1 Chapter 01 Measurement And Error

2 Summary Instrument – a device or mechanism used to determine the present value of a quantity Measurement – a process of comparing an unknown quantity with an accepted standard quantity. Standard – an instrument or device having a recognized permanent (stable) value that is used as a reference.

The Measurement Process A process in which the physical parameters change the numbers significantly by using specific instruments. Each number means that the gain must be followed by the unit because the unit can show the characteristics of the physical parameters of the survey, as meters for length, kg for weight, etc 3

Measurement and calibration insruments Before making measurements Factors that need to be identified before measurement is: i.Procedure / method of measurement - Identify the parameters to be measured. For example, determine whether the current or voltage parameters. - The best way to measure, for example, if the height of the fluid in a glass tube should be measured, make sure 4

Measurement and calibration insruments glass tube in upright position. - The quantity depends on the purpose of measurement. To obtain high-quality measurement, usually more samples of measurement should be made to ensure the quality. - How to record the measurements. Measurement results shall be recorded in a neat and easy to understand. For example, using tables and graphs. 5

Measurement and calibration insruments ii. Criteria / parameter nature - must know the parameters measured. For example, alternating current (ac), or (dc), and other frequencies. iii. Quality measurement - Time and cost allocation. The time and cost effectiveness will contribute the quality of measurement. 6

Measurement and calibration insruments iii. Potential tools. The equipment selected must be of high quality and meet the specifications of measurement. For example, if the desired results are to three decimal places, therefore the measuring equipment must be able to meet the requrement. iv. Instruments - Instruments must be an appropriate choice for a measurement. For example, if they want to measure voltage, the instrument can be used is a voltage meter or an oscilloscope 7

Measurement and calibration insruments Current measurement Factors that have been identified during the measurement are: i) Quality - select the best tools - positions based data acquisition - insufficient number of data taken - whether the results are reliable? 8

Measurement and calibration insruments ii) Safety first Safety first - be aware of the possible effects electric shock - wary of the impact load excessive - determine the range and limits of the instrument for exceeded the instrument used not used - read the manual instruments 9

Measurement and calibration insruments iii) Sampling - look at the current parameter changes measurement - consider the value which should be selected when changing parameters -take adequate sample bilanagn and ensure that representative samples can accepted and recognized 10

Measurement and calibration insruments After measuring the The factors that need to be identified after the measurement process are: - Data must be analyzed mathematically / statistical - The results must be reported fully and accurately. 11

ELEMENTS OF MEASUREMENT SYSTEM a. Primary Sensing Element - A device which first receives energy from the measured medium and produce an output proportional to input quantity. - Output which is received from primary will send to data convertion element or data manipulation element or data transmission element. 12

ELEMENTS OF MEASUREMENT SYSTEM b. Variable Manipulation Element - used to manipulated the signal presented to it preserving the original nature of the signal. - a change in numerical value of the signal c. Data Presentation Element - The element that can be used for the presentation of data can be classified into three groups : indicators, illuminate display and recorders. 13

Measurement and Error a. Scale - Is a system of odered marks at fixed intervals used as a reference standard in measurement. - Example : a ruler with scales in inches and centimetres b. Range - The minimum and maximum input voltage limits within which a power supply will operate to specifications. 14

15 Error in Measurement There are various types of error in measurement: Gross error systematic error random error Absolute error Relative error

16 Error in Measurement What is the meaning of error? Please define

17 Error in Measurement Error - The difference or deviation from the true value of the measurable quantity.

18 % Error = true value - measured value   100% true value % Accurate = 100% - % Error Refer to the circuit below, calculate the error percentage and accurate percentage if meter ampiar show the value of 6 mA. 1K  15V mA 2K  Error in Measurement

19 Error in Measurement Errors are generally categorized under the following three (3) major headings: Gross Errors Systematic Errors Random Errors

20 Error in Measurement 1.Gross Error generally the fault of the person using the instruments such as incorrect reading, incorrect recording, incorrect use etc. They can be avoided only by taking care in using and reading all instruments.

