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# The accuracy of the measuring instruments

##### Author:vacorda    Source:vacorda.com    Date: 2018-09-28 15:10:50

It is an important indicator that the accuracy of the measuring instruments when we selecting them. Today we are going to tell you the details about the error how to generate, calculate and calibrate in several different working situations, hope it can help you choose the right instrument.

First, the definition of measurement error

The measurement error is the difference between the measured results minus the measured true value, referred to as the error. Because the true value (also called the theoretical value) cannot be accurately obtained, the actual value is actually agreed. The agreed true value needs to be characterized by the measurement uncertainty, so the measurement error cannot be accurately obtained.

Measurement Uncertainty: Indicates that the dispersibility of the measured value is reasonably given. It is related to the degree of knowledge of the person being measured, and is an interval obtained by analysis and evaluation.

Measurement error: It is the difference indicating that the measurement result deviates from the true value. It exists objectively but cannot be determined.

Second, the generation of errors

The error is divided into random error and systematic error.

The error can be expressed as: error = measurement result - true value = random error + systematic error

Therefore, any error can be decomposed into algebraic and systematic errors of systematic and random errors:

Systematic error: The measurement error caused by the inherent error of the measuring tool (or measuring instrument), the measurement principle or the theoretical flaw of the measuring method itself, the experimental operation and the psychological and physiological conditions of the experimenter itself.

Random error: Random error is also called accidental error. Even in the ideal case of completely eliminating systematic error, repeated measurement of the same measurement object repeatedly will cause measurement error due to various accidental and unpredictable uncertain factors. It is called random error.

Third, precision, precision and accuracy

Use the same measuring tool and method to measure multiple times under the same conditions. If the random error of the measured value is small, that is, the fluctuation of each measurement result is small, indicating that the measurement repeatability is good, that is, the measurement precision is good, and the stability is good. Therefore, measuring the magnitude of the accidental error reflects the precision of the measurement.

Instrument accuracy is referred to as accuracy, also known as accuracy. Accuracy and error can be said to be twin brothers, because of the existence of errors, the concept of precision. The accuracy of the meter is simply the accuracy of the meter's measured value close to the true value, usually expressed as a relative percentage error (also known as relative folding error).

Meter accuracy is not only related to absolute error, but also to the meter's measurement range. The absolute error is large, the relative percentage error is large, and the accuracy of the meter is low. If two instruments with the same absolute error have different measurement ranges, the relative percentage error of the meter with a large measurement range is small, and the accuracy of the meter is high. Accuracy is a quality indicator that is important for the instrument. It is commonly used to standardize and represent accuracy levels. The accuracy level is the maximum relative percentage error minus the sign and %. According to the national unified regulations, the grades are 0.05, 0.02, 0.1, 0.2, 1.5, etc. The smaller the number, the higher the accuracy of the instrumentation.

Fourth, the choice of application accuracy

In the actual application process, the range and accuracy of the instrument should be selected according to the actual situation of the measurement, and the instrument with a small accuracy level is not necessarily the best measurement effect. Take the application of the multimeter as an example, and use a multimeter with different accuracy to measure the error caused by the same voltage.

For example: there is a standard voltage of 10V, measured with two multimeters of 100V, 0.5 and 15V, and 2.5. Which one has a small measurement error?

Solution: Maximum absolute allowable error of the first block

ΔX1 = ±0.5% × 100V = ±0.50V.

The maximum absolute allowable error of the second table

ΔX2 = ± 2.5% × 15V = ± 0.375V.

Comparing X1 and X2, it can be seen that although the accuracy of the first block is higher than that of the second block, the error caused by the measurement of the first block is larger than the error caused by the measurement of the second block. Therefore, it can be seen that when the instrument is selected, the higher the accuracy, the better. Also choose the appropriate range. Only when the range is correctly selected can the potential accuracy be achieved.

Five, the accuracy of the calibration method

In addition to the nationally stipulated level, with the wide application of electronic technology, depending on the performance, there are several precision calibration methods as follows.

1. Display value ± X:

Applied in an electronic display instrument, indicating that there is an error of X words at the lowest bit of the current display value. If the display value is Y, the error X=X/Y×100%

2. Display the X% of the value:

Applicable in electronic display instrumentation, indicating that X% of the current display value is the current error range. If the display value is Y error ΔX=X%, the error value is ±X%×Y

3. Segmented range calibration:

For wide-range instruments, the instrument uses different error calibration methods in different measurement intervals. For example, when measuring 0.01~1 volt, the error is 5%. When measuring 1~10 volts, the error is 1%. For the segmentation calibration method, when applying the segmentation calibration instrument, it is necessary to select a suitable range and carefully check the error calculation and calibration method of the range.

4. Mathematical model error calibration:

The error calculation formula F(X) of the instrument is given, and the current error ΔX=F(Y) is calculated according to the current measurement result Y of the instrument and other relevant conditions. The corresponding relationship between the error result and the measured value measured by this method is mostly a curve. Since the error of each point of this method is different, the application should pay special attention and be carefully calculated.

To sum up, it is not difficult to conclude from the above conditions that the actual measurement accuracy of the results is different for different measurement values. When selecting, it is necessary to analyze the measurement conditions and the allowable error of the instrument at the measurement point. It is not necessarily the lowest level of instrumentation that will have the best measurement results. It is necessary to select the appropriate instrumentation and range according to the specific situation, so as to minimize the measurement error.

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