ELECTRONIC MEASUREMENT AND INSTRUMENTATION CHARACTERISTICS OF INSTRUMENTS
Syllabus of ESE Electronic Measurement and Instrumentation Principles of measurement, accuracy, precision and standards; Analog and Digital Systems for measurement, Measuring instruments for different applications; Static/ Dynamic Characteristics of Measurement Systems, Errors, Statistical Analysis and Curve Fitting; Measurement systems for non electrical quantities; Basics of Telemetry; Different types of transducers and displays; Data acquisition system basics
Measurement & Instrument Measurement is a process of calculating the value of unknown parameter with the help of an instrument. The various parameters that we measure are Voltage, Current, Temperature, Pressure, Resistance, Capacitance, Inductance, Frequency, etc Essential Characteristics of Instruments The operational power consumption of the instrument should be negligible so that it doesn't consume any power The instrument should not alter the ambient conditions of the circuit in which it has been introd uced .
CHARACTERISTICS OF MEASUREMENT SYSTEMS The instrument characteristics must be known to choose an instrument that most suite to a particular measurement application. The performance characteristics may be broadly divided into two groups e Static Characteristics The set of characteristics defined for the instruments which are used to measure the quantities that are slowly varying with time or mostly constant Dynamic Characteristics The set of characteristics defined for the instruments which are used to measure the quantities that are rapidly changing with time.
STATIC CHARACTERISTICS OF INSTRUMENTS Accuracy: It is the closeness with which an instrument reading approaches the true value of the variable under measurement. Accuracy is always defined in terms of error Precision: It is a measure of repeatability, i.e. for a given variable, it is defined as a measure of the degree to which successive measurements differ from one another. Thus, the term accurate means one which is conforming to the truth ana precise means one which is sharply defined.
STATIC CHARACTERISTICS OF INSTRUMENTS Precision is composed of two characteristics: o Conformity Number of significant figures to which a measurement can be made Consider for example, that a resistor whose true value is 1,395,682 is measured by an ohmmeter which consistently and repeatedly reads 1.4 MQ. But there is an error in the value of the resistor owing to the limitation of the scale reading of the instrument and this is called as precision error. . Therefore, conformity is a necessary but not the necessary condition for precision because of the lack of the significant figures obtained.
STATIC CHARACTERISTICS OF INSTRUMENTS Error: It is the algebraic difference between the result of the measurement and the true value of the quantity being measured. Drift Error: It is the variation in the instrument output over a period of time which is not caused by any change of input; it may be caused by temperature variation and the instability of components. Uncertainty: The part of the result of a measurement which states the range of values within which the true value is estimated to lie. It is often expressed in the symmetric form x. . Resolution: This is the smallest increment in the measurement which can be detected with certainty by the instrument.
STATIC CHARACTERISTICS OF INSTRUMENTS hreshold: It is the minimum input which is necessary to cause a detectable change from the zero output. Sensitivity: It is the ratio of the change in the magnitude of instrument output to the corresponding change in the quantity under meaurement. Dead Zone: It is the largest change in the input quantity to which the instrument does not respond (i.e. the output doesn't change) Zero Stability: It is the ability of an instrument to return to zero reading when the measurand has returned to zero. Linearity: It is the closeness to which a curve between input & output approximates a straight line. Idealized Stnaight ling Outgmat Actusi Curve v iation Input
DYNAMIC CHARACTERISTICS OF INSTRUMENTS Speed of Response: It is defined as the rapidity with which a measurement system responds to changes in the measured quantity. Rise Time: It is defined as the time required for the output of a system to rise from 10% to 90% value for an over-damped system and from 0% to 100% value for an under-damped system. Settling Time: It is defined as the time required for the output to settle in the specified error band.