Selecting and measuring voltage and current ranges with a multimeter
The accuracy levels of multimeters are generally divided into several levels such as 0.1, 0.5, 1.5, 2.5, 5, etc. The calibration of the accuracy (precision) level of DC voltage, current, AC voltage, current and other gears is represented by the percentage of the maximum absolute allowable error △X and the full scale value of the selected range. Expressed by the formula: A%=(△X/full scale value)×100%... 1
(1) Error caused by using multimeters with different accuracies to measure the same voltage
For example: There is a 10V standard voltage, and it is measured with two multimeters at 100V level and 0.5 level and 15V level and 2.5 level. Which meter has the smallest measurement error?
Solution: From Equation 1: First meter measurement: Maximum absolute allowable error
△X1=±0.5%×100V=±0.50V.
Second meter test: 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 meter is higher than that of the second meter, the error caused by the measurement using the first meter is larger than the error caused by the measurement using the second meter. Therefore, it can be seen that when choosing a multimeter, the higher the accuracy, the better. With a multimeter with high accuracy, you also need to choose an appropriate measuring range. Only by correctly selecting the measuring range can the potential accuracy of the multimeter be unleashed.
(2) Error caused by measuring the same voltage with different ranges of a multimeter
For example: the MF-30 multimeter has an accuracy of level 2.5. It uses the 100V and 25V gears to measure a 23V standard voltage. Which gear has the smallest error?
Solution: The maximum absolute allowable error of 100V block:
X(100)=±2.5%×100V=±2.5V.
The maximum absolute allowable error of 25V block: △X (25) = ±2.5% × 25V = ±0.625V. It can be seen from the above solution:
Use the 100V gear to measure the 23V standard voltage. The value on the multimeter is between 20.5V and 25.5V. Use the 25V gear to measure the 23V standard voltage. The value on the multimeter is between 22.375V and 23.625V. Judging from the above results, △X (100) is greater than △X (25), that is, the error of the 100V block measurement is much larger than the error of the 25V block measurement. Therefore, when a multimeter measures different voltages, the errors produced by measuring with different ranges are different. Under the condition that the measured signal value is satisfied, a gear with a small range should be selected as much as possible. This improves measurement accuracy.
(3) The error caused by measuring two different voltages with the same range of a multimeter
For example: the MF-30 multimeter has an accuracy of 2.5. It uses the 100V gear to measure a standard voltage of 20V and 80V. Which gear has the smallest error?
Solution: Maximum relative error: △A% = maximum absolute error △X/measured standard voltage adjustment × 100%, maximum absolute error at 100V block △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 errors of the measured voltages of 20V and 80V, we can see that the former has a much larger error than the latter. Therefore, when using the same range of a multimeter to measure two different voltages, the one that 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 measurement errors be reduced.
