Zirconium Dioxide – A More Effective Approach to Oxygen Sensing

There are many applications where the level of oxygen in an environment needs to be measured, from the industrial, automotive, logistics/transportation and agriculture sector to aerospace and healthcare. In some applications a relatively low level of accuracy is deemed acceptable, however, when a high degree of precision is required, oxygen sensing devices based on a zirconium dioxide (ZrO2) active element are predominantly specified.  

It must be noted, however, that conventional ZrO2 technology still has certain operational restrictions that engineers need to ensure they are fully aware of. This blog details how SST Sensing (a brand of PST) has employed a more sophisticated, multi-faceted methodology to overcome them.

Zirconia Oxygen Sensor Applications

ZrO2 based sensors are commonly used in combustion control. By monitoring the partial pressures in exhaust gases emitted from industrial boiler flues, the presence of excess oxygen is identified. There must always be some excess oxygen in the flue (to prevent carbon monoxide compounds forming), and if this level is too high it means the boiler is heating up fresh air and therefore not operating efficiently (energy is being needlessly expended). By having continuous access to data on the oxygen content of the flue, adjustments can be made to the fuel/air ratio to optimise the combustion process. This saves money and lowers the effect on the environment.

In passenger jets, zirconia sensors are instrumental in preventing the build-up of oxygen in the headspace of fuel tanks. On-board inert gas generation (OBIGG) systems are used to eliminate oxygen, so that the headspace has an elevated concentration of nitrogen (which is inert) and the risk of explosions can be avoided.

In server rooms and document archives, ZrO2 sensors play a role in establishing hypoxic (low oxygen) environments as a fire prevention measure. These devices can provide nitrogen generators with data on the oxygen level present, so that (in areas where staff are not present) a reduction in oxygen levels can be achieved.

Lowering oxygen levels can also help to prolong the life of perishable goods (such as fruit and vegetables) as they are transported over long distances. The emissions testing of vehicles is another application that requires the use of high accuracy oxygen sensor technology.

ZrO2 Sensor Types

Oxygen sensing devices with active elements in SST Sensing's ZrO2 are generally classified according to the two different techniques used to determine oxygen levels. In both cases, this is a direct consequence of the properties that ZrO2 exhibits when placed 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 650oC, 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 proportional 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).

There are many sensors on the market that are based on one of these techniques. Ion pump sensors have temperature sensitivity problems associated with them, which means they cannot be used in certain application environments. Also, they rely on small diameter capillary holes that can become easily clogged when placed in locations where large particulates are present in high volumes (like industrial boilers). This places acute constraints on their working lifespan.

The performance of sensors based on the Nernst Effect is also, to some extent, impacted upon by high temperatures. In addition, a known reference gas sample normally needs to be integrated into the sensing system, which can make their installation unfeasible in some applications.

Combining These Techniques

In contrast to devices based on the two different sensing techniques outlined above, PST has developed its own distinctive sensing mechanisms that brings together attributes of both. The company’s sensors each have an arrangement where cyclic pressurisation/evacuation is applied (via oxygen ion pumping) to a sealed chamber between two pieces of ZrO2. The pressure change is simultaneously monitored (via the Nernst Effect) and by measuring the time period needed to reach the desired pressure change, the oxygen partial pressure can be accurately determined.

The Zirconia range of oxygen sensors supplied by SST Sensing have the advantage that they do not necessitate inclusion of a reference gas. This means that they can be deployed in more space constrained application settings. Furthermore, they do not have the temperature issues that other oxygen sensing devices are troubled by. This allows them to support much higher operating conditions (with 400°C as standard and the scope to extend this to 1000oC if appropriate thermal management is employed).

They support lifespans of up to 10 years (depending on the application environment), with negligible maintenance or calibration requirements. Their innate ruggedness dispenses with the need to include complex temperature control sub-systems. Also, the pressurisation/evacuation cycle that characterises their operation supplies valuable diagnostic information, enabling the health of the device to be examined.