RU57012U1 - SENSOR FOR MEASURING THE CONCENTRATION OF COMPONENTS OF ANALYZED GAS MEDIA - Google Patents

Abstract

The proposed utility model relates to the field of analytical instrumentation and can be used as a sensor in gas analyzers for measuring the concentration of oxygen and carbon monoxide in gas analysis systems and environmental monitoring. The purpose of the utility model is to expand the scope. The proposed sensor consists of two potentiometric solid electrolyte cells and an additional coulometric solid electrolyte cell made of a solid electrolyte composition of 0.8ZrO ₂ + 0,15Y ₂ O ₃ . A common electrode of finely dispersed platinum of potentiometric cells is deposited on the inner surface of the capillary and is a reference for both cells. On the outer surface of the capillary above the reference electrode, two measuring electrodes isolated from each other are applied. The first electrode is similar to the reference electrode made of finely dispersed platinum and has a high catalytic activity with respect to the combustion reaction. The second measuring electrode is made of non-catalytic material, which is a composition based on silicon carbide and copper oxide and has a low reduction ability. The capillary together with the electrodes forms two potentiometric solid electrolyte cells with a common reference electrode. The reference oxygen concentration is created using an additional solid electrolyte cell operating in the coulometric mode. On the inner and outer surfaces of a cone made of solid electrolyte, electrodes are made of finely dispersed platinum: inner - anode, outer - cathode.

The capillary and cone are connected using high-temperature dielectric glue in such a way that their inner space forms a common inner chamber, in which the reference electrode of potentiometric cells and the anode of the coulometric cell are located. The cathode of the coulometric cell and the measuring catalytic and non-catalytic electrodes are located in the analyzed medium. The required temperature in the working area of the potentiometric cells is created by the heating device, and its control is carried out by a thermocouple placed between the catalytic and non-catalytic measuring electrodes. The coulometric cell creates an oxygen concentration of 100% in the common inner chamber, which is a reference. By measuring the potential difference between the measuring catalytic, measuring non-catalytic and reference electrodes, it is possible to determine the concentration of oxygen and carbon monoxide in the analyzed medium. The proposed sensor is illustrated in the drawing.

Description

The utility model relates to the field of analytical instrumentation and can be used as a sensor in gas analyzers for measuring the concentration of oxygen and carbon monoxide in gas analysis systems and environmental monitoring.

A known sensor used in the infrared method of gas analysis (V.P. Kholod and others. "Methods and tools for analytical control of automobile exhaust", TsNIITEI instrument making, M., 1980).

The infrared gas analyzer sensor uses the ability to selectively absorb infrared (IR) radiation by the analyzed mixture: the main elements of the exhaust gases (nitrogen oxides, carbon monoxide and carbon dioxide, hydrocarbons) are capable of absorbing infrared radiation of a certain wavelength, while pure gases (nitrogen, hydrogen) , helium) do not possess such ability.

The types of molecules can be identified by the absorption spectrum, and the concentration of this component can be determined by the absorption intensity. Since each gas has an individual absorption spectrum, it becomes possible to selectively analyze the concentration of this component. Such a sensor uses the non-dispersive infrared method (NDIC) - without decomposing the beam into a spectrum. The test gas is passed under slight excess pressure through an analytical chamber located on the path of the measuring beam. On the path of the second beam in the comparison chamber is pure gas, which does not absorb radiation. The radiation from two incandescent spirals connected in series after passing through the analytical chamber and the comparison chamber falls into two receiving chambers, which are filled with the measured component and separated by a stretched metal membrane. Incident radiation

absorbed selectively, only in areas corresponding to the characteristic absorption bands of gas in the receiving chambers. The difference in the energy absorbed by the cameras depends only on the concentration of the measured component in the analytical chamber. The absorbed energy creates a temperature and pressure difference in both receiving chambers. Due to the rotation of the diaphragm disk, the corresponding light fluxes are modulated, which causes periodic pressure changes in the receiving chambers. The pressure difference causes the membrane to bend and the related change in capacitance of the capacitor formed by the movable membrane and the stationary electrode.

Due to the fact that the absorption spectra of carbon monoxide and carbon dioxide overlap somewhat in the sensor design, filter cuvettes filled with an interfering component of carbon dioxide are installed on the path of both rays. In the filtering cells, all the radiation is absorbed, the wavelength of which is the same for the measured and interfering components.

The disadvantages of the sensor include:

- the complexity of the design;

- the need to create excess pressure of the analyzed gas mixture;

- when working, it is necessary to ensure the identity of the light flux in a two-beam optical system, sufficient intensity and stability of the radiation flux over time;

- when measuring, the content of only one component is determined - carbon monoxide.

Also known is a sensor for analyzing the oxygen content in the gas mixture and monitoring the completeness of fuel combustion (Volkov AN et al. "EMF of electrochemical solid electrolyte cells with electrodes having different catalytic activity". Electrochemistry, T.XXV, issue 11, 1989 .).

