Classification of Electrical Instruments

Electrical Measuring Instruments – Definitions, Types

               In this post we will discuss about Classification of Electrical Instruments. But before proceeding further, it good to refresh the definitions and terms used in measurements, Instruments.

Definitions Used in Electrical Measuring Instruments:

True Value:
It is the exact value or the perfectly correct value in any measuring scheme.
It is defined as the average of infinite values taken when the average deviation due to various contributing factors approach zero.
True value is the one which we cannot reach my experiments. In actual practice true value is usually taken from a laboratory standard or obtained with all possible error-cancelling provisions.

Accuracy:
We cannot say any measurement will be exactly correct.
The term Accuracy is used to express how much is near the measured value to the true value. When we say the readings obtained are very accurate, it means the readings are true for all practical purposes.

Precision:
It tells the method which gives the best possible accuracy. When we say the instrument is precise instrument, it means that the instrument will give uniformly equal readings repeatedly, for a given quantity measurement.
In short, precision refers to the ability of an instrument to give consistent readings.

Error:
Using an electrical instrument what we measure is termed as actual value. The perfect reading is the true value. So error is defined as the difference between the measured value and true value.
Error =  Measured Value – True value.

Sensitivity:
It means the ability to feel or realise readily and accurately the slight change in the input quality. When an instrument is reacting to even a slight difference in the input quantity we can say that insturment is very sensitive.

Resolution:
In instrumentation, the resolution means the smallest changed in the input signal that can be detected by the instrument. The resolution is usually expressed as a fraction or percentage or full-scale value.

Instrument Efficiency:
It is defined as the ratio of measured quantity to the power taken by the instrument at full scale deflection.

Types of Electrical Measuring Instruments:

Electrical Instruments are basically classified into two types namely:

  1. Absolute Instruments
  2. Secondary Instruments

Absolute Instruments:
Definition:
Absolute instruments are defined as the instruments that give deflections in terms of physical constants and not measurable on a graduated scale.

  • The value of the electrical quantity to be measured are given by these instruments.
  • The quantity are measured in terms of constants and from the deflection of the instruments only.
  • Tangent galvanometer, Reyleigh current balance and absolute electrometer are some of the examples of absolute instruments.

Secondary Instruments:
Definition:
They are defined as the instruments that give a ready measure of the quantities with the help of graduated scales.

  • The value of the electrical quantity to be measured is determined from the deflection of these instruments.
  • With an absolute instrument these instruments are calibrated. ie, the secondary instruments are compared with known standards or absolute instruments.
  • All the electrical measuring instruments belong to this category.

There are three categories of secondary instruments.
1. Indicating instruments
2. Recording instruments
3. Integrating instruments

1. Indicating Instruments:
The value of the electrical quantity is indicated by these instruments at the time when it is being measured. Pointers moving over the scale give the indication.
Ammeters, Voltmeters and wattmeters are the examples of these instruments.

2. Recording Instruments:
A continuous record of variations of the electrical quantity over a long period of time is given by these instruments. It has a moving system which carries an inked pen which rests tightly on a graph chart.
Graphic recorders and galvanometer recorders are the examples of these instruments.

3. Integrating instruments
The total amount of either electricity or electrical energy supplied over a period of time is measured by these instruments.
Ampere hour meters, watt-hour meters, energy meters are the few examples of these instruments.

Based on Effects Used:
Every electrical instrument needs a force on the moving system to indicate or represent the quantity being measured. This force may be due to one of the several effects as;
Magnetic effect and Electromagnetic effect
Chemical Effect
Thermal Effect
Electrostatic Effect
Induction Effect
The secondary electrical instruments will be classified based on the above mentioned effects also.

Read More:
Wattmeter Reading Errors
How to connect the wattmeter to the circuit?
Cathode Ray Oscilloscope Tutorial
Types of wattmeters

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