Sensors and measuring devices for determining the composition and properties of substances

The main feature of the classification of control devices and automation equipment is their role in automatic regulation and control systems in terms of information flow.

The tasks of technical means of automation in general are:

• obtaining primary information;

• her transformation;

• its transmission;

• processing and comparison of the received information with the program;

• formation of command (control) information;

• transmission of command (control) information;

• using command information to control the process.

Sensors for properties and composition of substances play a leading role in the automatic control system, they serve to obtain primary information and largely determine the quality of the entire automatic control system.

Let's establish some basic concepts.What is measurement, properties, composition of medium? The properties of the environment are determined by the numerical values ​​of one or more physical or physico-chemical quantities that can be measured.

Measurement is a process of revealing through an experiment the quantitative ratio of a certain physical or physico-chemical quantity characterizing the properties of the test medium and the corresponding amount of the reference medium. An experiment is understood as an objective process of active impact on the tested environment, produced with the help of material means under fixed conditions.

The composition of the environment, i.e. the qualitative and quantitative content of its constituent components, can be determined from its known dependence on the physical or physico-chemical properties of the environment and on the quantities that characterize them, subject to measurement.

As a rule, the properties and composition of the medium are determined indirectly. By measuring various physical or physico-chemical quantities characterizing the properties of the environment, and knowing the mathematical relationship between these quantities, on the one hand, and the composition of the environment, on the other, we can estimate its composition to a greater or lesser degree of accuracy.

In other words, in order to choose or build a measuring device, for example, to determine the complete composition of a multicomponent medium, it is necessary, first, to establish what physical or physico-chemical quantities characterize the properties of this medium and, secondly, to to find shape dependencies

ki = f (C1, C2, … Cm),

where ki — concentration of each component of the environment, C1, C2, ... Cm — physical or physico-chemical quantities characterizing the properties of the environment.

Accordingly, the device used to control the composition of the medium can be calibrated in units of the concentration of a certain component or properties of the medium, if there is an unambiguous relationship between them within some limits.

NSDevices for automatic control of the physical and physico-chemical properties and composition of substances are devices that measure separate physical or physico-chemical quantities that unambiguously determine the properties of the environment or its qualitative or quantitative composition.

However, experience shows that for the implementation of automatic regulation or control of a sufficiently studied technological process, it is not necessary to have complete information on the composition of intermediate and final products and on the concentration of some of their components at any moment. Such information is usually required when creating, learning and mastering processes.

When the optimal technological regulations have been developed, unambiguous relationships between the course of the process and the measurable physical and physico-chemical quantities characterizing the properties and composition of the products have been established, then the process can be carried out, device scale calibration directly in those quantities which he measures, for example, in units of temperature, electric current, capacitance, etc., or in units of the specified property of the medium, for example, color, turbidity, electrical conductivity, viscosity, dielectric constant, etc. n.

The main methods for measuring physical and physico-chemical quantities that determine the properties and composition of the environment are discussed below.

The existing historically established product nomenclature includes the following main groups of devices:

• gas analyzers,

• liquid concentrators,

• density meters,

• viscometers,

• hygrometers,

• mass spectrometers,

• chromatographs,

• pH meters,

• solinometers,

• sugar meters etc.

These groups, in turn, are subdivided according to the measurement methods or according to the analyzed substances. The extreme conventionality of such a classification and the possibility of assigning structurally identical devices to different groups make it difficult to study, select and compare devices.

Direct measurement devices include those that determine the physical or physico-chemical properties and composition of the directly tested substance. In contrast, in combined devices, the sample of the test substance is exposed to influences that significantly change its chemical composition or its state of aggregation.

In both cases, preliminary preparation of the sample in terms of temperature, pressure and some other parameters is possible. In addition to these two main classes of devices, there are also those in which both direct and combined measurement can be performed.

Direct measurement instruments

In direct measurement devices, the physical and physico-chemical properties of the medium are determined by measuring the following quantities: mechanical, thermodynamic, electrochemical, electrical and magnetic, and finally wave.

