Measuring Instruments | GATE Notes & Videos for Electrical Engineering - Electrical Engineering (EE) PDF Download

 Page 1


 
 
 
 
 
  
INTRODUCTION TO MEASURING INSTRUMENTS 
 
 
1.1 Definition of instruments 
 
An instrument is a device in which we can determine the magnitude or value of the 
quantity to be measured. The measuring quantity can be voltage, current, power and energy etc. 
Generally instruments are classified in to two categories. 
 
 
Instrument 
 
 
 
Absolute Instrument Secondary Instrument 
 
 
 
1.2 Absolute instrument 
 
An absolute instrument determines the magnitude of the quantity to be measured in terms of the 
instrument parameter. This instrument is really used, because each time the value of the 
measuring quantities varies. So we have to calculate the magnitude of the measuring quantity, 
analytically which is time consuming. These types of instruments are suitable for laboratory use. 
Example: Tangent galvanometer. 
 
1.3 Secondary instrument 
 
This instrument determines the value of the quantity to be measured directly. Generally these 
instruments are calibrated by comparing with another standard secondary instrument. 
 
Examples of such instruments are voltmeter, ammeter and wattmeter etc. Practically 
secondary instruments are suitable for measurement. 
 
 
Secondary instruments 
 
 
 
 
 
Indicating instruments Recording Integrating Electromechanically Indicating 
instruments 
 
 
 
 
 
 
 
 
 
Page 2


 
 
 
 
 
  
INTRODUCTION TO MEASURING INSTRUMENTS 
 
 
1.1 Definition of instruments 
 
An instrument is a device in which we can determine the magnitude or value of the 
quantity to be measured. The measuring quantity can be voltage, current, power and energy etc. 
Generally instruments are classified in to two categories. 
 
 
Instrument 
 
 
 
Absolute Instrument Secondary Instrument 
 
 
 
1.2 Absolute instrument 
 
An absolute instrument determines the magnitude of the quantity to be measured in terms of the 
instrument parameter. This instrument is really used, because each time the value of the 
measuring quantities varies. So we have to calculate the magnitude of the measuring quantity, 
analytically which is time consuming. These types of instruments are suitable for laboratory use. 
Example: Tangent galvanometer. 
 
1.3 Secondary instrument 
 
This instrument determines the value of the quantity to be measured directly. Generally these 
instruments are calibrated by comparing with another standard secondary instrument. 
 
Examples of such instruments are voltmeter, ammeter and wattmeter etc. Practically 
secondary instruments are suitable for measurement. 
 
 
Secondary instruments 
 
 
 
 
 
Indicating instruments Recording Integrating Electromechanically Indicating 
instruments 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1.3.1 Indicating instrument 
 
This instrument uses a dial and pointer to determine the value of measuring quantity. The pointer 
indication gives the magnitude of measuring quantity. 
 
 
1.3.2 Recording instrument 
 
This type of instruments records the magnitude of the quantity to be measured continuously over 
a specified period of time. 
 
1.3.3 Integrating instrument 
 
This type of instrument gives the total amount of the quantity to be measured over a specified 
period of time. 
 
1.3.4 Electromechanical indicating instrument 
 
For satisfactory operation electromechanical indicating instrument, three forces are 
necessary. They are 
 
(a) Deflecting force 
 
(b) Controlling force 
(c)Damping force 
 
1.4 Deflecting force 
 
When there is no input signal to the instrument, the pointer will be at its zero position. To deflect 
the pointer from its zero position, a force is necessary which is known as deflecting force. A 
system which produces the deflecting force is known as a deflecting system. Generally a 
deflecting system converts an electrical signal to a mechanical force. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.1 Pointer scale 
 
 
 
 
 
Page 3


 
 
 
 
 
  
INTRODUCTION TO MEASURING INSTRUMENTS 
 
 
1.1 Definition of instruments 
 
An instrument is a device in which we can determine the magnitude or value of the 
quantity to be measured. The measuring quantity can be voltage, current, power and energy etc. 
Generally instruments are classified in to two categories. 
 
 
Instrument 
 
 
 
Absolute Instrument Secondary Instrument 
 
 
 
1.2 Absolute instrument 
 
An absolute instrument determines the magnitude of the quantity to be measured in terms of the 
instrument parameter. This instrument is really used, because each time the value of the 
measuring quantities varies. So we have to calculate the magnitude of the measuring quantity, 
analytically which is time consuming. These types of instruments are suitable for laboratory use. 
Example: Tangent galvanometer. 
 
