CHAPTER 1.1 INTRODUCTION

Transcription

1 CHAPTER 1 Introduction To measurements 1.1 INTRODUCTION Man is, in general, curious in nature and this trait, coupled with his imagination, skill and intuition has seen rapid strides in the field of measurement over the last few decades. During the early days of human civilisation, barter trade system was in vogue as measurement and it was used as a means to quantify the exchange of goods. Although very crude and unscientific as the system was, human beings had to do with this system because of non-availability of scientific and reliable forms of measurements. As human civilisation continued to evolve and progress, man tried to improve upon the then existing systems in order to have more meaningful exchange of trade. Man s continued stride for betterment in measurement systems gave rise to industrial revolution in the last century, followed by electronic revolution a few decades later. These developments, followed by the introduction of microprocessors in 70s and digital computers, have led to hitherto unseen and unimaginable measurement techniques which are very accurate, reliable and at the same time cheap. Measurement has become an all pervading one, be it chemical, aeronautical, automobiles or manufacturing systems of all sorts. As consumers are demanding higher and higher limits of accuracy coupled with cost control, measurement techniques are reaching the sky to achieve the above. The success on the above fronts have been made possible because of the ability and a quest for betterment in measuring the state, condition and the characteristics of physical systems with sufficient and dependable degree of accuracy. Techniques and technologies employed nowadays in manufacturing/process industries have undergone a sea change with the introduction of new technologies like robotics, continuous condition based monitoring (CBM), remote control, etc. 1.2 elements of a measuring system Fig. 1.1: Basic elements of a measuring system 1

2 2 Measurement techniques in industrial instrumentation The basic elements of a measuring system consists of three stages I, II and III shown in figure 1.1. Stage I consists of sensor/transducer part, stage II the signal processing part and the last or final stage may be indicator, recorder or it may include controller also. Stage I gives an output which is a function of the measurand or input parameter. An ideal transducer should give an output which is proportional to the input parameter variations, but should be insensitive to every other possible input. Thermocouples, strain gauges, liquid-in-glass thermometers are examples of primary transducer. Measurands, which are normally measured are temperature, pressure, flow, level, velocity, acceleration, ph, humidity, force, torque, etc. Stage II is the intermediate stage and may include variable conversion element and signal processing circuitry. For force and displacement measurements, a strain gauge is used which gives an output in the form of a varying resistance. This is then converted into a voltage change by employing a bridge circuit. The signal processing circuit increases the amplitude or power of the signal (transducer s output) or both; it all depends on the transducer output. This enhances the sensitivity and resolution of measurement. This stage also has several other functions like filtering, integration, differentiation and sometimes telemetering also. Final or stage III provides an indication, record or utilised for control of the process. The measurand may be presented as an indicator (as in a pressure gauge) or a digital output (as in pulse counting). 1.3 Application areas of measuring instruments Varied application areas exist for measuring instruments. These are: System parameter informations Perform mathematical manipulations Design studies Monitoring Simulation of system conditions Verification of physical phenomena/scientific formulae Testing of materials, product specifications and adherence to standards Quality control Automatic control of a process/operation System Parameter Informations System parameter informations: By determining various system parameters, healthiness of a process can be ascertained. In fact, condition-based monitoring leads to a continuous assessment and subsequent corrective actions can be taken, e.g., the health of a patient. Perform Mathematical Manipulations A simple pocket calculator performs the functions of addition, subtraction, multiplication, division, integration, differentiation, logarithmic calculations, etc. In addition, several instruments are designed to perform functions like signal sampling, linearisation, averaging, ratio control, etc. Design Studies Before launching a product in the market, extensive design studies are made on the drawing board. Thus, before formally launching a car/aircraft, its prototype is developed taking the help of design data. It is then tested under working conditions and various operating parameters are noted. Any deviations from expected parameter values are corrected at the design stage before marketing the product. Thus, experiments supplement design data in the intermediate stage to ensure acceptability of the final product by the discerning customers.

