The process of measuring is essentially that of
comparing some unknown value with a value which
is assumed to be known.
1) latter one in standard.
2) And measuring system is instrument.
the static characteristics are
obtained in one form or another by a process called
Calibration procedures involve a comparison of
the particular instrument with either
a) a primary standard.
b) a secondary standard with a higher accuracy
than the instruments be calibrated.
c) an instrument of known accuracy.
Accuracy may be defined as the ability of
a device or a system to respond to a true value of a
measured variable under reference conditions.
Precision is defined by the degree of exactness
for which an instrument is designed or intended to perform.
It is the closeness of agreement among a
number of consecutive measurements of the output for the
same value of the input, under the same operating
It is the closeness of agreement among
repeated measurements of the output for the same value of
the input, made under the same operating conditions over a
period of time.
It is an undesired change or a gradual variation in
output over a period of time that is unrelated to changes in
input and operating conditions.
Span: If in a measuring instrument the highest point of
calibration is y units and the lowest point x units.
Then the instrument range y units
The instrument span is given by
Sensitivity: Sensitivity can be defined as the ratio of a
change in output to the change in input which causes it.
Resolution: The smallest increment in input (the quantity
being measured) which can be detected with certainty by an
instrument is its resolution.
Dead zone: Dead zone is the largest range of values of a
measured variable to which the instrument does not
Static Error : the numerical difference between the true
value of a quantity and its value as obtained by
Mistakes: These are errors due to human mistakes such as
careless reading, mistakes in observations, incorrect
application of a correction, improper application of
instruments and computational errors.
1)Instrumental Error: Instrumental errors are the errors
inherent in measuring instruments because of their
mechanical structure, such as friction in bearings of various
moving components, irregular spring tension.
2)Environmental Error: it is due to conditions external to
the measuring device including conditions in the area
surrounding the instrument such as the effect of change
in temp , humidity, barometric pressure , or magnetic or
Random Error: the cause of such error is unknown or not
determinable in the ordinary process of making
measurements. Such errors are normally small and follows
the laws of chance.
SOURCES OF ERROR
Insufficient knowledge of process parameters and
Error caused by people who operate instrument
Certain design limitations.
TYPES OF INSTRUMENTS
Absolute instruments are those which give the value of the
electrical quantity to be measured, in terms of the constant of
the instruments and their deflection only, e.g. tangent
Secondary instruments are those which have been
precalibrated by comparison with an absolute instrument. The
value of the electrical quantity to be measured in these
instruments can be determined from the deflection of the
Without calibration of such an instrument, the deflection is
Different types of secondary instruments:
Indicating instruments are those which indicate the instantaneous
value of the electrical quantity being measured, at the time at
which it is being measured. Their indications are given by pointers
moving over calibrated dials(scale), e.g. ammeters,voltmeters and
Recording instruments are those which give a continuous record
of variations of the electrical quantity over a selected period of
time. The moving system of the instrument carries an inked pen
which rests tightly on a graph chart. E.g. recording voltmeters
used in supply station.
Integrating instruments are those which measure and register the
total quantity of electricity (in ampere-hour) or the total amount of
electrical energy(in watt-hours or kilowatt-hours) supplied to a
circuit over a period of time, e.g. ampere-hour meters, energy
ESSENTIALS OF INDICATING INSTRUMENTS
It is the torque which deflects the pointer on a calibrated
scale according to the electrical quantity passing through the
instrument. The deflecting torque causes the moving system
and hence the pointer attached to it to move from zero
position to indicate on a graduated scale the value of
electrical quantity being measured.
It is the torque which controls the movement of the pointer
on a particular scale according to the quantity of electricity
passing through it. If deflecting torque were acting alone, the
pointer would continue to move indefinitely and would swing
over to the maximum deflected position irrespective of the
magnitude of current (or voltage or power) to be measured.
