Zirconia oxygen sensor

The zirconia oxygen analyzer is suitable for measurements of ppm to % stages of oxygen in a gas or mixture of gases. Vacuum gauge is an electrochemical galvanic mobile using a higher temperature ceramic sensor made up of stabilised zirconium oxide.

In an instrument the zirconia cell is mounted in a temperature managed furnace with the needed electronics to method the signal from the detection cell. Generally measurements are shown directly by way of a electronic show as oxygen focus above the variety .01ppm to 100%.

The principle guiding Systech’s zirconia oxygen analyzer

The zirconia mobile is a high temperature ceramic sensor. It is an electrochemical galvanic cell comprising of two electrically conducting, chemically inert, electrodes connected to both side of a solid electrolyte tube. This is demonstrated schematically in Determine one under.

The tube is fully gas restricted and made of a ceramic (stabilised zirconium oxide) which, at the temperature of operation, conducts electricity by indicates of oxygen ions. (Be aware: In sensors of this sort, the temperature has to be previously mentioned 450°C prior to they turn out to be active as an electrolyte conductor). The prospective big difference across the mobile is presented by the Nernst equation.

Exactly where:

E is the possible distinction (volts)

R is the fuel constant (8.314 J mol-one K-one)

T is the complete temperature (K)

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

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

The Nernst equation can as a result be decreased to:

Hence, if the oxygen partial strain at a single of the electrodes is known and the temperature of the sensor is controlled, then oxygen measurement of the potential distinction amongst the two electrodes enables the unknown partial strain to be calculated.

Notice

The partial pressure of the gas is equal to the molar concentration of the part in a fuel mixture instances the whole force of the gas combination.

PO2 = CO2 P2

where:

PO2 = Oxygen partial stress

CO2 = Molar focus of oxygen

P2 = Overall pressure

Case in point

For atmospheric air:

CO2 = 20.nine%

P2 = 1 ambiance

PO2 = (.209/100) x one

PO2 = .209 atmospheres

Principle of Operation

The zirconia cell 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.

Figure 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 single side of the mobile give a catalytic surface for the change in oxygen molecules, O2, to oxygen ions, and oxygen ions to oxygen molecules. Oxygen molecules on the large concentration reference fuel aspect of the cell gain electrons to turn out to be ions which enter the electrolyte. Concurrently, at the other electrode, oxygen ions drop electrons and are unveiled from the area of the electrode as oxygen molecules.

The oxygen content material of these gases, and for that reason the oxygen partial pressures, is diverse. Therefore, the charge 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 quantity of ions moving in every single path across the electrolyte will depend on the price at which oxygen is ionised and enters the electrolyte at every single electrode. The mechanism of this ion transfer is complicated, but it is known to involve vacancies in the zirconia oxide lattice by doping with yttrium oxide.

The result of migration of oxygen ions throughout the electrolyte is a internet movement of ions in 1 path relying on the partial pressures of oxygen at the two electrodes. For example in the Nernst equation:

If P1>P2 ion movement 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.

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.