SENSORS & TRANSDUCERS FEATURE
Zirconium oxide based sensors for OXYGEN MEASUREMENT
fuel vapours occupying the head space of fuel tanks can bring about a possible risk of explosion. As a preventative measure, modern airliner models generally have mechanisms installed, in which oxygen sensors are an integral part, whereby a percentage of the oxygen is purged from the headspace of the fuel tank and subsequently replaced by additional nitrogen (which has an inert nature). ZrO2
based oxygen sensors are normally
categorised by the two fundamentally different methods they utilise for measuring the level oxygen. Though both have their merits, as we will see, they also have certain limitations that engineers need to be aware of.
ION PUMP OXYGEN SENSORS ZrO2
partly dissociates when temperatures exceed 650o
Patrick Shannon of SST Sensing considers recent advances in zirconium oxide (ZrO2
T
here are numerous reasons for monitoring the oxygen levels within
a given environment. In some cases there are relatively low expectations in terms of the accuracy required, whilst, for others, substantial precision is certain to be mandated. Having proved themselves to be the most suitable option, sensing devices that rely on a zirconium oxide (ZrO2
) active element
are widely deployed in such higher end applications. There are still some limitations associated with this type of oxygen sensing however, at least in its common form, and emerging technology is needed to address these.
SENSING OXYGEN The list of tasks now encompassed by ZrO2
Examples of Zirconia oxygen sensor devices
) based sensors, discussing both ion pump and Nernst effect devices based sensors is considerable.
They may provide a way to maintain safety - ensuring that factory operatives are not put in danger, by helping to initiate reductions in the output of nitrogen oxides. In places were potential flammable items are present, such as the high density electronics systems found in server farms, or storage sites where large amounts of paper based material is being kept, 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. Restricting the oxygen levels in freight containers can help to prolong the lifespan of perishable goods while they are being stored or transported, so the oxygen levels present need to be determined with exactitude by such devices. Emissions tests on automobiles is another key area where a high performance sensor is essential. In a large proportion of cases 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 an industrial boiler’s flue, it can be ascertained if there is too much oxygen content present. This can be used to indicate if the boiler is operating efficiently or not. If excess oxygen is being discharged, it will be necessary to alter the fuel/air ratio, so that the combustion process avoids wasting energy and harmful emissions are mitigated. Likewise, in passenger aircraft
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
above 650o 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.This again limits their scope, as they cannot be deployed in settings where they will be exposed to intense heat. The Nernst effect sensors are also blighted by temperature issues, their performance being extremely temperature dependent. Furthermore, they will
INSTRUMENTATION | FEBRUARY 2017 17
exhibits at temperatures
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