1 Measuring Instruments

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In this type of instruments no calibration or comparison with other

instruments is necessary.

They are mostly used as means of standard measurements and are

maintained lay national laboratories and similar institutions.

Some of the examples of absolute

instruments are:

* Tangent galvanometer

* Raleigh current balance

* Absolute electrometer.


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(B) Secondary Instrument :

The instrument which measures an electrical

quantity on a calibrated scale is termed as secondary instrument. Most of

the measuring instruments which are generally used are secondary

instruments

They are often calibrated by comparing them with either some absolute

instruments or with those which have already been calibrated.


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According to Working Principle:1.Electromagnetic Instrument: Moving coil and moving iron instruments belong to this category.

2.Electro dynamic Instrument: Dynamo type ammeter, voltmeter and wattmeter belong to this category

3.Electro thermal Instrument: Hot wire ammeter and thermocouple meters belong to this category.

4.Electro induction Instrument: Watt meters and energy meters belong to this category.

5.Electrochemical Instrument: D.C. ampere-hour meter and watt-hour meters belong to this category.

6.Electrostatic Instrument: Certain voltmeters belong to this category.

FORCES ACTING IN AN INSTRUMENT

Following forces are necessary for working of an instrument1.Deflecting force

2.Controlling force

3.Damping force.


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Deflecting force:

The force acting on the moving system of an instrument

produce a torque which moves the moving system, is called deflecting force

or deflecting torque.

The deflection of the pointer is proportional to the deflecting

torque, and which itself is proportional to the current passed through the

instrument.

Controlling Force:This force opposes the action of deflecting force, and

the pointer will rest in a position where the two force will neutralize each

other.

The controlling force stops the pointer to move

beyond the actual value of electrical quantity under measurement. It alsobrings the pointer back to zero position after disconnecting the meter

from the circuit. Thus no indicating instrument can work without a

controlling force.


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The controlling force can be developed in the following two ways:

1.Spring control :

2.Gravity control :1.Spring Control :

The spindle of the moving system rests on two pivots.

A hair spring made of phosphor bronze wire is attached to the spindle.


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Gravity Control:In this system, the pointer should rotate in a vertical plane.

The control is obtained by attaching a control weight to the pointer in

such a way that it produces a controlling torque because of change in its

position,

A balance weightis also attached to the pointer. Both the weights should

be adjustable.

The controlling torque is proportional to the sine of the angle of

deflection and hence the scale will not be a uniform scale.


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The various methods adopted for damping are:

1.Air friction damping

2.Fluid friction damping

3.Eddy current damping


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In air friction damping a light aluminum piston is attached to the moving

part of the instrument.

The piston moves in an air chamber which is closed at its one end. It will

have a small clearance with the air chamber.

The system works on the compression and suction of the air. Now days,

two aluminum vanes are attached to the moving part, which rotate in

sector shaped air-chambers.

1.Air Friction Damping:


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Fluid Friction Damping:

This system is similar to the air friction damping, but it employs a liquid in place of

air.

The liquid should have a high viscosity so as to produce sufficient damping force.

The system is not suitable for portable instruments, hence it is out dated now.


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Eddy Current Damping:

In this system, a circular thin disc of copper or aluminum is mounted on

the spindle. A damping magnet is placed on side of the disc.

The magnet is place in such a way that the rotating disc will cut the

magnetic lines of force of the magnet.

Thus, eddy currents will be induced in the disc which will oppose its

motion, and thus a damping force is produced in the instrument.


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Permanent Magnet Moving Coil (PMMC) Meter

A coil of fine wire is suspended in a magnetic field produced by permanent

magnet.

According to the fundamental law of electromagnetic force, the coil will rotatein the magnetic field when it carries an electric current by electromagnetic (EM)

torque effect.

A pointer which attached the movable coil will deflect according to the amount

of current to be measured which applied to the coil.


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Uniform scale.

Eddy current damping, so very effective & reliable.

No hysteresis loss.

Low power consumption as the driving power is small.

No effect of stray magnetic field.

High torque/weight ratio, require small operating current.

Very accurate & reliable.

The scale covered upto 270 degree.


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Can not be used for AC measurement.

Costlier in comparison to moving iron instruments.

Friction & temperature might introduce some error.

Errors due to ageing of control springs & permanentmagnets.


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E i f A R


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Extension of Ammeter Range:

Since the coil winding in PMMC meter issmall and light, they can carry only small

currents (A-1mA). Measurement of large current requires a shunt external r esistor to

connect with the meter movement, so only a fraction of the total current will passes

through the meter.

Vsh = Vm

IshRsh= ImRm

But Ish = IT Im

Ish

RmIm

shR

mT

RmImsh

II

R


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Example:If PMMC meter have internal resistance of 10 and full scale range of 1mA.

Assume we wish to increase the meter range to 1A.

we must connect shunt resistance with the PMMC meter of Rsh

ImTI

RmImshR


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Example 1:- A moving coil ammeter has a full

scale deflection of 50 Amp and a coil resistance

of 1000 . What will be the value of the shuntresistance required for the instrument to be

converted to read a full scale reading of 1 Amp.

0.05

Example 2 :- The full scale deflection current of

an ammeter is 1 mA and its internal resistance is

100 . If this meter is to have scale deflection at5 A, what is the value of shunt resistance to be

used.

R sh = 0.020004 ohm


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VOLTMETER


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ExampleConvert a 50 micro A, 3kPMMC into a 0-10VDC voltmeter.

The voltmeter looks like this:

A PMMC meter with a coil resistance 100 and a full scale deflection currentof 100A is to be used in the voltmeter circuit as shown in Fig.42.6. The

voltmeter ranges are to be 50. Determine the required value of resistances

for each range.

(i) For 50V - 0.. 4999M

(ii) For 100V - 0.9999M

(iii)For 150V - 1.4999M


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A moving coil ammeter gives full scale deflection with 15 mA and has aresistance of 5 . Calculate the resistance to be connected in (a)Parallel to enable the instrument to read upto 1 A (b) Series to enableit to read up to 10 V.

Rsh = 0.07614 Rse = 661.6667


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Moving-iron InstrumentsThe deflecting torque in any moving-iron instrument is due to forces on a small

piece of magnetically soft iron that is magnetized by a coil carrying the

operating current.(i) Attraction Type (ii) Repulsion Type


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REPULSION TYPE

Repulsive forces will act when two similarly magnetized iron pieces are

placed near to each other.


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Sensitivity: It is defined by the change in the output or response of the

instrument for a unit change of input or measured variable.

Resolution: Resolution is the smallest change in a measured variable (or

measurand ) to which the instrument will respond.

Terms used :

Error: It is the deviation of the measured (or indicated) value from the

true (or expected) value of a quantity.

In other words, error is the difference between the measured value and

the true value of the unknown quantity. It is also called absolute error are

maximum possible error.

A = Am At

whereAm = measured value of the quantity

At = true value of the quantity


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