BASIC PRINCIPLES OF MEASUREMENTS FOR ME.pptx

Mechanical engineering

1. BASIC PRINCIPLES OF MEASUREMENTS

2. INTRODUCTION TO MEASUREMENT Measurement plays an important role in every branch of scientific research and industrial processes interacting basically with the control systems. Measurement involves precise measurement and recording of physical, chemical, mechanical properties. The measurement involves always to control.

3. Measurement is just think of man made product. Example: consider a vehicle. The following are to be measured: Speed of Travel: This is measured for the safety of the person driving the vehicle by using Speedometer. Distance travelled: This is measured to know the number of kilometers the vehicle has travelled by using Odometer. Fuel level: This is measured to know the quantity of fuel available in the tank of the vehicle that is being is used. Engine temperature: This is measured to know the temperature of the engine for safety. Measuring coolant temperature.

4. What is Instrumentation Instrumentation is a branch of engineering science that deals with techniques used for measurement, the measuring devices used and the problems are associated with the techniques used for measurement. Instrument: The instrument serves as an extension of human facilities and enables the man to determine the value of unknown quantity or variable .

5. What is Measurement It is defined as the comparison between a predefined standard and an unknown quantity. The physical quantity which is to be measured is termed as Measurand. Measurement involves the use of instruments as physical means of determining the variables.

6. BLOCK DIAGRAM OF MEASUREMENT Standard (Known quantity) Measurand (Input signal magnitude) Comparison process Result (Numerical value)

7. Measurement System Instruments consisting single unit are sufficient to give output reading corresponding to the given input( measurand ). More complex measurement situations an instrument with several elements need to used. These elements may consists of transducing elements which convert the input to analogue form. The results of the measurement for the purpose of display and control. All the above set of elements constitute a measurement System.

8. GENERAL CONCEPT IN MEASUREMENT The following are the general concept of measurement 1. Measurement Methods 2. Application of measuring system/ instruments

9. 1. Measurement methods Measurement of methods are classified in to two types Direct comparison method Indirect comparison method a) Direct comparison method: In this method the measurand ( unknown quantity) is directly compared with the standard. The measurement is a Number and a Unit. In this method is used for measuring physical quantities such as Time, Mass, Length etc. EX: Measuring the Length of wire. Unit of length is Meter

10. b) Indirect comparison method: In many applications indirect comparison method is used for the Measurent system. In this measurement system by using Transducer. By using transducer convert one form to another form (analogous signal) without changing the information content. The analogous signal is then processed and sent to the end devices, which gives the result of measurement. In this method, the input signal is converted to some other form and then compared with the standard.

11. Methods of Measurent can also be classified as Primary Measurement Secondary Measurement Tertiary Measurement 1) Primary Measurement: Only subjective information is provided in this method. These measurements are made by direct observation , They do not involve any translation of information. EX: i ) One vessel is cooler than other. ii) One rod is longer than other

12. 2) Secondary Measurement: In this method the output result is obtained by one translation. EX: i ) Conversion of measurand in to length. ii) Bellows gauge iii) Spring loaded with weight iv) Bourdon’s Gauge Pressure Length primary Signal Secondary signal Input Or Measurand TRANSLATION OUPUT OR MEASURED VALUE

13. 3) Tertiary Measurement: In this Method the output result is obtained by two translations EX: Electric Tachometer Input (Measurement) is converted in voltage (first translation) and this voltage is converted to length( Second translation). Speed voltage Length primary signal Secondary signal Tertiary signal Input Or Measurand Tachometer First Translation Volt meter Second Translation output Or Measurand

14. b) Application of measuring system or instruments Measuring instruments are used in the following three areas Monitoring of Processes and operations Controlling of processes and operations Experimental engineering analysis

15. a) Monitoring of Processes and operations: Some of the measuring instruments perform the function of monitoring EX: Thermometer, Pressure gauge, Water, Gas and Electric meters, Odometer and Tachometer. b) Control of processes and operations: The Instrument is used as a component of an automatic control system. EX: A Home heating system and a Refrigeration system. c) Experimental Engineering Analysis: There are two methods to solve solving engineering problems namely i ) Theoretical methods ii) Experimental methods

