Instrumentation & Measurement AAiT. Chapter 1. Basic Concepts of Measurement and Instrumentation

Transcription

1 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. Compiled by Yidnekachew M. Page 1 of 20

2 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: Compiled by Yidnekachew M. Page 2 of 20

3 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 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 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 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 Compiled by Yidnekachew M. Page 3 of 20

4 characterize materials, devices and components. In general, these requirements may be called analysis. 1.4 Basic requirements for a meaningful measurement The standard used for comparison purposes must be accurately defined and should be commonly accepted. The apparatus used and the method adopted must be provable (verifiable). 1.5 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 platinum iridium 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 x 10 6 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/ 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 Compiled by Yidnekachew M. Page 4 of 20

5 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. Table 1.1 Definitions of standard units Table 1.2 Fundamental and derived SI units (a) Fundamental units Compiled by Yidnekachew M. Page 5 of 20

6 (b) Supplementary fundamental units Compiled by Yidnekachew M. Page 6 of 20

7 (c) Derived units Compiled by Yidnekachew M. Page 7 of 20

8 1.6 Methods of Measurement Measurement of any quantity involves two parameters: the magnitude of the value and unit of measurement. For instance, if we have to measure the temperature we can say it is 10 degree C. Here the value 10 is the magnitude and C which stands for Celsius is the unit of measurement. Similarly, we can say the height of wall is 5 meters, where 5 is the magnitude and meters is the unit of measurement. There are two methods of measurement: Direct Method Indirect method Direct Method: In the direct method of measurement, we compare the quantity directly with the primary or secondary standard. Say for instance, if we have to measure the length of the bar, we will measure it with the help of the measuring tape or scale that acts as the secondary standard. Here we are comparing the quantity to be measured directly with the standard. Even if you make the comparison directly with the secondary standard, it is not necessary for you to know the primary standard. The primary standards are the original standards made from certain standard values or formulas. The secondary standards are made from the primary standards, but most of the times we use secondary standards for comparison since it is not always feasible to use the primary standards from accuracy, reliability and cost point of view. There is no difference in the measured value of the quantity whether one is using the direct method by comparing with primary or secondary standard. The direct comparison method of measurement is not always accurate. In above example of measuring the length, there is limited accuracy with which our eye can read the readings, which can be about 0.01 inch. Here the error does not occur because of the error in the standards, but because of the human limitations in noting the readings. Similarly, when we measure the mass of Compiled by Yidnekachew M. Page 8 of 20

9 any body by comparing with some standard, it s very difficult to say that both the bodies are of exactly the same mass, for some difference between the two, no matter how small, is bound to occur. Thus, in direct method of measurement there is always some difference, however small, between the actual value of the quantity and the measured value of the quantity. Indirect Method: There are number of quantities that cannot be measured directly by using some instrument. For instance we cannot measure the strain in the bar due to applied force directly. We may have to record the temperature and pressure in the deep depths of the ground or in some far off remote places. In such cases indirect methods of measurements are used. In the indirect method of measurements some transducing devise, called transducer, is used, which is coupled to a chain of the connecting apparatus that forms the part of the measuring system. In this system the quantity which is to be measured (input) is converted into some other measurable quantity (output) by the transducer. The transducer used is such that the input and the output are proportional to each other. The readings obtained from the transducer are calibrated to as per the relations between the input and the output thus the reading obtained from the transducer is the actual value of the quantity to be measured. Such type of conversion is often necessary to make the desired information intelligible. The indirect method of measurements comprises of the system that senses, converts, and finally presents an analogues output in the form of a displacement or chart. This analogues output can be in various forms and often it is necessary to amplify it to read it accurately and make the accurate reading of the quantity to be measured. The majority of the transducers convert mechanical input into analogues electrical output for processing, though there are transducers that convert mechanical input into analogues mechanical output that is measured easily. 1.7 Elements of a measurement system Each measurement system consists of five elements. These elements could all be in one item or could be all in separate five items. They could be adjacent to each other or they could be separated by a distance. Some simple systems might not contain all of the components. The components of a typical system are shown in Figure 1. Compiled by Yidnekachew M. Page 9 of 20