3 Introduction To Measurements 3 Monitoring Weather forecasting is made possible by employing instruments like barometers, anemometers, thermometers, etc. Again, by measuring flow, temperature, pressure, etc. in a process plant, an operator would be able to take corrective actions. Temperature of a greenhouse can be taken care of by a thermometer. Simulation of System Conditions A prototype of an aircraft is tested in controlled air streams generated in a wind tunnel that simulates the flow, turbulence conditions that the aircraft would actually face in its flights. Such simulation testing helps improve the design parameters of different parts of an aircraft. Again, a passenger car is tested under simulated road conditions to determine its worthiness/acceptability by customers. Verification of Physical Phenomena/Scientific Formulae Experimental studies are at times undertaken where substantial theoretical backgrounds are unavailable. Postulates proposed by scientists are verified experimentally to ascertain their veracity. Thus, Coulomb s postulate that friction between two dry surfaces is proportional to the normal reaction and independent of area of contact was experimentally verified and since then is known as Coulomb s law. Testing of Materials, Product Specifications and Adherence to Standards To improve longevity, reliability of product parts and a product as a whole, standard organisations specify material and product standards which the manufacturers of such parts/ products must adhere to. Again, as accuracy in measurement goes on improving, the specifications/ standards are also changed from time to time. These ensure less downtime for parts/products. Quality Control Each and every part of a machine is extensively put under quality control tests so that the defective parts are outright rejected. This ensures that the end product is flawless and reliable over a considerable length of time. The machine parts normally tested are for blow holes, cracks, etc, apart from some long duration tests, such as creep, fatigue, etc. Automatic Control of a Process/Operation The block diagram of a closed loop control system is shown in figure 1.2. This is one of the most important application areas of a measuring instrument. The deviation (called the error signal) between the reference input and the measured value is fed to a controller, the output of which drives a servo valve and an actuator. This closed loop system thus helps in maintaining the value of the measurand to a value in close proximity to the reference input value. Chapter 1 Fig. 1.2: Block diagram of a closed loop control system (for monitoring and control)

4 4 Measurement techniques in industrial instrumentation The automatic control system has widespread use in process industries like oil refineries, fertiliser plants, power plants, textile industries as also in sophisticated systems like radar tracking system, missile guidance, automatic aircraft landing, etc. 1.4 Classification of instruments It is based on method of energy conversion, nature of output signal and mode of operation. Different types of instruments are: Manual/automatic type Active/passive type Null/deflection type Analog/digital type Contact/non-contact type They are discussed as under: Manual/Automatic Type An instrument, which requires human intervention and operation for recording/displaying the value of the input (measurand), is manual in nature. Thermocouples and RTDs employed for measurement of temperature employ null type potentiometers and are examples of manual type. Employing an automatic self-balancing feedback device in the above will lead to an automatic type of instrument Active/Passive Type Active or power operated instruments require an auxiliary power source, such as electrical supply, compressed air, etc, for their operation. Examples of such instruments are linear variable differential transformer (LVDT), a float-type water tank level indicator, etc. When an LVDT is used for displacement measurements, its output changes, but it needs an auxiliary power across its primary for it to be operational. Similarly, in a water tank level indicator, the position of the wiper along the potentiometer changes when the water level changes. The output is taken from the wiper as shown in figure 1.3. In passive type instruments, the force/energy required for driving their indicator/output are derived from the measurand. Examples are a Bourdon gauge, a mercury-in-glass thermometer, To output (indicator) a Pitot tube, etc. Auxiliary power For active instruments, resolution can be Wiper source varied considerably while the same for passive ones can be varied in a limited manner. For example, in the case of a Bourdon type pressure gauge (a passive instrument), resolution can be increased to some extent by making the pointer longer such that the pointer tip can move through a longer arc. But obviously, there is a practical limit to an increased pointer length. In Float Tank Fig. 1.3: An example of an active instrument

