Measurement Bravien 2

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July 19, 2014

[2.0 Understand the classification, Essential Constructional

features of Electrical Measuring Instrument parts.]

By Md. Irshad (+91 9066753011)

2.1 Classify the measuring instruments

Basic classification of measuring instruments:

1. Mechanical instruments:- They are very reliable for static and stable

conditions. The disadvantage is they are unable to respond rapidly to

measurement of dynamic and transient conditions.

2. Electrical instruments:- Electrical methods of indicating the output of

detectors are more rapid than mechanical methods. The electrical system

normally depends upon a mechanical meter movement as indicating device.

3. Electronic instruments:- These instruments have very fast response.For example a cathode ray oscilloscope (CRO) is capable to follow dynamic

and transient changes of the order of few nano seconds (10-9 sec).

Other classification of instruments:-

1. Absolute instruments or Primary Instruments:- These instruments gives

the magnitude of quantity under measurement in terms of physical constants of

the instrument e.g. Tangent Galvanometer. These instruments do not requirecomparison with any other standard instrument

• These instruments give the value of the electrical quantity in terms of absolute

quantities (or some constants) of the instruments and their deflections.

• In this type of instruments no calibration or comparison with other

instruments is necessary.

• They are generally not used in laboratories and are seldom used in practice by

electricians and engineers. 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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2. Secondary instruments:-These instruments are so constructed that the

quantity being measured can only be determined by the output indicated by theinstrument. These instruments are calibrated by comparison with an absolute

instrument or another secondary instrument, which has already been calibrated

against an absolute instrument.

Working with absolute instruments for routine work is time consuming since

every time a measurement is made, it takes a lot of time to compute the

magnitude of quantity under measurement. Therefore secondary instruments are

most commonly used.

• They are direct reading instruments. The quantity to be measured bythese instruments can be determined from the deflection of the

instruments.

• They are often calibrated by comparing them with either some absolute

instruments or with those which have already been calibrated.

• The deflections obtained with secondary instruments will be

meaningless untill it is not calibrated.

• These instruments are used in general for all laboratory purposes.

• Some of the very widely used secondary instruments are: ammeters,

voltmeter, wattmeter, energy meter (watt-hour meter), ampere-hourmeters etc.

Classification of Secondary Instruments:

(a) Classification based on the various effects of electric current (or

voltage) upon which their operation depend. They are:

• Magnetic effect: Used in ammeters, voltmeters, watt-meters,

integrating meters etc.

• Heating effect: Used in ammeters and voltmeters. • Chemical effect: Used in dc ampere hour meters.

• Electrostatic effect: Used in voltmeters.

• Electromagnetic induction effect: Used in ac ammeters,

voltmeters, watt meters and integrating meters.

Generally the magnetic effect and the electromagnetic

induction effect are utilized for the construction of the commercial



July 19, 2014



By Md. Irshad (+91 9066753011)

instruments. Some of the instruments are also named based on the

above effect such as electrostatic voltmeter, induction instruments,

etc.

(b) Classification based on the Nature of their Operations

We have the following instruments.

• Indicating instruments: Indicating instruments indicate,

generally the quantity to be measured by means of a pointer which

moves on a scale. Examples are ammeter, voltmeter, wattmeter

etc.

• Recording instruments: These instruments record continuously

the variation of any electrical quantity with respect to time. In

principle, these are indicating instruments but so arranged that a

permanent continuous record of the indication is made on a chart

or dial. The recording is generally made by a pen on a graph paper

which is rotated on a dice or drum at a uniform speed. The amount

of the quantity at any time (instant) may be read from the traced

chart. Any variation in the quantity with time is recorded by these

instruments. Any electrical quantity like current, voltage, power

etc., (which may be measured lay the indicating instruments) may

be arranged to be recorded by a suitable recording mechanism.

• Integrating instruments: These instruments record the

consumption of the total quantity of electricity, energy etc., during

a particular period of time. That is, these instruments totalize

events over a specified period of time. No indication of the rate or

variation or the amount at a particular instant are available from

them. Some widely used integrating instruments are: Ampere-hour

meter: kilowatthour (kWh) meter, kilovolt-ampere-hour

(kVARh) meter.

(c) Classification based on the Kind of Current that can be

Measurand.

Under this heading, we have:

• Direct current (dc) instruments



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• Alternating current (ac) instruments

• Both direct current and alternating current instruments (dc/ac

instruments).

