How to choose the multimeter range and measurement error analysis
There will be some errors when measuring with a multimeter. Some of these errors are the maximum absolute errors allowed by the accuracy class of the meter itself. Some are human errors caused by improper adjustment and 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 in use: 1. Place the multimeter horizontally and perform mechanical zero adjustment before measurement; 2. Keep the eyes vertical to the pointer when reading; 3. When measuring resistance, zero-adjustment must be performed every time the gear is changed. When it is not adjusted to zero, a new battery should be replaced; 4. When measuring resistance or high voltage, do not hold the metal part of the test lead with your hands, so as to avoid shunt of human body resistance, increase measurement error or electric shock; 5. When measuring resistance in RC circuit, it is necessary to Turn off the power in the circuit and discharge the electricity stored in the capacitor before taking the measurement. After excluding human reading errors, we do some analysis on other errors.
1. Multimeter voltage and current range selection and measurement error
The accuracy level of the multimeter is generally divided into 0.1, 0.5, 1.5, 2.5, 5 and other levels. For DC voltage, current, AC voltage, current and other gears, the calibration of the accuracy (accuracy) grade is expressed 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) Using multimeters with different accuracy to measure the error of the same voltage
For example: There is a 10V standard voltage, and it is measured with two multimeters of 100V, 0.5 and 15V, and 2.5. Which meter has the smallest measurement error?
Solution: Obtained from formula 1: The first meter is measured: the maximum absolute allowable error
△X1=±0.5%×100V=±0.50V.
The second table measurement: 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 of the first watch is larger than that 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 select the appropriate range. Only by selecting 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 type multimeter, its accuracy is 2.5, choose 100V gear and 25V gear to measure a 23V standard voltage, which gear has the smallest error?
Solution: The maximum absolute allowable error of 100V block is:
X(100)=±2.5%×100V=±2.5V.
The maximum absolute allowable error of 25V block: △X(25)=±2.5%×25V=±0.625V. From the above solution it can be seen that:
Use the 100V gear to measure the 23V standard voltage, and the indicated value on the multimeter is between 20.5V and 25.5V. Use the 25V gear to measure the 23V standard voltage, and the indication value on the multimeter is between 22.375V and 23.625V. From the above results, △X (100) is greater than △X (25), that is, the error of the 100V measurement is much larger than that of the 25V measurement. Therefore, when a multimeter measures different voltages, the errors produced by different ranges are different. In the case of satisfying the value of the measured signal, the small range should be selected as much as possible. This improves 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 type multimeter, its accuracy is 2.5, use 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%, the maximum absolute error of 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 error of the measured voltage of 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 range value has higher accuracy. Therefore, when measuring the voltage, the measured voltage should be indicated at more than 2/3 of the range of the multimeter. Only in this way can the measurement error be reduced.
2. Range selection and measurement error of electrical barrier
Each range of the electrical barrier can measure the resistance value from 0 to ∞. The scale of the ohmmeter is a non-linear, uneven inverse scale. It is expressed as a percentage of the arc length of the ruler. And the internal resistance of each range is equal to the center scale of the arc length of the scale multiplied by the multiplying factor, which is called "center 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 points in the center of the scale. Its accuracy is expressed as:
R%=(△R/center resistance)×100%……2
(1) When measuring the same resistance with a multimeter, 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 R×l00 block is 2.5kΩ. The accuracy rating is 2.5. Use it to measure a standard resistance of 500Ω, and ask the R×10 block and the R×100 block to measure, which has the largest error? Solution: From formula 2, we get:
R×l0 block maximum absolute allowable error △R(10)=center resistance×R%=250Ω×(±2.5)%=±6.25Ω. Use it to measure 500Ω standard resistance, the indication 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)=center resistance×R％2.5kΩ×(±2.5)％=±62.5Ω. Use it to measure 500Ω standard resistance, the indication 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 when different resistance ranges are selected, the errors generated by the measurement are very different. 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.
