Measurement and Instrumentation: Important Interview Questions Part-2

1. What is multimeter?

Ans. It is an instrument which can be used to measure voltage, current and resistance. The instrument is provided with a selector switch which can be set for different modes of operation like measurement of voltage, current and resistance and also for various range of these qualities.

2. What are the advantages of moving iron instruments?

Ans.

1. Cheap and Robust: the instruments are robust owing to simple construction and also that there are no current-carrying moving part.

2. High accuracy.

3. Universal use: These instruments can be used for both AC and DC.

3. Why electrodynamometer type of instruments are used as transfer instruments?

Ans. A transfer instrument is one that may be calibrated with a DC source and then used without modification to measure AC. This requires the transfer type instrument to have same accuracy for both DC and AC which the electrodynamometer instruments have.

4. What is the torque equation of the electrodynamometer type of instruments? How is scale constructed for these instruments?

Ans. The torque equation of the electrodynamometer type of instruments is: 

,where θ varies from -45° to 45° from the positive of zero mutual inductance and as changes in the value of M over this range is not large and is practically constant over this range, the instrument shows almost a square law response, i.e.

5. What are the advantage of electrodynamometer type instruments?

Ans. 1. They have no hysteresis and eddy current errors as the coils are air cored.

1. They have precision grade accuracy upto 500 Hz. Lower grade instruments can be used for a frequency range of 15 to 1000 Hz.
2. Electrodynamometer type of instrument are very useful where rsm value is to be measured irrespective of waveforms.

6. What are the advantages of electrostatic instruments?

Ans. 1. They draw almost zero power form mains in case of DC and negligible power in case of AC.

1. They have no frequency or waveform errors.
2. There are no errors caused by stray magnetic field.
3. they are suited for high voltages.

7. what are the advantages of instrument transformers?

Ans. 1. When instrument are used in conjunction with instrument transformers, their readings do not depend upon their constants (R, L, C) as is the case with shunts and multipliers.

1. Current transforms have been standardized at 5 A secondary winding current and voltage transformers from 100 to 120 V secondary winding voltage.
2. With the standardization of C.T. and P.T. secondary winding rating, it is possible to standardize instruments around these rating and therefore reduction in cost of instruments.

8. How electrodynamometer type of instruments are used to measure power?

Ans. The two coils are connected in different circuits for measurement of power. The fixed coil are connected in series with load and so carry the current in the circuit. The moving coil is connected across the voltage and therefore carries a current proportional to the voltage. In this way fixed coils are called current coils of wattmeter and moving coil is called pressure coil of the wattmeter.

9. What is the effect of the inductance of the pressure coil circuit on wattmeter reading ?

Ans. On lagging loads the wattmeter will read high, as the effect of the inductance of the pressure coil circuit is to bring the pressure coil current more nearly into phase with the load current than would be the case if this inductance were zero. Very serious errors may be introduced by the pressure coil inductance at low power factors unless special precautions are taken. The wattmeter will read low when the load power factor is leading as in that case the effect of pressure angle between load current and pressure coil current.

10. What modifications are made in wattmeters for power factor measurements?

Ans. 1. Low power factor meters are designed to have a small control torque so that they give full scale control definition for power factors as low as 0.1.

1. The pressure coil circuit is designed to have a low value of resistance so that the current flowing through it is increases to give an increased operation torque.
2. A compensating coil is used in the instruments to connecting for the error caused by power loss in the pressure coil circuit.
3. Inductance of pressure coil circuit is compensated by connecting a capacitor across a part of series resistance in the pressure coil circuit.

11. How does power factor decides the readings of phase power using two wattmeter method?

Ans. P₁ + P₂ = 3 VI cos ɸ, P₁ – P₂ = √3 VI sin ɸ.

If power factor is pow then reading of P₂ would be such that P₁ – P₂ = √3 VI sin ɸ and P₁ + P₂ = 3 VI cos ɸ. In case of cos ɸ = 0.5; P₂ = 0 and of power factor is less reading of P₂ will be negative. Both equations is must be satisfied for any value of power factor so far low values of power factor one wattmeter reads negative.

12. How meter constant is defined for energy meter?

Ans. The total number of revolutions made by a watt-hour meter in a given interval of time is proportional to the energy supplied. Meter constant is defined as the number of revolution made per kilowatt-hour. Meter constant is marked on the meter.

13. Why braking torque is provided in energy meters?

Ans. In energy meter, the speed of the moving system is controlled by a braking system. The braking system consists of a permanent magnet so placed that it induced an eddy current in some parts of the moving system. The moving system attains a steady speed when the driving torque is equal to the braking torque.

14. What is creeping and how is it prevented in energy meters?

Ans. In energy meters, a slow but continuous rotation is obtained even when there is no current flowing through the current coil and only pressure coil is energized. This is called creeping. The major cause of creeping is over compensation for friction.