The sensor consists of a housing placed in the heater. A solid electrolyte test tube is sealed in the housing, in the working area of which the temperature is maintained with an accuracy of ± 5 ° С. Temperature control in the analysis zone is carried out by a thermocouple. As a solid electrolyte, a thin-walled tube made of zirconia stabilized with a weight of 15 is used. % yttrium oxide. A porous platinum electrode, called a reference electrode, is applied at the inner end of the tube, to which air or other gas with a known partial pressure of oxygen is supplied. Measuring electrodes are deposited on the outer surface of the test tube working area. One of the measuring electrodes, similar to the reference one, is made of porous platinum and has a high catalytic activity with respect to the combustion reaction, the second measuring electrode is made of non-catalytic material, which is a composition based on silicon carbide and copper oxide.

Gas for analysis is supplied under excess pressure to the working area of the solid-state electrolyte tube, and then discharged into the atmosphere. EMF meters are connected to the terminals of the electrodes, which measure the potential difference between the reference and catalytic, reference and non-catalytic, catalytic and non-catalytic electrodes. By applying a gas with a known partial oxygen pressure to the reference electrode from the potential difference between the reference and non-catalytic measuring electrodes, the partial pressure of oxygen in the analyzed gas can be determined. By measuring the potential difference between the catalytic and non-catalytic measuring electrodes, it is possible to determine the amount of oxygen used to oxidize the combustible components of the gas mixture, and in accordance with the oxidation reaction, the quantitative content of combustible impurities in the analyzed gas can be determined.

Disadvantages of the sensor:

- elements of the gas and electrical circuits require careful sealing;

- it is necessary to supply gas with a known partial pressure of oxygen to the reference electrode;

- gas for analysis must be supplied under excess pressure.

The closest in technical essence is the flue gas oxygen sensor (RF Patent No. 2099697 G 01 No. 27/419) that allows measuring the concentration of oxygen in flue gases.

The oxygen sensor contains a sensitive element made in the form of a solid electrolyte potentiometric cell with a working and comparative electrodes to which an EMF meter is connected, and an additional solid electrolyte cell hermetically connected to the potentiometric solid electrolyte cell with a dielectric composition. The working electrode of the potentiometric cell and one of the electrodes of the additional cell are in contact with the analyzed medium, and the comparative electrode of the potentiometric cell and the second electrode of the additional cell are located in the inner chamber, common for both cells and connected to the analyzed medium by a capillary from 10 to 100 mm long and with an internal diameter of 0.1 to 1.5 mm. The electrodes of the additional cell are connected to the current source in polarity plus the source is applied to the internal electrode, and a constant current of 10 to 100 mA is passed through the cell.

The sensor in question has the following disadvantages. Firstly, it is designed to measure only one component - oxygen. Secondly, all electrodes are made of finely dispersed platinum, and down conductors are made of platinum wire. This material has a high catalytic activity, therefore, if there are reducing agents in the composition of the analyzed gas, some of the oxygen goes to the oxidation of the combustible components of the gas mixture. As a result, the accuracy of measuring the concentration of oxygen in the analyzed gas is reduced.

The essence of the utility model is to eliminate the above disadvantages.

The figure shows a drawing of the proposed sensor.

In the inner channel of capillary 1, made of a solid electrolyte of composition 0.85ZrO ₂ + 0.15Y ₂ O ₃ , a porous electrode of finely dispersed platinum is deposited, into which a platinum wire is burned, which is the electrode outlet. This electrode 2 is a reference (comparative). Measuring electrodes are deposited on the outer surface of the capillary. One of the measuring electrodes 3, similarly to the reference electrode, is made of finely dispersed platinum and has high catalytic activity with respect to the combustion reaction, the second measuring electrode 4 is made of non-catalytic material, which is a composition based on silicon carbide and copper oxide. The conclusions from the electrodes 3 and 4 are made respectively by a wire of platinum and rhodium. The capillary together with the electrodes is the first and second potentiometric solid electrolyte cell (PTEY-1 and PTEY-2).

An additional solid electrolyte cell is made in the form of a cone 5 from the composition given above, and is a coulometric solid electrolyte cell (CTEC). Finely dispersed platinum is deposited on the inner and outer surfaces, which forms the electrodes. The inner electrode 6 of the cell is the anode, and the outer 7 is the cathode. From each electrode, platinum wire leads are made.

The capillary and cone are connected with a high-temperature dielectric glue in such a way that their inner space forms a common inner chamber 8, in which the reference electrode 2 of the potentiometric cells and the anode 6 of the coulometric cell are located. The inner chamber 8 is connected using a capillary 1 with the analyzed medium. Capillary 1 is selected with a length of 100 mm and an inner diameter of 1 mm.

