When the sensor is immersed in the melt, due to the difference in the chemical potentials of oxygen in the phases on both sides of the solid electrolyte, i.e. in the reference electrode and in the melt under study, an emf arises between them. The results of emf measurements can be recorded on the potentiometer tape, together with the emf of the thermoreceiver, they can be fed to the microprocessor device, and the results of calculating the oxygen activity in the melt are displayed on a digital display. The results of emf measurements can be entered into a computer for process control. Currently, zirconium dioxide ZrO2 stabilized with MgO, Y2O3 or CaO is used as a solid electrolyte. The results of studies on the effect of stabilizing additives MgO, CaO and Y2O3 on the properties of a solid electrolyte based on ZrO2 indicate some advantages of using MgO. The EMF readings of the electrochemical sensor with ZrO2 (MgO mole fraction 9%) during measurements in metal and slag were more stable than when using the ZrO2(CaO) electrolyte. It is believed that MgO additives provide better material resistance to sudden temperature changes. At the same time, the results of EMF measurements performed in slags using oxygen sensors from different companies with a change in the MgO content in the solid electrolyte within 7-15% were almost identical. Electrolytes based on HfO2 and ThO2 remain promising for use in oxygen sensors. If the electron conductivity parameter Pe for ZrO2 stabilized with 14% (mol.) CaO at 1600 °C is 1.2 10-10 Pa, and for ZrO2 stabilized with 2.4% (mol.) MgO — 5.2 10-11 Pa, then for HfO2 with CaO additives and for ThO2 stabilized with Y2O3, the electron conductivity parameter is 6.9⋅10-13 Pa and 1.2 10-13 Pa, respectively.
In oxygen sensors for short-term measurements in liquid metals, mixtures of Cr + Cr2O3 and Mo + MoO2 powders are mainly used as reference electrodes, the thermodynamic parameters of which are specified in the work. In single measurements and long-term continuous measurements in laboratory conditions, a gas electrode, in particular air with a current collector made of a noble metal, for example, platinum-rhodium wire, successfully served as a reference electrode. In the practice of electrochemical measurements, sensors with metal-metal oxide mixtures are much more common than those with a gas reference electrode, since in the second case the sensor design is more complicated and the measurements require greater care. Long-term electrochemical measurements with sensors with Cr + Cr2O3 and Mo + MoO2 reference electrodes at 1600 °C showed the stability of the measured emf for at least 2 hours, only when the equilibrium oxygen pressures above the reference electrode and the melt in which the measurements are taken are equal. For oxygen sensors with a Cr + Cr2O3 reference electrode and a thin-walled solid electrolyte used for short-term measurements in liquid steel, when the equilibrium oxygen pressures above the reference electrode and the metal differ significantly, the duration of reliable emf measurements is limited to 1 min. Oxygen transfer through the solid electrolyte disrupts the equilibrium in the reference electrode, resulting in a change in emf with constant oxidation of the metal melt.
In practice, current collectors made of steel, molybdenum, iridium, and platinum are mainly used.
To date, three main types of oxygen sensors have been developed for measurements in liquid metals in order to determine the concentration of oxygen dissolved in the melt (Fig. 2).