To mechanical values first of all, the density and specific gravity of the medium are determined using instruments based on float, gravity, hydrostatic and dynamic measurement methods.This also includes determining the viscosity of the medium, measured with various viscometers: capillary, rotary, based on the falling ball methods and others.

From thermodynamic quantities the heat effect of the reaction, measured with thermochemical devices, the coefficient of thermal conductivity, which is measured with thermoconductive devices, the ignition temperature of petroleum products, the vapor pressure, etc. have found application.

Extensive development to measure the composition and properties of liquid mixtures as well as some resulting gases electrochemical devices… They include above all conductometers and potentiometersdevices designed to determine the concentration of salts, acids and bases by changing electrical conductivity decisions. These are the so-called conductometric concentrators or contact and non-contact conductometers.

Found very widely distributed pH meters — devices for determining the acidity of the medium by the potential of the electrode.

The electrode potential shift due to polarization is determined in galvanic and depolarizing gas analyzers, serving to control the content of oxygen and other gases, the presence of which causes depolarization of the electrodes.

It is one of the most promising polarographic measurement method, which consists in the simultaneous determination of the release potentials of various ions on the electrode and the limiting current density.

The measurement of moisture concentration in gases is achieved by means of coulometric method, where is defined rate of electrolysis of wateradsorbed from the gas through a moisture-sensitive film.

Devices based on for measuring electric and magnetic quantities.

Gas ionization with simultaneous measurement of their electrical conductivity, is used to measure low concentrations. Ionization can be thermal or under the influence of various radiations, in particular radioactive isotopes.

Thermal ionization is widely used in flame ionization detectors of chromatographs… Ionization of gases by alpha and beta rays is widely used in chromatographic detectors (so-called "argon" detectors), as well as in alpha and beta ionization gas analyzersbased on the difference in the ionization cross sections of different gases.

The test gas in these instruments passes through an alpha or beta ionization chamber. In this case, the ionization current in the chamber is measured, which characterizes the content of the component. Determining the dielectric constant of a medium is used to measure the content of moisture and other substances by means of various types capacitive moisture meters and dielectric meters.

The dielectric constant a sorbent film washed by a gas stream is used, characterizing the concentration of water vapor in it dielometric hygrometers.

The specific magnetic sensitivity makes it possible to measure the concentration of paramagnetic gases, mainly oxygen, by means of thermomagnetic, magnetoeffusion and magnetomechanical gas analyzers.

Finally, the specific charge of particles, which together with their mass is the main characteristic of a substance, is determined by time-of-flight mass spectrometers, high-frequency and magnetic mass analyzers.

Measurement of wave quantities — one of the most promising directions in instrument building, based on the use of the effect of the interaction of the tested environment with different types of radiation. So, the intensity of absorption from the environment ultrasonic vibrations makes it possible to estimate the viscosity and density of the medium.

Measuring the velocity of propagation of ultrasound in a medium gives an idea of ​​the concentration of individual components or the degree of polymerization of latexes and other polymeric substances. Almost the entire scale of electromagnetic oscillations, from radio frequencies to X-rays and gamma radiation, is used in sensors for the properties and composition of substances.

They include the most sensitive analytical instruments that measure the intensity of absorption of energy from electromagnetic oscillations in the short-wavelength, centimeter and millimeter ranges, based on electromagnetic and nuclear magnetic resonance.

The most widely used are devices that use the interaction of the environment with light energy. in the infrared, visible and ultraviolet parts of the spectrum… Both the integral emission and absorption of light and the intensity of the characteristic lines and bands of the emission and absorption spectra of substances are measured.

Devices based on the optical-acoustic effect are used, operating in the infrared region of the spectrum, suitable for measuring the concentration of polyatomic gases and vapors.

Refractive index of light in the medium used to determine the composition of liquid and gaseous media by refractometers and interferometers.

