Broadly, the analog instrument (and for that matter digital instruments) may be classified according to the quantity they measure. For example, an instrument meant for measurement of current is classified as an Ammeter while an instrument that measures voltage is classified as a Voltmeter. In addition to above instruments, we have wattmeters, power factor meters, frequency meters etc.
Electrical instruments may also be ‘classified according to the kind of current that can be measured by them. Electrical instruments may be classified as instruments for :
- direct current (d.c.)
- alternating current (a.c.) and
- both direct and alternating current instruments (d.c./a.c.), these instruments are appropriately called universal instruments.
Analog instrument depend for their operation on one of the many effects produced by current and voltage and thus can be classified according to which of the effects is used for their working.
Analog instruments are also classified as :
- Indicating,
- Recording and
- Integrating.
Indicating instruments are those instruments which indicate the magnitude of a quantity being measured. They generally make use of a dial and a pointer for this purpose. Ordinary voltmeters, ammeters and wattmeters belong to this category.
The analog indicating instruments may be divided into two groups:
- electromechanical instruments,
- electronic instruments.
Electronic instruments are constructed by addition of electronic circuits to electromagnetic indicators in order to increase the sensitivity and input impedance.
Recording instruments give a continuous record of the quantity being measured over a specified period. The variations of the quantity being measured are recorded by a pen (attached to the moving system of the instrument; the moving system is operated by the quantity being measured) on a sheet of paper fixed or moving.
For example, we may have a recording voltmeter in a sub-station which keeps record of the variations of supply voltage during the day.
Integrating instruments totalize events over a specified period of time. The summation, which they give is the product of time and an electrical quantity. Ampere hour and watt hour (energy) meters are examples of this category.
The integration (summation value) is generally given by a register consisting of a set of a pointers and dials.
The analog instrument may also be classified on the basis of method used for comparing the unknown quantity (measured) with the unit of measurement. The two categories of instruments based upon this classification are :
- Direct measuring instruments : These instruments convert the energy of the measurand directly into energy that actuates the instrument and the value of the unknown quantity is measured or displayed or recorded directly. The examples of this class of instruments are ammeters, voltmeters, wattmeters and energy meters.
- Comparison instruments : These instruments measure the unknown quantity by comparison with a standard. (Direct measuring instruments are the most commonly used in engineering practice because they are the most simple and inexpensive. Also their use makes the measurement possible in the shortest time.) and a.c. bridges.
Comparison type instruments are used in cases where a higher accuracy of measurement is required.
Electrical instruments may also be classified according to their accuracy class. The limits of intrinsic error in the measured quantity for instruments for various classes of accuracy are :
| Accuracy class | 0.2 | 0.5 | 1.0 | 1.5 | 2.5 | 5 |
| Limit of error (percent) | ±0.2 | ±0.5 | ±1.0 | ±1.5 | ±2.5 | ±5 |
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