By Craig Mitchell, SST Sensing
There are a plethora of different applications where the level of oxygen in a given environment needs to be determined – covering everything from the industrial, automotive, logistics/transportation and agriculture sectors right through to aerospace and healthcare. Though in some circumstances a relatively low level of accuracy is deemed acceptable, when a high degree of precision is called for oxygen sensors based on a zirconium oxide (ZrO2) active element are predominantly specified – and this has been the case for many years. It must be noted though, that conventional ZrO2 technology still has certain operational restrictions that engineers need to ensure they are fully aware of. The following article will look at how a more sophisticated, multi-faceted methodology has been employed in order to overcome them.
Combustion control is among the key areas where ZrO2 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. Although there must always be some excess oxygen in the flue (so as to prevent carbon monoxide compounds forming), if this level is too high it means that the boiler is heating up fresh air and therefore not operating efficiently (as energy is being needlessly expended). By having constant access to data on the oxygen content of the flue, adjustments can be made to the fuel/air ratio in order to optimise the combustion process – thereby saving money and also lowering the effect on the environment. In passenger jets, ZrO2 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 remove oxygen, so that the headspace has an elevated concentration of nitrogen (which is inert) and the risk of explosions can be circumvented. In server rooms and document archives, ZrO2 sensors have a part to play in the establishing of 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 realised. Lowering the quantity of oxygen present 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 ZrO2 active elements are generally classified in accordance with the two different techniques they use for determining oxygen levels. In both cases, these are a direct result of the properties that ZrO2 displays when placed at temperatures above 650oC. 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 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).
There are numerous sensors currently 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 deployed in certain application environments. Also they rely on small diameter capillary holes that can easily become 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, SST Sensing 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 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. This means that these sensors are able to address safety critical applications.