The measurement of the intensity of rotation of the plane of polarization of light by solutions of optically active substances is used to determine their concentration by polarimeters.

Methods for measuring the density and composition of various media, based on the various applications of the interaction of X-ray and radioactive radiation with the medium, have been widely developed.

Combined devices

In a number of cases, the combination of direct determination of the physical and physico-chemical properties of the environment with various auxiliary operations preceding the measurement can significantly expand the measurement possibilities, increase the selectivity, sensitivity and accuracy of simple methods. We call such devices combined.

Ancillary operations include primarily absorption of a gas from a liquid, vapor condensation and liquid evaporationallowing the use of methods for measuring the concentration of liquids in the analysis of gases, such as conductometry, potentiometry, photocolorimetry, etc.and vice versa, to measure the concentration of the liquids used methods for gas analysis: thermal conductometry, mass spectrometry, etc.

One of the most common sorption methods is chromatography, which is a combined measurement method in which the determination of the physical properties of the test medium is preceded by the process of its chromatographic separation into its constituent components. This simplifies the measurement process and dramatically expands the limits of the possibilities of direct measurement methods.

The ability to measure the total composition of complex organic mixtures and the high sensitivity of the devices have led to the rapid development of this direction in analytical instruments in recent years.

A practical application has been found in industry gas chromatographsconsisting of two main parts: a chromatographic column designed to separate the test mixture and a detector used to measure the concentration of the separated components of the mixture. There is a wide variety of designs for gas chromatographs, both in terms of the thermal regime of the separation column and the principle of operation of the detector.

In isothermal mode chromatographs, the temperature of the column thermostat is kept constant during the analysis cycle; in chromatographs with temperature programming, the latter changes over time according to a predetermined program; in thermodynamic mode chromatographs, during the analysis cycle, the temperature of different parts of the column changes along its length.

In principle, a chromatographic detector can be used any device for determining the physical and physico-chemical properties of a given substance. Its design is even simpler than that of other analytical instruments, since the concentrations of the already separated components of the mixture must be measured.

Currently widely used detectors based on measuring gas density, thermal conductivity (the so-called "catarometers"), the thermal effect of the combustion of the products ("thermochemical"), the electrical conductivity of the flame into which the test mixture enters ("flame-ionization"), the electrical conductivity of the gas ionized by radioactive radiation ("ionization -argon") and others.

Being very universal, the chromatographic method gives the greatest effect when measuring the concentration of impurities in complex hydrocarbon mixtures with a boiling point of up to 400 — 500 ° C.

Chemical processes that bring the medium to parameters that can be measured in simple ways can be used with almost all direct measurement methods. The selective absorption of individual components of a gas mixture by a liquid makes it possible to measure the concentration of the test substances by measuring the volume of the mixture before and after absorption. The operation of volume-manometric gas analyzers is based on this principle.

Different color reactions, preceding the measurement of the effect of the interaction with the substance of the light emission.

This includes a large group of so-called strip photocolorimeters, in which the measurement of the concentration of gas components is carried out by measuring the degree of darkening of a strip on which a substance which gives a color reaction with the test substance has previously been applied. This method is widely used to measure microconcentrations, in particular dangerous concentrations of toxic gases in the air of industrial premises.

Color reactions are also used in liquid photocolorimeters to increase their sensitivity, to measure the concentration of colorless components in liquids, etc.

It's promising measuring the luminescence intensity of liquidscaused by chemical reactions. One of the most common analytical chemical methods is titration... The titration method consists in measuring physical and physico-chemical quantities inherent in a liquid medium that is exposed to external chemical or physical factors.

At the moment of transition of quantitative changes to qualitative ones (the end point of titration), the consumed amount of substance or electricity corresponding to the concentration of the measured component is recorded. Basically, it is a cyclic method, but there are different versions of it, up to continuous. The most widely used as indicators of the end point of the titration are potentiometric (pH-metric) and photocolorimetric sensors.

Arutyunov OS Sensors for the composition and properties of matter