DEFINITIONS OF SOME STATIC CHARACTERISTICS
1.Accuracy:
Accuracy is the closeness with which the instrument reading approaches the true value of the variable under measurement. Accuracy is determined as the maximum amount by which the result differs from the true value. It is almost impossible to determine experimentally the true value. The true value is not indicated by any measurement system due to the loading effect, lags and mechanical problems (e.g., wear, hysteresis, noise, etc.).
Accuracy of the measured signal depends upon the following factors:
- Intrinsic accuracy of the instrument itself
- Accuracy of the observer
- Variation of the signal to be measured
- Whether or not the quantity is being truly impressed upon the instrument.
2.Precision:
Precision is a measure of the reproducibility of the measurements, i.e., precision is a measure of the degree to which successive measurements differ from one another. Precision is indicated from the number of significant figures in which it is expressed. Significant figures actually convey the information regarding the magnitude and the measurement precision of a quantity. More significant figures imply greater precision of the measurement
3.Resolution:
If the input is slowly increased from some arbitrary value it will be noticed that the output does not
change at all until the increment exceeds a certain value called the resolution or discrimination of the
instrument. Thus, the resolution or discrimination of any instrument is the smallest change in the
input signal (quantity under measurement) which can be detected by the instrument. It may be
expressed as an accrual value or as a fraction or percentage of the full scale value. Resolution is
sometimes referred as sensitivity. The largest change of input quantity for which there is no output
of the instrument is called the dead zone of that instrument.
The sensitivity gives the relation between the input signal to an instrument or a part of the instrument
system and the output. Thus, the sensitivity is defined as the ratio of output signal or response of the
instrument to a change of input signal or the quantity under measurement.
4.Speed of Response:
The quickness of an instrument to read the measured variable is called the speed of response.
Alternately, speed of response is defined as the time elapsed between the start of the measurement
to the reading taken. This time depends upon the mechanical moving system, friction, etc.
The measuring instruments may be classified as follows:
1.Absolute and Secondary Instruments:
Absolute Instruments:
The instruments of this type give the value of the measured in terms of instrument constant and its
deflection. Such instruments do not require comparison with any other standard. The example of this
type of instrument is tangent galvanometer, which gives the value of the current to be measured in
terms of tangent of the angle of deflection produced, the horizontal component of the earth’s
magnetic field, the radius and the number of turns of the wire used. Rayleigh current balance and
absolute electrometer are other examples of absolute instruments. Absolute instruments are mostly
used in standard laboratories and in similar institutions as standardizing.
Secondary Instruments:
These instruments are so constructed that the deflection of such instruments gives the magnitude of
the electrical quantity to be measured directly. These instruments are required to be calibrated by
comparison with either an absolute instrument or with another secondary instrument, which has
already been calibrated before the use. These instruments are generally used in practice.
Measuring instruments (i.e. secondary instruments) may be classified according to their function as:
1.Indicating instruments
2.Integrating instruments
3.Recording instruments
4.Controlling instruments
1. Indicating Instruments: These instruments indicate the magnitude of electrical quantity at the time when it is being measured. The indication are given by a pointer moving over a scale. Ammeters, Voltmeter, Watt meter, Frequency meter are the example of these instruments.
These days another class of indicating instruments has come up which is known as Digital meter. In this can a particular value of quantity is indicated on the dial in the form of illuminated digits.
2.Recording Instruments: These instruments keep a continuous record of the variations of the magnitude of an electrical quantity to be observed over a definite time. Generally in this case an inked pointer marks a continuous graph on a paper. Such instruments are generally used in power houses.
3. Integrating Instruments: These instruments measure the total quantity of electricity or electrical energy(in KWH) in a given period. The ampere hour meter and kilometer hour meter fall into class.
In such instruments, there are sets of dials or gear which registers the total quantity of electricity or the total amount of electricity energy supplied to a circuit in a given time.
4.Controlling Instruments: These type of instruments are preferably used in the field of industrial controls. In this case, the information is used by the instruments to control the original measured quantity. The Examples of such controlling devices are thermostats for controlling the temperature etc.
2.Analog and Digital Instruments
1. Analog Instruments
The signals of an analog unit vary in a continuous fashion and can take on infinite number of values
in a given range. Fuel gauge, ammeter and voltmeters, wrist watch, speedometer fall in this category.
