errors of measurement and systematic errors

errors of measurement and systematic errors

1. errors of measurement and systematic errors

2. What is a measurement? A measurement tells us about a property of something. A measurement gives a number to that property. Measurements are always made using an instrument of some kind. The result of a measurement is normally in two parts: a number and a unit of measurement, e.g. ‘How long is it? ... 2 metres .’ www.theapprentiice.com

3. What is Error …? Error in science does not mean the terms of mistake . • Error in a scientific measurement means the different between the individual result and true value. • Errors cannot be eliminated although the measurement is being done very carefully . • The total value of error is made up of a number of error source.

4. THE ROLE OF ERROR Repeated measurement will contribute the discrepancy or random errors. The discrepancy can only be obtained when there are differences between the readings and the true value. The smaller the random errors, the greater the precision. • If the individual readings are the same, there still an error called uniform error or systematic error.

5. Random Error a) A component of the error of measurement which, in the course of a number of measurements of the same measurand , varies in an unpredictable way. b) The mean of a large number of measurement influenced by random errors matches the true value. c) It can be evaluate by study the repeated measurement values.

6. Systematic Error a) The exist of the error is known by inference. b) A component of the error of measurement which, in the course of a number of measurements of the same measurand , remains constant or varies in a predictable way c) The mean of a large number of measurements influenced by systematic errors deviates from the true value. d ) It can be evaluate by comparing the measurement results with a higher standard, which is measurement.

7. Sources of Measurement Errors Dynamic error Characterised by frequency and phase response of the system for periodic variations in the measured input. Loading error It is the difference between the value measured before and after the measurement system is measured.

8. Static error It is cause by physical nature of various components of the measuring system. Characteristic error It is the deviation of measurement under constant environmental conditions from the theoretical predicted performance.

9. Elastic deformation It is divided into two ; a)Error cause from reflection when end gage is used for setting or measure. b)Error cause from deflection due to self weight of the object.

10. Parallax Any instrument that using pointer and scale may have parallax error because the gap between pointer and scale is different at any reading angle. Contact pressure While measuring, the pressure at contact causes some penetration causing error in measurement.

11. Backlash Due to backlash in gears and screw threads, some motion is lost to overcome backlash Hysteretic It is a source of errors in electrical instruments. Ascending values are observed when decrease current or voltage.

12. Avoidable error The errors occurred due to non-alignment of workpiece centre, improper of measuring instruments, etc. Human Error Difficult to detect. It can be include a tendency to read high or low using a wrong instrument. Human training is the best way to prevent these error

13. Errors in Technique and Experimental Error If wrong techniques is used. Example: Calibration technique for vernier is used for micrometer. Education helps to prevent these errors. Computational Error Can be random or continuous, but, once an error has started, it usually establishes itself in the computation. This error is affected by environmental, fatigue and instrumentation.

14. Chaotic Error Extreme disturbances that ruin or hide the measurement results. This error include vibration, shock, extreme noise and etc.

15. Uncertainty of measurement www.theapprentiice.com

16. What is uncertainty of measurement? The uncertainty of a measurement tells us something about its quality. Uncertainty of measurement is the doubt that exists about the result of any measurement. www.theapprentiice.com

17. Expressing uncertainty of measurement Since there is always a margin of doubt about any measurement, we need to ask ‘How big is the margin?’ and ‘How bad is the doubt?’ Thus, two numbers are really needed in order to quantify an uncertainty. One is the width of the margin, or interval. The other is a confidence level, and states how sure we are that the ‘true value’ is within that margin. For example: We might say that the length of a certain stick measures 20 centimetres plus or minus 1 centimeter, at the 95 percent confidence level. This result could be written: 20 cm ±1 cm, at a level of confidence of 95%. The statement says that we are 95 percent sure that the stick is between 19 centimeters and 21 centimeters long. www.theapprentiice.com

18. Error versus uncertainty It is important not to confuse the terms ‘error’ and ‘uncertainty ’. Error is the difference between the measured value and the ‘true value’ of the thing being measured. Uncertainty is a quantification of the doubt about the measurement result . Whenever possible we try to correct for any known errors: for example, by applying corrections from calibration certificates . But any error whose value we do not know is a source of uncertainty . www.theapprentiice.com

19. Why is uncertainty of measurement important? to make good quality measurements to understand the results. You may be making the measurements as part of a: calibration - where the uncertainty of measurement must be reported on the certificate test - where the uncertainty of measurement is needed to determine a pass or fail or to meet a tolerance - where you need to know the uncertainty before you can decide whether the tolerance is met ... or you may need to read and understand a calibration certificate or a written specification for a test or measurement. www.theapprentiice.com