Basic classification of measuring instruments:
1. Mechanical instruments:- They are very reliable for static and stable conditions. The disadvantage is they are unable to respond rapidly to measurement of dynamic and transient conditions.
2. Electrical instruments:- Electrical methods of indicating the output of detectors are more rapid than mechanical methods. The electrical system normally depends upon a mechanical meter movement as indicating device.
3. Electronic instruments:- These instruments have very fast response. For example a cathode ray oscilloscope (CRO) is capable to follow dynamic and transient changes of the order of few nano seconds (10-9 sec).
Other classification of instruments:-
1. Absolute instruments or Primary Instruments:- These instruments gives the magnitude of quantity under measurement in terms of physical constants of the instrument e.g. Tangent Galvanometer. These instruments do not require comparison with any other standard instrument
• These instruments give the value of the electrical quantity in terms of absolute quantities (or some constants) of the instruments and their deflections.
• SSome of the examples of absolute instruments are:
* Tangent galvanometer
* Raleigh current balance
* Absolute electrometer
2. Secondary instruments:-These instruments are so constructed that the quantity being measured can only be determined by the output indicated by the instrument. These instruments are calibrated by comparison with an absolute instrument or another secondary instrument, which has already been calibrated against an absolute instrument.
with absolute instruments for routine work is time consuming since every time a
measurement is made, it takes a lot of time to compute the magnitude of
quantity under measurement. Therefore secondary instruments are most commonly
• They are direct reading instruments. The quantity to be measured by these instruments can be determined from the deflection of the instruments.
• The deflections obtained with secondary instruments will be meaningless untill it is not calibrated.
Classification of Secondary Instruments:
(a) Classification based on the various effects of electric current (or voltage) upon which their operation depend. They are:
• Magnetic effect: Used in ammeters, voltmeters, watt-meters, integrating meters etc.
(b) Classification based on the Nature of their Operations
We have the following instruments.
• Recording instruments: These instruments record continuously the variation of any electrical quantity with respect to time. In principle, these are indicating instruments but so arranged that a permanent continuous record of the indication is made on a chart or dial. The recording is generally made by a pen on a graph paper which is rotated on a dice or drum at a uniform speed. The amount of the quantity at any time (instant) may be read from the traced chart. Any variation in the quantity with time is recorded by these instruments. Any electrical quantity like current, voltage, power etc., (which may be measured lay the indicating instruments) may be arranged to be recorded by a suitable recording mechanism.
• Integrating instruments: These instruments record the consumption of the total quantity of electricity, energy etc., during a particular period of time. That is, these instruments totalize events over a specified period of time. No indication of the rate or variation or the amount at a particular instant are available from them. Some widely used integrating instruments are: Ampere-hour meter: kilowatthour (kWh) meter, kilovolt-ampere-hour
(c) Classification based on the Kind of Current that can be Measurand.
Under this heading, we have:
• Direct current (dc) instruments
• Alternating current (ac) instruments
• Both direct current and alternating current instruments (dc/ac instruments).
(d) Classification based on the method used.
Under this category, we have:
• Direct measuring instruments: These instruments converts the energy of the measured quantity directly into energy that actuates the instrument and the value of the unknown quantity is measured or displayed or recorded directly. These instruments are most widely used in engineering practice because they are simple and inexpensive. Also, time involved in the measurement is shortest. Examples are Ammeter, Voltmeter, Watt meter etc.
• Comparison instruments: These instruments measure the unknown quantity by comparison with a standard. Examples are dc and ac bridges and potentiometers. They are used when a higher accuracy of measurements is desired.