1.3 Secondary instrument 
 
This instrument determines the value of the quantity to be measured directly. Generally these 
instruments are calibrated by comparing with another standard secondary instrument. 
 
Examples of such instruments are voltmeter, ammeter and wattmeter etc. Practically 
secondary instruments are suitable for measurement. 
 
 
Secondary instruments 
 
 
 
 
 
Indicating instruments Recording Integrating Electromechanically Indicating 
instruments 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1.3.1 Indicating instrument 
 
This instrument uses a dial and pointer to determine the value of measuring quantity. The pointer 
indication gives the magnitude of measuring quantity. 
 
 
1.3.2 Recording instrument 
 
This type of instruments records the magnitude of the quantity to be measured continuously over 
a specified period of time. 
 
1.3.3 Integrating instrument 
 
This type of instrument gives the total amount of the quantity to be measured over a specified 
period of time. 
 
1.3.4 Electromechanical indicating instrument 
 
For satisfactory operation electromechanical indicating instrument, three forces are 
necessary. They are 
 
(a) Deflecting force 
 
(b) Controlling force 
(c)Damping force 
 
1.4 Deflecting force 
 
When there is no input signal to the instrument, the pointer will be at its zero position. To deflect 
the pointer from its zero position, a force is necessary which is known as deflecting force. A 
system which produces the deflecting force is known as a deflecting system. Generally a 
deflecting system converts an electrical signal to a mechanical force. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.1 Pointer scale 
 
 
 
 
 
 
  
 
 
1.4.1 Magnitude effect 
 
When a current passes through the coil (Fig.1.2), it produces a imaginary bar magnet. When a 
soft-iron piece is brought near this coil it is magnetized. Depending upon the current direction 
the poles are produced in such a way that there will be a force of attraction between the coil and 
the soft iron piece. This principle is used in moving iron attraction type instrument. 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.2 
 
If two soft iron pieces are place near a current carrying coil there will be a force of repulsion 
between the two soft iron pieces. This principle is utilized in the moving iron repulsion type 
instrument. 
 
1.4.2 Force between a permanent magnet and a current carrying coil 
 
When a current carrying coil is placed under the influence of magnetic field produced by a 
permanent magnet and a force is produced between them. This principle is utilized in the moving 
coil type instrument. 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.3 
 
1.4.3 Force between two current carrying coil 
 
When two current carrying coils are placed closer to each other there will be a force of repulsion 
between them. If one coil is movable and other is fixed, the movable coil will move away from 
the fixed one. This principle is utilized in electrodynamometer type instrument. 
 
 
 
 
 
 
 
 
Page 4


 
 
 
 
 
  
INTRODUCTION TO MEASURING INSTRUMENTS 
 
 
1.1 Definition of instruments 
 
An instrument is a device in which we can determine the magnitude or value of the 
quantity to be measured. The measuring quantity can be voltage, current, power and energy etc. 
Generally instruments are classified in to two categories. 
 
 
Instrument 
 
 
 
Absolute Instrument Secondary Instrument 
 
 
 
1.2 Absolute instrument 
 
An absolute instrument determines the magnitude of the quantity to be measured in terms of the 
instrument parameter. This instrument is really used, because each time the value of the 
measuring quantities varies. So we have to calculate the magnitude of the measuring quantity, 
analytically which is time consuming. These types of instruments are suitable for laboratory use. 
Example: Tangent galvanometer. 
 
1.3 Secondary instrument 
 
This instrument determines the value of the quantity to be measured directly. Generally these 
instruments are calibrated by comparing with another standard secondary instrument. 
 
Examples of such instruments are voltmeter, ammeter and wattmeter etc. Practically 
secondary instruments are suitable for measurement. 
 
 
Secondary instruments 
 
 
 
 
 
Indicating instruments Recording Integrating Electromechanically Indicating 
instruments 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1.3.1 Indicating instrument 
 
This instrument uses a dial and pointer to determine the value of measuring quantity. The pointer 
indication gives the magnitude of measuring quantity. 
 
 
1.3.2 Recording instrument 
 
This type of instruments records the magnitude of the quantity to be measured continuously over 
a specified period of time. 
 