16. Generalised Measurement System and its Elements Measurement system Input signal Output signal ( Measurand ) (Measurement) The main functions of an instrument are follows Getting the information Processing the information and Presenting the information to a human observer

17. STANDARDS OF MEASUREMENT Introduction: The result of measurement to be meaningful one of the conditions to be satisfied is that the standard being used for comparison should have common acceptability. International level, progress has been made ta adapt uniform standards of measurement. The term STANDARD represents the physical object which serves to preserve the value of some unit. Or Standard is the physical representation of the unit of measurement .

18. Classification of Measurement Standards Measurement of standards classified in to four types International Standards Primary Standards Secondary Standards Working Standards

19. International Standards: These Standards are maintained by the International Bureau of Weights and Measurements at France. These standards represent the units of measurement of various physical quantities. The highest possible accuracy is maintained. For the purpose of day to day comparison and calibration, these standards are not available.

20. b) Primary Standards: The ‘National laboratories or Standard Organisations’ at various parts of the world maintained these standards. The ‘National Physical Laboratory’ at delhi maintains these standards in india . Primary Standards are maintained for three reasons i ) To check the secondary reference standard ii) To calibrate the secondary reference standard iii) To certify the secondary reference standard The primary standards are the basis of refercence the following factors are considered Material used should highly resist change in dimension due to low/high temperatures. Material characteristics should should not get effected due to environmental changes. machining operations done on the material should yield accuracy.

21. c) Secondary standards: The Regional laboratories and industrial measurement laboratories maintain these standards. These standards are derived from the primary standards. d) Working standards: These standards are used by the worker to check/test the manufactured products. Working standards are to be checked and certified against the primary or the secondary standards.

22. Measuring Instruments Measurement means getting to know about physical quantities such as Length, Weight, Temperature, Pressure, Force. For measurement an instrument is necessary. A measuring instrument is a system/device that has many components a particular operation/function. Measuring instruments have specific names and are used to obtain dimensional or surface measurement. Instruments are also used to variables in a system. Each instrument works on different principles.

23. The variables are two types Variables constant with time Variables varying with time Variables constant with time: Ex: measurement of weight of an object. ii) Variables varying with time: Ex: measurement of pressure inside an Ic engine

24. Classification of Measuring Instruments: Measuring Instruments are mainly classified in to Based on their Applications Mode of operation Manner of energy conversion nature of out put signal

25. Instruments are classified as follows Mechanical instruments Electrical instruments Electronic instruments Deflection and Null type instruments Analog and Digital type instruments Self generating and power operated instruments Automatic and power operated instruments Contacting and Non Contacting type instruments Remote indicating measuring instruments Intelligent instruments

26. Mechanical instruments: In this instruments do not require an external power source. These instruments are simple in construction. These instruments do not respond quickly to Dynamic and Transient conditions. They can’t measure the frequency of 50Hz and lesser. b) Electrical instruments: In these instruments quick out put compare to mechanical instruments. Mechanical devices are used to indicate the records.

27. c) Electronic instruments: These instruments are respond quickly to dynamic and transient conditions. These instruments are Light in weight and very compact. Consumes less power and highly reliable and costs less . Remote indication is possible. Non contact measurement is also possible.

28. d) Deflection and Null type instruments: Deflection type instrument: EX: Wheatstone Bridge Circuit (Electrical circuit) Advantages and Limitations: Simple Construction. Dynamic Respond is Good. Loading of the source which results in loading error.

29. ii) Null type instruments: EX: Beam Balance (Mechanical Device) Advantages and limitations: Do not Alter the variable being measured. These instruments are slow. Dynamic response is poor.

