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Zirconia oxygen sensor

The zirconia oxygen analyzer is suited for measurements of ppm to % amounts of oxygen in a fuel or mixture of gases. The zirconia cell is an electrochemical galvanic mobile employing a higher temperature ceramic sensor containing stabilised zirconium oxide.

In an instrument the zirconia cell is mounted in a temperature managed furnace with the required electronics to process the sign from the detection mobile. Typically measurements are displayed right through a electronic exhibit as oxygen concentration more than the variety .01ppm to 100%.

The theory driving Systech’s zirconia oxygen analyzer

The zirconia mobile is a substantial temperature ceramic sensor. It is an electrochemical galvanic cell comprising of two electrically conducting, chemically inert, electrodes hooked up to either side of a solid electrolyte tube. This is proven schematically in Determine one under.

The tube is fully fuel restricted and manufactured of a ceramic (stabilised zirconium oxide) which, at the temperature of operation, conducts electrical power by indicates of oxygen ions. (Note: In sensors of this type, the temperature has to be earlier mentioned 450°C just before they turn out to be energetic as an electrolyte conductor). The likely big difference throughout the mobile is given by the Nernst equation.



Exactly where:

E is the possible distinction (volts)
R is the gasoline continual (eight.314 J mol-1 K-1)
T is the complete temperature (K)
F is the Faraday continual (96484 coulomb mol-1)
P1 & P2 are the partial pressures of the oxygen on possibly aspect of the zirconia tube

The Nernst equation can therefore be decreased to:



Therefore, if the oxygen partial stress at one of the electrodes is known and the temperature of the sensor is controlled, then oxygen measurement of the likely difference among the two electrodes permits the unfamiliar partial pressure to be calculated.

Note
image

The partial force of the fuel is equal to the molar focus of the element in a gas mixture occasions the total pressure of the fuel mixture.

PO2 = CO2 P2

exactly where:

PO2 = Oxygen partial strain
CO2 = Molar concentration of oxygen
P2 = Whole stress

Example

For atmospheric air:

CO2 = 20.9%
P2 = one environment

PO2 = (.209/one hundred) x 1

PO2 = .209 atmospheres

Principle of Procedure
The zirconia mobile employed by Systech Illinois is manufactured of zirconium oxide stabilised with yttrium oxide as the ceramic with porous platinum electrodes. This cell is demonstrated in Determine one.



Determine one: 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 facet of the mobile provide a catalytic floor for the adjust in oxygen molecules, O2, to oxygen ions, and oxygen ions to oxygen molecules. Oxygen molecules on the large concentration reference fuel aspect of the mobile achieve electrons to turn into ions which enter the electrolyte. Concurrently, at the other electrode, oxygen ions shed electrons and are unveiled from the area of the electrode as oxygen molecules.

The oxygen content material of these gases, and as a result the oxygen partial pressures, is diverse. Therefore, the rate at which oxygen ions are created and enter the zirconium oxide electrolyte at every electrode differs. As the zirconium oxide permits mobility of oxygen ions, the amount of ions moving in each and every path throughout the electrolyte will depend on the price at which oxygen is ionised and enters the electrolyte at each electrode. The mechanism of this ion transfer is sophisticated, but it is acknowledged to involve vacancies in the zirconia oxide lattice by doping with yttrium oxide.

The consequence of migration of oxygen ions across the electrolyte is a internet stream of ions in 1 path relying on the partial pressures of oxygen at the two electrodes. For instance in the Nernst equation:



If P1>P2 ion movement will be from P1 to P2 i.e. a positive E.M.F.
If P1If 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°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.



Vacuum gauge 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.

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.

Any combustible gas, 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.