5 Introduction To Measurements 5 the case of an active instrument, resolution can be increased by increasing say, the supply voltage. But in such a case, heat dissipation is a restraining factor. In general, passive instruments are generally cheaper and simpler in construction than their active counterparts Null/Deflection Type Null type instruments can be manual or automatic type. A dead weight tester is a null type instrument where the applied external pressure is balanced by adjusting weights placed on a pan. Another example of a null type instrument is that the thermo-emf developed in a thermocouple is balanced (nulled) by a potentiometer. A common Bourdon type pressure gauge is an example of a deflection type instrument. In this type of instrument, the physical effect produced by the measurand (pressure, temperature, etc.) is opposed by the moving parts of the instrument, the displacement of which is thus a measure of the measurand. Deflection type instruments are simple in construction and have good dynamic response. However, they interfere with the measurand and hence accuracy is low. Null type instruments are slow in response and hence their dynamic response is poor. But their accuracy is much more and they are normally used for calibration purposes Analog/Digital Type An analog instrument gives an output which is continuous or stepless in nature. Thus, the output can have an infinite number of values within the range of the instrument s span. Examples of analog instruments are a Bourdon type pressure gauge, a mercury-in-glass thermometer, a U-tube manometer, etc. Although theoretically an analog instrument can assume an infinite number of values, practically it is dependent on the scale length and its graduations. In digital instruments, the measurand (physical parameter or variable) is expressed digitally, i.e., by numbers. Thus, the display is discrete and it varies in steps. The measurand is sensed first and quantised (i.e., assigned a particular analog value) and then it is digitalised, say by means of an ADC (Analog to digital converter). It is then electronically processed, calibrated and the measurand is displayed digitally. Digital instruments have a number of advantages like ease in processing, noise immunity and direct compatibility with microprocessors/microcomputers, apart from data coding, error detection, error correction capability, if needed. Digital instruments are more reliable, requires less calibration but is a little more costly Contact/Non-contact Type A contact type instrument is kept in the measuring medium. A clinical thermometer is an example of a contact type instrument. Non-contact type instruments are placed at a distance from the measuring medium. An example is an optical pyrometer which measures very high temperature from a considerable distance. A variable reluctance tachometer, which measures the rpm of a rotating device, is also an example of a non-contact type of instrument. Chapter 1

6 6 Measurement techniques in industrial instrumentation 1.5 Measurement standards During the early days of civilisation, very crude methods for units of measurements were used. For example, human torso like foot or hand were used as standard for measurement of length. As human civilisation evolved over the years, better and accurate methods for different standards were tried out and improved upon. In 1960, a standard meter was defined in terms of wavelengths of the radiation from Krypton-86 in vacuum. Later on, in 1983, it was redefined as the length of path travelled by light in an interval of 1/ seconds. Similar improvements were also effected for other kinds of measurement units. For different types of applications and functions, different standards are specified. These are: International Standards, Primary Standards, Secondary Standards and Working Standards International Standards These are standards conforming to highest possible accuracy achievable using very advanced measurement technologies. These are maintained by International Bureau of Weights and Measures at Se vre s in France. These standards are maintained under recommended environment conditions and are not available to the ordinary user. International standards for Kilogram (M), length (L) and time (T) are prototype kilogram, wavelength of Krypton-86 orange-red lamp in vacuum and cesium clock respectively Primary Standards These are maintained by national laboratories/standard organisations. These are calibrated independently by absolute measurements and are used to calibrate and check secondary standards. Primary standards are also not available to the ordinary user for use Secondary Standards These are maintained by industrial units to act as basic reference standards. These are periodically checked and calibrated against primary standards. Secondary standards are available to the ordinary user for calibration and checking of their instruments Working Standards Working standards are commercially available in the market after their certification by primary or secondary standards. As for example, the industry working standard for length is the precision gauge block made of steel of specified compositions. This has a accuracy tolerance of 0.25 to 0.5 micron range. As other working standards, standard cells and standard resistors are available conforming to environmental specifications. These standards are very widely used for calibration of laboratory/field instruments, for checking products, etc. Table 1.1 shows definitions of standard units, while table 1.2 gives the details of fundamental, supplementary and derived quantities, their units and symbols.

