100 Most Important MCQ Of Measurement and Instrumentation with explanation

Answer.4. Any of the above

Explanation:-

Basically, there are three types of Force or torque act upon the Measuring Instrument

1. Deflecting Torque
2. Controlling Torque
3. Damping Torque

Deflecting Torque: In order to move the pointer from its zero position on the scale deflecting torque is required. The deflecting torque works on the moving system which the pointer is attached. Obviously, the magnitude of deflecting torque produced is proportional to the magnitude of the quantity being measured, say, the current I flowing through the instrument. A deflecting torque is required to overcome the inertia, damping effect and controlling effect of the moving system.

This deflecting torque can be produced by any of this effects of current (or of voltage) such as

1. Magnetic effect:-  Suitable for A.C and D.C moving iron instruments such as voltmeter and Ammeter
2. Electrostatic effect:- It utilizes the forces between electrically charged conductors. It is suitable only for both AC and DC Voltmeter
3. Electromagnetic effect:- It utilizes the magnetic effect of electric current. It Is suitable for A.C instrument only such as Ammeter, Energy meter, Wattmeter, Voltmeter, and Watt-hour meter.
4. Thermal effect:- It utilizes the heating effect. It is suitable for both A.C and D.C such as Ammeter and Voltmeter.
5. Chemical Effect:- It is used for D.C instrument such as D.C ampere-hour meter.

Answer.3. A condenser of suitable value is used in parallel with the swamp resistance

Explanation:-

Error due to changes in frequency: In certain type of instruments, like in moving iron instruments, the change in frequency causes error due to the change in magnitude of eddy currents setup in the metal portion of the instruments. Changes in frequency cause changes in reactance of the operating coil (X = 2πf ).  At high frequencies, the meter will give a reading lower than the actual value and vice-versa. This type of error can be eliminated only if the impedance of the coil is independent of the frequency. This is achieved by having a capacitance in shunt with the swamping resistance of the coil as shown in Fig. 

As shown in Fig. let R and L be the resistance and inductance of the coil of the moving-iron instrument and let r(R>>r) be the swamping resistance connected in series to eliminate the temperature errors. Let C be the values of the capacitance to be shunt-connected so that the impedance remains unaltered. Then, the total impedance Z which remains independent of the frequency is given as

If Z is to be independent of the frequency, then it must not contain any j terms or the j terms should be equal to zero.i.e

But generally

(ωCr)² << 1

∴ ω − ωCr² = 0

C = L/r²

Answer.4. All of the above

Explanation:-

The permanent magnet moving coil (PMMC)-type instrument is the basic dc measuring instrument. In the instruments, a permanent magnet, generally of horseshoe type, creates a magnetic field in which a coil of fine wire of the number of turns is placed. The coil is wound on a very light aluminum drum and is pivots on jewel bearings so that the coil is free to move when current flows through it. The current carrying coil placed in the magnetic field experiences a torque and tries to turn. its free turning is restricted by spring tension attached to its shaft. The moving coil produces a defecting torque which is opposed by the control torque produced by the spring action. A simplified diagram of a PMMC type instrument is shown in the figure.

Its operating principle is the same as that of the D’Arsonval galvanometer. When a current flow through the coil, a magnetic field is produced which reacts with the magnetic field produced by the permanent magnet.

The current to be measured or a definite fraction of it proportional to the voltage to be measured is passed through the coil. A deflecting torque is produced on account of the reaction of the permanent magnetic field with the magnetic field produced by the coil. The direction of deflecting torque can be found by applying Flemming’s left-hand rule.

Advantages of PMMC Type instruments

1. In moving-coil instruments, the power consumption is very small.
2. Torque to weight ratio is very high.
3. Moving-coil instruments have uniformly graduated scales.
4. The same instrument can be used both as an ammeter and a voltmeter by a suitable shunt or series-resistance (multiplier).
5. Moving-coil instruments are free from errors due to external magnetic fields since the working magnetic field strength itself is very high.
6. Very large scale deflection (300°) is possible using the “circular scale “type of instruments.
7. Damping employed in moving-coil instruments is perfect since an eddy current damping is used.
8. The torque-weight ratio is very high.
9. Due to the application of intense polarized or unidirectional field, the stray magnetic field has no effect.

