Notes

Zirconia oxygen sensor

The zirconia oxygen analyzer is suited for measurements of ppm to % ranges of oxygen in a gasoline or combination of gases. The zirconia cell is an electrochemical galvanic mobile utilizing a large temperature ceramic sensor made up of stabilised zirconium oxide.


In an instrument the zirconia cell is mounted in a temperature controlled furnace with the necessary electronics to method the signal from the detection cell. Generally measurements are displayed immediately through a digital display as oxygen focus over the selection .01ppm to a hundred%.


The theory driving Systech’s zirconia oxygen analyzer


The zirconia cell is a substantial temperature ceramic sensor. It is an electrochemical galvanic mobile comprising of two electrically conducting, chemically inert, electrodes hooked up to both facet of a sound electrolyte tube. This is shown schematically in Determine 1 under.


The tube is entirely gas restricted and produced of a ceramic (stabilised zirconium oxide) which, at the temperature of operation, conducts electrical power by signifies of oxygen ions. (Note: In sensors of this kind, the temperature has to be over 450°C just before they turn into energetic as an electrolyte conductor). The possible difference throughout the mobile is given by the Nernst equation.




The place:


E is the possible difference (volts)

R is the gasoline consistent (8.314 J mol-1 K-1)

T is the complete temperature (K)

F is the Faraday constant (96484 coulomb mol-one)

P1 & P2 are the partial pressures of the oxygen on possibly side of the zirconia tube


The Nernst equation can as a result be decreased to:




As a result, if the oxygen partial force at one of the electrodes is known and the temperature of the sensor is controlled, then oxygen measurement of the potential variation between the two electrodes permits the unfamiliar partial pressure to be calculated.


Notice


The partial pressure of the fuel is equal to the molar concentration of the component in a gasoline combination times the whole strain of the gas mixture.


PO2 = CO2 P2


in which:


PO2 = Oxygen partial stress

CO2 = Molar concentration of oxygen

P2 = Whole force


Example


For atmospheric air:


CO2 = twenty.nine%

P2 = one atmosphere


PO2 = (.209/100) x 1


PO2 = .209 atmospheres


Theory of Procedure

The zirconia mobile used by Systech Illinois is produced of zirconium oxide stabilised with yttrium oxide as the ceramic with porous platinum electrodes. This cell is revealed in Figure one.




Figure 1: Enlarged cross sectional representation of the zirconia substrate


Molecular oxygen is ionised at the porous platinum electrodes.


PtO → Pt + ½ O2

½ O2 + 2e- → O2–


The platinum electrodes on every single facet of the mobile supply a catalytic area for the adjust in oxygen molecules, O2, to oxygen ions, and oxygen ions to oxygen molecules. Oxygen molecules on the substantial concentration reference gas facet of the mobile obtain electrons to grow to be ions which enter the electrolyte. Simultaneously, at the other electrode, oxygen ions shed electrons and are released from the area of the electrode as oxygen molecules.


The oxygen content of these gases, and therefore the oxygen partial pressures, is various. For that reason, the fee at which oxygen ions are created and enter the zirconium oxide electrolyte at every single electrode differs. As the zirconium oxide permits mobility of oxygen ions, the variety of ions transferring in each and every route across the electrolyte will rely on the fee at which oxygen is ionised and enters the electrolyte at every electrode. The system of this ion transfer is complicated, but it is acknowledged to entail vacancies in the zirconia oxide lattice by doping with yttrium oxide.


The end result of migration of oxygen ions throughout the electrolyte is a internet movement of ions in a single path depending upon the partial pressures of oxygen at the two electrodes. For example in the Nernst equation:




If P1>P2 ion flow will be from P1 to P2 i.e. a good E.M.F.

If P1
<p2 ion="" flow="" will="" be="" from="" p2="" to="" p1="" i.e.="" a="" negative="" e.m.f.<br="" />If P1=P2 there will be no net ion flow i.e. a zero E.M.F.


In the zirconia analyzer, the Nernst equation is written




The zirconia analyzer uses air as a reference, a constant oxygen concentration of 20.9%, and the zirconia cell is mounted inside a furnace whose temperature is controlled to 650&deg;C (923 K).


Thus, our Nernst equation further reduces to:




The zirconia analyzer electronically calculates the oxygen partial pressure, and therefore oxygen concentration, of a sample gas with unknown oxygen concentration. This is accomplished by measuring the potential, E, produced across the zirconium cell electrodes, substituting for E in the Nernst equation and anti-logging to obtain PO2. The cell potential output is shown in Figure 2.




Figure 2 Graph of cell potential vs. oxygen concentration of zirconia cell.


By anti-logging the equation, the output signal can be displayed directly on a digital readout meter as oxygen concentration in ppm or %.


Calibration

As the zirconia instrument uses an absolute measurement principle once built and factory calibrated, it does not require any further factory calibration.


Factory calibration consists of calibration of the electronics to accept the millivolt input signal from the detection cell and checking that the instrument then reads correctly on air, 20.9%. The instrument is then further checked for correct reading on ppm oxygen content in nitrogen.

<img width="371" src="https://www.avcray.com/Avcray-Oxygen-dew-point-Analyzers.png" />


Applications of zirconia oxygen analyzers


The zirconia analyzers may be used for measurement of oxygen at any level between 0-100% in gases or gas mixtures.


The only restriction on the instrument’s usage is that the gas to be measured must not contain combustible gases or any material that will poison the zirconium oxide detection cell.


Vacuum gauge , e.g. CO, H2, hydrocarbons such as methane, in the sample gas entering the instrument will combine with any oxygen in the sample gas in the furnace due to the high temperature at which the furnace is kept. This will actually reduce the amount of oxygen in the sample gas and cause the instrument to give an incorrect low reading.


Materials that will poison the detection cell are:


Halogens e.g. Chlorine

Halogenated Hydrocarbons e.g. Methylchloride

Sulphur containing compounds e.g. Hydrogen Sulphide

Lead containing compounds e.g. Lead Sulphide

Gases or gas mixtures containing any of the above are not suitable for oxygen determination with a zirconia type oxygen analyzer.


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