What Is an Oxygen Sensor?
An oxygen sensor is a sensor that measures the concentration of oxygen in the atmosphere of a measurement space.
In an isolated and enclosed space, it is extremely important to measure oxygen concentration in order to maintain vital activities. Oxygen sensors play a major role in this oxygen concentration measurement.
It is said that when oxygen levels fall below 15%, people have difficulty breathing; when oxygen levels fall below 7%, brain function is impaired; and when oxygen levels fall below 4%, death occurs. Oxygen sensors are needed in various scientific and industrial fields, and a wide range of oxygen concentration measurement conditions and systems have been developed to meet the requirements of these fields.
A typical sensor is the zirconia sensor. Zirconia sensors are also used for product control in semiconductor manufacturing, energy conservation in automobiles, and exhaust gas purification.
Uses of Oxygen Sensors
Measurement of oxygen concentration is performed in two major roles: first, to detect and monitor oxygen concentration in order to prevent oxygen deficiency, which can save human lives. Secondly, they are used in controlling oxygen concentration in the production process of industrial products, etc.
1. Prevention of Oxygen Deprivation
Examples of applications for acid deprivation prevention include safety management at civil engineering sites, such as tunnels and underground construction, and oxygen inhalation management and hyperbaric resuscitation in the medical field. Products for these applications can be portable or wall-mounted.
2. Oxygen Concentration Control
Oxygen concentration control is used in the chemical, ceramics, and metals industries for process control in the manufacture of industrial products. One of the characteristics of industrial product manufacturing is that they are often used in high-temperature environments during heat treatment processes.
An example familiar to us in our daily lives is its use in automobile and motorcycle engines. By detecting the oxygen concentration in the exhaust gas, they play a role in adjusting the fuel concentration.
Principle of Oxygen Sensors
The measurement principles of oxygen sensors include galvanic cell type, zirconia individual electrolyte type, magnetic type, and wavelength tunable semiconductor laser spectroscopy type.
1. Galvanic Cell Type
The galvanic cell type has a simple structure and is used in portable oxygen meters. It consists of gold and lead electrodes, a resin membrane, and an electrolyte solution, and uses a mechanism in which an electric current corresponding to the oxygen concentration flows when oxygen passes through the membrane and dissolves in the electrolyte solution.
2. Zirconia Individual Electrolyte Method
The zirconia individual electrolyte method uses zirconia as a solid electrolyte. Zirconia can conduct oxygen negative ions (O2-) in a solid state, and the ions are conducted from a gas with high oxygen concentration (high O2 pressure side) to an atmosphere with low oxygen concentration (low O2 pressure side).
In a zirconia electrolyte oxygen sensor, electrodes are placed on the O2 high-pressure side and the O2 low-pressure side, respectively, and they are electrically connected to each other. The O2- electrode on the low O2 pressure side receives electrons from the transmitted O2-.
The electrons released at the low O2 pressure side (negative electrode) flow back to the high O2 pressure side (positive electrode). The electromotive force generated between the electrodes can be used to determine the partial pressure of oxygen at each electrode using a relationship called the Nernst equation, as shown below.
E= (RT/4F) – 1n (PA/PB)
Where R is the gas constant, T is the temperature, F is Faraday’s constant, and PA and PB are the oxygen partial pressures on the high and low O2 pressure sides, respectively. The temperature is measured by a thermocouple installed in the zirconia, and PA is based on the oxygen partial pressure in a normal atmosphere.
Other Information on Oxygen Sensors
Deterioration of Oxygen Sensors
Zirconia oxygen sensors used in the manufacturing process of industrial products need to be carefully monitored for deterioration. They are used in high-temperature environments, and a variety of gases can also degrade or affect the zirconia cell. Reducing gases, such as halogens, are also degradation factors.
In automotive applications, oxygen sensors can degrade or fail, causing an increase in toxic substances in the exhaust gases. Also, fuel consumption can deteriorate when fuel is more concentrated than necessary; when O2 sensors deteriorate, parts should be replaced at a dealer or service facility.