(d) Classification based on the method used.

Under this category, we have:

• Direct measuring instruments: These instruments converts the

energy of the measured quantity directly into energy that actuates

the instrument and the value of the unknown quantity is measured

or displayed or recorded directly. These instruments are most

widely used in engineering practice because they are simple and

inexpensive. Also, time involved in the measurement is shortest.Examples are Ammeter, Voltmeter, Watt meter etc.

• Comparison instruments: These instruments measure the

unknown quantity by comparison with a standard. Examples are dc

and ac bridges and potentiometers. They are used when a higher

accuracy of measurements is desired.

2.2 Compare analog with digital type of instruments.

Analog Digital

1. Low precision High precision

2. Continuous stepless deflection Numerical readout

3. More flexible Limited flexibility, different instruments for

various ranges and applications

4. Frequency response is large, from DC or

low-frequency range to high-frequencyrange

Frequency response can also be very high

5. Parallax error in readout is possible No such error due to digital display

6. Not convenient for readout Convenience in readout, viz. in low light, from a

distance, etc.

7. Errors due to friction, spring tension canarise.

No such errors

8. No direct PC or µP or µC compatibility Compatibility with microprocessors (µP),

microcontrollers (µC), and personal computers(PC) is possible



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By Md. Irshad (+91 9066753011)

2.3 Distinguish between absolute and secondary instruments

Absolute instruments are those which give the value of the quantity to be measured, interms of the constants of the instrument and their deflection only . No previous calibration or

comparison is necessary in their case. The example of such an instrument is tangentgalvanometer, which gives the value of current, in terms of the tangent of deflection produced by

the current, the radius and number of turns of wire used and the horizontal component of earth’s

field.

Secondary instruments are those, in which the value of electrical quantity to be measured

can be determined from the deflection of the instruments, only when they have been pre-

calibrated by comparison with an absolute instrument. Without calibration, the deflection of suchinstruments is meaningless.

2.4 What are indicating,recording,integrating instruments.Give

example for each.

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. Ordinary ammeters, voltmeters and wattmeters belong to

this class.

Recording instruments are those, which, instead of indicating by means of a pointer and ascale the instantaneous value of an electrical quantity, give a continuous record or the variationsof such a quantity over a selected period of time. The moving system of the instrument carries an

inked pen which rests lightly on a chart or graph that is moved at a uniform and low speed in a

direction perpendicular to that of the deflection of the pen. The path traced out by the pen presents a continuous record of the variations in the deflection of the instrument.

I ntegrating instruments always record the unknown quantity in an integrated manner

indicating the total cumulative value of the measurand at any instant of time. For e.g., energy

meters and ampere-hour meters.



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By Md. Irshad (+91 9066753011)

2.5 State the need, purpose and methods of deflecting, controlling

and damping Torque in measuring instruments

Deflecting Torque

The deflecting or operating torque (Td ) is produced by utilizing one or other effects like

magnetic, electrostatic, electrodynamic, thermal or inductive etc. The actual method of torque production depends on the type of instrument. This deflecting torque causes the moving system

(and hence the pointer attached to it) to move from its ‘zero ’ position i.e. its position when the

instrument is disconnected from the supply.

Controlling Torque

The deflection of the moving system would be indefinite if there were no controlling or restoring

torque. This torque oppose the deflecting torque and increases with the deflection of the moving

system. The pointer is brought to rest at a position where the two opposing torques are equal.The deflecting torque ensures that currents of different magnitudes shall produce deflections of

the moving system in proportion to their size. Without such at torque, the pointer would swing

over to the maximum deflected position irrespective of the magnitude of the current to be

measured. Moreover, in the absence of a restoring torque, the pointer once deflected, would notreturn to its zero position on removing the current. The controlling or restoring or balancing

torque in indicating instruments is obtained either by a spring or by gravity as described below :

(a ) Spring Control

A hair-spring, usually of phosphor bronze, isattached to the moving system of the instrument

as shown in Fig. below With the deflection of the pointer, the spring is twisted in the opposite

direction. This twist in the spring producesrestoring 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. For example, in permanent-magnet, moving-coil

type of instruments, the deflecting torque is proportional to the current passing through them.