In order to prevent this creeping two diametrically opposite holes are drilled in the disc. The disc will come to rest with one of the holes under the edge of a pole of shunt magnet thus rotation is limited to a maximum of half a revolution.

15. What are primary and secondary transducers?

Ans. When there are two stages of transduction first stage transducer is called primary transducer and second stage transducer is called secondary transducer. In the case of a Bourdon’s tube, firstly pressure is converted into a displacement by Bourdon’s tube then the displacement is converted into an analogous voltage by LVDT. The Bourdon tube is called a ‘Primary transducer’ and LVDT is called a ‘Secondary transducer.’

16. What are active and passive transducers?

Ans. Active transducers are those which do not require an auxiliary power source to produce their output. They are also known as self-generating type since they develop their own voltage or current output. Passive transducers drive the power required for transduction from an auxiliary power source.

17. What is the inverse transducer?

Ans. An inverse transducer is defined as a device which converts an electrical quantity into a nonelectrical quantity. It is a precision actuator which has an electrical input and a low power nonelectrical output. A piezoelectric crystal acts as an inverse transducer.

18. Why the sensitivity of semiconductor strain gauge is high?

Ans. Semiconductor strain gauge has high gauge factor their action depends upon piezo-resistive effect. Their resistance charges due to charge in resistivity with strain.

19. What is the problem associated with semiconductor strain gauges?

Ans. 1. Semiconductor strain gauges are very sensitive to temperature.

1. Semiconductor strain gauges are more expensive and difficult to attach the object under study.

20. What are the requirements of conductor materials to be used in RTDs?

Ans. 1. The change in resistance of material per unit change in temperature should be as large as possible.

1. the material should have a high value of resistivity so that the minimum volume of material is used for the construction of RTD.
2. The resistance of materials should have a continuous and stable relationship with temperature. Platinum is the best in this regard.

21. How thermistors are different from RTDs and thermocouple?

Ans. In the case of thermistors, the negative temperature coefficient of resistance can be as large as several per cents per degree Celsius. This allows the thermistor circuit to detect very small changes in temperature which could not be observed with an RTD or a thermocouple.

22. What requirement a metal combination should fulfil in thermocouple?

Ans.  1. Should have a linear response and high sensitivity.

1. Should be physically strong to withstand high-temperature changes.
2. Should be able to withstand the effect of corrosive and reducing atmospheres.

23. What are the advantages of the thermocouple?

Ans. 1. Thermocouples are cheaper than RTDs.

1. They can be used for measuring high temperatures.
2. They can follow rapid temperature changes.

24. How LVDT works? What are its applications?

Ans. LVDT is a linear variable differential transformer. It measures the displacement of the core with respect to two secondary winding. Two secondary windings are connected so that output is the difference of the voltage induced in secondary windings. When there is no displacement, the voltage induced in two windings are equal and output is zero.

LVDT can be used in all application where displacements ranging from a fraction of an mm to a few cm have to be measured.

25. What are the advantages of LVDT?

Ans. 1. The LVDTs have a high range for measurement of displacement.

1. LVDT has less friction and high resolution.
2. Low power consumption effects.

26. What is the Piezo-electric effect?

Ans. A piezoelectric material is one in which an electric potential appears to cross the certain surface of a crystal if the dimensions of the crystal are changed by the application of a mechanical force. This potential is produced by the displacement of charges. The effect is reversible i.e. conversely, if a varying potential is applied to the proper axis of the crystal it will charge the dimension of the crystal thereby deforming it.

27. If the output of the sensor is very low then how can you measure?

Ans. If the output of the sensor is very low then it can measure by the use of an appropriate signal Conditioning circuits. This circuit takes the output of the sensing element and converts it into a form more suitable for further processing, usually a d.c voltage, d.c. current or a frequency signal.

Example: Deflection bridge, Amplifier, Oscillator.

28. Which machine we can use in place of ECG to measure the heart rate?

Ans. There are many instruments for measuring heart rate.

• An automated blood pressure unit also records heart rate.
• A pulse oximeter which is placed over the tip of the finger also record heart rate.
• A device called a Doppler may also used to record the heart rate.

29. What is the smart meter? How does it work? Which technology used in it?

Ans. A smart meter is an electronic device that records consumption of electric energy in intervals of an hour or less and communicates that information at least daily back to the utility for monitoring and billing. Smart meters enable two-way communication between the meter and the central system.

The smart meter uses a secure national communication network (called the DCC) to automatically and wirelessly send your actual energy usage to your supplier. This means households will no longer delay on estimated energy bills, have to provide their own regular readings, or have meter readers come into their homes to read the meter.

There are several benefits to smart meters:

1. More accurate bills
2. A better understanding of your usage
3. Faster and easier energy switching
4. Innovation energy tariffs.

30. What is the difference between storage and normal oscilloscope?

Ans. Digital storage oscilloscopes use digital memory Which can store data as long as required without degradation. They also allow complex processing of the signal by high-speed DSP circuit.

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