The choice of measuring instruments: types, classification, methodology and basic principles

Today, there are a large number of tools with which you can make measurements of various types: linear, weight, temperature, power, etc. The devices differ in accuracy, operating principle, purpose, and price.

In order to correctly perform the necessary work, you should carefully approach the choice of measuring instruments. They, in turn, are also divided into several types depending on the criteria under consideration.

Tool classification

Measuring instruments are tools and instruments that are used to perform measurements of physical quantities. For each of them, the errors specified in regulatory documents and technical regulations are determined.

Measuring instruments are divided into various types according to the following criteria:

• type of tool for work;
• principle of operation;
• comparison with the accepted standard;
• metrological application.

Tool types

The most common types of measuring instruments are listed below.

Measure - a measurement tool used to reproduce the desired size of the physical quantity in question. For example, a weight is used to reproduce the required mass. There are unambiguous and ambiguous measures, and in some cases whole stores of measures. An unambiguous measure is necessary to reproduce a value of only one size. Multiple-valued measures are used to determine several sizes of physical values ​​(for example, a choice is made of measuring instruments for linear dimensions, with which you can find out both centimeters and millimeters).

Standard - measures with a very high level of accuracy. They are used to control the correctness of measuring instruments.

A measuring transducer is a measuring tool that transforms a measurement information signal into another form. It is more convenient to transmit a signal for subsequent 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 transmitter is usually included in the complete design of the measuring tool or is used with it.

A measuring device is a means for performing measurements, which is used to generate a signal in a form that is accessible for subsequent visualization by an observer. There are various classifications of these devices depending on a group of factors. By appointment, they are divided into universal, special, and control. According to the design device can be mechanical, optical, electrical and pneumatic. According to the degree of automation, they are divided into mechanized, manual devices, automatic and semi-automatic.

A measuring installation is a combination 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. Moreover, the entire measuring installation is usually stationary.

A measuring system is a set of instruments, the elements of which are connected by communication channels located within the entire controlled space. Its purpose is the measurement of one or more physical quantities that are available in the space under study.

Criterias of choice

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, one should take into account the limiting deviations, as well as the methods for carrying out the work and the methods for controlling them.

The main principle of choosing measuring instruments is their compliance with the set requirements for obtaining 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 minimal if possible.

Initial data

For the correct choice of measuring instruments, you must have the source data for the following items:

• nominal weight of measurement value;
• the magnitude of the difference between the largest and smallest values;
• information on the available working conditions for the measurement.

If it is necessary to choose 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 (instruments for taking measurements, 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. When selecting the final version, the following requirements are also taken into account:

• The working area of ​​the quantities that are necessary in the process of work.
• Overall dimensions of tools.
• The weight of the tools.
• Design features of measuring instruments.

In metrology, the choice of measuring instruments according to the accuracy criterion requires the availability of the following initial data:

• the composition of the variable parameters of the tools;
• the value of the tolerance of the error of the working tools, as well as the permissible values ​​of the total error when measuring the parameters;
• allowable failure probability values ​​for the measured parameters;
• rules for the distribution of deviations of parameters from their true values.

Standardized Measurements

When choosing tools, they usually take into account the priority of standardized tools for taking measurements. A standardized measuring instrument is such a tool that has been manufactured in accordance with the regulations of an international or special standard for the implementation of the type of work under consideration.

In accordance with this, the conditions for choosing measuring instruments depend on the specialization of the production at which the work is being carried out.

In the production of bulk products, usually automated modern means of measurement and control, designed for high productivity, are used. In mass production, various templates and control devices are used, according to which comparisons are made. In individual production, the selection of universal measuring instruments is carried out, with which you can perform various types of work.

terms of Use

The choice of measuring and control instruments is based on the technical regulations for the selected instruments in the conditions of their normal use and use.

Normal conditions are those conditions under which the values ​​of the factors influencing the result of the factors can be omitted due to their smallness. The described conditions are usually indicated in the instructions for the measuring instruments or are calculated during their calibration.

Distinctions should be made between the operating and limiting conditions of the measurements.

