Measurement is the basic means of acquiring knowledge about the parameters and variables involved in the operation of a physical system. Measurement generally involves using an instrument as a physical means of determining a quantity or variable.
An instrument or a measuring instrument is, therefore, defined as a device for determining the value or magnitude of a quantity or variable. The electrical measuring insturment, as its name implies, is based on electrical principles for its measurement function.
These days a number of measuring instruments, both analog as well as digital ones, are available for the measurement of electrical quantities like voltage, current, power energy, frequency, power factor, etc. First Analog devices are worth concerning.
Analog instruments may be divided into three groups:
(a) Electromechanical instruments;
(b) Electronic instruments which are often constructed by the addition of electronic circuits to electromechanical indicators thus increasing their sensitivity and input impedances; and
© Graphical instruments which are electromechanical and electronic instruments having a modified display arrangement so that a graphical trace, that is, a display of instantaneous values against time is obtained.
Measurement:- The measurement of a given quantity is an act or the of comparison between the quantity whose magnitude is unknown and a predefined standard. Since two quantities are compared the result is expressed in the form of numerical values.There are two methods of measurement
(i) Direct method
(ii) Indirect method.
Direct Measurement :- In direct method the unknown quantity is measured directly such as measurement of current by an ammeter, voltage by voltmeter, resistance by ohm meter, power by wattmeter etc.
Indirect Measurement:- In the indirect method of measurement the unknown quantity is determined by measuring other functionally related quantities and calculating the desired quantity rather than measuring it directly with an instrument.
For example resistance of a conductor may be determined by measuring voltage across the conductor V, and current flowing through the conductor I, and then calculating it by R=V/I
Important definitions relevant to instruments will be discussed first. Measurement work employs a number of terms which are defined below:
Measurand: The quantity or variable being measured is called measurand or measurement variable.
Accuracy: It is defined in terms of the closeness with which an instrument reading approaches the true or expected (desired) value of the variable being measured.
Precision: It is measure of the consistency of reproducibility (repeatability) of the measurement (i.e., the successive reading do not differ). For a given fixed value of an input variable, precision is a measure of the degree to which successive measurement differ from one another.
Sensitivity: It is defined by the change in the output or response of the instrument for a unit change of input or measured variable.
Resolution: Resolution is the smallest change in a measured variable (or measurand) to which the instrument will respond.
True or Expected Value:
The true or expected value of a quantity to be measured may be defined as the average of an infinite number of measured values when the average deviation due to the various contributing factors tends to zero.
It also refers to a value of the quanity under consideration that would be obtained by a method (known as exemplar method) agreed upon by experts. In other words, it is the most probable value that calculations indicate and one should expect to measure.
Note that the value of the unknown obtained by making use of primary standards and measuring instruments is considered to be its ture value.
Error:
It is the deviation of the measured (or indicated) value from the true (or expected) value of a quantity. In other words, error is the difference between the measured value and the true value of the unknown quantity. It is also called absolute error are maximum possible error.
Correction:
The difference between the true value and the measured value of the sought quantity is defined as the reading correction or simply correction. That is, correction is negative or error. Thus,
Therefore addition of correction in measured value gives the true (or accurate or expected) value.
Bandwidth :
The bandwidth of an instrument relates to the maximum range of frequency over which it is suitable for use. It is normally quoted in terms of 3 dB (dB = decibel) point. For an amplifier, it is the range of frequencies between which the gain or amplitude ratio is constant to within 3 dB (this corresponds 30% reduction in gain).
Significant Figures:
An indication of the precision of the measurement is obtained from the number of significant figures in which it is expressed. Significant figures convey actual information regarding the magnitude and the measurement precision of a quantity. The more is the significant figures, the greater will be the precision of measurement.
For example, if a resistor is specified as having a resistance of 105 Ω and 105.3 Ω, than in 105 Ω there are three significant figures whereas in 105.3 Ω there are four. The later with more significant figures, expresses a measurement of greater precision than the former.