In the spring control method, a hair-spring, usually of phosphorbronze, attached to the moving system is used. With the deflection of
the pointer, the spring is twisted in the opposite direction. This twist in
the spring produces a restoring torque which is directly proportional
to the angle of deflection of the moving system. The pointer comes to
a position of rest (or equilibrium) when the deflecting torque (Td) and
controlling torque (Tc) are equal.
Tc ∞ θ
To give a controlling torque which is directly proportional to the angle of
deflection of the moving system, the number of turns of the spring
should be fairly large so that the deformation per unit length is small.
The stress in the spring must be limited to such a value that there is no
permanent set. Springs are made of materials which are
Not subject to much fatigue
Low in specific resistance
Have low temperature coefficient of resistance.
Gravity control is obtained by attaching a small weight to the moving
system in such a way that it produces a restoring or controlling torque
when the system is deflected.
Tc ∞ Sinθ
Thus, controlling torque in a gravity control system is proportional to the sine
of the angle of deflection.
The degree of control is adjusted by screwing the weight up or down on the
The advantages of gravity control system, as compared to spring control,
are given below:
It is cheap
It is unaffected by temperature
It is not subjected to fatigue or distortion, with time.
The disadvantages of gravity control system, as compared to spring control,
are given below:
It gives a cramped scale.
The instrument has to be kept vertical.
If the moving system is acted upon by deflecting and controlling torques alone,
then pointer, due to inertia, will oscillate about its final deflected position for quite
sometime before coming to rest. This is often undesirable because it makes
difficult to obtain quick and accurate readings. In order to avoid these oscillations
of the pointer and to bring it quickly to its final deflected position, a damping
torque is provided in the indicating instruments.
There are three types of damping:
Air friction damping:
air friction damping uses either aluminium piston or vane, which is attached to or
mounted on the moving system and moves in an air chamber at one end.
Fluid friction damping:
In fluid friction damping, a light vane (attached to the moving system) is dipped
into a pot of damping oil. The fluid produces the necessary opposing (or
damping) force to the vane. The vane should be completely submereged in the
The disadvantage of this type of damping is that it can only be used in the vertical
Eddy Current Damping:
Eddy-current damping uses a conducting material which moves in a magnetic
field so as to cut through the lines of force, thus setting up eddy currents. Force
always exists between the eddy current and magnetic field which is always
opposite to the direction of motion. This is most efficient type of damping and is
largely used in permanent magnet moving coil instruments.
This type of instrument based on the principle that
when a current carrying conductor is placed in a
magnetic field, a force acts on the conductor,
which tends to move it to one side and out of the
It consists of a powerful U-shaped permanent
magnet made of Alnico, and soft iron pole pieces
bored out cylindrically.
A soft iron core is fixed between the magnetic
poles whose functions are i) to make the field
uniform, and ii) to decrease the reluctance of the
air path between the poles and hence increase
the magnetic flux.
Surrounding the core is a rectangular coil of many
turns wound on a light aluminium or copper
frame, supported by delicate bearings.
A light pointer fixed to the frame moves on the
calibrated scale according to the amount of
electricity passed through the coil.
The aluminiun frame provides not only support for
the coil but also a damping torque by the eddy
currents induce in it.
When the instrument is connected in the
circuit to measure current or voltage,
the operating current flows through the
coil. Since the coil is carrying current
and is placed in the magnetic field of
the permanent magnet , a mechanical
force acts on it. As a result , the pointer
attached to the moving system moves
in a clockwise direction over the
graduated scale to indicate the value of
current or voltage being measured.
If the current in the coil is reversed, the
deflecting torque also be reversed
since the direction of the permanent
magnet is same. Consequently, the
pointer will try to deflect below zero.
Deflection in this direction is prevented
by a spring “stop”. Since the deflecting
torque reverses with the reversal of
current in the coil, such instrument can
be used to measure direct current and
Low power consumption.
Uniform scale extendable over an arc of 270º or so.
High torque weight ratio.
No hysteresis loss.
Very effective and efficient eddy current damping.
Not effected much by stray and magnetic fields due to strong operating field.
Costlier compared to moving iron instruments, due to delicate construction and
accurate machining and assembly of various parts.
