Measurement methods ( measurement techniques ) are a set of rules and operations, the implementation of which provides indicators with a known error. According to the provisions of the Federal Law No. 102, measurements must be carried out by methods certified in the established manner.
Factors Affecting Error
Deviation depends not only on the metrological characteristics of the measuring instruments. Of no small importance are operator errors, shortcomings in the selection and preparation of samples, the conditions under which measurements are made, and other factors. Accordingly, measurement techniques (MVI) are created in relation to specific conditions using specific tools.
This statement, however, does not mean that methods should be developed for each laboratory. Nevertheless, if the type of measuring instrument used in the laboratory is adjusted to the certified MVI, the influence factors are in the specified range, the operator has the established qualifications, then the physical indicators in this situation will be measured with a known error.
Influencing factors include:
- humidity and ambient temperature and the environment in which the measurement is made;
- frequency and voltage of the power supply network;
- a magnetic field;
- vibration and so on.
GOST GSI
Measurement methods , according to state standard, include the following sections and structural elements:
- Title.
- Scope of application.
- Normative references.
- Terms and Definitions.
- Abbreviations and notation.
- Error requirements or attributed characteristics of deviations.
- Measurement methods and conditions.
- Safety requirements, environmental protection measures, operator qualifications.
- Measurement preparation activities.
- Taking measurements.
- Processing the results.
- Precision control.
- Applications
Authorized bodies
In accordance with GOST, measurement procedures are created and certified in the manner established by Rosstandart. MVI verification is carried out:
- GNMTS (The main scientific metrological center);
- territorial bodies of the HMS (State Metrological Service);
- other organizations with accreditation and empowered to conduct certification.
Verification of methods used outside the scope of state metrological supervision, enterprises organize and conduct according to the rules established by them.
Creation of MVI
The development of the measurement methodology is carried out in accordance with the initial parameters and includes:
- The choice of method, measuring instruments, excipients, sequence of operations, algorithm for calculating the final indicators.
- Creating a draft document on the measurement methodology .
- Metrological certification.
Initial requirements include:
- The purpose of the measurement methodology .
- Norms of error.
- Measurement conditions.
- Characteristics of the measured object.
The appointment must indicate:
- The name (if necessary, a detailed name is given) of the quantity and its characteristics.
- Restrictions on the scope of MVI by departmental affiliation, characteristics and types of facilities, etc.
Error standards should be set in the form of parameters specified in regulatory documents, with reference to the regulatory and technical act in which they are provided (if any).
The measurement conditions are set in the form of a range of indicators of influencing quantities (factors): electrical, mechanical, climatic and so on.
The characteristic of the object is set by the limiting values of those parameters whose deviation from the nominal indicators affects the error.
The choice of means and method of measurement in the measurement procedure is carried out in accordance with current regulatory and technical documents. If there are no NTDs, the calculation of the error characteristics or the results of their experimental study is taken as the basis.
Classification
Certified measurement procedures are divided into groups in accordance with the methods of obtaining results:
- Direct methods. When using them, obtaining the desired value is carried out on the basis of experimental data.
- Indirect methods. In this case, the final value is set taking into account direct measurements of quantities having a certain dependence on the measured object. These methods are used when it is not possible to use direct methods. For example, the calculation of the density of a solid is carried out on the basis of the results of measuring its volume and mass.
According to the conditions in which measurements are made, the measurement procedures are divided into:
- Contact. They are based on the interaction of the sensing element of the measuring device and the object. A simple example is the determination of body temperature using a thermometer.
- Non-contact. These methods are based, respectively, on the lack of contact between the object and the sensitive element of the measuring device. For example, calculating the distance using a radar in a blast furnace - determining the temperature with a pyrometer, etc.
Depending on the selected method of comparing the parameter to be measured with a SI unit, there are:
- Direct method. In such cases, the value is determined by the reading device. For example, it can be a voltmeter, ammeter, thermometer, etc. A measure that reflects a unit of measurement does not take part in the process. This task in SI (measurement system) performs the scale.
- Comparison method. In this case, the measured parameter is compared with the indicator, which is reproduced by the measure. For example, the mass on a lever scale is determined by balancing weights.
Types of comparison methods
Among the main methods we can distinguish:
- Zero method. When using it, the resulting effect of the influence of the values on the comparison device is brought to 0. For example, the strength of the electrical resistance of the bridge is determined by its absolute balancing.
- Method of coincidence. When using it, the difference that arises between the indicators of the sought and reproduced measure is measured when the marks on the scales (for example, vernier caliper and vernier) coincide or periodic signals.
- Substitution method. It is based on a comparison with a measure. The measured parameter is replaced by a known value. It is reproduced by measure. The conditions remain unchanged. For example, weighing is carried out by alternately moving the mass and weights on one cup of balance.
Wastewater analysis: measurement procedure (PNA F 14.1: 2: 4.135-98)
This MVI allows you to determine the content of elements in certain ranges in the sample solution without dilution.
PND F 14.1: 2: 4.135-98 establishes a methodology for performing measurements of mass concentration :
- silicon;
- barium;
- aluminum;
- beryllium;
- boron;
- thallium;
- sodium;
- arsenic and other elements.
