Unit-3.

quantities. The term has its origins in the art and science of scientific instrument-making Instrumentation involves both measurement and control functions. In technology, the development and use of precise measuring equipment. Although the sensory organs of the human body can be extremely sensitive and responsive, modern science and technology rely on the development of much more precise measuring and analytical tools for studying, monitoring, or controlling all kinds of phenomena. Instrumentation is the essential process control in industry. In industrial control a wide number of variables temperature, move, level, pressure, and distance can be sensed together. All of these can be interdependent factors in one processing need a sophisticated microprocessor system for total control. Instrumentation is the science of automated measurement and control. Instruments are used to measure and control the condition of process streams as they pass through a Plant. Principles of Instrumentation  Accessibility (operator's posture and patient positioning).  Visibility, illumination, and retraction.  Selection of proper instrument.  Condition of instruments (sharpness).  Maintaining a clean field.  Instrument stabilization.  Instrument activation. Instrument Instrument is a device or mechanism used to determine the present value of a quantity under observation Instruments are used to measure and control process variables such as: Temperature; Flow; Level; Pressure; Quality. The instrument used for measuring the physical and electrical quantities is known as the measuring instrument. Measurement Measurement is the process of determining the amount, degree, capacity by comparison (direct or indirect) with the accepted standards of the system units being used. The term measurement means the comparison between the two quantities of the same unit. Classification of Measuring Instruments The instrument used for measuring the physical and electrical quantities is known as the measuring instrument. The term measurement means the comparison between the two quantities of the same unit. The magnitude of one of the quantity is unknown, and it is compared with the predefined value. The result of the comparison obtained regarding numerical value. Active and Passive Instruments Diligence, Determination, and Discipline are Keys to Excellent Success Instruments are divided into active or passive ones according to whether the instrument output is entirely produced by the quantity being measured or whether the quantity being measured simply modulates the magnitude of some external power source. This is illustrated by examples. Passive Instrument An example of a passive instrument is the pressure-measuring device shown in Figure below. The pressure of the fluid is translated into a movement of a pointer against a scale. The energy expended in moving the pointer is derived entirely from the change in pressure measured: there are no other energy inputs to the system. Active Instrument An example of an active instrument is a float-type petrol tank level indicator as sketched in Figure below. Here, the change in petrol level moves a potentiometer arm, and the output signal consists of a proportion of the external voltage source applied across the two ends of the potentiometer. The energy in the output signal comes from the external power source: the primary transducer float system is merely modulating the value of the voltage from this external power source. In active instruments, the external power source is usually in electrical form, but in some cases, it can be other forms of energy such as a pneumatic or hydraulic one. One very important difference between active and passive instruments is the level of measurement resolution that can be obtained. With the simple pressure gauge shown, the amount of movement made by the pointer for a particular pressure change is closely defined by the nature of the instrument. Diligence, Determination, and Discipline are Keys to Excellent Success Comparison between Active and Passive Instruments While it is possible to increase measurement resolution by making the pointer longer, such that the pointer tip moves through a longer arc, the scope for such improvement is clearly restricted by the practical limit of how long the pointer can conveniently be. In an active instrument, however, adjustment of the magnitude of the external energy input allows much greater control over measurement resolution. While the scope for improving measurement resolution is much greater incidentally, it is not infinite because of limitations placed on the magnitude of the external energy input, in consideration of heating effects and for safety reasons. In terms of cost, passive instruments are normally of a more simple construction than active ones and are therefore cheaper to manufacture. Therefore, choice between active and passive instruments for a particular application involves carefully balancing the measurement resolution requirements against cost. Difference between Active and Passive Instruments Diligence, Determination, and Discipline are Keys to Excellent Success Types of Measuring Instrument The measuring instrument categorised into three types;  Electrical Instrument  Electronic Instrument  Mechanical Instrument The mechanical instrument uses for measuring the physical quantities. This instrument is suitable for measuring the static and stable condition because the instrument is unable to give the response to the dynamic condition. The electronic instrument has quick response time. The instrument provides the quick response as compared to the electrical and mechanical instrument. The electrical instrument is used for measuring electrical quantities likes current, voltage, power, etc. The ammeter, voltmeter, wattmeter are the examples of the electrical measuring instrument. The ammeter measures the current in amps; voltmeter measures voltage and