Some error arise due to the ageing of control springs and the permanent magnet.
Use limited to d.c. only.
Scale length of meter can be increased from 120º and 240º or even 270º or 300º.
PMMC instruments can be used as dc ammeter. And its range can be increased
by using a large number of turns in parallel with the instrument.
The range of this instrument, when used as a dc voltmeter, can be increased by
using a high resistance in series with it.
MOVING IRON INSTRUMENTS
Moving Iron instruments depend for their action upon
the magnetic effect of current, and are widely used as
indicating instruments. In this type of instrument , the
coil is stationery and the deflection is caused by a
soft-iron piece moving in the field produced by the
There are two types of moving iron instruments:
Fig shows the constructional
details of an attraction type
MI instrument. It consists
of a cylindrical coil or
solenoid which is kept
fixed. An oval-shaped softiron piece. The controlling
torque is provided by an
aluminium vane, attached
to the spindle, which
moves in a closed air
When the instrument is connected in the circuit to measure
current or voltage, the operating current flowing through
the coil sets up a magnetic field. In other words, the coil
behaves like a magnet and therefore it attracts the soft iron
piece towards it. The result is that the pointer attached to
the moving system moves from zero position. The pointer
will come to rest at a position where deflecting torque is
equal to the controlling torque. If current in the coil is
reversed, the direction of magnetic field also reverses and
so does the magnetism produce in the soft iron piece.
Hence, the direction of the deflecting torque remains
unchanged. For this reason, such instruments can be used
for both d.c. and a.c. measurements.
Fig shows the constructional details
instrument. It consists of two soft-iron
pieces or vanes surrounded by a
fixed cylindrical hollow coil which
carries the operating current. One of
these vanes is fixed and the other is
free to move. The movable vane is
cylindrical shape and is mounted
axially on a spindle to which a pointer
is attached. The fixed vane, which is
wedge-shaped and has a larger
radius, is attached to the stationery
coil. The controlling torque is provided
by one spiral spring at the top of the
instrument. It may be noted that in
this instrument, springs do not
provide the electrical connections.
Damping is provided by air friction
due to the motion of a piston in an air
When current to be measured or current proportional to the
voltage to be measured flows through the coil, a magnetic field is
set up by the coil. This magnetic field magnetises the two vanes
in the same direction i.e. similar polarities are developed at the
same ends of the vanes. Since the adjacent edges of the vanes
are of the same polarity, the two vanes repel each other. As the
fixed vane cannot move, the movable vane deflects and causes
the pointer to move from zero position. The pointer will come to
rest at a position where deflecting torque is equal to controlling
torque provided by the spring. If the current in the coil is
reversed, the direction of deflection remains unchanged. It is
because reversal of the field of the coil reverses the
magnetisation of both iron vanes so that they repel each other
regardless which way current flows through the coil. For this
reason, such instruments can be used for both d.c. and a.c.
Cheap, robust and give reliable service.
Usable in both a.c. and d.c. circuits.
Have non-linear scale.
Cannot be calibrate with high degree of precision for d.c. on
account of the affect of hysteresis in the iron vanes.
Deflection up to 240º only may be obtained with this instrument.
This instrument will always have to be put in the vertical position
if it uses gravity control.
Errors with MI instruments:
Due to hysteresis when used in a.c. and d.c.
Due to stray magnetic fields when used both in a.c. and d.c.
Due to frequency variation when used in a.c.
Due to waveforms effect when used in a.c.
Applications of MI instruments:
As an ammeter:
It may be constructed for full-scale deflection of 0.1 to 30A
with out the use of shunts or current transformers. To
obtain full-scale deflection with currents less than 0.1A, it
requires a coil with a large number of fine wire turns, which
results in an ammeter with a high impedance.
As an voltmeter:
The MI voltmeter is a fairly low impedance instrument,
typically, 50Ω/V for a 100V instrument. The lowest full
scale is of the order of 50V.The range of the instrument,
when used as a voltmeter, can be extended by using a
high non-inductive resistance R connected in series with it.
