Zirconia oxygen sensor

The zirconia oxygen analyzer is appropriate for measurements of ppm to % amounts of oxygen in a gasoline or mixture of gases. The zirconia cell is an electrochemical galvanic mobile using a high temperature ceramic sensor made up of stabilised zirconium oxide.

In an instrument the zirconia mobile is mounted in a temperature managed furnace with the necessary electronics to method the signal from the detection mobile. Generally measurements are displayed directly by way of a digital display as oxygen focus above the range .01ppm to 100%.

The principle guiding Systech’s zirconia oxygen analyzer

The zirconia cell is a higher temperature ceramic sensor. It is an electrochemical galvanic mobile comprising of two electrically conducting, chemically inert, electrodes connected to possibly aspect of a solid electrolyte tube. This is shown schematically in Determine 1 below.

The tube is fully fuel tight and manufactured of a ceramic (stabilised zirconium oxide) which, at the temperature of operation, conducts electrical energy by means of oxygen ions. (Observe: In sensors of this sort, the temperature has to be earlier mentioned 450°C just before they become active as an electrolyte conductor). The possible difference across the cell is provided by the Nernst equation.

Where:

Electrochemical Cells Oxygen Sensor is the potential variation (volts)

R is the gas continuous (eight.314 J mol-one K-one)

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 both side of the zirconia tube

The Nernst equation can as a result be decreased to:

Therefore, if the oxygen partial force at 1 of the electrodes is acknowledged and the temperature of the sensor is managed, then oxygen measurement of the possible variation between the two electrodes enables the mysterious partial force to be calculated.

Observe

The partial force of the gasoline is equal to the molar concentration of the element in a fuel combination occasions the whole pressure of the fuel mixture.

PO2 = CO2 P2

the place:

PO2 = Oxygen partial pressure

CO2 = Molar focus of oxygen

P2 = Overall force

Illustration

For atmospheric air:

CO2 = 20.9%

P2 = one environment

PO2 = (.209/a hundred) x one

PO2 = .209 atmospheres

Theory of Operation

The zirconia cell utilised by Systech Illinois is made of zirconium oxide stabilised with yttrium oxide as the ceramic with porous platinum electrodes. This mobile is proven in Determine 1.

Determine 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 each facet of the mobile supply a catalytic surface for the adjust in oxygen molecules, O2, to oxygen ions, and oxygen ions to oxygen molecules. Oxygen molecules on the high focus reference fuel facet of the mobile obtain electrons to grow to be ions which enter the electrolyte. Simultaneously, at the other electrode, oxygen ions get rid of electrons and are released from the surface of the electrode as oxygen molecules.

The oxygen content material of these gases, and consequently the oxygen partial pressures, is various. As a result, the charge at which oxygen ions are created and enter the zirconium oxide electrolyte at each and every electrode differs. As the zirconium oxide permits mobility of oxygen ions, the variety of ions transferring in every single course across the electrolyte will count on the fee at which oxygen is ionised and enters the electrolyte at each electrode. The system of this ion transfer is complex, but it is recognized to require vacancies in the zirconia oxide lattice by doping with yttrium oxide.

The end result of migration of oxygen ions throughout the electrolyte is a web flow of ions in one path depending on the partial pressures of oxygen at the two electrodes. For example in the Nernst equation:

If P1>P2 ion circulation will be from P1 to P2 i.e. a optimistic 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.

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.

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.