Metrology Assignment

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2. METROLOGY AND INSTRUMENTATION PRINCIPLES OF MEASUREMENTToday, the techniques of measurement are of immense importance in most facets of human civilization. Present-day applications of measuring instruments can be classified into three major areas. The first of these is their use in regulating trade, and includes instruments which measure physical quantities such as length, volume and mass in terms of standard units.The second area for the application of measuring instruments is in Monitoring functions. These provide information which enables human beings to take some prescribed action accordingly. Whilst there are thus many uses of instrumentation in our normal domestic lives, the majority of monitoring functions exist to provide the information necessary to allow a human being to control some industrial operation or process. In a chemical process for instance, the progress of chemical reactions is indicated by the measurement of temperatures and pressures at various points, and such measurements allow the operator to take correct decisions regarding the electrical supply to heaters, cooling water flows, valve positions, etc. One other important use of monitoring instruments is in calibrating the instruments used in the automatic process control systems.Use as part of automatic control systems forms the third area for the application of measurement systems. The characteristics of measuring instruments in such feedback control systems are of fundamental importance to the quality of control achieved. The accuracy and resolution with which an output variable of a process is controlled can never be better than the accuracy and resolution of the measuring instruments used. This is a very important principle, but one which is often inadequately discussed in many texts on automatic control systems. Such texts explore the theoretical aspects of control system design in considerable depth, but fail to give sufficient emphasis to the fact that all gain and phase margin performance COLLEGE OF ENGINEERING THIRUVANATHAPURAM 2 3. METROLOGY AND INSTRUMENTATIONcalculations, etc., are entirely dependent on the quality of the process measurements obtained. Measuring Equipments A measuring instrument exists to provide information about the physical value of some variable being measured. In simple cases, an instrument consists of a single unit which gives an output reading or signal according to the magnitude of the unknown variable applied to it. However, in more complex measurement situations, a measuring instrument may consist of several separate elements. These components might be contained within one or more boxes, and the boxes holding individual measurement elements might be either close together or physically separate. Because of the modular nature of the elements within it, a measuring instrument is commonly referred to as a measurement system, and this term is used extensively to emphasize this modular nature.Common to any measuring instrument is the primary transducer: this gives an output which is a function of the measurand (the input applied to it). For most but not all transducers, this function is at least approximately linear. Some examples of primary transducers are a liquid-in-glass thermometer, a thermocouple and a strain gauge. In the case of a mercury- in-glass thermometer, the output reading is given in terms of the level of the mercury, and so this particular primary transducer is also a complete measurement system in itself. In general, however, the primary transducer is only part of a measurement system. The types of primary transducers available for measuring a wide range of physical quantities widely available.The output variable of a primary transducer is often in an inconvenient form and has to be converted to a more convenient one. For instance, the displacement-measuring strain gauge has an output in the form of a varyingCOLLEGE OF ENGINEERING THIRUVANATHAPURAM 3 4. METROLOGY AND INSTRUMENTATIONresistance. This is converted to a change in voltage by a bridge circuit, which is a typical example of the variable conversion element.Signal processing elements exist to improve the quality of the output of a measurement system in some way. A very common type of signal processing element is the electronic amplifier, which amplifies the output of the primary transducer or variable conversion element, thus improving the sensitivity and resolution of measurement. This element of a measuring system is particularly important where the primary transducer has a low output. For example, thermocouples have a typical output of only a few mill volts. Other types of signal processing element are those which filter out induced noise and remove mean levels, etc.The observation or application point of the output of a measurement system is often some physical distance