Zirconia oxygen sensor

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

Inside of an instrument the zirconia mobile is mounted in a temperature controlled furnace with the necessary electronics to method the signal from the detection mobile. Typically measurements are exhibited straight via a digital screen as oxygen concentration in excess of the variety .01ppm to one hundred%.

The principle behind Systech’s zirconia oxygen analyzer

The zirconia mobile is a higher temperature ceramic sensor. It is an electrochemical galvanic cell comprising of two electrically conducting, chemically inert, electrodes attached to either side of a solid electrolyte tube. This is demonstrated schematically in Figure one below.

The tube is entirely fuel limited and created of a ceramic (stabilised zirconium oxide) which, at the temperature of operation, conducts electrical power by signifies of oxygen ions. (Be aware: In sensors of this variety, the temperature has to be above 450°C before they become energetic as an electrolyte conductor). The potential difference throughout the cell is provided by the Nernst equation.

Where:

E is the potential difference (volts)

R is the gasoline consistent (eight.314 J mol-one K-1)

T is the absolute temperature (K)

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

P1 & P2 are the partial pressures of the oxygen on either aspect of the zirconia tube

The Nernst equation can consequently be reduced to:

Hence, if the oxygen partial strain at one of the electrodes is recognized and the temperature of the sensor is managed, then oxygen measurement of the potential difference between the two electrodes allows the mysterious partial stress to be calculated.

Observe

The partial stress of the fuel is equivalent to the molar focus of the ingredient in a gasoline combination times the complete force of the gas combination.

PO2 = CO2 P2

in which:

PO2 = Oxygen partial stress

CO2 = Molar concentration of oxygen

P2 = Complete strain

Illustration

For atmospheric air:

CO2 = twenty.9%

P2 = one ambiance

PO2 = (.209/a hundred) x 1

PO2 = .209 atmospheres

Theory of Operation

The zirconia cell utilised 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 facet of the mobile offer a catalytic floor for the change in oxygen molecules, O2, to oxygen ions, and oxygen ions to oxygen molecules. Oxygen molecules on the high concentration reference gasoline aspect of the cell obtain electrons to grow to be ions which enter the electrolyte. Simultaneously, at the other electrode, oxygen ions drop electrons and are introduced from the surface of the electrode as oxygen molecules.

The oxygen content of these gases, and for that reason the oxygen partial pressures, is different. For that reason, the charge at which oxygen ions are produced and enter the zirconium oxide electrolyte at each and every electrode differs. As the zirconium oxide permits mobility of oxygen ions, the amount of ions relocating in each route across the electrolyte will count on the price at which oxygen is ionised and enters the electrolyte at each and every electrode. The mechanism of this ion transfer is sophisticated, but it is known to include vacancies in the zirconia oxide lattice by doping with yttrium oxide.

Dew-Point Transmitters of migration of oxygen ions throughout the electrolyte is a net circulation of ions in a single course depending upon the partial pressures of oxygen at the two electrodes. For illustration in the Nernst equation:

If P1>P2 ion stream 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.