Today, there are a large number of tools with which you can make measurements of different types: linear, weight, temperature, power, etc. The devices differ in accuracy, principle of operation, purpose, and price.
In order to properly perform the necessary work, you should carefully consider the choice of measuring instruments. They, in turn, are also divided into several types depending on the criteria considered.
Classification of instruments
Measuring instruments are tools and instruments that are used to measure physical quantities. For each of them, the errors specified in the regulatory documents and technical regulations are determined.
Measuring instruments are divided into different types according to the following criteria:
- view tool for work;
- working principle;
- comparison with the accepted standard;
- metrological application.
Types of tools
The most common types of measuring instruments are listed below.
Measure is a measuring instrument used to reproduce the desired size of the considered physical quantity. For example, a weight is used to reproduce the required mass. There are single-valued and multi-valued measures, and in some cases entire stores of measures. An unambiguous measure is necessary to reproduce the value of only one size. Multi-valued measures are used to determine several sizes of physical values (for example, they select measuring instruments for linear dimensions, with which you can find out both centimeters and millimeters).
Reference - measures with a very high level of accuracy. They are used to control the correctness of measuring instruments.
Measuring transducer is a measuring instrument that transforms the measurement information signal into another form. This makes it easier to transmit the signal for further processing and storage. But the converted signal cannot be perceived by the observer without the use of a special tool. For visualization, the signal must be transmitted to the indicating device. Therefore, the transducer is usually included in the complete design of the measuring tool or used in conjunction with it.
Measuring device - a means for making measurements that is used togenerating a signal in a form that is available for subsequent visualization by an observer. There are various classifications of these devices depending on a group of factors. By purpose, they are divided into universal, special, and control. According to the constructive device, they can be mechanical, optical, electrical and pneumatic. According to the degree of automation, they are divided into mechanized, manual devices, automatic and semi-automatic.
Measuring installation is a set of tools and auxiliary elements combined to perform a specific function. The purpose of the parts of such an installation is to generate information signals in a form that will be convenient for the observer to perceive. In this case, the entire measuring installation is usually stationary.
Measuring system - a set of tools, the elements of which are connected by communication channels located within the entire controlled space. Its purpose is to measure one or more physical quantities that exist in the space under study.
Selection criteria
When choosing measuring instruments, it is necessary first of all to take into account the accuracy that will need to be achieved when performing work. It is indicated in the regulatory documents or in the technical documentation for the part.
In addition, when choosing a tool for measurement, limit deviations should be taken into account, as well as methods for carrying out work and ways to control them.
The main principle of the choice of measuring instruments is their compliance with the set requirements forobtaining reliable results in compliance with the accuracy specified by the regulations. In addition, it is important to take into account material and time costs: they should be as minimal as possible.
Initial data
For the correct choice of measuring instruments, it is necessary to have initial data on the following points:
- nominal weight of measurement value;
- value of the difference between the highest and lowest values;
- information about available working conditions for measurements.
If it is necessary to select a measurement system, taking into account the accuracy factor, it is necessary to calculate the error. It is calculated as the sum of the errors of all possible sources (devices for measuring, value converters, standards) in compliance with the laws established for each of the sources.
At the first stage, the choice of measuring instruments for accuracy in accordance with the requirements of the work is carried out. When selecting the final version, the following requirements are also taken into account:
- Working area of the values that are needed in the process of work.
- Dimensions of tools.
- Tool weight.
- Design features of the measuring instrument.
In metrology, the choice of measuring instruments according to the criterion of accuracy requires the presence of the following initial data:
- composition of changeable tool parameters;
- the value of the error tolerance of working tools, as well as the allowable values of the totalmeasurement errors;
- permissible values of the probability of occurrence of failures for the measured parameters;
- rules for the distribution of parameter deviations from their true values.
Standardized measurements
The choice of instruments usually takes into account the priority of standardized means for performing measurements. A standardized measuring instrument is such an instrument that was manufactured in accordance with the regulations of an international or special standard for the performance of the type of work in question.
In accordance with this, the conditions for choosing measuring instruments depend on the specialization of the production where the work is being carried out.
In the production of mass products, automated modern measurement and control tools are usually used, designed for high productivity. In serial production, various templates and control devices are used, according to which comparisons are made. In individual production, universal measuring instruments are chosen, with which you can perform various types of work.
Terms of Use
The choice of measuring and control instruments is carried out on the basis of the technical regulations for the selected instruments under the conditions of their normal operation and use.
