According to the dependence on the quantity being measured, the errors of MI are divided into additive and multiplicative

The additive (absolute) errors don`t depend on the quantity being measured. They remain constant in all the measurement range (Fig. 8)

Fig. 8: The additive errors of MI: a is the additive error value, x_max is the upper limit of the instrument range

The example of the additive error is the error of an instrument, which pointer was not adjusted on the zero scale mark before beginning of the work.

The multiplicative (absolute) errors vary in proportion to the quantity being measured. (Fig.9)

Fig. 9: The multiplicative errors of MI:  вхor -вхare the multiplicative error values

It may be, for example, an error because of mismatch of a meter movement shaft and the deflectional instrument scale arc center.

Self – examination questions

1. What is the measurement error? What is the purpose of the errors classification?
2. How are the errors classified according to the components of measurement?
3. What is the methodical error of the measurement? What examples of these errors for electric and non-electric measurements can you adduce?
4. What is the instrumental error? What examples of those errors can you give?
5. What is the sence of the error from interaction? What are the rules of selection of the voltmeters and ammeters to decrease this error?
6. What are the external and subjective errors? What examples of these errors can you give?
7. What is the difference between the intrinsic and the complementary errors? What examples of these errors can you give?
8. What are the reasons and what is the difference between the MI errors – static, dynamic and the MI error in dynamic regime?
9. What are the reasons and what is the difference between the random, systematic and gross measurement errors? What examples of these errors can you give?
10. What is the absolute error of measurement? With what units is it represented? What examples of the absolute errors of different measurements can you give? Is it possible to characterize a measurement quality only with its absolute error?
11. What is the correction? How can we get it and how can it be used? What examples of the correction usage can you give? What is the danger of confusing the terms “absolute error” and “correction”?
12. What is the relative error of the measurement? With what units is it represented? Is it possible to compare the different measurements quality by using only their relative errors? What examples can you give?
13. What is the accuracy of measurements? How is it numerically represented? What examples of the measurement accuracy can you give? Are the following phrases correct: “distance is known with the accuracy to a mm” and “time is known with the accuracy to a second”? Why?
14.  Why can’t the deflectional instrument’s quality be characterized by the relative error or the accuracy of the measurements, which are performed with it? How can it be proved with a numeric example?
15. What is the fiducial error of MI? Is it possible to compare the different measuring instruments quality by using this error value?
16. What is the fiducial value of MI? What examples of the fiducial values for different instruments can you give?
17. What is the additive error? What examples of the additive errors can you give?
18. What is the multiplicative error? What examples of the multiplicative errors can you give?

METROLOGICAL CHARACTERISTICS STANDARDIZATION. ACCURACY CLASSES OF MEASUREMENT INSTRUMENTS

The proper use of the MIs is possible only with the presence of the sufficient volume of information on their metrological features. For this purpose, the nominal values of the most important characteristics and the permissible deviations from them are prescribed in the normative – technical documentation on MI.

Metrological characteristics standardization is the process of establishment of the nominal values and maximal permissible deviations of the MI real metrological characteristics from their nominal values.

The most important metrological characteristics of MIs may be unified into the following groups.

4.1. The characteristics intended for the measurement results determination (before any corrections are applied):

-the conversion function (static characteristic) for the transducers and for the measurement instruments having an unnamed scale;

 - the value of a single-valued material measure or the values of the multi-valued material measures;

 - the scale factor of the instruments or the multi-valued material measure;

- the format of the output code, the least significant digit and the code word length for the digital measuring instruments.

4.2. The characteristics of MI errors:

- the characteristics of the systematic component of the total error;

- the characteristics of the random component of the total error;

- the characteristics of the total error.

4.3. The characteristics of the MI sensitivity towards the influencing quantities:

 - the influence functions;

 - the changes of MI metrological performance caused by the changes in the influencing quantities within the specified limits.