The working conditions are usually considered the measurement conditions under which the values ​​of the factors influencing 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, inside which the existing error leads to normal or change the values ​​of the working tools.

The limiting conditions are usually called the maximum and minimum values ​​of the actual and influencing quantities, maintained by the measuring instrument without major damage and the deterioration of its working properties and characteristics.

When choosing measuring and control instruments for their use in working conditions, the interrelation of instrument readings and influencing quantities should be taken into account. Based on this, it is necessary to introduce amendments to the final readings of measuring instruments or to use corrective devices and instruments.

According to normative documents, amendments are determined by metrological characteristics normalized for conditions at the workplace.

Appointment of devices

The choice of measuring instruments is based on the study of the differences between two cases of their use:

• making measurements of device parameters;
• control over the measurement of device parameters.

In the first case, during the work it is necessary to achieve a value less than the limit of measurement error. 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 permissible values.

Inaccuracies

One of the main criteria for the choice of measuring instruments in metrology is the ratio of the values ​​of the margin of permissible absolute error or error (Δ) and the tolerance field of the quantity to be measured (D).

The ratio in this case should correspond to the following expression:

Δ ≤ 0.333 D.

The margin of error can be represented in relative values ​​(relative measurement error). In this case, it should be less than or equal to 33.3% of the total value of the tolerance field, if there are no other special restrictions.

Measurement errors specified in the regulations are the maximum permissible errors. They include all elements of the work, which may depend on the selected measuring instruments, installation standards, temperature changes, etc.

Selection technique

The technique of measuring instruments is divided into three types.

The approximate technique is widely used in the approximate selection of instruments for measurement, as well as in conducting control and examination for compliance with regulatory, design and technological regulations. To do this, perform the following steps:

1. The permissible part size is determined in accordance with GOST.
2. The possible measurement error is calculated. It is taken as 25% of the total size allowed.
3. A random element of the possible measurement error is calculated, which can be detected with almost all types of measurements.
4. According to the reference tables, the choice of measuring instrument is made depending on the type of part. The maximum possible error, which is a metrological indicator of any measuring device, should not exceed a random element of a possible measurement error.
5. The metrological table contains the characteristics of the selected device for making measurements.

The calculation method is used when choosing devices for single and small-scale production, measuring sampling parameters with a statistical method of control, conducting experiments, and also rechecking defective parts. It includes the following steps:

1. The permissible part size is determined according to GOST.
2. The possible measurement error is calculated. In this methodology, for calculation, it is necessary to use the table of the ratio of the possible measurement error and tolerances of the parts.
3. A random element of the possible measurement error is calculated, similar to the value in the previous method.
4. According to the reference tables, a tool is selected depending on the type of part.
5. The metrological table contains the characteristics of the selected device for making measurements.

Tabular methodology is used when choosing measurement tools for large-scale and mass production. This method can be carried out if the work on the manufacture of parts includes measurements, and not control using calibers.

1. The permissible part size is determined according to GOST, depending on the accuracy class.
2. The possible measurement error is calculated based on statistical data from past periods.
3. A random element of the possible measurement error is calculated, similar to the previous values.
4. According to the reference tables, a tool is selected depending on the type of part.
5. The metrological table contains the characteristics of the selected device for making measurements.

Thus, it can be noted that the methods of choosing measuring instruments depend on the type of production on which work will be carried out.

Making a choice

The selection and purpose of measuring instruments is carried out by departments that are involved in the development of:

• Normative documentation on the parameters of the choice of measuring instruments during laboratory research, quality control of manufactured products, operation of already manufactured products, its constituent elements and materials.
• Technological processes of product standardization, measurement of its constituent elements and materials.
• Projects for servicing instruments and equipment for making measurements.

The selection of measurement tools and methods according to the available source data is carried out by qualified employees. They should be familiar with the basics of physical measurements, with the methods of designing and using the results and measurement errors, as well as with the principles of normalizing metrological parameters and calculating instrument errors from them.

To carry out measurements during the manufacturing process, special workers are appointed who are responsible for the measuring instruments.

In conclusion, we can say that the right choice of instrument for measurements from the currently available assortment is the key to effective production and reduce the number of defective products.