1.3.3 Integrating instrument 
 
This type of instrument gives the total amount of the quantity to be measured over a specified 
period of time. 
 
1.3.4 Electromechanical indicating instrument 
 
For satisfactory operation electromechanical indicating instrument, three forces are 
necessary. They are 
 
(a) Deflecting force 
 
(b) Controlling force 
(c)Damping force 
 
1.4 Deflecting force 
 
When there is no input signal to the instrument, the pointer will be at its zero position. To deflect 
the pointer from its zero position, a force is necessary which is known as deflecting force. A 
system which produces the deflecting force is known as a deflecting system. Generally a 
deflecting system converts an electrical signal to a mechanical force. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.1 Pointer scale 
 
 
 
 
 
 
  
 
 
1.4.1 Magnitude effect 
 
When a current passes through the coil (Fig.1.2), it produces a imaginary bar magnet. When a 
soft-iron piece is brought near this coil it is magnetized. Depending upon the current direction 
the poles are produced in such a way that there will be a force of attraction between the coil and 
the soft iron piece. This principle is used in moving iron attraction type instrument. 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.2 
 
If two soft iron pieces are place near a current carrying coil there will be a force of repulsion 
between the two soft iron pieces. This principle is utilized in the moving iron repulsion type 
instrument. 
 
1.4.2 Force between a permanent magnet and a current carrying coil 
 
When a current carrying coil is placed under the influence of magnetic field produced by a 
permanent magnet and a force is produced between them. This principle is utilized in the moving 
coil type instrument. 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.3 
 
1.4.3 Force between two current carrying coil 
 
When two current carrying coils are placed closer to each other there will be a force of repulsion 
between them. If one coil is movable and other is fixed, the movable coil will move away from 
the fixed one. This principle is utilized in electrodynamometer type instrument. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.4 
 
1.5 Controlling force 
 
To make the measurement indicated by the pointer definite (constant) a force is necessary which 
will be acting in the opposite direction to the deflecting force. This force is known as controlling 
force. A system which produces this force is known as a controlled system. When the external 
signal to be measured by the instrument is removed, the pointer should return back to the zero 
position. This is possibly due to the controlling force and the pointer will be indicating a steady 
value when the deflecting torque is equal to controlling torque. 
 
T
d
  ? T
c 
(1.1) 
 
1.5.1 Spring control 
 
Two springs are attached on either end of spindle (Fig. 1.5).The spindle is placed in jewelled 
bearing, so that the frictional force between the pivot and spindle will be minimum. Two springs 
are provided in opposite direction to compensate the temperature error. The spring is made of 
phosphorous bronze. 
 
When a current is supply, the pointer deflects due to rotation of the spindle. While spindle is 
rotate, the spring attached with the spindle will oppose the movements of the pointer. The torque 
produced by the spring is directly proportional to the pointer deflection? . 
 
T
C
  ? ? 
(1.2) 
 
The deflecting torque produced T
d
 proportional to =I‘. When T
C
 ? T
d
 , the pointer will come to 
a steady position. Therefore 
? ? I (1.3) 
 
 
 
 
 
 
 
 
 
 
Page 5


 
 
 
 
 
  
INTRODUCTION TO MEASURING INSTRUMENTS 
 
 
1.1 Definition of instruments 
 
An instrument is a device in which we can determine the magnitude or value of the 
quantity to be measured. The measuring quantity can be voltage, current, power and energy etc. 
Generally instruments are classified in to two categories. 
 
 
Instrument 
 
 
 
Absolute Instrument Secondary Instrument 
 
 
 
1.2 Absolute instrument 
 
An absolute instrument determines the magnitude of the quantity to be measured in terms of the 
instrument parameter. This instrument is really used, because each time the value of the 
measuring quantities varies. So we have to calculate the magnitude of the measuring quantity, 
analytically which is time consuming. These types of instruments are suitable for laboratory use. 
Example: Tangent galvanometer. 
 
1.3 Secondary instrument 
 
This instrument determines the value of the quantity to be measured directly. Generally these 
instruments are calibrated by comparing with another standard secondary instrument. 
 
Examples of such instruments are voltmeter, ammeter and wattmeter etc. Practically 
secondary instruments are suitable for measurement. 
 