7 Introduction To Measurements 7 Table 1.1: Definition of Standard Units Physical Quantity Standard Unit Definition Length metre The length of path travelled by light in an interval of 1/ seconds Mass kilogram The mass of a platinum iridium cylinder kept in the International Bureau of Weights and Measures, Sévres, Paris Time second cycles of radiation from vaporized caesium-133 (an accuracy of 1 in or 1 second in years) Temperature kelvin The temperature difference between absolute zero and the triple point of water is defined as kelvin Current ampere One ampere is the current flowing through two infinitely long parallel conductors of negligible cross-section placed 1 metre apart in vacuum and producing a force of Newtons per metre length of conductor. Luminous intensity candela One candela is the luminous intensity in a given direction from a source emitting monochromatic radiation at a frequency of 540 terahertz (Hz ) and with a radiant density in that direction of mw/steradian. (1 steradian is the solid angle which, having its vertex at the centre of a sphere, cuts off an area of the sphere surface equal to that of a square with sides of length equal to the sphere radius) Matter mole The number of atoms in a kg mass of carbon-12 Chapter 1 Table 1.2: Fundamental, Supplementary and Derived Units (a) Fundamental units Quantity Standard unit Symbol Length metre m Mass kilogram kg Time second s Electric current ampere A Temperature kelvin K Luminous intensity candela cd Matter mole mol (b) Supplementary fundamental units Quantity Standard unit Symbol Plane angle radian rad Solid angle steradian sr

8 8 Measurement techniques in industrial instrumentation (c) Derived units Quantity Standard unit Symbol Derivation formula Area square metre m 2 Volume cube metre m 3 Velocity metre per second m/s Acceleration metre per second squared m/s 2 Angular velocity radian per second ras/s Angular acceleration radian per second squared rad/s 2 Density kilogram per cubic metre kg/m 3 Specific volume cubic metre per kilogram m 3 /kg Mass flow rate kilogram per second kg/s Volume flow rate cubic metre per second m 3 /s Force newton N kg m/s 2 Pressure newton per square metre N/m 2 Torque newton metre Nm Momentum kilogram metre per second kg m/s Moment of inertia kilogram metre squared kg m 2 Kinematic viscosity square metre per second m 2 /s Dynamic viscosity newton second per sq. metre N s/m 2 Work, energy, heat joule J N m Specific energy joule per cubic metre J/m 3 Power watt W J/s Thermal conductivity watt per metre kelvin W/m K Electric charge coulomb C A s Voltage, e.m.f., pot. diff. volt V W/A Electric field strength volt per metre V/m Electric resistance ohm W V/A Electric capacitance farad F A s/v Electric inductance henry H V s/a Electric conductance siemen S A/V Resistivity ohm metre W m Permittivity farad per metre F/m Permeability henry per metre H/m Current density ampere per square metre A/m 2 Magnetic flux weber Wb V s Magnetic flux density tesla T Wb/m 2 Magnetic field strength ampere per metre A/m Frequency hertz Hz s 1 Luminous flux lumen lm cd sr Luminance candela per square metre cd/m 2 Illumination lux lx lm/m 2 Molar volume cubic metre per mole m 3 /mol Molarity mole per kilogram mol/kg Molar energy joule per mole J/mol

9 Introduction To Measurements 9 Review Questions 1. Draw the basic elements of a measuring system and discuss the individual blocks. 2. Mention the different application areas of a measuring instrument. Discuss any two of them. 3. State the basis on which instruments are classified and mention their types. Discuss the active and passive types of instruments. 4. What are the different standards of measurements? Discuss them. Fill in the blanks: 1. During the early part of civilisation, human hand was used as a measurement standard for The elements of a measuring system, in general, consist of... stages. 3. A closed loop automatic system maintains the value of the measurand very close to the... value. 4. Prototype of aircrafts are tested under... conditions. 5. A clinical type thermometer is an example of... type instrument. 6. LVDT is an example of... type instrument. Tick the correct answer: 1. Signal processing circuitry enhances the sensitivity/resolution/both of measurement. 2. An active instrument requires/does not require an auxiliary power source for their operation. 3. A Bourdon type pressure gauge is an example of a passive/active instrument. 4. In the case of an active instrument, resolution can be increased by increasing/decreasing the supply voltage. 5. Null type instruments are fast/slow in response. 6. A clinical thermometer is a contact/non-contact type instrument. 7. International standard of measurement is more/less accurate than working standard. Chapter 1