Measurement and instrumentation. Module 1: Measurements & error analysis

Transcription

1 Measurement and instrumentation Module 1: Measurements & error analysis

2 Watch the following video and answer the questions

3 How would you The length of an eraser? measure...? Inches Feet Yards Meters

4 How would you measure...? The amount of water that will fill a tub? Cups Pints Gallons

5 How would you measure... the length of your finger?! Yards Centimeters Feet Meters

6 How would you measure...? the weight of this backpack when filled with books? Grams Ounces pounds Kilograms

7 Measurement You are making a measurement when you! Check you weight! Read your watch! Take your temperature! Weigh a cantaloupe!!! What kinds of measurements did you make today?

8 Some Tools for Measurement

9 Objectives Define 'measurement' Give examples of measurement applications in the physical world Identify measurement elements Differentiate between the types of measurement Differentiate between systematic and random errors Perform spastically error analysis on a set of measurements. Define the major characteristics of an instrument/ device and determine them through experiments Describe the concept of 'calibration'.

10 Introduction to Measurement

11 Introduction to measurements All physical parameters are measurable, and the first step in any engineering process is measurements. The concept of measurement is so deep rooted that today there are attempts to measure even abstract quantities like intelligence.

12 Introduction to measurements If we cannot measure something, it is pointless to try to control it, this 'something' is referred to as 'physical quantity'.

13 Introduction to measurements Measurements usually takes one of the following forms especially in industries: Physical dimension of an object. Count of objects Temperature of an object Fluid pressure/ flow rate /volume. Electrical voltage, current, resistance. Machine position/ speed and so on,

14 Example of industrial measurements

15 What is Measurements??

16 What is Measurements?? Measurement is: The process of determining the magnitude of a physical quantity such as length or mass, relative to a standard unit of measurement, such as the meter.

17 What is Measurements?? Measurement is: It could also be defined as 'the comparison between a standard and a physical quantity to produce a numeric value' Look at the figure below

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19 Introduction to measurements The measuring devices could be sensors, transducers or instruments.

20 a ruler provides measurement of length thermometer gives an indication of the temperature a light dependent resistor (LDR) is used to measure light intensity.

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22 xamples of measuring instruments

23 (1) Identify the measurement elements

24 Skill 1: identify the measurement elements Observe the examples in the figures given. And identify the physical quantity, measuring device, numeric value, and the standard unit of measurement. Write your answers in the table provided

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27 Try to give me

28 Online activity

29 Online activity Watch the following web link, and complete the table below: learningsteps/fcdlc/launch.html

30 Online activity

31 Measurements types

32 Measurements types The two basic methods of measurements are: 1- Direct measurements 2-Indirect measurements.

33 Direct measurements In direct measurements, the physical quantity or parameter to be measured is compared directly with a standard. The examples in the following figure illustrate the direct measurements method.

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36 Indirect measurements Here the physical quantity (measured) is converted to an analog form that can be processed and presented.

37 Indirect measurements For example, the mercury in the thermometer expands and contracts based on the input temperature, which can be read on a calibrated glass tube. A bimetallic thermometer produces a mechanical displacement of the tip of the bimetallic strip,as a function of temperature rise.

38 Indirect measurements

39 Measurements Errors and Analysis

40 Measurements Errors and Analysis The term error in a measurement is defined as Error = instrument reading - true value

41 Error types

42 Error types Measurements errors may be classified into two categories: Systematic error Random error Next

43 Systematic errors These errors affect all the readings in a particular fashion.

44 Systematic errors Systematic errors may arise due to different reasons such as, the zero error of the instrument, the shortcomings of the sensor, improper reading of the instrument due to the improper position of the person's head/eye (parallax error), the environment effect, and so on. Systematic errors can be corrected by calibration.

45 Systematic errors

46 Systematic errors The major feature of systematic errors is that the sources of errors can be traced and reduced by carefully designing the measuring system and selecting its components.

47 Random errors Random errors are errors that result in obtaining different measured values when repeated measures of a physical quantity are taken.

48 Random errors A example of random error is measuring the mass of gold on an electronic scale several times, and obtaining readings that vary in a random fashion.

49 Random errors The reasons for random errors are not known and therefore, they cannot be avoided. They can only be estimated and reduced by statistical operation.

50 Error analysis

51 Error analysis The two most important statistical calculations that could be used for the analysis of random errors are average or mean and the standard deviation.