21 Error in Measurement 2.Systematic Error Devided into two different categories:- i) Instrumental errors are errors inherent in measuring instrument because of their electrical and mechanical structure.Example : stretching of the spring. Instrumental error may be avoided by :- 1) selecting a suitable instrument for the particular measurement application. 2) Calibrating the instrument against a standard. ii) Environmental errors are due to condition external to the measuring device, including conditions in the area surrounding the instrument. Example :temperature, humidity and air pressure.

22 Error in Measurement 3.Random Errors These errors are due to unknown causes and occur even when all systematic errors have been accounted for. This variation cannot be corrected by any method of calibration or other known method of control. Random errors may be avoided by increasing the number of reading and using statistical means to obtain the best approximation of the true table of the quantity under measurement.

23 ABSOLUTE ERROR AND RELATIVE ERROR If resistor is known to have a resistance of 500Ω with a possible error of  50Ω, the  50Ω is an Absolute error. Relative error – is a percentage or as a fraction of the total resistance. Example: Measured voltage = 20 V  0.1% (Relative error) = 20 V  0.02V (Absolute error) Percentages are usually employed to express errors in resistances and other electrical quantities.

24 Error in Measurement Absolute errors. Absolute errors maybe defined as the difference between the expected value of the variable and the measured value of the variable, or e = Yn – Xn where: e = absolute error. Yn = expected value. Xn = measured value

25 Error in Measurement Example 2 The expected value of the voltage across a resistor is 5.0 V. However, measurement yields a value of 4.9 V. Calculate: a) absolute error b) % error c) % accuracy

26 CHARACTERISTIC OF MEASUREMENT Accuracy – The degree of exactness of a measurement compared to the expected value Precision – A measure of consistency, or repeatability of measurements Resolution – The smallest change in measured quantity that can be observed.

27 CHARACTERISTIC OF MEASUREMENT Significant figure – An indication of the measurement is obtained from the number of significant figures in which the result is expressed.Significant figures convey actual information regarding the magnitude and the measurement precision of a quantity.The more significant, the greater the precision of measurement.

28 Measurement Standards Measurement Standards are classified in four level :- International standards. Primary standards. Secondary standards. Working standards.

29 International standards. Are defined by international agreement, and are maintained at the International Bureau of Weight and Measure in France. These are as accurate as it is scientifically possible to achieve. They may be used for comparison with primary standard, but otherwise unavailable for any application.

30 Primary Standard Are maintained at institutions in various countries around the world, such as the National Bureau of Standards in Washington. These are as accurate as it is scientifically possible to achieve. They are also constructed for the greatest possible accuracy, and their main function is checking the accuracy of secondry standard.

31 Secondary standard Are periodically checked at the institutions the maintain primary standards. Are employed in industry as reference for calibrating high accuracy equipment and component and for verify the accuracy of working standards.

32 Working Standard The working standards are the principal tools of a measurement laboratory.They are used to checked and calibrated general laboratory instrument for accuracy and performance or to perform comparison measurement industrial application.

33 Summary Secondary standards are more accurate than working standard and are used throughout industry for checking working standards and for calibrating high accuracy equipment. Primary standards are more accurate than secondary standards.They are maintained to the highest possible accuracy by national institutions as references for calibrating secondary standards. International Standards are maintained by international agreement and may be used for checking primary standards.

EXAMPLES 1 A voltmeter, having a sensitivity of 1,000  /V, reads 100 V on its 150 V scale when connected across an unknown resistor in series with a milliammeter. When the milliammeter reads 5 mA, calculate a) apparent resistance of the unknown resistor, 34

b) actual resistance of the unknown resistor c) error due to the loading effect of the voltmeter. 35

EXAMPLE 2 Repeat the above example, if the milliammeter reads 800 mA and the voltmeter reads 40 V on its 150 V scale. 36