Measurement methods (measurement methods) are a set of rules and operations, the implementation of which provides indicators with a known error. According to the provisions of Federal Law No. 102, measurements must be carried out by methods certified in the prescribed manner.
Factors affecting the error
The deviation depends not only on the metrological characteristics of measuring instruments. Of no small importance are operator errors, shortcomings in the selection and preparation of samples, the conditions in which measurements are made, and other factors. Accordingly, measurement procedures (MP) are created in relation to specific conditions using specific tools.
This statement, however, does not mean that each laboratory should develop its own methods. However, if the laboratory uses a type of measuring instrument that is aligned with the certified MVI, the influencing factors are in the specified range,the operator has the established qualifications, then physical indicators in this environment will be measured with a known error.
Influencing factors should include:
- humidity and temperature of the ambient air and the environment in which the measurement is made;
- frequency and mains voltage;
- magnetic field;
- vibration and so on.
GOST GSI
Measurement methods, according to the state standard, include the following sections and structural elements:
- Name.
- Scope.
- Normative references.
- Terms and definitions.
- Abbreviations and symbols.
- Uncertainty requirements or assigned deviation characteristics.
- Methods and conditions of measurements.
- Requirements for safety, environmental protection measures, operator qualifications.
- Measurement preparation activities.
- Measuring.
- Processing results.
- Control accuracy.
- Applications.
Competent authorities
In accordance with GOST, measurement procedures are created and certified in the manner prescribed by Rosstandart. MVI verification is carried out:
- GNMC (Main Scientific Metrological Center);
- territorial authorities of the HMS (State Metrological Service);
- other organizations that have accreditation and have the right to conduct certification.
Verification of methods used outside the scope of the statemetrological supervision, enterprises organize and carry out according to the rules established by them.
Creation of MVI
Development of measurement methodology is carried out in accordance with the initial parameters and includes:
- Choice of method, measuring instruments, excipients, sequence of operations, algorithm for calculating totals.
- Creating a draft document for the measurement procedure.
- Metrological certification.
Initial requirements include:
- Assignment of the measurement technique.
- Standards of error.
- Measurement conditions.
- Characteristics of the measured object.
The appointment must include:
- Name (if necessary, a detailed name is given) of the quantity and its characteristics.
- Restrictions on the scope of application of MVI by departmental affiliation, characteristics and types of objects, 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).
Measurement conditions are set as a range of indicators of influencing quantities (factors): electrical, mechanical, climatic, and so on.
The characteristic of an object is set by the limiting values of those parameters, the deviation of which from the nominal indicators affects the error.
The choice of means and method of measurement inthe measurement technique is carried out in accordance with the 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 methods for performing measurements are divided into groups in accordance with the methods for obtaining results:
- Direct methods. When using them, the desired value is obtained on the basis of experimental data.
- Indirect methods. In this case, the final value is set taking into account direct measurements of quantities that have 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 body is based on the results of measuring its volume and mass.
According to the conditions in which measurements are made, measurement methods are divided into:
- Contact. They are based on the interaction of the sensitive element of the measuring device and the object. A simple example would be taking body temperature with a thermometer.
- Contactless. These methods are based, respectively, on the absence 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 parameter comparison method,to be measured, with SI unit, allocate:
- 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 (system of measurement) is performed by the scale.
- Comparison method. In this case, the measured parameter is compared with the indicator that is reproduced by the measure. For example, the mass on a balance scale is determined by balancing the weights.
Types of comparison methods
Among the main methods are:
- Null method. When used, the net effect of the magnitudes on the comparator is reduced to 0. For example, the electrical resistance strength of a bridge is determined by its absolute balancing.
- Coincidence method. When using it, the difference that occurs between the indicators of the desired and reproducible measure is measured when the marks on the scales (for example, calipers and vernier) or periodic signals coincide.
- Replacement 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 scale pan.
Wastewater analysis: measurement technique (PND F 14.1:2:4.135-98)
This MVI allows you to determine the content of elements in certain ranges in a sample solution withoutdilution.
PND F 14.1:2:4.135-98 establishes the methodology for performing mass concentration measurements:
- silicon;
- barium;
- aluminum;
- beryllium;
- boron;
- thallium;
- sodium;
- arsenic and other elements.
If necessary, it is possible to determine the content of oxides of various elements in samples of waste, drinking, natural water by calculation.
