Zirconia oxygen sensor

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

Inside an instrument the zirconia mobile is mounted in a temperature controlled furnace with the needed electronics to process the signal from the detection mobile. Typically measurements are exhibited directly via a digital exhibit as oxygen focus more than the variety .01ppm to a hundred%.

The principle powering Systech’s zirconia oxygen analyzer

The zirconia mobile is a large temperature ceramic sensor. It is an electrochemical galvanic mobile comprising of two electrically conducting, chemically inert, electrodes attached to possibly side of a sound electrolyte tube. This is demonstrated schematically in Determine one under.

The tube is entirely gasoline restricted and manufactured of a ceramic (stabilised zirconium oxide) which, at the temperature of operation, conducts electric power by indicates of oxygen ions. (Note: In sensors of this type, the temperature has to be over 450°C just before they become active as an electrolyte conductor). The prospective variation throughout the mobile is presented by the Nernst equation.

Exactly where:

E is the likely difference (volts)

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

T is the absolute temperature (K)

F is the Faraday constant (96484 coulomb mol-1)

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

The Nernst equation can for that reason be lowered to:

Thus, if the oxygen partial strain at one of the electrodes is known and the temperature of the sensor is managed, then oxygen measurement of the potential difference among the two electrodes permits the unfamiliar partial force to be calculated.

Observe

The partial pressure of the gas is equivalent to the molar focus of the part in a fuel combination moments the total pressure of the gas combination.

PO2 = CO2 P2

where:

PO2 = Oxygen partial force

CO2 = Molar focus of oxygen

P2 = Whole pressure

Case in point

For atmospheric air:

CO2 = twenty.nine%

P2 = one ambiance

PO2 = (.209/100) x 1

PO2 = .209 atmospheres

Principle of Operation

The zirconia mobile employed by Systech Illinois is produced of zirconium oxide stabilised with yttrium oxide as the ceramic with porous platinum electrodes. This mobile is proven 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 aspect of the mobile offer a catalytic surface area for the change in oxygen molecules, O2, to oxygen ions, and oxygen ions to oxygen molecules. Oxygen molecules on the higher focus reference fuel side of the cell achieve electrons to turn out to be ions which enter the electrolyte. Concurrently, at the other electrode, oxygen ions drop electrons and are released from the area of the electrode as oxygen molecules.

The oxygen articles of these gases, and consequently the oxygen partial pressures, is distinct. For that reason, the price at which oxygen ions are created and enter the zirconium oxide electrolyte at each electrode differs. As the zirconium oxide permits mobility of oxygen ions, the number of ions shifting in every single course throughout the electrolyte will count on the charge at which oxygen is ionised and enters the electrolyte at each electrode. The mechanism of this ion transfer is complex, but it is identified to involve vacancies in the zirconia oxide lattice by doping with yttrium oxide.

The result of migration of oxygen ions across the electrolyte is a internet movement of ions in a single path dependent 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 constructive 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.

Electrochemical Cells Oxygen Sensor 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.