2. Digital Instruments
Signals varying in discrete steps and taking on a finite number of different values in a given range are
digital signals and the corresponding instruments are of digital type. Digital instruments have some
advantages over analog meters, in that they have high accuracy and high speed of operation. It
eliminates the human operational errors. Digital instruments can store the result for future purposes.
A digital multimeter is the example of a digital instrument.
3.Deflection and Null Output Instruments:
In a deflection-type instrument, the deflection of the instrument indicates the measurement of the
unknown quantity. The measured quantity produces some physical effect which deflects or produces
a mechanical displacement in the moving system of the instrument. An opposite effect is built in the
instrument which opposes the deflection or the mechanical displacement of the moving system. The
balance is achieved when opposing effect equals the actuating cause producing the deflection or the
mechanical displacement. The deflection or the mechanical displacement at this point gives the value
of the unknown input quantity. These types of instruments are suited for measurement under
dynamic condition. Permanent Magnet Moving Coil (PMMC), Moving Iron (MI), etc., type instruments
are examples of this category.
In null-type instruments, a zero or null indication leads to determination of the magnitude of the
measured quantity. The null condition depends upon some other known conditions. These are more
accurate and highly sensitive as compared to deflection-type instruments. A dc potentiometer is a
null- type instrument.
OPERATING TORQUES:
Three types of torques are needed for satisfactory operation of any indicating instrument. These are
- Deflecting torque
- Controlling torque
- Damping torque
1.Deflecting Torque/Force:
Any instrument’s deflection is found by the total effect of the deflecting torque/force, control torque/
force and damping torque/force. The deflecting torque’s value is dependent upon the electrical signal
to be measured; this torque/force helps in rotating the instrument movement from its zero position.
The system producing the deflecting torque is called the deflecting system.
2.Controlling Torque/Force:
The act of this torque/force is opposite to the deflecting torque/force. When the deflecting and controlling torques are equal in magnitude then the movement will be in definite position or in equilibrium. Spiral springs or gravity is usually given to produce the controlling torque. The system which produces the controlling torque is called the controlling system.
The functions of the controlling system are
- To produce a torque equal and opposite to the deflecting torque at the final steady position of the pointer in order to make the deflection of the pointer definite for a particular magnitude of current.
- To bring the moving system back to its zero position when the force causing the instrument moving system to deflect is removed.
The controlling torque in indicating instruments is almost always obtained by a spring, much less commonly, by gravity.
Damping Torque/Force:
A damping force generally works in an opposite direction to the movement of the moving system.
This opposite movement of the damping force, without any oscillation or very small oscillation brings
the moving system to rest at the final deflected position quickly. Air friction, fluid friction and eddy
currents provide the damping torque/force to act. It must also be noted that not all damping force
affects the steady-state deflection caused by a given deflecting force or torque. With the angular
velocity of the moving system, the intensity of the damping force rises; therefore, its effect is greatest
when it rotates rapidly and zero when the system rotation is zero. In the description of various types
of instruments, detailed mathematical expressions for the damping torques are taken into
consideration.
When the deflecting torque is much greater than the controlling torque, the system is called
underdamped. If the deflecting torque is equal to the controlling torque, it is called critically damped.
When deflecting torque is much less than the controlling torque, the system is under overdamped
condition.
There are three systems of damping generally used. These are as follows:
- Air-friction damping
- Fluid-friction damping
- Eddy-current damping
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.
Air Friction Damping:
In this method, a light aluminum piston is attached to the moving system and moves in an air chamber
closed at one end. The cross-section of this chamber may be either circular or rectangular. The
clearance between the piston and the sides of the chamber should be small and uniform. If the piston
is moving rapidly into the chamber, the air in the closed space is compressed and the pressure
opposes the motion of the piston (and, therefore, of the whole moving system).
Eddy current damping :
An aluminum circular disc is fixed to the spindle. This disc is made to move in the magnetic field produced by a permanent magnet.
Eddy-Current Damping Torque of Metal Former shows a metallic former moving in the field of a permanent magnet.
Fluid-Friction Damping:
In this type of damping, a light vane, attached to the spindle of the moving system, dips into a pot of
damping oil and should be completely submerged by the oil. The frictional drag in the disc is always
in the direction opposing motion. There is no friction force when the disc is stationary. In the second
system, increased damping is obtained by the use of vanes.
