An instrument used for measurement of quantity (in terms of magnitude, etc) either it can be physical or electrical is said to a Measuring Instrument. Depending upon the type of quantity being measured there are three types of measuring instruments,
- Electrical Instruments
- Electronic Instruments
- Mechanical Instruments
The mechanical instruments can be used for measuring a physical quantity. These instruments have a poor response to time-varying quantities.
In this article let us see about various types of electrical measuring instruments. The main quantities in electrical are current, voltage, and power. By measuring these quantities we can also determine other electrical quantities and the performance of any electrical component.
The instruments used for measuring current, voltage, and power are ammeter, voltmeter, and wattmeter respectively. Depending on the types of work for which they are employed. Measuring instruments are classified as shown below.
Absolute Instruments :
These instruments are rarely used except in standard laboratories as standardizing instruments. These instruments don't require calibration or comparison with any other instrument. These instruments give the magnitude of the quantity to be measured in terms of the deflection and the instrument constant.
A good example of absolute instruments is the tangent galvanometer. In tangent galvanometer, the current to be measured is given by the product of galvanometer constant K, and tangent of deflection obtained during measurement. Mathematically, current I = K tanθ A, where K = galvanometer constant and θ = angle of deflection.
Secondary Instruments :
Secondary measuring instruments are the widely used measuring instruments. These instruments must be calibrated before being used. The measurement of the magnitude of the electrical quantity to be measured is obtained directly by taking the deflection from the instrument.
Secondary instruments are further classified into three groups depending on the end device used for measuring the desired quantity. They are,
- Indicating instruments
- Integrating instruments
- Recording instruments
Indicating Instruments :
This type of instrument will indicate the measuring quantities only at a particular instant. It cannot store any previous values. The indication of the measuring values are done with a pointer and scale in case of analog meters and with LCD (liquid crystal display) or LED (light-emitting diode) display in case of digital circuits. Examples of indicating instruments are voltmeters, ammeters, wattmeters, speedometers, etc.
For example, consider a dc voltmeter which has the range of 0-75 V. If the meter terminals are connected to dc supply of 50 V it will indicate 50 V, then if we change the supply to 20 V the pointer will immediately fall to 20 V point irrespective of the previous (50 V) value. So, they will just indicate the values instantaneously.
Recording Instruments :
Recording instruments by the name itself indicate that they record the values measured. These instruments are constructionally similar to indicating instruments but, the only variation is that the pointer and scale are replaced by a lightweight metal pencil and a carbon paper in the case of an analog meter. An example of these instruments is the power consumption recording meters at power stations.
Let us consider, the recording meters used at power stations. At the power stations, the operator must know the load demand at all times to operate the generators optimally.
For this, they use a recording instrument that records the power consumption and its variations, with respect to time throughout the day. By knowing the load demand variations the generator's input can be controlled to get maximum efficiency.
This instrument consists of a wattmeter and a metallic pencil (which moves continuously according to the power consumption and time) on carbon paper which gives a print on a graph sheet kept behind the carbon paper. Hence, the load variations throughout the day are found.
Instead of using this instrument, if we use an indicating instrument then a person should manually note down those load variations at each and every instant of time throughout the day which is a difficult task.
Integrating Instruments :
These instruments give the integration of the inputs applied over a particular period of time. Constructionally, there is a special arrangement of gears that drives the rings on which the numbers from O to 9 are marked. This is in the case of analog meters. While in the case of digital meters, a counter takes care of the integration. Examples of these instruments are speedometers, watt-hour meters, counters, etc.
Consider the watt-hour (energy) meter employed for domestic purposes. The disc inside the meter will rotate with a speed proportional to the power consumed at that particular instant of time.
The number of revolutions made by the disc is counted continuously by the special gear arrangement and it is displayed. So, the amount of power consumed is being added (i.e., integrated) over the specified period of time and thus, the reading gives the energy consumption during the period of consideration.
Depending upon the method of measurement or result obtained by the instrument. Measuring instruments are classified into two different types. They are,
- Deflection type instruments
- Null type instruments
Deflection Type Instruments :
The deflection type instruments are the direct reading instruments. They produce the deflection due to the physical action of the quantity being measured. The controlling system of the instrument opposes this deflection. When both the forces or torques are balanced, a steady deflection is obtained.
A PMMC, MI, electro-dynamometer, etc are examples of deflection type instruments. The various effects used in different types of deflection instruments are magnetic effect, electromagnetic induction effect, heating effect, chemical effect, and electrostatic effect.
Null Type Instruments :
Unlike deflection type instruments, in null type instruments, the quantity being measured can be determined from the null (i.e., zero) deflection. Initially, in this type of instrument also, a deflection will be produced due to the physical action of the quantity being measured. By properly adjusting other parameters of the circuit, the instrument is brought back to zero deflection.
The magnitude of the quantity being measured is calculated by observing the changes in circuit conditions or parameters. These type of instruments are more accurate, as it does not consume any power from the source being measured during zero deflection. Some of the examples of Null type instruments are tangent galvanometer, potentiometer, etc.