Oxygen Sensor Technology and its Ongoing Evolution

In many applications within the automotive, agricultural, industrial, storage/transportation and medical sectors, oxygen levels have to be accurately established. While the scope of applications is increasing, the performance requirements also continue to increase.

According to Micromarket Monitor, the global oxygen sensor business is estimated to be worth more than $2.6 billion yearly by 2022. If this prediction proves to be correct, then the market for such technology would have nearly doubled in the course of 10 years.

Key Oxygen Monitoring Applications

The following text provides a brief picture of various applications where it is important to establish (and subsequently regulate) the oxygen level.

  • Combustion Control –If oxygen levels in the flue of industrial boilers (whether they are biomass, oil, or gas fueled) are monitored, then the amount of surplus oxygen present in the exhaust gas (Figure 1) can be determined. While some amount of excess oxygen has to be present in the flue to prevent the production of harmful carbon monoxide, surplus amounts of this gas indicate that the boiler is heating fresh air and hence expending energy unnecessarily. This surplus oxygen data can be used to adjust the fuel/air ratio so that a ‘sweet spot’ is reached, where combustion is performed in the most economical way and with as little pollution as possible, thus resulting in better energy savings and also reducing the ecological impact.

Example of a Boiler Control Application

Figure 1. Example of a Boiler Control Application

  • Preservation of Perishable Goods – The presence of oxygen can degrade organic materials such as fruits and vegetables. If the oxygen levels in storage facilities and refrigerated freight containers are reduced, the commercial worth of these products can be extended while they are being stored or transported, as shown in Figure 2.

Example of Controlled Atmosphere Application

Figure 2. Example of Controlled Atmosphere Application

  • Fire Prevention – In document archives, server rooms, and warehouses where priceless historical paintings or artifacts are stored, fire poses a great risk and the outcomes of an incident can be disastrous, both in terms of safety and loss of precious data/property. In such situations, especially where personnel are not working in the area concerned, the most effective preventative measure is to create a hypoxic (low oxygen) environment. If oxygen sensors are used to control nitrogen generators, the partial pressure of oxygen in the area can be reduced to a level where people are able to work easily. However, the outbreak of fire is almost impossible.
  • Altitude Training – In the sporting world, it is a well-known fact that acclimatization to high altitude conditions can significantly improve athletes’ performance. In other words, athletes’ bodies become accustomed to the reduced oxygen levels, by increasing the mass of hemoglobin and red blood cells and also altering their muscle metabolism. When these athletes take part in events at lower altitudes, they will stand to benefit from a higher concentration of red blood cells for a period of 10 to 14 days and thus have a competitive advantage. However, it is not practical to relocate to high altitude training facilities. One alternative solution to this is to train in altitude simulation tents or rooms, where the oxygen’s partial pressure is lowered to the levels experienced at high altitude. Once again, oxygen sensors that control nitrogen generators allow the replication of high altitude environments.
  • On Board Inert Gas Generation (OBIGG) – When fuel is burned by airliners, the mixture or air and fuel vapors present in the head space of their fuel tanks presents a potentially explosive environment. In order to reduce the chances of mid-air explosions, OBIGG systems are now deployed in most modern airliners. Low pressure air from the plane’s exterior is compressed and a large amount of oxygen is removed using a wide range of technologies. The residual gas, which contains mainly nitrogen and is hence inert, can be used to fill the head space of the fuel tank. Thus, the risk of fire and explosion is prevented. The entire process is controlled using oxygen sensors.

In industrial plants, oxygen monitoring can be performed to reduce the output of nitrogen oxides (NOx) as well as to inspect the output emissions during vehicle testing. Oxygen monitoring is also beneficial in agriculture to improve the production of crops (for instance, in the growth of mushrooms) and to speed up compositing.

Zirconia Oxygen Sensors

Zirconia (ZrO2)-based oxygen sensors are suitable for all the above applications. They use one of two different sensing mechanisms to find out oxygen levels. These sensing mechanisms are divided in the following way:

  • Oxygen Ion Pump Sensors – ZrO2 partly dissociates at temperatures greater than 650 °C and produces mobile oxygen ions within the material. These ions typically move at random within the crystal lattice, and when a DC voltage is applied across the material, they can be steered through the ZrO2 piece. This releases an amount of oxygen at the anode proportional to the transported charge (which is known as an electrochemical pumping action). While several sensors available on the market leverage the speed of oxygen ion pumping to infer the oxygen, they largely depend on small capillary holes which tend to clog in ‘dirty’ applications (where an increased amount of larger particulates is present) and are highly sensitive to temperature.
  • Nernst Effect Cells – Again, at temperatures greater than 650 °C and when there is an oxygen pressure difference across a ZrO2 piece, a voltage (the Nernst voltage) is produced across it. This voltage is proportional to the ratio of the partial oxygen pressures on both sides of the material. The effect is highly dependent on temperature and often needs a known reference gas to be available which is not impractical in many applications.

SST sensors are different from other competing devices because instead of using one or other of these sensing mechanisms, they use both mechanisms at the same time. Using the principle of oxygen ion pumping, the sensing cell operates by pressurizing and evacuating a sealed chamber between two ZrO2 pieces. Using the Nernst effect, the pressure change is monitored simultaneously. The time taken to obtain the required pressure changes is directly proportional to the oxygen partial pressure of interest.

Therefore, SST’s Oxygen Zirconia sensors offer the following functional benefits:

  • These devices do not need access to a reference gas, and hence they can be used in a much wider range applications.
  • The dynamic operating principle (with concurrent ion pumping and Nernst measurement) leads to cyclical output waveform. This ‘heartbeat’ signal makes it possible to continuously monitor the sensor health and is the key reason for using these sensors in safety critical applications.
  • SST’s dynamic cell is different from other ZrO2 cells and has significantly reduced temperature sensitivity, thus preventing the necessity for costly temperature control of the cell.

These devices are robust and can endure up to 20 g vibrational forces. They have a normal working temperature range covering from -100 °C to +400 °C (enabling them to be deployed in the most complex operational settings). Through thermal management of the gas stream to the sensor, customers can extend the allowable gas temperature to more than 1000 °C. The devices are supplied in compact form factors and provide lifespan of up to 10 years. When recalibration has to be done, the new design that has been utilized considerably simplifies the process. Just a span adjustment is required and there is no need for zero calibration. The span gas may come from a calibrated source, but in majority of applications fresh air is more than sufficient. This helps obtain automatic and effectively free calibration.

SST Sensing has manufactured a wide range of high precision oxygen sensors. It also supports the electronics required to control them (customers can even design their own circuits and SST’s team of engineers are available to provide advice on this).

Conclusion

As observed before, oxygen monitoring system can be implemented for various purposes and each will have its own particular characteristics and many associated technical issues to overcome. Customers can work with an experienced oxygen sensor manufacturer that provides a wide range of products. They can find the best fit and benefit from the company’s extensive application knowledge to implement a sensing system that is completely optimized for the required job.

This information has been sourced, reviewed and adapted from materials provided by SST Sensing Ltd.

For more information on this source, please visit SST Sensing Ltd.

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