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MEASURING ERRORS. INTRODUCTION. There is no perfect measure. Measured values should never simply read and recorded. True Value : It is the theoretical value of the measured, which is free of errors.
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INTRODUCTION • There is no perfect measure. Measured values should never simply read and recorded. • True Value: It is the theoretical value of the measured, which is free of errors. • Exact value: It is the value of the measured that can be obtained with the highest measuring accuracy attainable in practice. • Actual value: It is the value of the measured which obtained through measurements with the permissible error.
Error: It is the difference between the actual value, and the true value. As the true value could not be determined, error could not be determined. • Uncertainty: It is the best estimate of error. • Measured values are expressed as • Actual value ± uncertainty • Errors are Classified as :- • - Gross error. • - Systematic error. • - Random error.
GROSS ERRORS • This class of errors mainly cover human mistakes • Misreading of the scale. • Error in recording the results. • Calculating error. • This type of error could not be treated mathematically, but can be avoided by taking many readings of the same quantity be different operators. Take at least three separate readings.
SYSTEMATIC ERROR (BIAS) • This error is constant or of a systematic nature. • Environmental Errors • These include the effects of temperature, humidity, dust, vibration, or external magnetic and electrostatic field. • This error may be eliminated by make an arrangement to keep these conditions within the specified limits. For example, Standard measuring temperature = 20°C. • Sensing Errors • Sensing capability of the observer may affect the accuracy of readings.
Parallax Errors • This error arise on account of pointer and the scale not being in the same plane, and could be minimized by using mirrored scale bent pointer, bevel the scale edge.
Loading Errors • Connection of the instrument in the system for measurement purpose may alter the exiting condition and the measured. For example; • On measuring the temperature of a hot liquid flow in a pipe by using a thermometer, Fig. 4.1. The existence of the thermometer may cause some losses of heat to the surround. This may affect the temperature of the fluid.
Fig.4.1 Measuring the temperature of a fluid by a thermometer
In case of using a voltmeter to measure the potential difference in a very high resistance circuit, the interference of the voltmeter may alter the circuit resistance and the potential difference, Fig.4.2 Fig. 4.2 Measuring the potential difference by a voltmeter
4.3.5.Zero Error • Measuring instruments must be initialized to set its reading to the zero reading. Poor or incorrect initial setting of the instrument will lead to an inaccurate measure. • 4.3.6.Misalignement (Cosine) Error • This error arise when the axis of measured is not aligned with the axis of the instrument, Fig.4.3.
Error due to Misuse of the Instrument • For example, closing the micrometer by turning the thimble instead of the ratchet causing excessive measuring force, and results in an error. • Errors due to Shortcoming of the Instrument • Defective mechanical or electrical component, wear of gear, deflection of a lever will introduce errors in the measured values. This error could be detected by periodic calibration.
RANDOM ERRORS (RESIDUALSS) • Experimental results show variations from one reading to another. • These errors are due to small-undefined factors, which change or fluctuate from one measure to another. • lack of definition of the measured may introduce such errors. • To offset these errors, a large number of reading are taken and a statistical analysis is applied to the results to obtain the actual value of the measured and the value of the uncertainty.