If necessary, it is possible to determine the content of oxides of different elements in the samples of wastewater, drinking, natural water by calculation method.
The methodology for measuring the mass concentration of substances is based on determining the radiation intensity of atoms and ions of the corresponding element excited in an argon plasma.
Research mechanism
A peristaltic pump and atomizer are used to introduce a sample solution (sample) into an atomic emission spectrometer. The solution in the form of small drops (in the form of an aerosol) enters the chamber. Through a burner tube in an argon stream, an aerosol is injected into an inductively coupled plasma.
During the entire time the sample is in it (approx. 2–3 ms), cycles of evaporation and atomization, ionization and excitement pass. The radiation emitted by ions and atoms is focused by a spectrometer on the entrance slit. Further, it is separated by wavelengths by a diffraction grating (dispersing element).
A polychromator spectrometer allows simultaneous multi-element research. In this case, monochromatic radiation, after passing through diffraction by the grating, falls on the exit slit. At the output, a fixed number of PMTs (photoelectronic multipliers) is set. Each of them registers radiation of a specific wavelength at its output.
In an atomic emission spectrometer with an Echelle optical system, radiation is separated (decomposed) by a diffraction grating and a prism. At the output, the spectral image is obtained two-dimensional.
The functions of the registrar are performed by CID (semiconductor matrix detector). The number of recording pixels in it exceeds 250 thousand. As a result, in one measurement, you can perform multi-element analysis and register the most sensitive lines of each element.
Measurement method example: sample mineralization
Analysis of wastewater samples containing visible suspended particles (sediment) is carried out in two ways.
The first is a study in open vessels. A sample of wastewater containing sludge or suspended particles is mixed. After that, 100 cubic meters are taken into a heat-resistant glass (or flask). see samples.
If it is necessary to determine the dissolved forms of substances, the samples are pre-filtered. A membrane or paper filter can be used for this.
At the same time, a blank test is being prepared. It uses de-initialized or double-distilled water instead of waste water.
Concentrated nitric acid (2 cc) and hydrogen peroxide (1 cc) are added to the analyzed and blank samples.
The tanks are heated for two hours, avoiding boiling. As a result, the solution is evaporated to about 25 cubic meters. cm.
After cooling, the samples are brought with de-ionized or double-distilled water to the initial volume (100 cc).
If the suspension remains, it is removed (by filtration) in a dry dish.
Microwave decomposition
As in the previous case, the sample containing suspended particles should be mixed. With a measuring cylinder, you need to take 50 cm 3 of the sample and place it in the fluoroplastic cylinder.
After that, concentrated nitric acid (2 cm 3 ) is added to the sample. The mixture is placed in a fume hood for 15-30 minutes.
The fluoroplastic cylinder is inserted into the autoclave (apparatus for heating) of the microwave oven. In doing so, follow the operating instructions for the equipment and observe safety precautions.
Devices for heating are placed in the furnace; The sample decomposition program is installed.
The chilled autoclaves are gently shaken. This is necessary so that the contents are thoroughly mixed. After that, to balance the pressure, open the lid.
Qualitatively decomposed mixture after removal of nitrogen oxides is a yellowish or colorless transparent solution. Insoluble particles on the walls of the liner should be absent.
The solution was cooled to room temperature, then transferred to a flask with a capacity of 50 cm 3 . The walls of the fluoroplastic liner are washed with double-distilled or deionized water (in small portions).
Certification
It is carried out for those MVI, which are used in the areas of state metrological supervision. Certification of measurement procedures is also carried out to monitor the status of technically complex systems (GOST 22.2.04).
MVI, which are applied outside the areas of dissemination of state control and supervision, are certified according to the rules defined at the enterprise or in the branch department.
The key goal of the procedure is to confirm the possibility of taking measurements in the order and with an error not exceeding the indicators stated in the document on the methodology.
Certification is carried out by metrological services and other structures authorized to carry out the functions of ensuring the uniformity of measurements.
Verification is carried out on the basis of the results of the examination of materials and documents drawn up during the development of the MVI. These include technical / experimental research materials.
Documents for certification
The list of securities includes:
- Initial requirements for the creation (development) of MVI.
- The draft document regulating the methodology.
- The program and the results of the calculated / experimental estimation of the error characteristics.
Positive result
In the case of establishing compliance of MVI with the provisions of the regulatory document, the latter shall be approved in the established manner. It (except for the state standard) indicates that the MVI is certified. At the same time, the organization (enterprise) is indicated, the metrological service of which conducted an audit. GNMC or HMS organ may be indicated.
Registration MVI
Certified methods are subject to accounting. For this, the Federal Register of Measurement Techniques was created. It consists of several sections.
The procedures regulated by the standard and certified, intended for use in the areas of distribution of metrological state control and supervision, must be registered without fail.
To make measurements in the register, the developer sends to VNIIMS (All-Russian Research Institute of Metrological Service) a document on the MVI with an attached copy of the certification certificate.
No registration fee is charged.
Each method is assigned a code when entered into the registry. It includes the abbreviation FR (Federal Register), section number (one digit), measurement type code (two digits), registration date (year) and account serial number (five digits). For example: FR.1.37.1998.00004.