Detailed explanation of multimeter range selection and measurement error
There will be certain errors when measuring with a multimeter. Some of these errors are the maximum absolute errors allowed by the accuracy level of the instrument itself. Some are human errors caused by improper adjustment and use. If you correctly understand the characteristics of multimeters and the causes of measurement errors, and master the correct measurement techniques and methods, you can reduce measurement errors.
Human reading error is one of the reasons that affects measurement accuracy. It is unavoidable but can be minimized. Therefore, special attention should be paid to the following points during use:
1. Before measurement, place the multimeter horizontally and perform mechanical zero adjustment;
2. Keep your eyes perpendicular to the pointer when reading;
3. When measuring resistance, zero adjustment must be performed every time you change gears. If it cannot reach zero, replace the battery with a new one;
4. When measuring resistance or high voltage, do not hold the metal part of the test lead with your hands to avoid shunting of human body resistance, increasing measurement error or causing electric shock;
5. When measuring the resistance in an RC circuit, cut off the power supply in the circuit and discharge all the electricity stored in the capacitor before measuring again. After excluding human reading errors, we conduct some analysis on other errors.
1. Multimeter voltage and current range selection and measurement error
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.
2. Range Selection and Measurement Error of Electrical Barrier
Each range of electrical resistance can measure resistance values from 0 to ∞. The scale of the ohmmeter is a non-linear, uneven inverted scale. It is expressed as a percentage of the arc length of the ruler. Moreover, the internal resistance of each range is equal to the central scale number multiplied by the arc length of the ruler, which is called the "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 is in 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 choosing different ranges
For example: MF-30 multimeter, the center resistance of Rxl0 block is 250Ω; the center resistance of R×l00 block is 2.5kΩ. The accuracy level is level 2.5. Use it to measure a standard resistance of 500Ω, and ask if you use R×l0 block or R×100 block to measure, which one has the larger error? Solution: From Equation 2:
The maximum absolute allowable error of R×l0 block is △R(10)=center resistance×R%=250Ω×(±2.5)%=±6.25Ω. Use it to measure the 500Ω standard resistance, and the indication value of the 500Ω standard resistance is between 493.75Ω and 506.25Ω. The maximum relative error is: ±6.25÷500Ω×100%=±1.25%.
The maximum absolute allowable error of R×l00 block is △R (100) = center resistance × R% 2.5kΩ × (±2.5)% = ±62.5Ω. Use it to measure the 500Ω standard resistance, and the indication value of the 500Ω standard resistance is between 437.5Ω and 562.5Ω. The maximum relative error is: ±62.5÷500Ω×100%=±10.5%.
Comparison of the calculation results shows that the measurement errors vary greatly when different resistance ranges are selected. Therefore, when selecting the gear range, try to keep the measured resistance value at the center of the arc length of the range scale. The measurement accuracy will be higher.
