An analog instrument is one in which the output or display is a continuous function of time and bears a constant relation to its input.
The analog instruments find extensive use in present-day applications although digital instruments are increasing in number and applications.
The areas of application which are common to both analog and digital instruments are fairly limited at present.
Hence, it can safely be predicted that the analog instruments will remain in extensive use for a number of years and are not likely to be completely replaced by digital instruments for certain applications.
Types of Analog Instruments
Broadly, the analog instruments (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. Thus 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 (DC)
- Alternating Current (AC)
- Direct and Alternating Current (DC/AC)
- Indicating instruments
- Recording instruments
- Integrating instruments
1. Indicating Instruments
Indicating instruments are those instruments that 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 the addition of electronic circuits to electromagnetic indicators in order to increase the sensitivity and input impedance.
2. Recording Instruments
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 carried by it rotating drum.
For example, we may have a recording voltmeter in a substation which keeps record of the variations of supply voltage during the day.
3. Integrating Instruments
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 at pointers and dials.
The analog instruments may also be classified on the basis of method used for comparing the unknown quantity (measurand) with the unit of measurement. The two categories of instruments based upon this classification are:
- Direct Measuring Instruments
- Comparison Instruments
a) Direct Measuring Instruments
Direct measuring 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 metres.
b) Comparison Instruments
Comparison 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
The examples of comparison type instruments are DC and AC bridges.