Td ∝ I

and for spring control Tc ∝ θ As Tc = Td

θ ∝ I



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By Md. Irshad (+91 9066753011)

Since deflection θ is directly proportional to current I , the spring-controlled instruments have a

uniform or equally-spaced scales over the whole of their range as shown in Fig. (a) . To ensure

that controlling torque is proportional to the angle of deflection, the spring should have a fairlylarge number of turns so that angular deformation per unit length, on full-scale deflection, is

small. Moreover, the stress in the spring should be restricted to such a value that it does not produce a permanent set in it.

Springs are made of such materials which

(i ) are non-magnetic

(i i ) are not subject to much fatigue

(i i i ) have low specific resistance-especially in cases where they are used for leading current in or

out of the instrument

(iv ) have low temperature-resistance coefficient.

Gravity Control

Gravity control is obtained by attaching a small adjustable weight to some part of the moving

system such that the two exert torques in the opposite directions. The usual arrangements is

shown in Fig.It is seen from Fig. (a) that the controlling or restoring torque is proportional

to the sine of the angle of deflection i.e.

T c∝ sin θ

The degree of control is adjusted by screwing the weight up or down the carrying system

If T d∝ I

then for position of rest

T d = T c



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By Md. Irshad (+91 9066753011)

or I ∝ sin θ (not θ)

It will be seen from Fig. (b) that as θ approaches 90º, the distance AB increases by a relatively

small amount for a given change in the angle than when θ is just increasing from its zero value.

Hence, gravity-controlled instruments have scales which are not uniform but are crampedor crowded at their lower ends

As compared to spring control, the disadvantages of gravitycontrol are :

1. it gives cramped scale

2. the instrument has to be kept vertical.

However, gravity control has the following advantages :

1. it is cheap

2. it is unaffected by temperature

3. it is not subjected to fatigue or deterioration with time.

Damping Torque

A damping force is one which acts on the moving system of the instrument only when i t is

moving and always opposes its motion. Such stabilizing or demping force is necessary to bring

the pointer to rest quickly, otherwise due to inertia of the moving system, the pointer will

oscillate about its final deflected position for quite some time before coming to rest in the steady

position.

Air Friction Damping



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By Md. Irshad (+91 9066753011)

The arrangement shown in Fig. (a) consists of a light aluminium vane which moves in aquadrant (sector) shaped air chamber. The chamber also carries a cover plate at the top. The

vane is mounted on the spindle of the moving system. The aluminium vane should not touch

the air-chamber walls otherwise a serious error in the deflection of the instrument will beintroduced. Now, with the motion, the vane displaces air and thereby a damping force is

created on the vane that produces a torque (damping) on the spindle. When the movement is

quicker the damping force is greater; when the spindle is at rest, the damping force is zero.

• The arrangement of Fig. (b) consists of a light aluminium piston which is attached to the

moving system. This piston moves in a fixed chamber which is closed at one end. Either

circular or rectangular chamber may be used. The clearance (or gap) between the piston andchamber walls should be uniform throughout and as small as possible. When the piston

moves rapidly into the chamber the air in the closed space is compressed and the pressure of

air thus developed opposes the motion of the piston and thereby the whole moving system. If

the piston is moving out of the chamber, rapidly, the pressure in the closed space falls and the pressure on the open side of the piston is greater than that on the opposite side. Motion is thus

again opposed. With this damping system care must be taken to ensure that the arm carrying

the piston should not touch the sides of the chamber during its movement. The friction whichotherwise would occur may introduce a serious error in the deflection.

The air friction damping is very simple and cheap. But care must be taken to ensure that the

piston is not bent or twisted. This method is used in moving iron and hot wire instruments

Fluid Friction Damping



July 19, 2014



By Md. Irshad (+91 9066753011)

This form is damping is similar to air friction damping. The action is the same as in the air

friction damping. Mineral oil is used in place of air and as the viscosity of oil is greater, the

damping force is also much greater. The vane attached to the spindle is arranged to move inthe damping oil.

• It is rarely used in commercial type instruments. • The oil used must fulfill the following requirements.

* It should not evaporate quickly* It should not have any corrosive effect on metals.

* Its viscosity should not change appreciably with temperature.

* It should be good insulator

(A) In Fig. (a) a disc attached to the moving system is immersed in the fluid (dampingoil). When the moving system moves the disc moves in oil and a frictional drag is

produced. For minimizing the surface tension affect, the suspension stem of the disc

should be cylindrical and of small diameter.

(B) In the arrangement of Fig.(b) a number of vanes are attached to the spindle. These

vanes are submerged in oil and moves in a vertical plane. This arrangement provides

greater damping torque.

