Standards and their classification

            A standard is a physical representation of a unit of measurement. It is a piece of equipment having a known measure of physical quantity. They are used for the measurements of other physical quantities by comparison methods.

            Standards of measurements can be classified in to;

                                            i.            International standards

                                          ii.            Primary standards

                                        iii.            Secondary standards

                                        iv.            Working standards

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1. International standards

        These are defined on the basis of international agreement. They represent the units of measurements which are closest to the possible accuracy attainable with present day technological and scientific methods. International standards are checked and evaluated regularly against absolute measurements in terms of the fundamental units. These standards are maintained at the International Bureau of Weights and Measures and are not available to the ordinary user of measuring instruments for the purposes of calibration or comparison.

            The international unit of length was defined in 1960 in terms of the wavelength of Krypton-86. The metre is equal to 1,650,763.73 wavelengths in vaccum of the orange-red light radiation of the Krypton-86 atom in its transition between levels 2 p₁₀ and 5d₅.

            In 1983, the metre was redefined. The metre is now, the length travelled by light in vaccum in a time interval of 1/299792458 sec.

2. Primary standards (Absolute standards)

The Primary standards are the absolute standards which can be used as the ultimate reference standards. These standards are maintained by National Standards Laboratories in different parts of the world. The primary standards which represent the fundamental units are independently calibrated by absolute measurements at each of the national laboratories. One of the main functions of the primary standards is the verifications and calibration of secondary standards.

The primary standards are very few in number. These standards have the highest possible accuracy and stability.

The primary unit of mass is a prototype kilogramme kept at National Physical Laboratories of every country. This has an accuracy of 1 part in 10⁸.

3. Secondary standards

    The secondary standards are the basic reference standards used in industrial measurement laboratories. They are sent periodically to the national standards laboratories for calibration and comparison against primary standards. The secondary standards are sent back to the industry by the National laboratories with a certification regarding their measured values in terms of primary standards.

            The secondary standards of mass are kept by industrial laboratories. These standards have an accuracy of 1 ppm and are checked against the primary standards.

4. Working standards

These standards are used to check and calibrate general laboratory instrument for their accuracy and performance.

The working standards of mass and length are available in a wide range of values so that, they suit any kind of application. The working standards of mass have an accuracy of 5 ppm and are checked against the secondary standards. The working standards of length are usually precision gauge blocks made up of steel. These blocks have two parallel surfaces and the distance between the two surfaces is specified. They have an accuracy of 1 ppm.

Classification of Instruments

            Instruments can be broadly classified in to

1. Absolute instruments
2. Secondary instruments

Absolute instruments give the magnitude of the quantity under measurement in terms of physical constants of the instruments.

e.g:- Tangent galvanometer, Rayleigh’s current balance.

In secondary instruments, the quantity under measurement can only be measured by observing the output of the instrument. The secondary instruments should be calibrated by comparing with an absolute instrument or another secondary instrument which has already been calibrated against an absolute instrument.

e.g:- Voltmeter, pressure gauge.

The secondary instruments are  the commonly  used instruments compared to the absolute instruments.

Electrical measuring instruments may be classified according to their functions as;

(i)                 Indicating instruments (ii)Integrating instruments (iii) Recording instruments

1. Indicating instruments:-

These instruments directly indicate the value of the electrical quantity at the time when it is being measured. In these instruments, a pointer moving over a graduated scale directly gives the value of the electrical quantity being measured.

e.g:- Ammeter, voltmeter, wattmeter.

2. Integrating instruments

The instruments which measure the total quantity of electricity (in Ampere hours ) or electrical energy (in Watt hours) in a given time are called integrating instruments. In such instruments, there are a set of dials and pointers which register the total quantity of electricity or electrical energy supplied to the load.

 e.g:- Ampere- Hour meter, Watt-hour meter.

3. Recording instruments

Recording instruments give a continuous record of the variations of the electrical quantity to be measured. A recording instrument is merely an indicating instrument with a pen attached to its pointer. The pen rests lightly on a chart wrapped over a drum moving with a slow uniform speed. The motion of the drum is in a direction perpendicular to the direction of the pointer. The path traced out by the pen indicates the manner in which the quantity being measured, has varied during the time of the record.

e.g:- Recording voltmeters, Recording ammeters in supply stations.

………………………………………………………………………………………………….. Reference:-

(i)                 A.K Sawhney, Electrical and Electronic Instrumentation and Measurements, page no:- 181-182]

(ii)               V.K Mehta, Rohit Mehta, Basic Electrical Engineering, page no:- 768

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