## Wednesday, December 29, 2010

### 【 Weak current College 】 multimeter measuring range selection and measurement error analysis 】

Multimeter measurement will bring certain errors. Some of these errors is the accuracy class of the instrument itself, to the maximum absolute value. Some are adjusted, abuse of human error. A proper understanding of the characteristics of the multimeter and the cause of the error, correct the measurement techniques and methods, you can reduce the measurement error. People reading error is one of the reasons for measurement accuracy. It is inevitable, but you can minimize. Therefore, use to pay particular attention to the following points: 1 measurement front to the multimeter horizontally, mechanical zero; 2 readings when eyes to perpendicular with the pointer; 3, each measuring resistance change gear should be zero. -Not to zero to replace a new battery; 4 measuring resistance or voltage, not hand squeeze the table amount of metal parts, shunt human resistance, increase the measurement error or shock; 5 in measuring the electrical resistance of RC circuits, to cut off power in the circuit, and the capacitor storage of electrical discharge, and then do the measuring. In excluding a human reading errors, we do some other error.

1. a multimeter to voltage, current, retaining the quantum measurement error of choosing and

Multimeter for accuracy class is generally divided into 0.1, 0.5, 1.5, 2.5, 5, and so on several levels. DC voltage, current, AC voltage, current, and other gear, accuracy (accuracy) level of calibration is absolutely by the maximum permissible error △ X and the selected range full-scale value expressed as a percentage. With the formula: A% = (△ full-scale value X/) × 100% … 1

(1) use different multimeter measurement accuracy with a voltage of error arising
For example: a standard voltage 10V, 100V retaining, 0.5 and 15V retaining, 2.5 on two multimeter measurement, ask which table measuring errors?

Solution: from 1-it: the first table logging: the maximum absolute tolerance
△X1=±0.5％×100V=±0．50V。
The second block table logging: the maximum absolute tolerance
△X2=±2．5％×l5V=±0.375V。
Compare △ △ X2 x 1 and can be seen: Although the first table accuracy than the second block table accuracy is high, but first a table generated by the error of measurement than second block table generated by the error of the measurement. Therefore, you can see that in the selection of a multimeter, not accuracy, the higher the better. There is a high accuracy, but also of the multimeter selected appropriate quantum. Only the correct choice of the range, in order to play a multimeter potential accuracy.

(2) using a multimeter measurement of different ranges with a voltage of error arising
For example: MF-30-multimeter, its accuracy, for 2.5 100V retaining and 25V retaining a 23V standard voltage measurement, ask which retaining error?
Solution: 100V retaining maximum absolute tolerance:
X(100)=±2.5％×100V=±2.5V。
25V retaining maximum absolute tolerance: X (25) = ± 2.5% × 25V = ±15 0.625V. We know from the above equations:
23V with 100V retaining measurement standard voltage, the indication of the multimeter in 20.5V-25.5V. 23V with 25V retaining measurement standard voltage, the indication of the multimeter in 22.375V-23.625V. From the above results, △ X (100) greater than △ X (25), 100V retaining measurement error ratio measurement error 25V retaining. Therefore, a multimeter measuring different voltages, used different quantum measurement error is generated. In meet the measured signal values, try to choose the range of gear. This can improve the accuracy of measurement.

(3) using a multimeter measurement with a range of different voltage generated by the error
For example: MF-30-multimeter, its accuracy, for 2.5 100V retaining measuring a 20V and 80V standard voltage and asked which one stop error?
Solution: the maximum relative error: A% = maximum absolute value of X/measured △ the standard voltage adjustment × 100%, 100V retaining maximum absolute error △ X (100) = ± 2.5% × 100V = ± 2.5V.
For 20V, between the indication between 17.5V-22.5V. Its maximum relative error is: A (20) percent = (± 2.5V/20V) x 100% = ± 12.5 per cent.
For 80V, the indication is between 77.5V-82.5V. Its maximum relative error is:
A（80）％=±(2.5V/80V)×100％=±3.1％。
Comparing the measured voltage 20V and 80V maximum relative error can be seen: the former than the latter error. Therefore, using a multimeter measurement with a range of two different voltage, who is full of retaining values near, who's accuracy is high. Therefore, in the measurement voltage so that the measured voltage indicates multimeter measuring range of more than 2/3. The only way to reduce the measurement error.

2. Select a range of electric barrier and measurement errors

Electric stop each measurement range can be 0 ~ ∞ resistance value. Ohmmeter ruler scale is linear, non-uniform scale down. Use the ruler to arc length expressed as percentages. And the range of internal resistance equal to ruler arc length of Center ticks take rate, called the "Centre of resistance". In other words, the measured resistance equal to the selected block range Center resistance, the current flowing in the circuit is full of half of the current. Pointer indicates the center of the scale. Its accuracy is shown in the following type:
R% = (△ R/Center resistance) × 100% … 2
(1) using a multimeter measurement with a resistor, choose a different range of error arising
For example: MF-30-000Use the table, the center of its Rxl0 retaining 250 Ω resistor; resistance R×l00 retaining of Center for 2.5 k Ω. Accuracy grade-level 2.5. Use it to test a 500 Ω standard resistance, asked by R×l0 retaining and R× 100 block to measure, which error?: by 2-be:
R×l0 retaining maximum absolute tolerance △ R (10) = Center resistance × R% = 250 Ω × (± 2.5)% = ± 6.25 Ω. Use it to measure 500 Ω standard resistance, 500 Ω standard resistance value between 493.75 Ω ~ 506.25 Ω. Maximum relative error is: ± 6.25 ÷ 500 Ω × 100% = ± 1.25 per cent.
R×l00 retaining maximum absolute tolerance △ R (100) = Center resistance × R% 2.5 k Ω × (± 2.5)% = ± 62 .5µ Ω. Use it to measure 500 Ω standard resistance, 500 Ω standard resistance value between 437.5 Ω ~ 562.5 Ω. Maximum relative error is: ± 62.5 ÷ 500 Ω × 100% = ± 10.5 per cent.
Comparison by the results of a calculation shows that the choice of different resistance range, measurement error of varied widely. So, the choice of gear range, try to make the measured resistance value at a measuring range of Center ruler arc length. The measurement accuracy is high.