Normal conditions are those conditions under which the values of the factors influencing the result can be omitted due to their smallness. The described conditions are usually indicated in the instructions formeasuring instruments or calculated during their calibration.
Distinction should be made between operating and limit conditions for measurements.
Working conditions are usually considered the conditions for performing measurements, under which the values of the values of the influencing factors are included in the tolerance of the working areas. In this case, the working area is called the range of values of the magnitude of the influencing factor, within which the existing error is brought to normal or the values of working tools are changed.
Limit conditions are usually called the maximum and minimum values of the actual and influencing quantities that the measuring instrument can withstand without major damage and deterioration of its operating properties and characteristics.
When choosing measuring and control instruments for use under working conditions, one should take into account the relationship between instrument readings and influencing quantities. Based on this, it is necessary to introduce corrections into the final readings of measuring instruments or use corrective devices and instruments.
According to regulatory documents, the amendments are determined by metrological characteristics normalized for workplace conditions.
Instrument Assignment
The choice of measuring instruments is based on the study of the differences between the two cases of their use:
- product of measurements of device parameters;
- exercising control over the measurement of device parameters.
In the first case, during the work it is necessary to reach a value less than the limitmeasurement errors. In the second case, the devices are selected according to the condition that the probability of possible parameter errors should not be higher than the allowable values.
Errors
One of the main criteria for choosing measuring instruments in metrology is the ratio of the values of the limit of permissible absolute error or error (Δ) and the tolerance field of the value to be measured (D).
The ratio must correspond to the following expression:
Δ ≦ 0.333 D.
The margin of error can be represented in relative terms (relative measurement error). In such a case, it must be less than or equal to 33.3% of the total value of the tolerance field, unless there are other special restrictions.
Measurement errors specified in the regulations are the maximum allowable errors. They include all elements of work that may depend on the selected measuring instruments, setting standards, temperature changes, etc.
Selection Method
The method of measuring instruments is divided into three types.
Approximate methodology is widely used in the approximate selection of instruments for measurement, as well as in the control and examination for compliance with regulatory, design and technological regulations. To do this, perform the following actions:
- The permissible size of the part is determined according to GOST.
- Possible execution error is calculatedmeasurements. It is taken as 25% of the total allowable size.
- A random element of the possible measurement error is calculated, which can be detected in almost all types of measurements.
- According to the reference tables, the choice of means of measurement is made depending on the type of part. The maximum possible error, which is a metrological indicator of any measuring device, should not exceed the random element of the possible measurement error.
- The characteristics of the selected device for measurements are entered in the metrological table.
The calculation method is used when choosing devices for single and small-scale production, measuring sample parameters with a statistical method of control, conducting experiments, and also rechecking defective parts. It includes the following action steps:
- The allowable size of the part is determined according to GOST.
- The possible measurement error is calculated. In this method, for the calculation, it is necessary to use a table of the ratio of the possible measurement error and tolerances of parts.
- The random element of the possible measurement error is calculated, similar to the value in the previous method.
- According to the reference tables, the tool is selected depending on the type of part.
- The characteristics of the selected device for measurements are entered in the metrological table.
Tabular methodology is used when selecting measurement tools for high-volume and mass production. Thethe method can be carried out if the work on the manufacture of parts includes measurements, and not control using gauges.
- The permissible size of the part is determined according to GOST, depending on the quality of accuracy.
- Calculate possible measurement error based on historical data from past periods.
- The random element of the possible measurement error is calculated, similarly to the previous values.
- According to the reference tables, the tool is selected depending on the type of part.
- The characteristics of the selected device for measurements are entered in the metrological table.
Thus, it can be noted that the methods for choosing measuring instruments depend on the type of production where the work will be performed.
Making a choice
The selection and assignment of instruments for measurements is carried out by the development departments:
- Regulatory documentation for the parameters of the choice of measuring instruments during laboratory research, quality control of manufactured products, operation of already manufactured products, its components and materials.
- Technological processes of product standardization, measurement of its constituent elements and materials.
- Measurement instrument and equipment maintenance projects.
The choice of means and methods of measurements according to the available initial data is carried out by qualified employees. They should be well acquainted with the basics of physical measurements, with the methods of registration andthe use of measurement results and errors, as well as the principles of standardization of metrological parameters and the calculation of instrument errors from them.
Special workers responsible for measuring instruments are assigned to carry out measurements during the manufacturing process.
In conclusion, it can be said that the right choice of measuring instruments from the range available today is the key to efficient production and reducing the number of defective products.
