Why do errors occur and how are they classified?
Error source
In the measurement process, the causes of errors can be summarized as follows:
1. Measuring device error
(1). Standard gauge error
Instruments that reproduce the standard value in a fixed form, such as krypton 86 lamp tube, standard gauge block, standard linear ruler, standard battery, standard resistance, standard weight, etc., inevitably contain errors in the value reflected by themselves.
(2). Instrument error
All instruments and equipment used to directly or indirectly compare the measured quantity with the known quantity are called instruments or instruments, such as Abbe comparator, balance and other comparison instruments, pressure gauge, thermometer and other indicating instruments, which themselves have errors.
(3). Accessory error
The errors of the accessories and auxiliary tools of the instrument, such as the standard ring gauge of the length measuring instrument, the adjustment of the micrometer, the measuring rod, etc., will also cause measurement errors.
2. Environmental error
The error caused by the change of the measuring device and the measured itself caused by the inconsistency between various environmental factors and the specified standard state, such as the error caused by temperature, humidity, air pressure (causing the disturbance of various parts of the air), vibration (caused by external conditions and measuring personnel), illumination (causing parallax), gravitational acceleration, electromagnetic field, etc. Generally, the error of the instrument under the specified normal working conditions is called the basic error, and the increased error beyond this condition is called the additional error.
3. Method error
Errors caused by imperfect measurement methods, such as errors caused by approximate measurement methods. For example, measure the circumferential length s of the main shaft with a steel tape, and then calculate the diameter d = s / π of the main shaft. Due to the different values of the approximate number π, errors will be caused.
4.Personnel error
Due to the limitation of the surveyor's resolution, the physiological changes of visual organs caused by work fatigue, reading errors caused by inherent habits, and errors caused by temporary negligence caused by mental factors.
In short, when calculating the accuracy of measurement results, we must comprehensively analyze the error sources of the above four aspects, strive not to omit and repeat, and pay special attention to those factors that have a great impact on the error.
Error classification
According to the characteristics and properties of errors, errors can be divided into systematic errors, random errors and gross errors.
1. Systematic error
Under the same conditions, when the same value is measured for many times, the absolute value and symbol remain unchanged, or when the conditions change, the error changes according to a certain law is called systematic error. For example, the error caused by the inaccuracy of the standard value and the inaccuracy of the instrument scale.
Systematic errors can be classified according to the following methods:
(1). Score according to the degree of mastering the error
The defined systematic error refers to the systematic error whose absolute value and sign have been determined.
Undetermined systematic error refers to the systematic error whose absolute value and sign cannot be determined, but the error range can usually be estimated. 2. According to the law of error occurrence
Invariant systematic error refers to the systematic error with fixed absolute value and symbol.
Changing systematic error refers to the systematic error of absolute value of error and sign change. According to its variation law, it can be divided into linear system error, periodic system error and complex system error.
2. Random error
Under the same - measurement conditions, when the same value is measured many times, the error of absolute value and symbol changing in an unpredictable way is called random error. For example, the indication instability caused by the clearance and friction of transmission parts and the elastic deformation of connectors in instruments and meters.
3.Gross error
The error beyond the expectation under the specified conditions is called gross error, or "parasitic error". This error value is large and obviously distorts the measurement results, such as wrong mark, wrong reading or wrong number, defective instrument and negligent error caused by careless operation during measurement.
In short, there is no absolute limit between systematic error and random error. With the deepening of the understanding of the nature of errors and the development of testing technology, it is possible to separate some errors that used to be random errors as systematic errors, or treat some systematic errors as random errors.