In this lecture, we are going to learn about Measuring instruments, necessary requirements of any measuring instruments, and then we will see the classification of measuring instruments in brief in this lecture. we have added the links for detailed knowledge of the mechanisms used in any instruments. so let’s start with the basic definition of measuring instruments.
The measurements of a given quantity are the result of a comparison between the quantity to be measured and the definite standard. The instruments which are used for such measurements are called measuring instruments.
The three basic quantities in the electrical measurement are current, voltage, and power. The measurements of these quantities are important as it is used for obtaining the measurement of some other quantity or used to test the performance of some electronic circuits or components etc.
The instrument which measures the current flowing in the circuit is called an ammeter while the instrument which measures the voltage across any two points of a circuit is called a voltmeter. But there is no fundamental difference in the operating principle of analog voltmeter and ammeter.
The action of almost all the analog ammeters and voltmeters depends on the deflecting torque produced by an electric current. In an ammeter such torque is proportional to the current to be measured In voltmeters this torque is decided by a current which is proportional to the voltage to e measured.
Thus all the analog ammeters and voltmeters are basically current measuring devices. The instruments which are used to measure the power are called power meters or wattmeters.
Necessary Requirements of Measuring Instruments
The necessary requirements of measuring instruments are:
- With the introduction of the instrument in the circuit, the circuit conditions should not be altered. Thus the quantity to be measured should not get affected due to the instruments used.
- The power consumed by the instruments for their operation should be as small as possible.
Classification of Measuring Instruments
Electrical measuring instruments are mainly classified as:
|1. Indicating Instruments
|2. Recodring Instruments
|3. Integrating Instruments
1. Indicating Instruments:
- These instruments make use of a dial and pointer for showing or indicating the magnitude of an unknown quantity. Examples are ammeters, voltmeters, etc.
2. Recording Instruments:
- These instruments give a continuous record of the given electrical quantity which is being measured over a specified period.
- The examples are various types of recorders. In such recording instruments, the reading is recorded by drawing a graph. The pointer of such instruments is provided with a maker i.e. pen or pencil, which moves on graph paper as per the reading. The X-Y plotter is the best example of such an instrument.
3. Integrating Instruments:
- These instruments measure the total quantity of electricity delivered over a period of time. For example, a household energy meter registers a number of revolutions made by the disc to give the total energy delivered, with the help of a counting mechanism consisting of dials and pointers.
Essential Requirements of an Instruments
In the case of measuring instruments, the effect of an unknown quantity is converted into a mechanical force which is transmitted to the pointer which moves over a calibrated scale. The moving system of such an instrument is mounted on a pivoted spindle. For satisfactory operation of any indicating instruments, the following system must be present in an instrument.
- Deflecting system producing deflecting torque Td
- Controlling system producing controlling torque Tc
- Damping system producing damping torque.
We will see all the torque in the later chapter in a very detailed manner.