10 Figure 1: Components of a measurement system. Each of these components is discussed in more detail below. a. Sensor: The sensor is the element that gives an output that is proportional to the input applied to it. In general the output is in an electrical format as this is the most suitable format for later use (in processing, transmission and storage). The input format depends on the variable to be measured (e.g., temperature, pressure, humidity, ph, speed, acceleration, light ). Sensors usually have a near linear relationship, although this is not always the case. b. Signal Conditioning Element (SCE): This is also referred to sometimes as a variable conversion element: When the output variable of a primary sensor is in an unsuitable (or inconvenient) format, a signal conditioning element is used to convert it to a suitable form. For example, the change in resistance of a strain gauge cannot be directly measured Compiled by Yidnekachew M. Page 10 of 20

11 and thus a deflection type bridge circuit is used to convert it to a suitable voltage. Bridge circuits are examples of signal conditioning elements and are discussed in more detail in the coming Chapters. Another example is the amplification of a very weak signal such as a biomedical signal (such as that used in an electrocardiogram ECG). The combination of the sensor and the signal conditioning element (SCE) is called the transducer. By definition, a transducer is a device the converts from one form of energy to another. The term transducer is sometimes incorrectly used to mean sensor. c. Signal Processing Element (SPE): This component is needed to improve the quality of the signal. A very common example is filtering a signal that contains mains frequency noise (i.e., 50 Hz). Some of the examples of signal processing elements as used in a measurement system are: Remove the mean value from an a.c. signal (i.e., dc shift). Filter out induced noise (example 50 Hz hum/pick-up). Convert an analogue signal to a digital format. Convert a time signal into voltage (e.g., an ultrasonic level sensor). The combination of the sensor, SCE and SPE is called the transmitter. The output signal from the SPE could be in a number of formats: voltage, current, frequency or on/off (such as in a switch). In other words, the information about the variable to be measured will be contained in the voltage of the output signal, its current or its frequency. It could also just be a yes/no output signal (for example as given by a thermostat that gives a signal stating whether the variable measured is more or less than a set value). In the case of frequency for example, the value of the measured variable would be represented as a certain frequency deviation from a certain mean frequency. The voltage and current output usually follow a standard format (e.g V in case of voltage and 4-20 ma in case of current). Compiled by Yidnekachew M. Page 11 of 20

12 Use of voltage, current or frequency has implication in terms of the effect of noise. The effect of noise on current transmission and voltage transmission is discussed in more detail in the coming chapters. d. Signal Transmission: The signal is then transmitted to the final location where it is needed. Most modern measurement system could be distributed over a wide area, and hence transmission in this case is necessary. There are three reasons why the signal needs to be transmitted to a remote location: i. Convenience: It is easier for example to locate the final equipment in a warm ii. iii. office than on a the roof of the building where the transmitter is located. Inaccessibility: The transmitter may sometimes be located in an area that cannot be accessed or reach. The measured variable could be inaccessible because it is located in a narrow tunnel if it is located in a high position. Hazardous location: The transmitter might be located in an area that is accessible, but hazardous to humans. An example of the hazardous situation is where the measured variable is in a chemical or nuclear plant, or in an area with very high temperatures. Transmission can be done by a number of methods, some of which are: Cable Transmission: This is typically done by screened single core or multicore. This method suffers from the problem of losses and attenuation especially over long distances and from electromagnetic interference. The cable is screened to reduce noise interference. Where the distance is long and losses become excessive, repeaters are needed at regular distances to re-amplify the signal. Fiber optics: Fiber optic cables are now more widely used. They offer the following advantages (the first two being most important to measurement systems): They are resistant to interference by electric and magnetic fields. They have low losses over long distances (as opposed to copper cable that might need repeaters at long distances, e.g., 2 km). Compiled by Yidnekachew M. Page 12 of 20

13 They have a large bandwidth and can offer high speeds (up to Tera- Hz). This is not much of an issue in low speed sampling system used in most measurement systems and is more relevant to high speed communication and data systems. They offer electrical isolation (galvanic isolation) between the transmitter and receiver. In some cases this is necessary for safety reasons. The main disadvantage of fiber optic systems is their high cost. They also need special equipment for installation, testing and repair and they require highly trained and specialized technicians. Wireless transmission: This removes the need for cabling and can be very attractive in cases where the transmitter is placed in inaccessible or remote locations. However, it does suffer from the problem of obstacles interrupting the connection (e.g., reinforced concrete) and from attenuation. Most transmitter manufacturers offer wireless versions of their systems nowadays. Many of the home weather stations are equipped with a wireless connection. Display, recording or analysis: D/R/A or use in automatic feedback systems: This is where the final signal is utilized. One of the following actions is taken: It is either fed into the automatic feedback system. The signal is displayed, recorded or analyzed: The signal can either be displayed on a screen or industrial display, it could be recorded on a hard-disk for example over a period of days or months and it could be analyzed to understand trends or draw conclusions. Both actions can be taken simultaneously as well: We can feed the signal into an automatic feedback system and display it on a screen or record it. Not all measurement system will contain all of the five elements. In some cases it is difficult to identify the boundaries between different elements. Compiled by Yidnekachew M. Page 13 of 20