In a type A sensor, the solid electrolyte is a tube closed at one end. The outer diameter of the tube is 4-6 mm, the wall thickness is 0.8-1.5 mm, and the length is 25-40 mm. The tubular electrolyte is made from finely ground ZrO2 powder with stabilizing additives by molding, then sintered at temperatures from 1400 to 1900 C. The electrolyte must be tightly sintered and free of cracks. A metal-metal oxide mixture, usually Cr + Cr2O3 or Mo + MoO2, is placed in a solid electrolyte tube, then Al2O3 powder is poured in. Sensors of this type have found wide application in industrial conditions. A tubular sensor with a solid electrolyte insert of type 6, or a plug-type oxygen sensor, belongs to the first generation of electrochemical oxygen sensors that appeared abroad in the 1960s and were used for measurements in metal melts. A solid electrolyte of zirconium dioxide stabilized with MgO or CaO additives was welded into a quartz tube, ensuring the tightest possible connection of the tablet with the tube. This design showed good heat resistance when immersed in liquid steel, which initially contributed to its distribution, but oxygen sensors of this type turned out to be unreliable in operation. The main reason for the measurement errors was that silica was reduced when immersed in deeply deoxidized steel and enriched the metal layers near the sensor with oxygen. The error in measuring the emf of such an oxygen sensor became noticeable at an oxygen content in the metal of 0.005% and increased with increasing melt deoxidation. The type B pin oxygen sensor is made layered. A layer of fine-grained metal-metal oxide powder mixture 100-300 μm thick is applied to the pin, which can be molybdenum wire. When using plasma metallization technology, a metal powder, such as chromium, is used to form the layer, which after applying the layer in an oxidizing atmosphere turns into a mixture of metal and its oxide. A second layer of the same thickness of fine-grained stabilized zirconium dioxide or calcium zirconate CaZrO3 powder is applied to this layer, which serves as a reference electrode in the sensor. This layer, which can be applied by thermal metallization, is a solid electrolyte of the oxygen sensor. It must completely cover the reference electrode to avoid a short circuit when the sensor is immersed in a bath of liquid metal. Thermal metallization can be used to obtain solid electrolytes from materials with low resistance to thermal shocks, such as thorium dioxide ThO2. The pin probes are miniature. Their diameter is smaller than that of other types of oxygen sensors and is 1-2 mm. The response time is also shorter than that of other sensors. The manufacturing technology of pin sensors is less labor-intensive than that of tubular sensors; during thermal metallization, the processes of shaping and sintering occur in a few seconds. Careful electrochemical measurements in liquid iron with pin and tubular sensors with a change in oxygen activity in the melt from 5 10-5 to 0.030 showed good convergence of the results. The proportion of correct measurements for pin sensors was the same as for high-quality tubular sensors, i.e. ~ 90%. To increase the resistance to thermal shock when immersing tubular oxygen sensors in a metal melt, protective coatings are applied to the surface of the solid electrolyte. Such protective coatings are often a porous mass of solid electrolyte material. The reference electrode in such sensors is a mixture of Cr + Cr2O3, Mo + MoO2 or a gas with a known oxygen potential. It is believed that tubular sensors with protective coatings and gas reference electrodes can be used for long-term measurements of emf in liquid metals. Positive results in performing long-term measurements in liquid steel using such sensors have been obtained in laboratory conditions.
In Russia and abroad, tubular oxygen sensors with a closed end are currently produced for industrial measurements, which have high accuracy and reliability of measurements. Oxygen sensors manufactured since the early 80s allow determining the activity of oxygen dissolved in liquid steel to minimum values ​​of 10-4-5 10-5. One of the leading enterprises in the field of oxidation probe production is Eurasian Instruments LLC, which has been successfully operating on the market for 15 years. During its activity, the company has established itself as a reliable manufacturer that has mastered the production of probes of various types. The range of products of "Eurasian Instruments" includes probes designed to measure the oxidation of metal with different contents, both low and high. In addition, a universal probe has been developed that is suitable for almost all ranges of metal oxidation measurement.

The company also offers the ability to manufacture various types of sand body oxidation probe heads, including both standard and shortened models. In addition, different lengths of cardboard tubes are available, depending on the characteristics and methods of operation of these probes. This allows Eurasian Instruments to effectively meet the various requests of customers.

Work in the direction of oxidation measurement allowed the company to develop oxidation probes for both manual measurement and for use in manipulators. This makes the company's products a universal solution for various industries requiring high-quality oxidation measurements.

"Electrochemical control and calculations of steelmaking processes": monograph / S. N. Paderin, G. V. Serov, E. V. Shilnikov, A. V. Alpatov, - M .: Publishing House MISiS, 2011. - 248 p.