Wattmeter are used for measuring the power. The classification of the electric instruments depends on the methods of representing the output reading. The response time of the electronic instrument is very high as compared to the electrical and mechanical device. Diligence, Determination, and Discipline are Keys to Excellent Success GENERAL CHARACTERISTICS OF MEASURING INSTRUMENTS The subject of performance criteria assumes major proportions when one is paced with the choice from commercially available ones of measuring instruments that would be most suited to a particular measurement task often, two performance characteristics come into consideration-the static and dynamic characteristics. The static characteristics define the performance yardstick for the measurement of such quantities which are constant, or vary only quite slowly the dynamic characteristic define the comparison between system input and system output for measured quantities that vary rapidly. It has been discovered in practice that the characteristics of one group sometimes influence that of the other. CLASSIFICATION OF INSTRUMENTS Instruments are broadly classified into three types such as indicating, recording and controlling instruments. These are given in the subsections below: INDICATING INSTRUMENTS These safer to the instruments in which the value of the measurand is indicated visually but not at all recorded. The instruments may be analogue or digital display types. a) Analogue Display Indication on this instrument is by means of a mechanical pointer. The final amplification is provided by the length of the pointer from its pivot to the scale friction may possibly develop at the errors very likely when this type of measuring instrument is used. Example of analogue display devices are bourdon tube pressure gauge, oscilloscopes moving coil meters, Huggen berger extensometer, scale and galvanometer in which an electrical signal is transmitted to mechanical movement of the pointer etc. Diligence, Determination, and Discipline are Keys to Excellent Success b) Digital Display This type of indicating instruments such that the final value of the measurement is expressed in numbers. Example are digital voltmeters and ammeters, digital voltmeters and ammeters, digital frequency, meters, digital thermometer etc. RECORDING INSTRUMENTS The operation of these instruments involves recording output of measurement on a chart, digital computer or data logger. Examples are D'Arsonval galvanometer (which has similar electro-mechanical movement as in analogue display out having the pointer replaced with a writing arm) pen recorders, Yt recorders, X-Y plotters, ultra violet light recorders. Also commonly used in the performance of instrumentation tasks is the Magnetic Tape Recorders. CONTROLLING INSTRUMENTS These instruments function in line with a fixed program. For instance, when a computer is programmed to perform a specific control function, it cease to be a generalpurpose device but it is dedicated instead to the control of a single piece of equipment, such an instrument is called controlling instrument or controller Robot arms, for example, work according to specified programs while there are certain instruments which execute these computer programs, these are controlling instruments or controllers. Different Types of Electrical Instrument Absolute Instrument The absolute instrument gives the value of measures quantities regarding the physical constant. The physical constant means the angle of deflection, degree and meter constant. The mathematical calculation requires for knowing the value of a physical constant. The tangent galvanometer is the examples of the absolute instruments. In tangent galvanometer, the magnitude of current passes through the coil determines by the tangent of the angle of deflection of their coil, the horizontal component of the earth magnetic field, radius and the number of turns of wire used. The most common applications of this type of instrument are found in laboratories.

Measuring instruments are the technical objects that are specially developed for the purpose of measuring specific quantities. A general property of measuring instruments is that their accuracy is known. Measuring instruments are divided into material measures, measuring transducers, indicating instruments, recording instruments, and measuring systems. A material measure is a measuring instrument that reproduces one or more known values of a given quantity. Examples of measures are balance weights, measuring resistors, measuring capacitors, and reference materials. Single-valued measures, multiple-valued measures, and collections of measures are distinguished. Examples of multiple-valued measures are graduated rulers, measuring tapes, resistance boxes, and so on. Multiple-valued measures are further divided into those that reproduce discrete values of the corresponding quantities, such as resistance boxes, and those that continuously reproduce quantities in some range, for example, a measuring capacitor with variable capacitance. Continuous measures are usually less accurate than discrete measures. When measures are used to perform measurements, the measurands are compared with the known quantities reproduced by the measures. The comparison is made by different methods, but so-called comparators are a specific means that are used to compare quantities. A comparator is a measuring device that makes it possible to compare similar quantities and has a known sensitivity. The simplest comparator is the standard equal-armed pan balance. In some cases, quantities are compared without comparators, by experimenters, with the help of their viewing or listening perceptions. For instance, when measuring the length of a body with the help of a ruler, the ruler is placed on the body and the observer fixes visually the graduations of the ruler (or fractions of a graduation) at the corresponding points of the body.