How to choose the range of multimeter and analysis of measurement error
There will be some errors when measuring with a multimeter. Some of these errors are the maximum measurement errors allowed by the accuracy class of the meter itself. Some are human errors caused by adjustment and improper use. Correctly understand the characteristics of the multimeter and the causes of measurement errors, and master the correct measurement techniques and methods, you can reduce the measurement errors.
Human reading error is one of the reasons that affect the measurement accuracy. It is unavoidable but can be minimized. Therefore, special attention should be paid to the following points during use: 1. Before measuring, place the multimeter horizontally and perform mechanical zero adjustment; 2. When reading, the eyes should be kept perpendicular to the pointer; When the adjustment is less than zero, replace with a new battery; 4. When measuring resistance or high voltage, do not pinch the metal part of the test lead with your hands, so as to avoid shunting of human body resistance, increase measurement error or electric shock; Cut off the power supply in the circuit, and discharge the electricity stored in the capacitor before measuring. After excluding the human-made reading errors, we conduct some analysis on other errors.
1. Multimeter voltage, current range selection and measurement error
The accuracy grades of multimeters are generally divided into several grades such as 0.1, 0.5, 1.5, 2.5, and 5. For DC voltage, current, AC voltage, current, etc., the calibration of the accuracy (accuracy) level is expressed by the percentage of the maximum allowable error △X and the full scale value of the selected range. Expressed by formula: A%=(△X/full scale value)×100%... 1
(1) Using a multimeter with different accuracy to measure the error generated by the same voltage
For example: There is a 10V standard voltage, and it is measured with two multimeters with 100V gear, 0.5 level and 15V level, 2.5 level. Which meter has the smallest measurement error?
Solution: From formula 1, we get: ** block meter measurement: * maximum ** allowable error
△X1=±0.5%×100V=±0.50V.
△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 of the first watch is larger than the error produced by the measurement of 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, choose 100V gear and 25V gear to measure a 23V standard voltage, which gear has the smaller error?
Solution: 100V block maximum measurement allowable error:
X(100)=±2.5%×100V=±2.5V.
The maximum allowable measurement error for 25V gear: △X(25)=±2.5%×25V=±0.625V. It can be seen from the above solution that:
Use the 100V block to measure the 23V standard voltage, and the indication on the multimeter is between 20.5V-25.5V. Use the 25V block to measure the 23V standard voltage, and the indication 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, its accuracy is 2.5, use the 100V block to measure a standard voltage of 20V and 80V, which block has the smaller error?
Solution: 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. The 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 use R×l0 gear or R×100 gear to measure, which error is larger? Solution: From formula 2:
R×l0 block maximum absolute allowable error △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%.
The maximum absolute allowable error of R×l00 block △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.
