1. What is measurement error? Why do measurement errors occur?
The difference between the measured value and the true value is the measurement error. The purpose of measurement is to obtain the true value of the measured value, although the true value exists objectively. However, in the actual measurement, because the measurement principle and method, measuring instrument (or equipment), measuring environment and the measurement itself are affected by many main and guest factors, it is difficult to measure the "true value" of the measured parameters. This is the main reason why measurement errors occur. This situation is universal in measurement.
2, according to the method of error value, error can be divided into: absolute error, relative error, reference error. According to the law of error, it can be divided into: systematic error, random error, negligence error. According to the use conditions of the instrument, it can be divided into: basic error and additional error.
3, What is absolute error, relative error, reference error?
Absolute error: The difference between the measured value and the true value.
Relative error: is the ratio of absolute error to the measured value, commonly used absolute error and the ratio of the instrument to indicate the value, expressed as a percentage.
Reference error: Ratio of absolute error to range, expressed as a percentage. The accuracy level of the instrument is divided according to the reference error.
4. The scale of a certain pressure gauge is 0-100kPa, and the measured value at 50kPa is 49.5kPa. Find the absolute error, relative error and reference error of the indicator value at 50kPa.
Solution: Absolute error =50-49.5 = 0.5kPa; Relative error =(0.5/50)× 100% = 1%; Reference error =(0.5/100)× 100% = 0.5%
5, What is systematic error, accidental error, inadvertent error? What are the characteristics of each, and what are the causes?
Systematic error: also known as regular error, its size and symbol do not change or change according to a certain law. Its main feature is that it is easy to eliminate or modify. The main reasons are the defects of the instrument itself, the incorrect method of using the instrument, the habit or bias of the observer, and the change of the single factor environmental conditions.
Accidental error: also known as random error, its appearance is completely random. Its main characteristics are not easy to detect, difficult to analyze, difficult to correct, but it obeys statistical laws. The reasons for this are complex, and it is caused by small changes in many complex factors.
Negligence error: also called gross error, its main feature is that there is no law to follow, and it is obviously inconsistent with the facts. The main cause of such errors is observer error or accidental interference from the outside world.
6. Method of solving systematic error
Method 1: (formula method)δ total =±(ΣCi2)1/2; In the formula, Ci is the maximum reference error of each unit instrument in the system; n is the number of unit instruments
Method 2: (system joint calibration method) that is, the standard signal value is added to the primary component end, the indication value is read in the secondary table, the reference error is calculated, and the maximum reference error is selected in each check point as the systematic error of the measurement system.
7, What is the basic error and additional error?
The basic error of the instrument refers to the maximum error of the instrument under standard working conditions under the specified reference working conditions, and the basic error of the general instrument is the allowable error of the instrument.
Additional error is an additional error that occurs when the instrument is used under non-specified reference operating conditions. Such as: power supply fluctuation additional error, temperature additional error, etc.
8. What is accuracy and accuracy level?
Accuracy refers to the degree to which the measured value agrees with the actual value. It is also known as precision or precision. Is the maximum allowable value of the basic error of the instrument.
Accuracy level is the level of the instrument according to the level of accuracy, which is also known as the accuracy level or accuracy level.
9, the accuracy level of the instrument is divided according to the reference error.
10, back difference (also called variation) is the maximum difference between the two corresponding outputs of the same input when the input goes up and down.
11, sensitivity is to express the sensitivity of the instrument to the change of the measured parameters. It refers to the ratio of the output increment to the input increment after the instrument has reached a steady state. The greater the ratio, the more sensitive the instrument. The accuracy of the measurement results of the instrument is not only related to the accuracy level of the instrument, but also to the (range) of the instrument.
12. Is the measuring range and measuring range of the instrument consistent? What's the difference?
The range of an instrument is the algebraic difference between the upper and lower limits of the instrument range. The measuring range of the instrument refers to the range in which the instrument can measure the variables on the side according to the specified accuracy. Numerically, they are sometimes the same and sometimes they are not.
13, zero error refers to the error when the input is zero under the specified parameter working conditions. If the input is the lower limit of the measurement range, it is called the starting point error.
14, stability refers to the ability of the instrument output to remain unchanged for a specified period of time when the input remains constant under the specified working conditions, usually measured by zero drift.
15, zero drift refers to the output change when the instrument enters a constant value (zero or the lower limit of the measurement range) under the reference working conditions.
16, when the instrument is used to measure the parameters to be measured, the indicator value of the instrument is always displayed after a period of time, which is called the reaction time of the instrument. If the instrument can not react the measured parameters in time, it will cause an error, which is called a dynamic error.
17, the size of the dynamic error is commonly expressed by time constant, full travel time and lag time.
18, the time constant refers to the time required for the output value of the instrument to reach 63.2% of its stable value when the input step changes. The full stroke time refers to the time required for the output to move from the lower limit to the upper limit, or the reverse stroke, when the full scale step changes. Usually 5% of the full range is used as the output lower limit, and 95% of the full range is used as the output upper limit.
19, MTBF refers to the average trouble-free time, which is an important indicator to measure the reliability of the instrument. MTTR is the average time to repair an instrument failure. The availability of A meter is the ability of the meter to have or maintain a specified function at a given time, and the availability formula is: A=MTBF/(MTBF+MTTR)×100%