This series resistance is known as „multiplier‟.
It works on dynamometer principle i.e. mechanical
force exists between two current carrying
conductors or coils.
Similar to PMMC
Portable, highest precision.
Fixed Coils: The operating field is
produced by the fixed coil which is
divided into two sections to give a
uniform field near the centre. The coil is
wound with fine wire when used as a
voltmeter and heavy wire when used as
an ammeter and wattmeter. The wire is
stranded, when necessary, to reduce
eddy current losses in conductors.
Moving Coil: The moving coil is
wound either as a self sustaining coil or
else on a non-metallic former. The use
of metallic former is avoided in order to
avoid the inducement of eddy currents
in it. Both moving coils and fixed coils
are air cored.
Moving System: The moving coil is supported by an aluminium spindle
and jewel bearings and carries a pointer moving over a graduated scale.
The entire movement is very solid and rigidly constructed in order to keep
mechanical dimensions stable and its calibration in tact.
Control system: The controlling torque is provided by two control
springs, which also act as leads to the moving coil.
Damping System: Air friction damping is used in these instruments and
may be either piston type or vane type. Eddy current damping cannot be
used in these instruments as introduction of a permanent magnet for the
purpose would distort the working magnetic field of the instrument.
Shielding: The operating magnetic field produced by the fixed coils in
these instruments is somewhat weaker (0.005-0.006T) in comparison to
that in instruments of other types. So it is essential to provide magnetic
shielding to this arrangement.
The complete assembly is surrounded by a laminated steel shield to
protect the instrument from external magnetic field which may affect the
operation of the instrument.
The operating principle of dynamometer type instruments is
the interaction between the currents in the moving coil,
mounted on a shaft, and the fixed coils.
W hen two coils are energized, their magnetic fields interact
and the resulting torque tends to rotate the moving coil. Since
there is no iron, the field strength is proportional to the current
in the fixed coil and, therefore, the deflecting torque is
proportional to the product of the currents in the fixed coil and
the moving coil.
When used as a wattmeter and, the fixed coil is the current
coil and the moving coil is the pressure coil. Thus the current
in the latter is proportional to the voltage applied. Hence, the
deflecting torque is proportional to the product of the voltage
and current(that is power).
If if and im are the currents flowing through the fixed and
moving coils respectively then
Deflecting torque, Td ∞ if im
In spring-controlled instruments
Controlling Torque, Tc ∞ θ, the deflection
But in steady deflected position
θ ∞ if im
or θ ∞ i2 (if=im=i)
Frictional Error: Due to Heavy moving parts.
Temperature Error: Temperature errors due to internal
Error due to stray magnetic fields
As the instrument has square-law response, so can be
used on both dc as well as on ac.
These instruments are free from hysteresis and eddy
current errors because of absence of iron in the
operating parts of the instrument.
Ammeters up to 10A and voltmeters up to 600V can be
constructed with precision grade accuracy.
Dynamometer type voltmeters are very useful for
accurate measurement of rms values of voltages
irrespective of waveforms.
Because of precision grade accuracy and same
calibration for dc and ac measurements these
instruments are used as transfer and calibration
The scale is not uniform as the instrument has
The magnetic field strength obtained in these
instruments, being small due to the absence of iron,
a large number of ampere turns are required on the
moving coil in order to obtain the necessary
deflecting torque. As a result, the moving system
becomes heavy and power loss becomes high.
Small torque-weight ratio.
High frictional losses.
As an Ammeter: When it is used as an ammeter of low range (about
0.2A), the moving and fixed coils are connected in series.
When this type of instrument is used to measure a heavy amount of
current(about 20A), the moving coil is usually connected in series with a
swamping resistance, across a shunt, together with the fixed coils. Fixed
coils are made of thick wires to carry larger currents.
As a Voltmeter: To use the instrument as a voltmeter, the coils are
wound with high resistance wire and fixed and moving coils are connected
in series with a high resistance.
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