The oxygen concentration in the medium is determined using two plates
Scientists from the Ural Federal University and the Institute of High Temperature Electrochemistry of the Ural Branch of the Russian Academy of Sciences have created a combined electrochemical amperometric-potentiometric oxygen sensor, which differs from existing sensors by an unusual design solution and increased reliability. The scientists published an article describing this invention in the highly rated journal Sensors and Actuators.
Oxygen sensors are used for determination of the O2concentration in the exhaust gases of cars, in the exhaust flue gases on boilers, in metallurgical processes. By the concentration of oxygen one can judge the ratio of fuel and air supplied to car engines and boilers, the oxygen content in exhaust gases, the composition of the atmosphere during technological processes in metallurgy, cement industry, and so on.
The sensor created by the employees of UrFU and IHTE, is a miniature structure 1.7 by 0.7 cm in size 1.5 mm thick, which consists of two plates glued to each other – electrolytes based on zirconium dioxide 0.91ZrO2 + 0.09 Y2O3, the most reliable from known solid electrolytes. On the outer and inner sides of the plates the electrodes are applied with current leads attached to them. Between the plates there is a cavity into which a capillary is inserted from the outside – a thin stainless steel tube with a diameter of 0.15 mm.
“One plate serves in order to pump out oxygen in the form of ions from the cavity through the electrolyte by applying voltage to it. In other words, this plate functions as an amperometric sensor. At a certain pumping current inside the cavity the oxygen content reaches its minimum values, therefore, the current cannot grow further, thus, we reach the so-called limiting current. The lower the oxygen concentration, the lower the current limit.
Thus, by the limiting current, which is measured by an ammeter, it is possible to determine the concentration of oxygen in an oxygen-containing atmosphere, whether it is air, mixtures of oxygen with nitrogen, inert gases, and so on, “explains the co-author of the article, leading researcher of the Laboratory of Materials and Devices for Friendly Environment of UrFU, head of the Laboratory of Electrochemical Devices based on Solid Oxide Proton Electrolytes, Institute of High Temperature, Ural Branch of the Russian Academy of Sciences, Anatoly Demin.
The second plate acts as a potentiometric sensor. It allows establishing the potential difference of the electrodes measured by a voltmeter, one of which is located inside the cavity, and the other in the studied atmosphere. The presence of a second plate in the design, a potentiometric sensor, is the essence of the innovation.
“The limiting current is determined primarily by the oxygen concentration, but at the same time it depends on the state and parameters of the capillary. Various particles can get inside it, which change the inner diameter of the channel. The channel becomes clogged – the current limit becomes low. Thus, the ingress of particles into the diffusion channel leads to distortion of the sensor readings, the true oxygen concentration in the measured medium becomes uncertain, – explains Anatoly Demin. –In order to prevent this from happening, we added a second plate to the conventional amperometric sensor, which serves as a potentiometric sensor. Its use ensures more reliable operation of the device. “
At oxygen concentrations below 20% the current limit is proportional to the concentration. For example, if, as a result of instrument calibration, it is known that the current limit is 10 milliamper at 20% oxygen concentration, then the 5 milliamper current limit indicates 10 percent oxygen. However, at a higher oxygen concentration, the limiting current increases to a greater extent than the concentration increases; therefore, the sensor is calibrated before use: the dependence of the oxygen concentration on the limiting current is plotted. And the presence of a potentiometric sensor in the design matters here.
“We get a picture when not only the limiting current depends on the oxygen concentration in the analyzed medium, as it happens with amperometric sensors, but also the potential difference of the electrodes of the potentiometric sensor. We build a second calibration curve that links the current limit and the readings of the potentiometric sensor. If during measurements the readings of the two calibration curves coincide, then there are no changes in the diffusion channel. If the readings for the limiting current and for the potentiometric sensor differ from the calibration curves, this indicates that the sensor began to work incorrectly and its readings cannot be trusted, ”says Anatoly Demin.
Under the influence of high temperature the cracks can form in the plates, and the leaks can appear in the sealant. Then oxygen will penetrate into the sensor cavity not only through the capillary, but also through leaks. And the readings of the amperometric and potentiometric sensors will begin to differ from the calibration curves. It will also mean that the sensor has stopped working normally and needs to be replaced.
Due to its unique properties, the sensor, created by the scientists from UrFU and IHTE, provides a fast and accurate response to gas mixtures containing oxygen in the concentration range from minimal to almost 90% (0.8–88%) and at operating temperatures between 400 and 700 оС.
“In the design we offer, it is convenient to carry out measurements: we pass a current through one plate, measure the voltage on the other plate and, by comparing these two data, we judge the oxygen concentration in the analyzed medium,” sums up Anatoly Demin.
Source: https://urfu.ru/ru/news/32466/