30. e) Analog and Digital type instruments: Analog type instruments: A signal is said to be Analog. Transducers are used. EX: Voltage - Voltmeter Pressure - Pressure Gauge ii) Digital Type Instruments: A signal is said to be Digital. Out put is displayed numerically as digits. Direct reading is required. Low voltage for operation. Digital value is to be coded.

31. f) Self generated and power operated instruments: Self Generating instruments: They donot require any external power source. Energy required is completely meet from the in put signal itself. Ex: Mercury in glass thermometer. Bourdon tube pressure gauge. Dail indicator. ii) Power operated instruments: External power source is used. External power source may be compressed air, Electrically, Hydraulic. EX: LVDT to measure the displacement.

32. g) Automatic and power operated instruments: i ) Automatic operated instruments: Do not depend on the operator’s service. Auxiliary devices are used to eliminate the human service. Readings are automatically. EX: Mercury thermometer. Automated null balance instruments. ii) power operated instruments: Depend on the Human operator. EX: Resistance thermometer. Null balance instruments.

33. h) Contacting and Non Contacting type instruments: i ) Contacting type instruments: Directly contact with measured medium. Ex: Mercury in glass thermometer Bourdon tube pressure gauge Thermocouple Voltmeter ii) Non-contacting instruments: Does not contact with measured medium. EX: Optical pyrometer Total radiation pyrometer (Measured temperature)

34. i ) Remote indicating measuring instruments: These instruments are used when the control on process is required globally. Two sensing elements are used. One sensing element will be at one place and the secondary sensing element will be at a different place. EX: Control of pressure and temperature on certain boilers an a regular basis. j) Intelligent Instruments: Micro processor is used. The microprocessor enables pre-programmed signal processing and application data manipulation algorithms to the measured variables.

35. Errors Introduction: Error is the difference between the measured value and the true value. Error = measured value - true value Error may be positive or negative. The instruments reads higher than the true value is called as positive error . The instruments reads Lower than the true value is called as Negative error. Error helps in reducing them and helps in finding the reliability of the results.

36. Types of Errors: Errors are classified as follows Systematic errors or Fixed errors i ) Calibration Errors ii) Human Errors(observation and operational) Loading Errors(System interaction errors) Errors of Techniques Experimental errors b) Random or Accidental errors Errors of Judgement Variation of judgements c) Illegitimate errors Blunder or Mistakes Computational errors Chaotic errors

37. Systematics Errors: These errors that occur due to the use of improper procedures/conditions . i ) Errors due to calibration: Calibration is a process of giving a known input to the measurement system and taking necessary action to see that the output of the measurement system. The instrument not calibrated properly, it will show reading with a higher of error, This is called as calibration error. ii) Human errors: There are two human errors a) Observation error b) Operational error

38. iii) Loading errors: The signal source is always altered by the act of measurement this effect is called Loading. The measured quantity loses energy due to the act of measurement error is introduced this is called loading error. iv) Errors of technique: Improper use of the exact technique for executing an operation leads to this type of error. v) Experimental errors: These errors are due to the equipment/instrument. The accuracy of an instrument is affected due to limitations in its design and construction.

39. b) Random errors: The sign may be negative or positive. The measured value for the same input to the instrument. These errors are caused due to defects in the elements of the measuring system such as friction and a very high tolerance in mating parts. i ) Error of judgement: These errors are making judgement of the results of an instrument. They are different in comparison with human errors in sign. ii) Variation of conditions: The environmental conditions such as pressure, temperature, humidity.

40. c) Illegitimate errors: These errors are due to blunders on the part of the person using the instrument. it may also faulty instrument, faulty judgement, improper use of instrument . Blunder or Mistakes: The instrument might commit a blunder in using the instrument or adopting the right procedure. ii) Computational errors: The Human being performing calculation might commit a mistake which leads to this error. iii) Chaotic errors: Random disturbances such as vibrations, noises, shocks.

