Range Selection of Multimeter and Measurement Error Analysis
The accuracy level of the multimeter is generally divided into several levels such as 0.1, 0.5, 1.5, 2.5, and 5. For DC voltage, current, AC voltage, current and other gears, the calibration of the accuracy (accuracy) level is expressed by the percentage of the maximum absolute allowable error △X and the full scale value of the selected range. Expressed by formula: A%=(△X/full scale value)×100%... 1
(1) Using multimeters with different accuracy to measure the error generated by the same voltage
For example: There is a 10V standard voltage, and two multimeters with 100V range, 0.5 level and 15V level, 2.5 level are used to measure. Which meter has the smaller measurement error?
Solution: From formula 1, we can get: the first meter measurement: the maximum absolute allowable error
△X1=±0.5%×100V=±0.50V.
The second meter test: the maximum absolute allowable error
△X2=±2.5%×l5V=±0.375V.
Comparing △X1 and △X2, it can be seen that although the accuracy of the first watch is higher than that of the second watch, the error produced by the measurement with the first watch is larger than the error produced by the second watch. Therefore, it can be seen that when choosing a multimeter, the higher the accuracy, the better. With a multimeter with high accuracy, it is necessary to choose an appropriate range. Only by choosing the correct range can the potential accuracy of the multimeter be brought into play.
(2) The error caused by measuring the same voltage with different ranges of a multimeter
For example: MF-30 multimeter, its accuracy is 2.5 grades, choose 100V gear and 25V gear to measure a 23V standard voltage, which gear has the smaller error?
Solution: The maximum absolute allowable error for 100V gear:
X(100)=±2.5%×100V=±2.5V.
The maximum absolute allowable error for 25V gear: △X(25)=±2.5%×25V=±0.625V. It can be seen from the above solution that:
Use the 100V gear to measure the 23V standard voltage, and the displayed value on the multimeter is between 20.5V-25.5V. Use the 25V block to measure the 23V standard voltage, and the indication value on the multimeter is between 22.375V-23.625V. From the above results, △X (100) is greater than △X (25), that is, the error of 100V block measurement is much larger than that of 25V block measurement. Therefore, when a multimeter measures different voltages, the errors generated by different ranges are different. In the case of meeting the value of the signal to be measured, the gear with the smallest measuring range should be selected as much as possible. This increases the accuracy of the measurement.
(3) The error caused by measuring two different voltages with the same range of a multimeter
For example: MF-30 multimeter has an accuracy of 2.5 grades. Use the 100V gear to measure a standard voltage of 20V and 80V. Which gear has the smaller error?
Solution: The maximum relative error: △A%=Maximum absolute error △X/measured standard voltage adjustment×100%, the maximum absolute error of 100V gear △X(100)=±2.5%×100V=±2.5V.
For 20V, its indication value is between 17.5V-22.5V. Its maximum relative error is: A(20)%=(±2.5V/20V)×100%=±12.5%.
For 80V, its indication value is between 77.5V-82.5V. Its maximum relative error is:
A(80)%=±(2.5V/80V)×100%=±3.1%.
Comparing the maximum relative error of the measured voltage 20V and 80V, it can be seen that the error of the former is much larger than that of the latter. Therefore, when using the same range of a multimeter to measure two different voltages, whoever is closer to the full scale value will have higher accuracy. Therefore, when measuring voltage, the measured voltage should be indicated above 2/3 of the multimeter's range. Only in this way can the measurement error be reduced.
2. Range selection and measurement error of electrical barrier
Each range of electrical resistance can measure the resistance value from 0 to ∞. The scale scale of an ohmmeter is a non-linear, uneven, inverted scale. It is expressed as a percentage of the arc length of the scale. Moreover, the internal resistance of each range is equal to the multiplier of the central scale number of the arc length of the scale, which is called "central resistance". That is to say, when the measured resistance is equal to the center resistance of the selected range, the current flowing in the circuit is half of the full scale current. The pointer indicates the center of the scale. Its accuracy is expressed by the following formula:
R%=(△R/center resistance)×100%……2
(1) When using a multimeter to measure the same resistance, the error caused by selecting different ranges
For example: MF-30 multimeter, the central resistance of the Rxl0 block is 250Ω; the central resistance of the Rxl00 block is 2.5kΩ. The accuracy class is 2.5. Use it to measure a 500Ω standard resistance, and ask whether to measure it with R×l0 gear or R×100 gear, which one has the larger error? Solution: From formula 2:
The maximum absolute allowable error of R×l0 block △R(10)=central resistance×R%=250Ω×(±2.5)%=±6.25Ω. Use it to measure 500Ω standard resistance, then the indicated value of 500Ω standard resistance is between 493.75Ω～506.25Ω. The maximum relative error is: ±6.25÷500Ω×100%=±1.25%.
R×l00 block maximum absolute allowable error △R(100)=central resistance×R%2.5kΩ×(±2.5)%=±62.5Ω. Use it to measure 500Ω standard resistance, then the indicated value of 500Ω standard resistance is between 437.5Ω～562.5Ω. The maximum relative error is: ±62.5÷500Ω×100%=±10.5%.
The comparison of the calculation results shows that the measurement error varies greatly when different resistance ranges are selected. Therefore, when selecting the gear range, try to make the measured resistance value in the center of the arc length of the range scale. The measurement accuracy will be higher.