 
Secondary instruments 
 
 
 
 
 
Indicating instruments Recording Integrating Electromechanically Indicating 
instruments 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1.3.1 Indicating instrument 
 
This instrument uses a dial and pointer to determine the value of measuring quantity. The pointer 
indication gives the magnitude of measuring quantity. 
 
 
1.3.2 Recording instrument 
 
This type of instruments records the magnitude of the quantity to be measured continuously over 
a specified period of time. 
 
1.3.3 Integrating instrument 
 
This type of instrument gives the total amount of the quantity to be measured over a specified 
period of time. 
 
1.3.4 Electromechanical indicating instrument 
 
For satisfactory operation electromechanical indicating instrument, three forces are 
necessary. They are 
 
(a) Deflecting force 
 
(b) Controlling force 
(c)Damping force 
 
1.4 Deflecting force 
 
When there is no input signal to the instrument, the pointer will be at its zero position. To deflect 
the pointer from its zero position, a force is necessary which is known as deflecting force. A 
system which produces the deflecting force is known as a deflecting system. Generally a 
deflecting system converts an electrical signal to a mechanical force. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.1 Pointer scale 
 
 
 
 
 
 
  
 
 
1.4.1 Magnitude effect 
 
When a current passes through the coil (Fig.1.2), it produces a imaginary bar magnet. When a 
soft-iron piece is brought near this coil it is magnetized. Depending upon the current direction 
the poles are produced in such a way that there will be a force of attraction between the coil and 
the soft iron piece. This principle is used in moving iron attraction type instrument. 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.2 
 
If two soft iron pieces are place near a current carrying coil there will be a force of repulsion 
between the two soft iron pieces. This principle is utilized in the moving iron repulsion type 
instrument. 
 
1.4.2 Force between a permanent magnet and a current carrying coil 
 
When a current carrying coil is placed under the influence of magnetic field produced by a 
permanent magnet and a force is produced between them. This principle is utilized in the moving 
coil type instrument. 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.3 
 
1.4.3 Force between two current carrying coil 
 
When two current carrying coils are placed closer to each other there will be a force of repulsion 
between them. If one coil is movable and other is fixed, the movable coil will move away from 
the fixed one. This principle is utilized in electrodynamometer type instrument. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.4 
 
1.5 Controlling force 
 
To make the measurement indicated by the pointer definite (constant) a force is necessary which 
will be acting in the opposite direction to the deflecting force. This force is known as controlling 
force. A system which produces this force is known as a controlled system. When the external 
signal to be measured by the instrument is removed, the pointer should return back to the zero 
position. This is possibly due to the controlling force and the pointer will be indicating a steady 
value when the deflecting torque is equal to controlling torque. 
 
T
d
  ? T
c 
(1.1) 
 
1.5.1 Spring control 
 
Two springs are attached on either end of spindle (Fig. 1.5).The spindle is placed in jewelled 
bearing, so that the frictional force between the pivot and spindle will be minimum. Two springs 
are provided in opposite direction to compensate the temperature error. The spring is made of 
phosphorous bronze. 
 
When a current is supply, the pointer deflects due to rotation of the spindle. While spindle is 
rotate, the spring attached with the spindle will oppose the movements of the pointer. The torque 
produced by the spring is directly proportional to the pointer deflection? . 
 
T
C
  ? ? 
(1.2) 
 
The deflecting torque produced T
d
 proportional to =I‘. When T
C
 ? T
d
 , the pointer will come to 
a steady position. Therefore 
? ? I (1.3) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.5 
 
Since, ? and I are directly proportional to the scale of such instrument which uses spring 
controlled is uniform. 
 
1.6 Damping force 
 
The deflection torque and controlling torque produced by systems are electro mechanical. 
Due to inertia produced by this system, the pointer oscillates about it final steady position before 
coming to rest. The time required to take the measurement is more. To damp out the oscillation 
is quickly, a damping force is necessary. This force is produced by different systems. 
 
(a) Air friction damping 
 
(b) Fluid friction damping 
 
(c) Eddy current damping 
 
1.6.1 Air friction damping 
 
The piston is mechanically connected to a spindle through the connecting rod (Fig. 1.6). The 
pointer is fixed to the spindle moves over a calibrated dial. When the pointer oscillates in 
clockwise direction, the piston goes inside and the cylinder gets compressed. The air pushes the 
piston upwards and the pointer tends to move in anticlockwise direction.