Using current collectors PTEY-1 and PTEY-2 are connected to EMF meters that are not part of the sensor, and the coulometric cell is connected to an external stabilized current source. The sensor is thermally stabilized by the heating device 9, the working temperature in the area of the electrodes of the potentiometric cells is maintained at a level of (700 ± 2) ° С, and the temperature is controlled by a thermocouple 10. All current collectors and thermocouple leads are insulated from each other by ceramic tubes 11. All elements of the sensor construction are fastened rings 12 and are fixed in a sleeve 13 made of stainless metal, which is isolated from the heating device by a non-metallic gasket 14. The potentiometric and coulometric cells are tsya sensitive sensor element.

In the measurement mode, the sensitive element is in the analyzed medium, washing the measuring electrodes (catalytic 3 PTEI-1 and non-catalytic 4 PTEI-2) and cathode 7 CTEI.

Under the action of a voltage from a stabilized current source, a current flows through the CTEC, which is transferred due to the oxygen-ion conductivity of the solid electrolyte by oxygen ions. Oxygen ions are discharged at the inner electrode (anode 6), forming oxygen molecules that are released into the gas phase of the inner chamber 8 of the sensing element. Capillary 1 impedes the flow of gas from the analyzed medium into the inner chamber of the sensing element. By selecting the size of the capillaries (along the length and inner diameter of the channel) and choosing the value of the dosing current through KTEY, the conditions are achieved under which the inner chamber of the sensing element is filled with pure oxygen at a pressure close to the pressure of the analyzed medium. Pure oxygen washes the reference electrode of potentiometric cells and a potential arises on it, proportional to the partial pressure of oxygen in the comparative medium. If the analyzed gas does not contain components capable of oxidation (in this case, CO), the measurement

the electrodes washed by the analyzed gas have a potential proportional to the partial pressure of oxygen in this gas.

The potential difference (EMF) between the reference and measuring electrode, in accordance with the Nernst formula, is equal to:

where E is the EMF;

R is the gas constant;

T is the temperature in the zone of the electrodes;

4 - the number of electrons involved in the electrochemical reaction;

F is the Faraday number;

- partial oxygen pressure on the reference and any of the measuring electrodes, respectively.

If combustible components are present in the analyzed gas, they will be oxidized on the surface of the measuring electrode 3 made of catalytic material; oxidation on the surface of the second measuring electrode 4, non-catalytic, will not occur. Then the potential difference between the reference 2 and the catalytic measuring 3 electrodes (E ₁ ) of the sensor, allows you to calculate the residual partial pressure of oxygen in the analyzed gas after oxidation of combustible components

From the potential difference between the reference and non-catalytic measuring electrodes (E ₂ ) of the sensor, it is possible to determine the total partial pressure of oxygen in the analyzed gas

The partial pressure of oxygen that went into oxidation will be equal

or

Where

Considering that the internal chamber of the sensor and the analyzed medium are connected by a capillary and are under the same pressure, in the above formulas the partial pressures can be replaced by concentrations.

In operating mode, the inner chamber is filled with pure oxygen, so the oxygen concentration

Based on the foregoing, you can write:

or

The potential difference between the two measuring electrodes (E ₃ ) can be expressed across and E ₃ as follows

For practical calculations, you can use the following ratio:

In the absence of oxidizable impurities in the analyzed medium, the ratio equal to 1, as their content increases, it will decrease. If you measure the total oxygen content at the same time in the analyzed gas, then in accordance with the reaction

can determine the concentration of CO.

The oxidation reaction shows that

Given that - volume fraction of oxygen in the analyzed medium it is possible to make an equation of the static characteristic of the sensor:

- for oxygen

- carbon monoxide

Experimental sensor studies have been carried out that confirm the possibility of measuring oxygen concentration up to 5% and carbon monoxide concentration up to 2.2%.

Claims (1)

A sensor containing a sensing element in the form of a solid electrolyte potentiometric cell with a working and comparative electrodes of finely dispersed platinum deposited respectively on the outer and inner surfaces of the capillary, an additional solid electrolyte cell with electrodes of finely dispersed platinum, hermetically connected to a potentiometric solid electrolyte cell with a dielectric electrodes, moreover and an additional solid electrolyte cell are located in medium, and the comparative electrode of the potentiometric cell and the second electrode of the additional cell in the inner chamber, common for both cells and connected to the analyzed medium by a capillary 100 mm long and 1 mm inner diameter, characterized in that an additional non-catalytic measurement is applied to the capillary from the outside an electrode of a second potentiometric solid electrolyte cell made of a composition based on silicon carbide and copper oxide and having a low reduction ability, and the morning electrode is a continuation of the comparative electrode of the first potentiometric cell and serves as a reference electrode, which makes it possible to measure the potential difference between the measuring catalytic, measuring non-catalytic and reference electrodes, using pure oxygen generated by an additional coulometric solid electrolyte cell as a comparative medium.