The zirconia oxygen analyzer is suitable for measurements of ppm to % levels of oxygen in a gasoline or combination of gases. The zirconia cell is an electrochemical galvanic cell employing a higher temperature ceramic sensor that contains stabilised zirconium oxide.
Within an instrument the zirconia cell is mounted in a temperature managed furnace with the essential electronics to procedure the sign from the detection cell. Normally measurements are displayed straight via a digital display as oxygen focus above the assortment .01ppm to 100%.
The concept powering Systech’s zirconia oxygen analyzer
The zirconia cell is a substantial temperature ceramic sensor. It is an electrochemical galvanic cell comprising of two electrically conducting, chemically inert, electrodes attached to possibly side of a strong electrolyte tube. This is shown schematically in Figure one underneath.
The tube is fully fuel tight and made 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 earlier mentioned 450°C prior to they turn out to be energetic as an electrolyte conductor). The likely distinction across the cell is presented by the Nernst equation.
In which:
E is the prospective distinction (volts)
R is the gas consistent (eight.314 J mol-one K-one)
T is the complete temperature (K)
F is the Faraday constant (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:
Thus, if the oxygen partial stress at one particular of the electrodes is acknowledged and the temperature of the sensor is controlled, then oxygen measurement of the likely variation between the two electrodes allows the mysterious partial pressure to be calculated.
Observe
The partial strain of the fuel is equivalent to the molar focus of the component in a gasoline mixture times the overall strain of the gasoline combination.
PO2 = CO2 P2
where:
PO2 = Oxygen partial pressure
CO2 = Molar concentration of oxygen
P2 = Total strain
Case in point
For atmospheric air:
CO2 = twenty.nine%
P2 = 1 environment
PO2 = (.209/one hundred) x one
PO2 = .209 atmospheres
Theory of Operation
The zirconia mobile utilized by Systech Illinois is created of zirconium oxide stabilised with yttrium oxide as the ceramic with porous platinum electrodes. This mobile is proven in Figure 1.
Determine one: Enlarged cross sectional illustration 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 cell supply a catalytic area for the modify in oxygen molecules, O2, to oxygen ions, and oxygen ions to oxygen molecules. Oxygen molecules on the large focus reference gas aspect of the cell achieve electrons to turn into ions which enter the electrolyte. Simultaneously, at the other electrode, oxygen ions get rid of electrons and are launched from the surface area of the electrode as oxygen molecules.
The oxygen content of these gases, and consequently the oxygen partial pressures, is different. Therefore, the price at which oxygen ions are made and enter the zirconium oxide electrolyte at every single electrode differs. As the zirconium oxide permits mobility of oxygen ions, the amount of ions moving in each and every course throughout the electrolyte will rely on the rate at which oxygen is ionised and enters the electrolyte at every single electrode. The system of this ion transfer is complex, but it is identified to require vacancies in the zirconia oxide lattice by doping with yttrium oxide.
The result of migration of oxygen ions throughout the electrolyte is a net movement of ions in one route based on 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 optimistic E.M.F.
If P1oxygen sensor will be from P2 to P1 i.e. a negative E.M.F.
If 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.