Method of measuring the mass concentration of substances is based on determining the intensity of radiation of atoms and ions of the corresponding element excited in argon plasma.
Research mechanism
A perist altic pump and nebulizer are used to introduce the sample solution (sample) into the atomic emission spectrometer. The solution in the form of small droplets (in the form of an aerosol) enters the chamber. The aerosol is injected into the inductively coupled plasma through a burner tube in an argon flow.
During the entire time the sample is in it (approx. 2-3 ms), cycles of evaporation and atomization, ionization and excitation pass. The radiation emitted by ions and atoms is focused by the spectrometer at the entrance slit. It is further separated by wavelength by a diffraction grating (dispersing element).
Spectrometer with polychromator allows you to perform simultaneous multi-element research. In this case, monochromatic radiation, having passed diffraction on the grating, enters the exit slit. At the output, a fixed number of PMTs (photoelectronicmultipliers). Each of them registers radiation of a specific wavelength at its output.
In an atomic emission spectrometer with an Echelle optical system, the separation (decomposition) of radiation is carried out by a diffraction grating and a prism. As a result, the spectral image is two-dimensional.
The functions of the recorder are performed by CID (semiconductor matrix detector). The number of recording pixels in it exceeds 250 thousand. As a result, a multi-element analysis can be performed in one measurement and the most sensitive lines of each element can be registered.
Example of measurement procedure: sample mineralization
Analysis of wastewater samples containing visible suspended particles (sediment) is carried out in two ways.
The first is open vessel research. A sample of waste water containing sediment or suspended particles is mixed. After that, 100 cubic meters are taken into a heat-resistant glass (or flask). see sample.
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.
A blank sample is being prepared at the same time. It uses deoinized or bidistilled water instead of waste water.
To the analyzed and blank samples are added concentrated nitric acid (2 cc) and hydrogen peroxide (1 cc).
The containers are heated for two hours without boiling. As a result, the solution is evaporated to about 25 cubic meters. see
Aftercooling, the samples are brought to the initial volume (100 cc) with deoinized or bidistilled water.
If a suspension remains, it is removed (by filtration) into a dry dish.
Microwave decomposition
As in the previous case, the sample containing suspended particles should be mixed. Take 50 cm samples with a measuring cylinder3 and place in a PTFE cylinder.
After that, concentrated nitric acid (2 cm3) is added to the sample. The mixture is placed in a fume hood for 15-30 minutes.
The PTFE cylinder is inserted into the autoclave (heating apparatus) of the microwave oven. In this case, you should be guided by the instruction manual for the equipment and observe safety precautions.
Apparatus for heating are placed in the oven; the sample digestion program is installed.
The cooled autoclaves are gently shaken. This is necessary so that the contents are thoroughly mixed. After that, to balance the pressure, open the lid a little.
Qualitatively decomposed mixture after removal of nitric oxides is a yellowish or colorless transparent solution. There should be no undissolved particles on the walls of the liner.
The solution is cooled to room temperature, then transferred to a 50 cm flask3. The walls of the fluoroplastic liner are washed with bidistilled or deionized water (small portions).
Attestation
It is carried out for those MVI whoare used in the areas of state metrological supervision. Certification of measurement methods is also carried out to control the state of technically complex systems (GOST 22.2.04).
MTI, which are used outside the scope of state control and supervision, are certified according to the rules defined at the enterprise or in the industry 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 for 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 compiled 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 MMI.
- Draft document regulating the methodology.
- Program and results of calculation/experimental estimation of error characteristics.
Positive result
In the case of establishing compliance of the MMI with the provisions of the regulatory document, the latter is approved in the prescribed manner. It (except for the state standard) indicates that the MVI is certified. In this case, the organization (enterprise) whose metrological service carried out the check is indicated. May be indicated by the GNMC or the GMS authority.
Registration of MVI
Certified methods are subject to accounting. For this, the Federal Register of Measurement Methods was created. It consists of several sections.
Regulated by the standard and certified methods intended for use in the areas of distribution of metrological state control and supervision must be registered without fail.
To be included in the register of measurement methods, the developer sends to VNIIMS (All-Russian Research Institute of Metrological Service) a document for MVI with a copy of the attestation certificate attached.
There is no registration fee.
Each technique is assigned a code when it is entered into the register. It includes the abbreviation FR (Federal Register), section number (one digit), measurement type code (two digits), registration date (year) and account number (five digits). For example: FR.1.37.1998.00004.
