Sensitivity, precision, accuracy and precision are often used in the measurement of physical quantities, but they are several concepts that are easily confused. Among these concepts, sensitivity is only for the experimental instrument, precision is only for the measurement, and precision and accuracy are for both the instrument and the measurement. According to the meaning and function of these concepts, the following two aspects are analyzed and explained.
1. Measuring the quality of measuring instruments
1. Sensitivity:
refers to the ability of the instrument to measure the smallest measurand. The smaller the measured minimum value, the higher the sensitivity of the instrument.
Sensitivity generally refers to balances and electrical instruments, but does not matter to rulers, vernier calipers, spiral micrometers, stopwatches, etc.
For example, the higher the sensitivity of the balance, the smaller the milligrams per division, that is, the smaller the mass required for the balance pointer to move from the balance position to one division of the dial.
Another example is the number "20kΩ/V" marked on the dial of the multimeter, which indicates the sensitivity. It means that when a voltage of 1V is applied across the meter, the total internal resistance RV of the meter (the internal resistance of the meter and the additional internal resistance) is required to make the pointer fully offset. The sum of resistances) is 20kΩ. The higher the number, the higher the sensitivity. This is because U=IgRV, that is, RV/U=1/Ig. Obviously, when RV/U is larger, it means that the full bias current Ig is smaller and the sensitivity is higher.
The sensitivity of the instrument is not as high as possible, because if the sensitivity is too high, the stability during measurement will be worse, and even difficult to measure, that is, the accuracy will be poor. Therefore, under the premise of ensuring accuracy, the sensitivity should not be too high.
2. Accuracy: For electrical instruments, it does not matter to other instruments.
The accuracy of the instrument is generally expressed by the accuracy level. For example, the accuracy level of an electric meter refers to the percentage value of the maximum relative error that occurs when the pointer is fully offset when measured under specified conditions.
The accuracy of an electric meter is 2.5, which means that the relative error does not exceed 2.5% of the full skewness, that is, the absolute error of the instrument = range × accuracy. Such as a DC ammeter with a range of 0.6A, its maximum absolute error = 0.6A × 2.5% = 0.015A. Obviously, when measuring the same measurand with different ranges of the same electric meter, the maximum absolute error is different. Therefore, when using the electric meter, there is a problem of selecting an appropriate range.
3. Precision: also referred to as precision, refers to the fineness and compactness of the structure of the instrument, generally refers to the minimum graduation value of the instrument.
General instruments have accuracy problems. For example, the minimum division of the scale is 1mm, and its accuracy is
1mm; the minimum division of a mercury thermometer is 0.20C, and its accuracy is 0.20C.
The smaller the minimum division of the instrument, the higher the accuracy and the higher the sensitivity.
For example, a thermometer with an accuracy of 0.10C is more sensitive and precise than a thermometer with a minimum division of 0.20C.
Under normal use, the precision of the instrument is high, and the accuracy is high. This shows that the accuracy of the instrument is the premise of accuracy, and what kind of accuracy is required, what kind of accuracy is required. This is why precision is often used to describe the accuracy of an instrument.
However, the accuracy of the instrument does not fully reflect its accuracy. For example, in a voltmeter of a certain specification, the additional resistance inside it deteriorates, which reduces its actual accuracy, but the accuracy remains unchanged. There is a difference between precision and accuracy.
2. Evaluate the pros and cons of measurement results
1. Precision: The precision of measurement refers to the degree to which the data sizes of each measurement are close to each other when a certain quantity is measured, which is a reflection of accidental errors.
The measurement precision is high, indicating that the measurement data are relatively close and concentrated. However, due to the uncertainty of the systematic error situation, high measurement precision does not necessarily mean high measurement accuracy.
2. Accuracy: The accuracy of the measurement refers to the degree to which the average value of the measurement data deviates from the true value, which is a reflection of the systematic error.
The accuracy of the measurement is high, indicating that the mean value of the measurement deviates less from the true value.
However, due to the uncertainty of the accidental error situation, that is, the data are not necessarily concentrated near the true value, and may be scattered. Therefore, high measurement accuracy does not necessarily mean high measurement precision.
3. Accuracy: The accuracy of the measurement refers to the degree to which the measurement data is concentrated near the true value.
The accuracy of the measurement is high, indicating that the average value of the measurement is close to the true value, and the data of each measurement are relatively concentrated, that is, the systematic error and accidental error of the measurement are relatively small, and the measurement is accurate and precise. Therefore, the accuracy of the measurement is the comprehensive evaluation of the measurement results.
4. Resolution: refers to the ability of the detection instrument to accurately detect the smallest change in the measured value. Accuracy is used to describe how accurate a physical quantity is, while resolution is used to describe scale division