By Patrick Shannon, SST Sensing
There are many reasons why establishing the oxygen levels within a given environment can be required and the degree of accuracy that needs to be adhered to in each case can be quite different. In certain circumstances much greater exactitude will be expected than in others. Having displayed numerous attributes that make them suitable for this purpose, sensing devices based a zirconium oxide (ZrO2) active element are widely deployed in applications where elevated accuracy is mandated. Nevertheless, there are still some shortcomings associated with the common used form of these devices and more sophisticated technology is needed to overcome these.
There are a vast number of uses recognized for ZrO2 based oxygen sensors. Automotive emissions tests are a familiar area where they are implemented. They are also invaluable within a safety context – for example protecting the operatives in industrial sites, by facilitating the reduction of nitrogen oxide emissions. In settings that have potential flammable materials present (like the high density electronics systems inside server farms, or depots where large amounts of paper are being stored), they can assist in the creation of hypoxic (low oxygen) environments that will reduce the risk of fires occurring (through the control of nitrogen generators). Limiting the oxygen levels in freight containers can help to extend the lifespan of perishable goods during storage/transportation, which thereby calls for a precise monitor mechanism.
Often the objective is optimizing the oxygen level so that physical processes (such as combustion) are executed at peak efficiency. By measuring the output from the flues of industrial boilers, for instance, it can be determined whether too much oxygen content is present. If it is shown excess oxygen is being emitted, adjustments to the fuel/air ratio can be undertaken. This will mean the boiler does not waste energy and its output is less harmful. Likewise, the vapors that occupy the head space of airliner fuel tanks can lead to danger of explosion. To prevent this, in modern airliners a percentage of the oxygen is expelled from the head space and replaced by additional nitrogen (which is inert). Oxygen sensors are instrumental in accomplishing this.
ZrO2 Oxygen Sensors
The ZrO2 based devices utilized for oxygen measurement can be classified by the two principal methods of operation they employ. Though both have certain advantages, engineers need to also consider their limitations.
Ion Pump Oxygen Sensors – As ZrO2 partly dissociates at 650oC, mobile oxygen ions are produced within the material when this temperatures is exceeded. The application of a DC voltage allows these ions to be driven through a piece of ZrO2. The ions will then liberate an amount of oxygen (which relates proportionally to the charge transported) when they reach the anode.
Nernst Effect Oxygen Sensors – An oxygen pressure difference across a piece of ZrO2 which is kept at a temperature above 650oC will cause a voltage (referred to as the Nernst voltage) to be generated. This is directly proportional to the ratio of the partial oxygen pressures present on either side of the material.
A variety of different sensors currently on the market rely on these sensing mechanisms. Because ion pump devices are dependent on capillary holes of small diameter, which are prone to clogging in applications where there are high volumes of relatively large particulates (such as in industrial boilers, etc.), they have a short operational lifespan. They are also prone to temperature sensitivity issues, which means they cannot be installed in certain demanding settings. Sensors that use the Nernst effect have temperature problems too (their performance being impacted on by exposure to intense heat). In addition, they will require a sealed reference gas within the sensing system, which can be impractical in space constrained environments.
Instead of using one of these sensing mechanisms in isolation, SST Sensing has followed a unique approach that combines both mechanisms together. The sensing cells of these devices operate via the successive pressurizing and evacuating of a sealed chamber between 2 pieces of ZrO2, using the principle of oxygen ion pumping. Simultaneously, the pressure change is monitored through the Nernst effect. From the time period taken for required pressure changes to occur, the oxygen partial pressure can be determined with a high degree of accuracy. SST Zirconia sensors eliminate the need for a reference gas, allowing them to have more compact housings. They do not have the temperature sensitivity drawbacks of alternative solutions – enabling them to support 400°C operation (with the capacity to extend this still further to 1000oC, if appropriate thermal management is implemented). The cyclic pressurizing/evacuating of the chamber can be used for diagnostic purposes too, enabling the health of the sensor to be checked on a continuously basis. These sensors can be placed in the most challenging of working environments, with the robustness needed to cope with high degrees of both shock and vibration. They offer lifespans of up to 10 years (depending on the operating conditions), with minimal maintenance and calibration being needed.