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FEATURE SENSORS & SENSING SYSTEMS


When monitoring oxygen levels, some applications may demand high levels of precision – so sensing devices that rely on a zirconium oxide (ZrO2


)


The cyclic output waveform distinguishes SST devices. It provides a ‘heartbeat’ that confirms the oxygen sensor is operating correctly


active element are widely deployed in these higher end applications. Patrick Shannon, SST Sensing, looks into their limitations and introduces a new technology designed to address demands


A new approach to oxygen measurement


Z rO2 based sensors can be used for a wide range of tasks. They can, for


example, ensure that factory operatives are not put in danger, by helping to initiate reductions in the output of nitrogen oxides. In places where potential flammable items are present – such as the high density electronics systems found in server farms – they could be involved in creating a hypoxic (low oxygen) environment to mitigate the risk of fires. Sensors can be instrumental here in


controlling nitrogen generators, so that the partial pressure of oxygen is lowered. In addition, restricting the oxygen levels in freight containers can help to prolong the lifespan of perishable goods, and here the oxygen levels need to be determined with exactitude. Emissions tests on automobiles is another key area where a high performance sensing solution will be obligatory. Often, it will be a question of optimising the oxygen level so that physical processes (such as combustion) can be carried out with maximum efficiency. For example, through monitoring the output from industrial boiler flues, it can be ascertained if there is too much oxygen content present, indicating if the boiler is operating efficiently or not. If excess oxygen is being discharged, it will be necessary to alter the fuel/air ratio. Likewise, in passenger aircraft, fuel vapours occupying the head space of fuel tanks can bring about a possible risk of explosion. So, modern airliner models generally have mechanisms installed whereby a percentage of the oxygen is purged from the headspace of the fuel tank and subsequently replaced by additional nitrogen. Oxygen sensors are an integral part of these.


CATEGORIES ZrO2


22


based oxygen sensors are normally categorised by the two fundamentally


NOVEMBER 2016 | DESIGN SOLUTIONS


different methods they utilise for measuring the level oxygen. Though both have their merits, they also have certain limitations that engineers need to be aware of. Ion Pump Oxygen Sensors: ZrO2


partly dissociates when temperatures exceed 650˚C. Mobile oxygen ions are consequently produced within the material. By application of a DC voltage, these ions can be driven through the piece of ZrO2


. The ions then liberate an


amount of oxygen upon reaching the anode and this relates proportionally to the charge transported. Nernst Effect Oxygen Sensors: Devices


based on this scientific phenomenon also make use of the properties ZrO2


exhibits


at temperatures above 650˚C. An oxygen pressure difference across a piece of this material 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 on either side of the material. There are a variety


of sensors currently available that rely on these mechanisms. Unfortunately ion pump sensors are dependent on capillary holes of small diameter and these are prone to clogging in applications where high volumes of relatively large particulates are present (such as industrial boilers, etc.). This significantly shortens their operational lifespan. In addition, there are issues associated with these devices in terms of temperature sensitivity, so they cannot be deployed in settings where they will be exposed to intense heat. The Nernst effect sensors are also blighted by temperature issues and they will require the incorporation of a sealed reference gas sample into the sensing


system – something that in many applications will prove to be impractical as it takes up too much space.


A NEW SOLUTION Rather than employing one of these sensing mechanisms in isolation, a new approach has been taken by SST Sensing which brings both mechanisms together. Its devices each have a sensing cell that operates through the successive pressurising and evacuating of a sealed


chamber between two pieces of ZrO2 using the principle of oxygen ion pumping. Simultaneously, the pressure change is monitored via the Nernst effect. From the time period taken to achieve the desired pressure changes, the oxygen partial pressure can be calculated to a high degree of precision. The SST Zirconia oxygen sensors dispense with the need for a reference gas, allowing them to have more compact housings and thus address a broader spectrum of applications where space constraints need to be factored in. They do not have the temperature sensitivity problems that are endemic in alternative solutions – enabling them to support 400°C operation (with the capacity to extend this still further to 1000˚C, when appropriate thermal management is put in place). This robustness means that the expense of complex temperature control apparatus can be circumvented. The novel cycle of pressurising (using ion pumping) and evacuating the chamber (so that Nernst measurement can be carried out) results in a cyclical output waveform. This signal allows diagnostic benefits to be derived, allowing the health


Examples of


Zirconia oxygen sensor devices


of the sensor to be continuously checked. These devices are thus highly suited to safety critical application scenarios. SST Zirconia sensors can be


implemented in the most uncompromising of operational environments, with the ability to deal with high degrees of shock and vibration. In addition, they offer lifespans of up to 10 years, depending on the operating conditions, with minimal maintenance and calibration requirements.


SST Sensing www.sstsensing.com 


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