Chapter 2: Measurement Errors Gross Errors or Human Errors – Resulting from carelessness, e.g. misreading, incorrectly recording 1 Systematic Errors – Instrumental Errors • Friction • Zero positioning – Environment Errors • Temperature • Humidity • Pressure – Observational Error Random Errors 2 Absolute Errors and Relative Errors Absolute Error e X t X m where X t : True Value X m : Measured Value Relative Error %Error Xt Xm 100% Xt 3 Accuracy, Precision, Resolution, and Significant Figures – Accuracy (A) and Precision • The measurement accuracy of 1% defines how close the measurement is to the actual measured quality. • The precision is not the same as the accuracy of measurement, but they are related. Accuracy 1 %Error x x Precision 1 n n xn xn x n 4 a) b) If the measured quantity increases or decreases by 1 mV, the reading becomes 8.936 V or 8.934 V respectively. Therefore, the voltage is measured with a precision of 1 mV. The pointer position can be read to within one-fourth of the smallest scale division. Since the smallest scale division represents 0.2 V, one-fourth of the scale division is 50 mV. – Resolution • – The measurement precision of an instrument defines the smallest change in measured quantity that can be observed. This smallest observable change is the resolution of the instrument. Significant Figures • The number of significant figures indicate the precision of measurement. 5 Example 2.1: An analog voltmeter is used to measure voltage of 50V across a resistor. The reading value is 49 V. Find a) Absolute Error b) Relative Error c) Accuracy d) Percent Accuracy Solution a) e X t X m 50V 49V 1V b) % Error Xt Xm 100% Xt 50V 49V 100% 2% 50V c) A 1 % Error 1 2% 0.98 d) % Acc 100% 2% 98% 6 Example 2.2: An experiment conducted to measure 10 values of voltages and the result is shown in the table below. Calculate the accuracy of the 4th experiment. No. (V) No. (V) 1 98 6 103 2 102 7 98 3 101 8 106 4 97 9 107 5 100 10 99 Solution xn x x x2 ... x10 n 10 98 102 101 97 100 103 98 106 107 99 10 101.1 1 Precision 1 xn xn 97 101.1 1 0.959 96% xn 101.1 7 Class of Instrument – Class of instrument is the number that indicates relative error. – Absolute Error e(range) Class range 100 – Relative Error %Error erange %Error erange Xt Xm 100% , xt true value 100% , xm measured value 8 Example 2.3 A class 1.0 Voltmeter with range of 100V, 250V, and 1,000V is used to measure voltage source with 90V. Calculate range of voltage and its relative errors Solution 1 100V 1V 100 100V 1V, 99V 101V a) e 100V 1V 100% 1.11% 90V 1 b) e 250V 250V 2.5V 100 250V 2.5V, 247.5V 252.5V %Error 2.5V 100% 2.77% 90V 1 c) e 1,000V 1,000V 10V 100 1,000V 10V, 990V 1,010V %Error %Error 10V 100% 11.11% 90V 9 Measurement Error Combinations – When a quantity is calculated from measurements made on two (or more) instruments, it must be assumed that the errors due to instrument inaccuracy combine is the worst possible way. – Sum of Quantities • Where a quantity is determined as the sum of two measurements, the total error is the sum of the absolute errors in each measurement. E V1 ΔV1 V2 ΔV2 giving E V1 V2 ΔV1 ΔV2 10 – Difference of Quantities • The error of the difference of two measurements are again additive E V1 ΔV 1 V2 ΔV 2 V1 V2 ΔV 1 ΔV 2 – Product of Quantities • When a calculated quantity is the product of two or more quantities, the percentage error is the sum of the percentage errors in each quantity P EI E ΔE I ΔI EI EΔ I I Δ E ΔEΔI since ΔEΔI is very small , P EI EΔ I I Δ E 11 EI IE 100% EI I E 100% I E percentage error % error in P % error in I % error in E Quotient of Quantities % error in E % error in E % error in I I Quantity Raised to a Power % error in AB B % error in A Example 2.4 An 820Ω resistance with an accuracy of 10% carries a current of 10 mA. The current was measured by an analog ammeter on a 25mA range with an accuracy of 2% of full scale. Calculate the power dissipated in the resistor, and determine the accuracy of 12 the result. Solution P I 2 R 10 mA 820 2 82 mW error in R 10% error in I 2% of 25 mA 0.5 mA 0.5 mA 100% 5% 10 mA %error in I 2 2 5% 10% %error in P %error in I 2 %error in R 10% 10% 20% Basics in Statistical Analysis Arithmetic Mean Value x1 x2 x3 ... xn x n • Minimizing the effects of random errors 13 14 – Deviation • Difference between any one measured value and the arithmetic mean of a series of measurements • May be positive or negative, and the algebraic sum of the deviations is always zero dn x n x • The average deviation (D) may be calculated as the average of the absolute values of the deviations. D d 1 d 2 d 3 ... d n n 15 – Standard Deviation and Probable of Error • Variance: the mean-squared value of the deviations d 12 d 22 ... d n2 n 2 • Standard deviation or root mean squared (rms) d 12 d 22 ... d n2 SD or σ n • For the case of a large number of measurements in which only random errors are present, it can be shown that the probable error in any one measurement is 0.6745 times the standard deviation: Probable Error 0 . 6745 16 Example 2.5 The accuracy of five digital voltmeters are checked by using each of them to measure a standard 1.0000V from a calibration instrument. The voltmeter readings are as follows: V1 = 1.001 V, V2 = 1.002, V3 = 0.999, V4 = 0.998, and V5 = 1.000. Calculate the average measured voltage and the average deviation. Solution V1 V2 V3 V4 V5 5 1.001 1.002 0.999 0.998 1.000 1.000V 5 d1 V1 Vav 1.001 1.000 0.001V Vav d 2 V2 Vav 1.002 1.000 0.002V d 3 0.999 1.000 0.001V d 4 0.998 1.000 0.002V d 5 1.000 1.000 0V d1 d 2 ... d 5 5 0.001 0.002 0.001 0.002 0 0.0012V 5 D 17