14 As an example a simple measurement system is the mercury-in-glass thermometer. In this case all the items are within the same instrument and it is in fact difficult to separate one component from another. The system only contains a sensor (effectively the mercury in the tube) and a display component (the scale on the glass). There is neither an SCE, SPE or transmission system. On the other hand, an example of a complicated system is a computer controlled remote system in a chemical plant. In this case the five components can be clearly identified. The system is distributed, and thus the transmission element in this case is necessary due to the distance between the variable of the process to be measured and the receiver (e.g., a computer). The computer receiving the signal would display it, record it and keep available for later analysis if needed. The signal could also be fed into an automatic control system (e.g., temperature control of the chemical reaction). 1.8 Measurement systems and measurement devices A measurement system is the generic term of an instrument or a complex system. A person using a thermometer to measure his body temperature represents a measurement system. This system is made of: the human observer, the thermometer and the measured variable (temperature) from the process (the human body). It is important to note that the human observer is part of the measurement system in this example. If the observer makes an error in reading the temperature from the thermometer scale, then an error results from the whole measurement system. 1.9 Overview of variables that are measured The following is a selection of the most widely measured quantities: a. Electrical parameters: The basic seven parameters are: voltage, current, resistance, capacitance, inductance, frequency and phase shift. Other electrical parameters that are effectively derived from the 7 above in terms of measurement are: power and power factor. b. Magnetic: One of the magnetic parameters that can be directly measured is the magnetic flux density. c. Environmental variables such as: Temperature, pressure and humidity. Compiled by Yidnekachew M. Page 14 of 20

15 d. Mechanical measurements such as: Mass, force, torque, length, area, volume/capacity, angle and surface roughness. e. Fluid measurements such as: Viscosity, level measurement and flow measurement. f. Motion measurement such as: Translational motion and rotational motion. g. Others: Sound pressure, gas sensing and PH in solutions Classification of Measuring Instruments I. Basic classification of measuring instruments: 1. Mechanical instruments:- They are very reliable for static and stable conditions. The disadvantage is they are unable to respond rapidly to measurement of dynamic and transient conditions. 2. Electrical instruments:- Electrical methods of indicating the output of detectors are more rapid than mechanical methods. The electrical system normally depends upon a mechanical meter movement as indicating device. 3. Electronic instruments:- These instruments have very fast response. For example a cathode ray oscilloscope (CRO) is capable to follow dynamic and transient changes of the order of few nano seconds (10-9 sec). II. Other classification of instruments: 1. Absolute instruments or Primary Instruments:- These instruments gives the magnitude of quantity under measurement in terms of physical constants of the instrument e.g. Tangent Galvanometer. These instruments do not require comparison with any other standard instrument These instruments give the value of the electrical quantity in terms of absolute quantities (or some constants) of the instruments and their deflections. In this type of instruments no calibration or comparison with other instruments is necessary. They are generally not used in laboratories and are seldom used in practice by electricians and engineers. They are mostly used as means of standard Compiled by Yidnekachew M. Page 15 of 20

16 measurements and are maintained lay national laboratories and similar institutions. Some of the examples of absolute instruments are: a. Tangent galvanometer b. Raleigh current balance c. Absolute electrometer 2. Secondary instruments:-these instruments are so constructed that the quantity being measured can only be determined by the output indicated by the instrument. These instruments are calibrated by comparison with an absolute instrument or another secondary instrument, which has already been calibrated against an absolute instrument. Working with absolute instruments for routine work is time consuming since every time a measurement is made, it takes a lot of time to compute the magnitude of quantity under measurement. Therefore secondary instruments are most commonly used. They are direct reading instruments. The quantity to be measured by these instruments can be determined from the deflection of the instruments. They are often calibrated by comparing them with either some absolute instruments or with those which have already been calibrated. The deflections obtained with secondary instruments will be meaningless untill it is not calibrated. These instruments are used in general for all laboratory purposes. Some of the very widely used secondary instruments are: ammeters, voltmeter, wattmeter, energy meter (watt-hour meter), ampere-hour meters etc Error in a Measurement System Any measurement system has an input variable which is the true value of the quantity to be measured and an output variable which is the measured value. This is shown in Figure 1. Ideally, we would aim to make these two values identical, but in practice this is not possible. Compiled by Yidnekachew M. Page 16 of 20

