5 Experimental Fluid Mechanics
377
The expression methods of error are: (1) absolute error represents the difference between the measured value and the real value, which is usually called
error; (2) relative error is to measure the accuracy of a certain measured value,
which is generally expressed by relative error. That is to say, the percentage of
the ratio of absolute error to the actual value to be measured is the relative
error; (3) the reference error is the ratio of the absolute error of the instrument to the measuring range; (4) the arithmetic mean error is the mean value
of the error of each measuring point; (5) the standard error is the root mean
square error. The standard error is not a specific error, and its size indicates the
dispersion degree of each observation value of equal precision measurement
set to its arithmetic mean value under certain conditions.
4. Accuracy of Measuring Instrument
The accuracy level of the measuring instrument is indicated by the maximum
reference error (also known as the allowable error), which is equal to the
percentage of the ratio of the maximum absolute error of the instrument to
the range of the instrument. Generally, standard instruments are used to calibrate lower level instruments. Therefore, the maximum absolute error is the
maximum absolute error between the calibrated instrument and the standard
instrument. The accuracy grade of the measuring instrument is uniformly
stipulated by the state. Remove the percentage sign from the allowable error,
and the remaining figures are called the accuracy grade of the instrument.
The accuracy level of the instrument is usually indicated on the panel of the
instrument by the number in the circle. For example, the allowable error
of a certain pressure gauge is 1.5%, and the accuracy grade of the electrical
instrument of this pressure gauge is 1.5, which is usually referred to as level
1.5 instrument for short.
5. Error properties
(1) Normal distribution of errors
If the system error and the negligence error are not included in the measurement series, the magnitude of the random error is found in a large number
of tests as follows: (1) The error with a small absolute value has more chances
than the error with a larger absolute value, that is, the probability of error
is related to the size of the error. This is the unimodality of the error; (2)
377
The expression methods of error are: (1) absolute error represents the difference between the measured value and the real value, which is usually called
error; (2) relative error is to measure the accuracy of a certain measured value,
which is generally expressed by relative error. That is to say, the percentage of
the ratio of absolute error to the actual value to be measured is the relative
error; (3) the reference error is the ratio of the absolute error of the instrument to the measuring range; (4) the arithmetic mean error is the mean value
of the error of each measuring point; (5) the standard error is the root mean
square error. The standard error is not a specific error, and its size indicates the
dispersion degree of each observation value of equal precision measurement
set to its arithmetic mean value under certain conditions.
4. Accuracy of Measuring Instrument
The accuracy level of the measuring instrument is indicated by the maximum
reference error (also known as the allowable error), which is equal to the
percentage of the ratio of the maximum absolute error of the instrument to
the range of the instrument. Generally, standard instruments are used to calibrate lower level instruments. Therefore, the maximum absolute error is the
maximum absolute error between the calibrated instrument and the standard
instrument. The accuracy grade of the measuring instrument is uniformly
stipulated by the state. Remove the percentage sign from the allowable error,
and the remaining figures are called the accuracy grade of the instrument.
The accuracy level of the instrument is usually indicated on the panel of the
instrument by the number in the circle. For example, the allowable error
of a certain pressure gauge is 1.5%, and the accuracy grade of the electrical
instrument of this pressure gauge is 1.5, which is usually referred to as level
1.5 instrument for short.
5. Error properties
(1) Normal distribution of errors
If the system error and the negligence error are not included in the measurement series, the magnitude of the random error is found in a large number
of tests as follows: (1) The error with a small absolute value has more chances
than the error with a larger absolute value, that is, the probability of error
is related to the size of the error. This is the unimodality of the error; (2)