What Is an Oxygen Monitor?
An oxygen monitor is a measuring instrument used to determine the concentration of oxygen in the air.
It is sometimes called an oxygen sensor or an oxygen meter. Oxygen monitors are needed because oxygen is an essential substance for human life.
Monitoring oxygen levels is extremely important, especially in confined work benches that are prone to oxygen deprivation. In addition, many scientific and industrial fields require accurate oxygen concentration control in terms of process control and equipment maintenance. Depending on the requirements of the scientific and industrial sectors, measurement systems are available for a variety of measurement conditions.
Typical examples include zirconia sensors, which are used for product control in semiconductor manufacturing, energy conservation in automobiles, and exhaust gas purification. For more information on oxygen monitors, please refer to the figure below.
Uses of Oxygen Monitors
Uses of oxygen monitors can be broadly classified into two categories: “for prevention of oxygen deprivation” and “for oxygen concentration control.”
1. Monitoring of Oxygen Concentration for the Purpose of Preventing Oxygen Deprivation (Detection and Monitoring)
Oxygen monitoring plays an extremely important role in maintaining vital activities in enclosed spaces. This is because it is said that if the oxygen concentration falls below 15%, a person will have difficulty breathing. If it falls below 7%, brain function will be impaired, and if it falls below 4%, death will occur. The equipment can be portable or wall-mounted.
2. Oxygen Concentration Control in Industrial Processes
In some industrial heat treatment processes in the chemical industry, ceramics, metals, etc., oxygen concentration must be kept low. The combustion process in industrial furnaces also requires monitoring and control of oxygen concentration to optimize combustion efficiency and the oxidation-reduction process.
Oxygen monitors for such industrial applications can be exposed to intense chemical reactions in high-temperature environments. Products that are resistant to harsh environments are required.
Principle of Oxygen Monitors
The two main operating principles of oxygen monitors are the “galvanic cell type” and the “zirconia solid electrolyte type.” Other types include “magnetic type” and “wavelength tunable semiconductor laser spectroscopy type.
1. Galvanic Cell Type
The galvanic cell consists of a resin membrane that allows oxygen from the outside world to pass through, gold (Au) and lead (Pb) electrodes, and an electrolyte (potassium hydroxide solution). The following reactions take place at each electrode:
- Anode: Pb + 2OH- → Pb2+ +H2O + 2e-
- Cathode: O2 + 2H2O + 4e- → 4H2O
Electrons emitted at the anode reach the cathode, where oxygen taken in from the air takes up the electrons emitted at the anode. Since the flow of electrons (current) is proportional to the oxygen concentration, the oxygen concentration can be measured by measuring the current. Since this reaction occurs spontaneously, no power supply is required to drive the sensor.
2. Zirconia Solid Electrolyte Method
This method uses a zirconia cell, taking advantage of the fact that zirconia exhibits the properties of a solid electrolyte at temperatures of 500°C or higher. Zirconia can conduct oxygen negative ions (O2-) in a solid state, and the ions are conducted from a gas with high oxygen concentration (in the air) to an atmosphere with low oxygen concentration (in an industrial furnace, for example).
This ionic conduction generates a potential difference, and electrodes are placed on the high O2 concentration side and the low O2 concentration side, respectively, generating an electromotive force. The relationship is just like the positive and negative electrodes of a battery.
- High O2 concentration side: O2 + 4e- → 2O2-
- Low O2 concentration side: 2O2- → O2 + 4e-
The electromotive force generated between the electrodes obeys the Nernst equation (see below), so the partial pressure of oxygen at each electrode can be determined.
- E= (RT/4F) -ln (PA/PB)
- (R: gas constant, T: temperature, F: Faraday constant, PA: oxygen partial pressure at high concentration (in air), PB: oxygen partial pressure at low concentration)
Temperature is measured by thermocouples attached to the zirconia. In an atmosphere of approximately 400°C or lower, the target gas is introduced into the device via a sampling tube, and the zirconia cell is heated to a predetermined temperature by a platinum heater or other means (sampling method). This is because zirconia requires a temperature of 500°C or higher to function as a solid electrolyte.
Types of Oxygen Monitors
Different products should be used for oxygen meters intended to prevent oxygen deprivation and those intended to maintain low oxygen concentrations in industrial processes.
1. Oxygen Monitor for the Purpose of Preventing Oxygen Deprivation
Portable and stationary oxygen analyzers designed to prevent oxygen deficiency use galvanic battery-powered oxygen analyzers. This type does not require a power supply to drive the sensor.
The life of the sensor is approximately 2 to 3 years. However, the usable environment is limited to an atmosphere similar to the general environment, and the accuracy is about ±0.5%O2. The instruments are available in portable and wall-mounted types, and some are explosion-proof.
2. Oxygen Monitor for Industrial Use
Zirconia type oxygen monitors are suitable for measuring oxygen concentration in high-temperature industrial processes such as industrial furnaces, etc. In atmospheres above 700°C, the direct insertion type is used, where the sensor part is inserted directly into the atmosphere.
For temperatures below 400°C, the sampling method is appropriate, whereby the ambient gas in the furnace is drawn in through a sampling tube, etc., and the zirconia cell is heated separately. The correct choice should be made according to the application.