat in a refrigerator or geyser determines the temperature of the relevant

    environment and accordingly switches off or on the cooling or heating mechanism to keep

    the temperature constant, i.e. to control the temperature. A single system sometimes may

    require many controls. For example, an aircraft needs controls from altimeters, gyroscopes,

    angle-of-attack sensors, thermo- couples, accelerometers, etc.

    Controlling a variable is rather an involved process and is therefore a subject of study by

    itself.

    1.3.3 Analysis

    Measurement are also made to

    test the validity of predictions from theories,

    build empirical models, i.e. relationships between parameters and quantities

    associated with a problem, and

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    characterize materials, devices and components.

    In general, these requirements may be called analysis.

    1.4 Measurement units The very first measurement units were those used in barter trade to quantify the amounts being

    exchanged and to establish clear rules about the relative values of different commodities. Such

    early systems of measurement were based on whatever was available as a measuring unit. For

    purposes of measuring length, the human torso was a convenient tool, and gave us units of the

    hand, the foot and the cubit. Although generally adequate for barter trade systems, such

    measurement units are of course imprecise, varying as they do from one person to the next.

    Therefore, there has been a progressive movement towards measurement units that are defined

    much more accurately.

    The first improved measurement unit was a unit of length (the meter) defined as 10-7 times the

    polar quadrant of the earth. A platinum bar made to this length was established as a standard of

    length in the early part of the nineteenth century. This was superseded by a superior quality

    standard bar in 1889, manufactured from a platinumiridium alloy. Since that time, technological

    research has enabled further improvements to be made in the standard used for defining length.

    Firstly, in 1960, a standard meter was redefined in terms of 1.65076373 x 106 wavelengths of the

    radiation from krypton-86 in vacuum. More recently, in 1983, the meter was redefined yet again

    as the length of path travelled by light in an interval of 1/299792458 seconds. In a similar

    fashion, standard units for the measurement of other physical quantities have been defined and

    progressively improved over the years.

    The early establishment of standards for the measurement of physical quantities proceeded in

    several countries at broadly parallel times, and in consequence, several sets of units emerged for

    measuring the same physical variable. For instance, length can be measured in yards, meters, or

    several other units. Apart from the major units of length, subdivisions of standard units exist

    such as feet, inches, centimeters and millimeters, with a fixed relationship between each

    fundamental unit and its subdivisions.

    The latest standards for defining the units used for measuring a range of physical variables are

    given in Table 1.1.

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