VTU eNotes On Electrical & Electronic Measurements & Instrumentation (Electrical And Electronics Engineering)
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CHAPTER 1 UNITS AND DIMENSIONS 1.1 ART OF MEASUREMENTS Measurement of a quantity is the result of a comparison between the unknown quantity and its predefined standard. A measuring system is required to quantify the parameters involved and establish clear rules about their relative values. Early systems of measurements were based on imprecise units, while the modern measurement systems are based on accurately defined units. Many standard measuring units have been established at different levels of measurements. The significance of electrical measurements and measuring instruments is evident from the rapid developments in the field of electrical engineering owing to the developments in their measuring devices. The measurement is meaningful only if the standards are accurately defined, reliable and commonly accepted, the methods used are well proven and the circuit conditions are little affected by the introduction of the measuring systems. The parameter under measurement is referred as measurand and it is always measured in terms of its numeric value. The process of measurement through a meter is referred as Instrumentation. Electrical measuring instruments are the most common devices used not only for the measurement of electrical quantities, but also for all other quantities, which could be transformed into an electrical signal. The usual quantities to be measured are current, voltage, power, energy, frequency, power factor, etc. The various circuit parameters also need to be measured such as resistance, inductance, capacitance, etc. An umpteen number and varieties of measuring instruments have been developed for measurement purposes. In other words, measurement and instrumentation are based on different methods depending upon the characteristic features of the quantity being measured.
1.2 METHODS OF MEASUREMENTS AND MEASURING INSTRUMENTS The measurement methods can be analog or digital methods, deflection or null methods, active or passive methods, direct or indirect methods and absolute or secondary methods. Measurement generally involves an instrument as a physical means of determining an unknown quantity or a variable called the parameter. The instrument is a means for determining the value or magnitude of the measurand. The instruments can also be divided into separate classes according to several criteria as, analog or digital instruments, deflection or null type instruments, power operated active or self generating passive instruments, contacting or non-contacting instruments, mechanical or electrical instruments and monitoring or control instruments.
Signals which vary continuously with the change in the measurand are analog signals and the devices producing them are analog instruments. The deflection type dynamometer type wattmeter is a good example of an analog instrument. As the input