away from the site of the primary transducer which is measuring a physical quantity, and some mechanism of transmitting the measured signal between these points is necessary. Sometimes, this separation is made solely for purposes of convenience, but more often it follows from the physical inaccessibility or environmental unsuitability of the site of the primary transducer for mounting the signal presentation/recording unit. Thesignal transmission elementhas traditionally consisted of single- or multi-cored cable, which is often screened to minimize signal corruption by induced electrical noise. Now, optical fiber cables are being used in ever increasing numbers in modem installations, in part because of theirlow transmission loss and imperviousness to the effects of electrical and magnetic fields.The final element in a measurement system is the point where the measured signal is utilized. In some cases, this element is omitted altogether because the measurement is used as part of an automatic control scheme, and the transmitted signal is fed directly into the control system. In COLLEGE OF ENGINEERING THIRUVANATHAPURAM4 5. METROLOGY AND INSTRUMENTATIONother cases, this element takes the form of either a signal presentation unit or a signal recording unit. These take many forms according to the requirements of the particular measurement application. PRECISION Precision is how close the measured values are to each other. The precision of a measurement is the size of the unit you use to make a measurement. The smaller the unit, the more precise the measurement. Precision depends on the unit used to obtain a measure. Consider measures of time, such as 12 seconds and 12 days. A measurement of 12 seconds implies a time between11.5 and 12.5 seconds. This measurement is precise to the nearest second, with a maximum potential error of 0.5 seconds. A time of 12 days is far less precise. Twelve days suggests a time between 11.5 and 12.5 days, yielding a potential error of 0.5 days, or 43,200 seconds! Because the potential error is greater, the measure is less precise. Thus, as the length of the unit increases, the measure becomes less precise. The number of decimal places in a measurement also affects precision. A time of 12.1 seconds is more precise than a time of 12 seconds; it implies a measure precise to the nearest tenth of a second. The potential error in12.1 seconds is 0.05 seconds, compared with a potential error of 0.5 seconds with a measure of 12 seconds. Although students learn that adding zeros after a decimal point is acceptable, doing so can be misleading. The measures of 12 seconds and 12.0 seconds imply a difference in precision. The first figure is measured to the nearest seconda potential error of 0.5 seconds. The second figure is measured to the nearest tentha potential error of 0.05 seconds. Therefore, a measure of 12.0 seconds is more precise than a measure of 12 seconds. COLLEGE OF ENGINEERING THIRUVANATHAPURAM5 6. METROLOGY AND INSTRUMENTATION Differing levels of precision can cause problems with arithmetic operations. Suppose one wishes to add 12.1 seconds and 14.47 seconds. The sum, 26.57 seconds, is misleading. The first time is between 12.05 seconds and12.15 seconds, whereas the second is between 14.465 and 14.475 seconds. Consequently, the sum is between 26.515 seconds and 26.625 seconds. A report of 26.57 seconds suggests more precision than the actual result possesses. The generally accepted practice is to report a sum or difference to the same precision as the least precise measure. Thus, the result in the preceding example should be reported to the nearest tenth of a second; that is, rounding the sum to 26.6 seconds. Even so, the result may not be as precise as is thought. If the total is actually closer to 26.515 seconds, the sum to the nearest tenth is 26.5 seconds. Nevertheless, this practice usually provides acceptable results. Measurements in industrial settings such as a rubber manufacturing plant must be both accurate and precise. Here a technician is measuring tire pressure. Multiplying and dividing measures can create a different problem. Suppose one wishes to calculate the area of a rectangle that measures 3.7 centimeters (cm) by 5.6 cm. Multiplication yields an area of 20.72 squareCOLLEGE OF ENGINEERING THIRUVANATHAPURAM 6 7. METROLOGY AND INSTRUMENTATIONcentimeters. However, because the first measure is between 3.65 and 3.75 cm, and the second measure is between 5.55 and 5.65 cm, the area is somewhere between 20.2575 and 21.1875 square centimeters. Reporting the result to the nearest hundredth of a square centimeter is misleading. The accepted practice is to report the result using the fewest number of significant digits in the original measures. Since both 3.7 and 5.6 have two significant digits, the result is rounded to two significant digits and an area of 21 square centimeters is reported. Again,