Important Two Marks & 16 Marks Question and Answers

TWO MARKS QUESTION AND ANSWER

1. List the functional elements of the measurement systems. (Dec 2009)

The three main functional elements of the measurement systems are:

i. Primary sensing element

ii. Variable conversion element

iii. Data presentation element

2. Write the different types of systematic errors. (Dec 2009)

These types of errors are divided into three categories:

i. Instrument Errors

ii. Environmental Errors

iii. Observational Errors

3. What is standard? What are the different types of standards? (May/June 2006)

A standard is a physical representation of a unit of measurement. The term standard is applied to a piece of equipment having a known measure of physical quantity.

Types of Standards

i. International Standards (defined based on international agreement)

ii. Primary Standards (maintained by national standards laboratories)

iii. Secondary Standards (used by industrial measurement laboratories)

iv. Working Standards (used in general laboratory)

4. Why must instruments be calibrated? (Nov/Dec 2008)

Calibration of all instruments is important since it affords the opportunity to check the instruments against a known standard and subsequently to find errors and accuracy.

Calibration Procedure involve a comparison of the particular instrument with either

i. a primary standard

ii. a secondary standard with a higher accuracy than the instrument to be calibrated a instrument of known accuracy

5. What is mean by accuracy of an instrument? (May/June 2007)

It is the closeness with an instrument reading approaches the true value of the quantity being measured.

6. Define: Calibration. (April/May 2005)

Calibration is defined as the process by which comparing the instrument with a standard to correct the accuracy.

7. Why the PMMC instruments are not used for a.c measurement? (Nov 2006)

When the PMMC instruments are connected to a.c, the torque reverse as the current reveres and the pointer cannot follow the rapid reversals. Hence the deflection corresponding to mean torque is zero thus making the PMMC instrument not suitable for a.c measurements.

8. What is the basic principle PMMC instrument?

A current carrying coil is placed in the magnetic field experiences a force proportional the current it carries.

9. For which type of measurements PMMC devices are suitable.

These are suitable only for D.C. In A.C torque produced on coil is reversing which cannot give accurate reading.

10. What are the sources of error in DC voltage measurement? (Apr/May 2005)

i. Friction in the moving system

ii. Heat generated changes the resistance of the working coil.

iii. The ageing if the magnet and control spring.

11. List the possible causes of errors in moving iron instruments.

i. Hysteresis error

ii. Temperature error

iii. Stray magnetic field error

iv. Frequency error

v. Eddy current error

12. Define instrumental error. (Dec 07)

These errors are arises due to inherent short comings in the instrument, misuse of the instruments and loading effects.

13. Define limiting errors. (Dec 07)

Instruments having analog meters are usually guaranteed to be accurate within certain percentage limits called limiting errors or guarantee errors.

Limiting error = accuracy* full scale value

14. State the essential torques required for successful operation of an instruments.

i. Deflecting torque

ii. Controlling torque

iii. Damping torque.

15. Which torque is absent in energy meter? Why? (May/June 2006)

The controlling torque is not present in energy meter. As the disc of energy meter has to rotor continuously and there is no need to reset its position any time, the controlling torque is not required.

16. Give the importance of iron loss measurement. (May/June 2007)

Many apparatus like transformer, generator, and motor etc. use magnetic materials for their construction. The design of transform core, armature for motor and generator is very important. To have high efficiency, the losses must be as minimum as possible. Hence from ideal designing point

17. Name the methods used in wattmeter calibration.

The methods used in wattmeter calibration are:

i. Comparing with standard wattmeter.

ii. Using voltmeter ammeter method.

iii. Using Potentiometer.

18. Name the errors caused in Dynamometer type wattmeter.

i. Error due to pressure coil inductance

ii. Error due to pressure coil capacitance

iii. Error due to methods of connection

iv. Error due to stray magnetic fields

v. Error due to eddy current.

19. State the disadvantages of Dynamometer type wattmeter.

i. Readings may be affected by stray magnetic fields.

ii. At low power factor it causes error.

20. State the advantages of Moving iron type instruments

i. Less expensive

ii. Can be used for both dc and ac

iii. Reasonably accurate.

21. State the advantages of instrument transformers. (April/May 2008)

i. Used for extension of range

ii. Power loss is minimum

iii. High voltage and currents can be measured.

22. State the disadvantages of PMMC instruments.

i. Cannot be used for ac measurements.

ii. Some errors are caused by temperature variations.

23. What are the constructional parts of current transformer?

i. Primary winding

ii. Secondary winding

iii. Magnetic core.

24. Name the errors caused in current transformer.

i. Ratio error

ii. Phase angle error

25. Define dynamic error.

Dynamic error is defined as the difference between the true value of the quantity changing with time and the value indicated by the measurement system if no static error is assumed. It is also called measurement error. It is one the dynamic characteristic

26. Define threshold.

Threshold is defined as the minimum value of the input at which the output starts (20 changing/increasing from zero..

27. Mention the functions performed by the measurement system.

The functions performed by the measurement system are

i. Indicating function

ii. Recording function

iii. Controlling function

28. State the function of measurement system.

The measurement system consists of a transducing element which converts the quantity to be measured in an analogous form the analogous signal is then processed by some intermediate means and is then fed to the end device which presents the results of the measurement.

 

PART-B QUESTIONS

1. Describe the functional elements of an instrument with its block diagram. (MAY 05, 07, 09, 10 DEC 07,09,10) (16)

2. What are the three categories of systematic errors in the instrument and explain in detail. (MAY 06, 07, DEC 05, 06, 07,09,11)

3. Explain in detail calibration technique and draw the calibration curve in general. (MAY 04, 11, DEC 06, 07)

4. Describe the construction and working of a permanent magnetic moving coil instruments. (DEC 04, 06 MAY 05, 07)

5. Explain the design of three phase wattmeter and give the reactive power measurement in 3 circuits. (MAY 08)

6. How B-H curve is determined for a ring specimen. (DEC 08, 11, 10, MAY 04, 06, 07, 11)

7. Explain the function of three phase wattmeter and energy meter. (MAY 08)

8. Explain with a neat sketch the construction and working principle of 10 induction type energy meter. (DEC 03, 06, 09, MAY 05, 08, 09, 11)

9. With a neat diagram explain the construction and working of electrodynamometer instruments. Also derive its torque equation. (MAY 06)

10. Give detailed notes on Instrument transformers. (DEC 05, 09, MAY 11)

11. Explain the block diagram of Digital oscilloscope with a neat sketch, (MAY 07, 11, DEC 05)

12. Explain the working principle of various types of DAC with neat sketches. (MAY 04, 06, 07)