By Craig Mitchell, SST Sensing
There are a plethora of applications where the oxygen level in a given environment needs to be determined. These extend from the industrial, automotive, logistics, transportation and agricultural sectors to aerospace and healthcare.
In some circumstances, a relatively low level of accuracy is acceptable. However, when greater precision is required, oxygen sensing devices based on a zirconium oxide (ZrOâ‚‚) active element are predominantly specified. This has been the case for many years.
Conventional ZrOâ‚‚ technology still has certain operational restrictions of which engineers need to be fully aware. This white paper examines how a more sophisticated, multifaceted methodology can overcome these restrictions.
Applications for ZrOâ‚‚ Oxygen Sensors
Combustion control is among the key areas where ZrOâ‚‚-based sensors are employed. By monitoring the partial pressures in exhaust gases emitted from industrial boiler flues, the presence of excess oxygen can be identified.
Some excess oxygen must remain in the flue to prevent carbon monoxide compounds from forming. However, if this level is too high, the boiler is heating fresh air and is therefore not operating efficiently. Energy is consequently being expended unnecessarily.
Constant access to oxygen-content data allows adjustments to be made to the fuel-to-air ratio. This helps optimise the combustion process, reduce operating costs and lower the effect on the environment.
In passenger jets, ZrOâ‚‚ sensors help prevent oxygen from accumulating in the headspace of fuel tanks. On-board inert gas generation systems are used to remove oxygen and increase the concentration of inert nitrogen. This helps reduce the risk of explosions.
In server rooms and document archives, ZrOâ‚‚ sensors help establish hypoxic, or low-oxygen, environments as a fire-prevention measure. The sensors provide nitrogen generators with oxygen-level data, allowing oxygen to be reduced in areas where staff are not present.
Reducing oxygen levels can also help prolong the life of perishable goods, such as fruit and vegetables, during transportation over long distances.
Vehicle emissions testing is another application that requires high-accuracy oxygen sensor technology.
ZrO2Â Sensor Types
Oxygen sensing devices with ZrO₂ active elements are generally classified according to the technique used to determine oxygen levels. Both techniques result directly from the properties displayed by ZrO₂ at temperatures above 650°C.
The techniques are:
- Ion Pumping – Since ZrO2 partly dissociates at 650oC, mobile oxygen ions are emitted from the material. Applying a DC voltage means that these ions (which would otherwise move randomly throughout the crystal lattice) can be driven through the piece of ZrO2 and made to subsequently liberate an amount of oxygen when they reach the anode. The amount of oxygen that is produced corresponds proportionally to the charge transported.
- The Nernst Effect – Above 650ÂoC, an oxygen pressure difference across a piece of ZrO2 will cause a voltage to be generated. This is known as the Nernst Voltage and is logarithmically proportionally to the ratio of the partial oxygen pressures on either side of the material. The relationship is defined via the following equation – where kB is Boltzmann constant; T is temperature (in Kelvins) e0 is elementary charge (i.e. 1.602 x 10-19 Coulomb) and ci is the ion concentration (in mol/kg).
Limitations of Conventional ZrOâ‚‚ Sensors
Numerous sensors currently available are based on one of these techniques. Ion-pump sensors can experience temperature-sensitivity problems, preventing their deployment in certain application environments.
They also rely on small-diameter capillary holes that can become clogged in locations containing high volumes of large particulates, such as industrial boilers. This places significant constraints on their working lifespan.
The performance of sensors based on the Nernst Effect can also be affected by high temperatures. In addition, a known reference gas sample normally needs to be integrated into the sensing system. This can make installation impractical in some applications.
Combining Ion Pumping and the Nernst Effect
In contrast to devices based on only one of the sensing techniques outlined above, SST Sensing has developed a distinctive sensing mechanism that combines attributes of both.
The sensors use an arrangement in which cyclic pressurisation and evacuation are applied to a sealed chamber between two pieces of ZrOâ‚‚ through oxygen-ion pumping. The pressure change is simultaneously monitored using the Nernst Effect.
By measuring the time required to reach the desired pressure change, the oxygen partial pressure can be accurately determined.
Advantages of SST Zirconia Oxygen Sensors
SST’s zirconia oxygen sensors do not require a reference gas. This allows them to be deployed in more space-constrained applications.
They also avoid the temperature-related issues experienced by other oxygen sensing devices. The sensors support gas temperatures up to 400°C as standard, with scope to extend this to 1000°C when appropriate thermal management is employed.
Depending on the application environment, they can provide operational lifespans of up to 10 years with negligible maintenance or calibration requirements. Their inherent ruggedness also removes the need for complex temperature-control subsystems.
The pressurisation and evacuation cycle that characterises their operation provides valuable diagnostic information. This enables the health of the device to be monitored and makes the sensors suitable for safety-critical applications.