52 Error analysis If we measure the same input variable a number of times, keeping all the other factors affecting the measurement the same, the measured value would differ in a random way. But fortunately, the deviations of the readings normally follow a particular distribution and random error may be reduced by taking the average or mean. The average / mean gives an estimate of the 'true' value.

53 Error analysis

54 Suppose the mass of gold was recorded at different instants as in the table, then the arithmetic mean would be the average of all the readings.

55 Error analysis The standard deviation, denoted by the letter ' ' tells us how much the individual readings deviate or differ from the average/ mean of the set of readings. The distribution curve for a number of readings of a same variable takes the nature of a histogram.

56 The following figure shows the normal (Gaussian) distribution curves for different set of readings.

57 The example below shows how to calculate the standard deviation.

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59 Solve the quiz on Plato

60 Conduct lab activity 1 page 17

61 Use the following table to calculate the resistor values using colour coding

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63 For more help, check the following link

64 To convert the units of measuring resistors value use the following example

65 To convert the units of measuring resistors value You can visit the following web link, to find out how to convert the different units of resistance. Convert-mega-ohm-to-kilo-ohm.htm!

66 Lab activity 1 I need a report Form each group after you finish your lab activity

67 Instrument Performance Evaluation

68 Instrument Performance Evaluation Any measuring instrument/ device has a set of specifications that inform the user of its function. These characteristics are described in the catalogue or data sheet provided by the manufacturer. One most be familiar with these parameters, the operation of the device /instrument, and method of eliminating errors.

69 Characteristics of instruments

70 Characteristics of instruments Accuracy: Accuracy of an instrument is how close a measured value is to the true value.

71 Accuracy Mathematically, it can be expressed as the maximum absolute deviation of the reading from the conventional true value (CTV). This is called the absolute accuracy.

72 Accuracy

73 Accuracy

74 Characteristics of instruments Precision: The ability of an instrument to give the similar reading when the same physical quantity is measured more than once is called precision.

75 Precision The closer together a group of measurements are, the more precise the instrument. A smaller standard deviation result indicates a more precise measurement.

76 Precision Mathematically, it can be expressed as the absolute maximum deviation from the average of readings.

77 Precision

78 Accuracy and Precision

79 Characteristics of instruments Bias: The difference between the conventional true value (CTV) and average value (VAV). Ideally, the bias should be zero.

80 Characteristics of instruments Range: The range of an instrument defines the minimum and maximum values that the instrument can measure.

81 Range The maximum reading of the instrument is : 120 F The minimum reading of the instrument is : 40 F Range = Maximum reading - Minimum reading = = 80 F

82 Characteristics of instruments Sensitivity: The sensitivity of a measuring instrument is the ability to detect small changes in the measured quantity. Sensitivity = Change in Output / Change in Input

83 Characteristics of instruments Linearity: Some measuring instrument/ devices output a signal that is proportional to the input physical quantity. These instrument are called linear devices.

84 Linearity Other instruments that don't have a proportional relationship between the output signal and the input signal are non-linear devices.

85 Linearity

86 A voltmeter is used for reading on a standard value of 50 volts, the following readings are obtained : 47, 52, 51, 48

87 If we re-calibrated the instrument to remove the bias, then the average reading = CTV. The new readings would then be 49, 51, 52, 48. Hence, after re-calibration, average = CTV = 50 volts.

88 Calibration

89 Calibration Calibration is a process of comparing the performance of an instrument/device with a standard.

90 Calibration All working instruments must be calibrated against some reference instruments which have a higher accuracy.

91 Calibration Calibration could be performed by holding some inputs constant, varying others, recording the output (s), and developing the input-output relations. The relationship between the input and the output is given by the calibration curve.

92 Calibration

93 Calibration There are several reasons why you need to calibrate a measurement device. Can you think of some of them??

94 Calibration One of the reasons is to reduce systematic errors. By comparing the actual input value with the output value of the system, the overall effect of systematic errors can be observed. The errors at those points are then made zero by adjusting certain components or by using software corrections

95 Calibration The other reason could be because the device cannot be used unless the relationship between the input radiance and the output variable is known.

96 Calibration The characteristics of an instrument change with time based on temperature and other environmental conditions. So, calibration process has to be repeated at regular intervals for accurate readings.

97 Conduct lab activity 2 page 21

98 Lab activity 2 I need a report Form each group after you finish your lab activity

99 Don't forget your homework