17 One of the main aims in designing a measurement system is to minimize the error between the true value and the measured value. In fact, a large percentage of this textbook addresses the issue of how to minimize the error between the true value and the measured value. The reason for this error developing could one of the following: a. Systematic Errors: These are errors that have a clear understood explanation within the measurement system. Systematic error can be sub-divided into: Static errors caused by the static characteristics of the measurement system (effectively the steady state characteristics). Dynamic errors caused by the dynamic response of the measurement system (transient response of the device). b. Random errors caused by unknown reasons. c. Internal and external noise disturbances Definition of Terms The following terms are often employed to describe the quality of an instruments reading. Range The region between the limits within which a quantity is measured, received or transmitted, expressed by starting the lower and upper range values. Example: 0 to 150 of, 20 to 200 psi. Span The algebraic difference between the upper and lower range values. For example: a) Range 0 to 150 of, span 150 of. b) Range -20 to 200 of, span 220 of. c) Range 20 to 150 psi, span 130 psi. Elevated Zero Range A range in which the zero value of the measured variable, measured signal, is greater than the lower range value. Example: Compiled by Yidnekachew M. Page 17 of 20

18 -25 to 50 psi. Suppressed Zero Range A range in which the zero value of the measured variable is less than the lower range value. Example: 20 to 100 psi. Measured Variable A quantity property or condition that is measured. Sometimes referred to as the measured. Example: Temperature, Pressure, rate of flow. Measured Signal The electrical, mechanical, pneumatic or other variable applied to the input of a device. It is the analog of the Measured Variable produced by a transducer. Example: In a thermocouple thermometer, the measured signal is an emf which is the electrical analog of the temperature applied to the thermocouple. In a flow meter, the measured signal may be a differential pressure which is the analog of the rate of flow through the orifice. In an electric tachometer system, the measured signal may be a voltage which is the electrical analog of the speed of rotation of the part coupled to the tachometer generator. Output Signal A signal delivered by a device, element or system. Accuracy The accuracy of an instrument indicates the deviation of the reading from a known value accuracy is typically expressed as: a. Percentage of full scale reading (upper range value). Example: A 100 Kpa pressure gage having an accuracy of ± 1 % would be accurate of ± 1 Kpa over the entire range of the gage. b. Percentage of span. Example: A pressure gage has span of 200 Kpa, Accuracy of ± 0.5%. Compiled by Yidnekachew M. Page 18 of 20

19 To one reading of 150 Kpa is taken, then the true value of measurement will be 0.5x200 between 150 ± = 150 ± 1 or 149 kpa and 151 kpa 100 c. Measured Variable Accuracy of ± 1 Kpa, over all ranges of the Instrument. d. Percentage of the actual reading. Thus, for a ± 2% of reading voltmeter, we would have an inaccuracy of ± 0.04 volts for a reading of 2 volts. Precision The difference between the instruments reported values during repeated measurements of the same quantity. Typically, this value is determined by statistical analysis of repeated measurement. Repeatability Is the ability of an instrument to reproduce the same measurement each time the same set of conditions is repeated. This does not imply that the measurement is correct, but rather that the measurement is the same each time. Sensitivity Poor Repeatability means poor Accuracy. Good Accuracy means good repeatability. Good Repeatability does not necessarily mean good Accuracy. The change of an instrument or transducer output per unit change in the measured quantity. A more sensitive instrument reading changes significantly in response to smaller changes in the measured quantity. Typically an instrument with higher sensitivity will also have better repeatability and higher accuracy. Compiled by Yidnekachew M. Page 19 of 20

20 True value True value of quantity may be defined as the average of an infinite no. of measured value. Static error It is defined as the difference between the measured value and true value of the quantity. Resolution The smallest increment of change in the measured valve that can be determined from the instrument readout scale. Dead Band In process instrumentation the range through which an input signal may be varied upon reversal of direction, without initiating an observable change in output signal. Dead band is usually expressed in percent of span. Hysteresis An instrument is said to exhibit hysteresis when there is a difference in readings depending on whether the value of the measured quantity is approached from above or below. Hysteresis results from the inelastic quantity of an element or device. In other word, it may be the result of mechanical friction, magnetic effects, elastic deformation, or thermal effects. Hysteresis is expressed in percent of span. Dead band term is included in the hysteresis. Compiled by Yidnekachew M. Page 20 of 20