Disadvantages of PMMC Type Instrument

• PMMC type of instrument can be operated in direct current only. In alternating current, the instrument does not operate because in the positive half the pointer experiences a force in one direction and in the negative half, the pointer experiences the force in the opposite direction. Due to the inertia of the pointer, it remains in its zero position.
• Compared to moving iron instruments, these instruments are costlier,
• Friction error due to Jewel and pivot suspension, temperature, aging of control springs, and permanent magnets might introduce errors.

Answer.3. A resistance of 0.502 Ω  must be connected parallel with the resistance of the ammeter

Explanation:-

As we know that in ammeter the resistance is connected is connected in parallel.

The resistance of milliammeter Rm = 10 ohms

Full scale deflection current Im = 5mA = 0.005A

To measure current upto 1 A. The Resistance of the shunt Rs 

R_s = I_mR_m ⁄ (I − I_m) = (0.005 × 10) ⁄ (1 − 0.005)

R_s = 0.5025 Ω

Answer.2. 2000

Explanation:-

Voltmeter sensitivity (ohm per volt ratings)

The sensitivity of a voltmeter is given in ohms per volt. It is determined by dividing the sum of the resistance of the meter (R_m) plus the series resistance (R_s), by the full-scale reading in volts. In equation form, sensitivity is expressed as follows:

Sensitivity (S) = (R_m + R_s) ⁄ V_fld

Now as per the given question

R_m= 400 KΩ

R_s = 100 KΩ

V_fld= 250 V

S = (400 + 100) ⁄ 250

S = (500 × 10³) ⁄ 250

S = 2000 ohm/volt

Answer:- 4. Conducting and Non-magnetic material

Explanation:-

Eddy Current Damping:

This is the most effective way of providing damping. It is based on the Faraday’s law and Lenz’s law. When a conductor moves in a magnetic field cutting the flux, e,m.f. got induced in it. And the direction of this e.m.f. is such as to oppose the cause producing it.

This force always acts in opposite direction to that of the cause producing it, that is, motion according to Lenz s law. This provides necessary damping torque. The magnitude of damping torque is directly proportional to the speed of the moving system.

In this method, a thin disc of non-magnetic conducting material like aluminum or copper is mounted on the spindle which carries the moving system and the pointer of the instrument. The disc is allowed to move along with the spindle inside the magnetic field provided by a permanent magnet. The disc is placed such that, when it rotates it cuts the magnetic flux between the poles of a permanent magnet Hence, eddy currents (eddy currents are currents produced on the surface of conducting materials are produced in the disc. According to Lenz’s law, the eddy currents produce a damping force in such a direction as to oppose the very cause of producing them. Since the cause here is the rotation of the disc, the eddy current forces, damp the movement of the disc and hence the pointer attached to the spindle. 

Answer.3. Permanent magnet type

Explanation:-

The PMMC type can be used for dc measurements only, and the induction type for ac measurements only.

Answer.4. None of the above

Explanation:-

Induction type wattmeter is used for the A.C measurement only whereas the Dynamometer type wattmeter is used for DC only.

Electrostatic type instrument does not come in the category of the wattmeter and it is used for voltage measurement.

Answer.1. Resistance is less

Explanation:-

Moving iron instruments are of two types: attraction type and repulsion type. In both types, the principle of operation is the force experienced by an iron piece in the presence of a magnetic field. The necessary magnetic field is produced by the ampere-turns of a current-carrying coil. When used as an ammeter the coil has fewer turns of thick wire so that the ammeter has low resistance.

When used as a voltmeter, the coil has the high impedance so as to draw a small current as possible since it is connected in parallel. Since the current through it would be small, it has to have a large number of turns to produce the necessary ampere-turns.

Answer.4.  Induction type

Explanation:-

Induction type instruments

• These instruments works on AC supply only. This is based on the principle of induction motor, therefore called induction type instrument.
• We can use these instruments as an ammeter, voltmeter, wattmeter and an energy meter.
• The scale of induction type instruments is quite uniform and extends over an angle of 300°.
• Currents up to 100 A can be handled by induction type instrument so that their power consumption is fairly large and cost relatively high.