Advantages of Fluid Friction Damping

1. The oil used for damping can also be used for insulation purpose in some forms of instrumentswhich are submerged in oil.

2. The clearance between the vanes and oil chamber is not as critical as with the air friction

clamping system.

3. This method is suitable for use with instruments such as electrostatic type where the

movementis suspended rather than pivoted.

4. Due to the up thrust of oil, the loads on bearings or suspension system is reduced thereby thereducing the frictional errors.

Disadvantages of Fluid Friction Damping

1. The instruments with this type of damping must be kept always in a vertical position.2. It is difficult to keep the instrument clean due to leakage of oil.

3. It is not suitable for portable instruments.

The fluid friction damping can be used for laboratory type electrostatic instruments.



July 19, 2014



By Md. Irshad (+91 9066753011)

Eddy Current Damping

Eddy current damping is the most efficient form of damping. The essential components in this

type of damping are a permanent magnet; and a light conducting disc usually of alumninum.When a sheet of conducting material moves in a magnetic field so as to cut through lines of

force, eddy currents are set up in it and a force exists between these currents and the magnetic

field, which is always in the direction opposing the motion. This force is proportional to themagnitude of the current, and to the strength of field. The former is proportional to the velocity

of movement of the conductor, and thus, if the magnetic field is constant, the damping force is

proportional to the velocity of the moving system and is zero when there is no movement of thesystem. Figure below shows two methods of applying this method of damping.

In Fig. (a) a thin disc of conducting, but non-magnetic material-usually copper of aluminium is

mounted on the spindle which carries the pointer of the instrument. When the spindle rotates, theedge of the disc cuts through the lines of force in the gap of a permanent magnet, and eddy

currents, with consequent damping, are produced. An arrangement similar to this is often used in

hotwire instruments.

Figure (b) shows the essential parts of a permanent-magnet, moving coil, instrument. The coil iswound on a light metal former in which eddy currents are induced when the coil moves in the

Permanent-magnet field. The directions of the eddy-current which in turn produce the damping

torque due to the motion of the coil (clockwise) are as shown in Fig.(b) and this will produce

damping forces as indicated in the figure.



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By Md. Irshad (+91 9066753011)

2.6 Name the material used and their significance for following

parts of measuring Instruments

a) Magnets

Instruments having permanent magnets as their main component, it is essential to ensure that the

strength of the permanent magnets be constant over a considerable time period. Materials used

for construction of such magnets are:• Alloys of cobalt, chromium and steel

• Almico (or Alcomax)

• Alloys of iron, nickel and aluminium

b) Discs

c) Suspension

The main requirement to be fulfilled by a supporting system is that the friction should be as

minimum as possible. The two commonly used methods for supporting the moving system of aninstrument are:

1.By pivoting and 2. By thread suspension.

Most instruments use the supports of first kind. In the case, the ends of the spindle are conical

and are made of hardened steel. The ends fits into jeweled bearings of conical shape made from

aluminium oxide. The contract area at the pivots should be as small as practicable. However a

very small area of contact leads to a very high bearing stress.

The thread suspension systems have limited applications in commercial instruments due tofollowing reasons:

• The instrument must be leveled and its axis must be vertical.• It must be protected against mechanical shocks.

The method is advantageous where the operating torques are small compared with weight of the



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moving system since the friction is completely avoided. Phosphor bronze strips are commonly

used for suspension

d) Coils

e) Pointers and Scales

Pointers and scales of instruments may be classified together into two groups:

• Instruments used for reading at considerable distance.

• Instruments used for precisions work at shoot range.

It is essential that the pointer must be light and must have small inertia constant so as to reduce

the load in the bearing of the moving system and to avoid high degree of damping. Its outline

must be bold with sharp pointer in the first type. We often use aluminium strip on tube for the pointer.

The scale of an instrument of first category is mostly printed on the enameled surface of a metal

plate, or on paper or card-board cemented rigidly to a metal backing plate.For the precision (work) in reading, a strip of mirror is mounted in an opening in the scale

beneath the pointer. The reading is taken by removing the parallax error between the position and

its image in the mirror.

f) SpringsInitially iron/ ferrous springs were used but now a days spring made of phosphore- bronze is

used as they are(i ) are non-magnetic

(i i ) are not subject to much fatigue

(i i i ) have low specific resistance-especially in cases where they are used for leading current in or

out of the instrument

(iv ) have low temperature-resistance coefficient

Measurement Bravien 2

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