41. PERFORMANCE CHARACTERISTICS OF INSTRUMENTS The instrument is designed for a specific task, subject of performance criteria plays an important role. The detailed specifications of the functional characteristics of any instrument are termed as its performance characteristics. There are two types 1) Static Characteristics 2) Dynamic Characteristics

42. 1) STATIC PERFORMANCE CHARACTERISTICS The input quantity which is to be measured constant or vary quite slowly with time. The static performance characteristics can be obtained by a process called static calibration. Static calibration is a process in which all the input parameters except one parameter kept constant. The range of out put values are obtained for a given range of input and relation is established between the input and the output.

43. Accuracy Precision Error Linearity Repeatability Back slash Reliability Drift Reproducibility Range Resolution Sensitivity Dead space Threshold Lag Hysteresis

44. Accuracy: The accuracy of measuring device may me defined as the extent to which the reading given by is closed to the exact value denoted with +1. Percentage of true value or relative error = Measured value-True value x 100 True value Percentage of full scale deflection or Error = Measured value-True value x 100 Maximum scale value

45. Error: The error is differ between measured value and true value of the quantity. Repeatability: It is the ability to display the same reading as long as the sensor element is fed with same signal . Reliability: The reliability of a measuring system is the probability that it will operate with an agreeable accuracy under the condition specify for its operation. Ex: Thermometer ERROR= MEASURED VALUE – TRUE VALUE

46. Reproducibility: the reproducibility of a measuring device is the ability of the same reading over a period of time. Ex: Speedometer Range: The range is instrument limit between the minimum and maximum. Resolution: It is the smallest change quantity measure that will produce observable in the reading of instrument Ex: Sensitive balance(Chemical balance) Sensitivity: It is an instrument its response any change in the quantity being measured. Static sensitivity = Change of output signal/Change of input signal

47. Dead space: It is a measuring is the range of values of the quantity being measured for with give no reading. Ex: Diesel tank of train engine Threshold: It is the minimum level of quantity that is being measured which has to be reached before the instrument response gives a detachable reading. Lag: it is the time interval between the time of display of the reading . Hysteresis: it is the characteristics which makes an instrument give different reading for the same value of a measured quantity depending upon weather the value reached by a continuously increasing or decreasing change.

48. Precision: It is defined as the ability of the instrument to reproduce certain set readings with in the given accuracy. Precision represents the degree of repeatability of several independent measurement of the input under same environmental conditions. Linearity: It is defined as the ability of an instrument to reproduce the input characteristics symmetrically and this can be expressed by the equation of a straight line.

49. Back lash: It is defined as the maximum distance or angle through which any part of an instrument moves in one direction without causing any movement in the adjacent part. Drift: It is defined as the variation of out put for a given input caused due to change in the sensitivity of the instrument to certain interfering inputs like temperature changes component instability.

50. 2) DYNAMIC PERFORMANCE CHARACETERISTICS The input quantity which is to be measured is not constant but varying rapidly with time. Some differential equations are used in dynamic performance characteristics. The performance criteria based on these dynamic relations are termed as dynamic performance characteristics.

51. Different types of dynamic inputs are a) Periodic input: This varies cyclically with time. EX: Harmonic motion. b) Transient input: This signal is of definite duration and after a certain perid of time it becomes Zero. EX: Forces due to explosion or transient. c) Random input: This type of input varies randomly with time, with no definite period of time and amplitude. EX: Pressure fluctuations in fluid flow due to turbulence.

52. a) Periodic signals b) Transient input c) Random input

53. Speed of Response: It is the rapidity with which an instruments responds to the changes the quantity to be measured. Measuring Lag: It is the retardation or delay in the response of an instrument to a change in the input signal. Fidelity: It is defined as the degree of closeness with which the system indicates the signal which is impressed on it. Dynamic error: The difference between the true value of the quantity which is varying with time and the reading of the if no static error is assumed.

54. Overshoot: An instrument is said to exhibit overshoot if the moving parts of it do not immediately come to rest in the final deflected position. Dead time: The time required for an instrument to begin responding to change in the measured quantity is called dead time . Dead zone: Dead zone is the largest change in the measurand to the does not respond.