Zirconium Oxide

Paramagnetic

Polarographic

Delta F Non-Depleting Coulometric

Principle of Operation
The primary element of the high-temperature electrochemical oxygen sensor is a cell made of ceramic zirconium oxide stabilized with an oxide of yttrium or calcium forming a lattice structure and maintained in a temperature-controlled environment.  The cell is coated on the inside and outside with porous platinum coatings that serve as electrodes on both sides of the lattice.  At high temperatures (above 1200ºF), the openings in the lattice permit the passage of oxygen ions.  As long as the oxygen partial pressures are equal on both sides of the lattice, the movement of ions is random within the lattice and there is no net flow through the lattice. When a sample gas is introduced on one side of the lattice, oxygen ions pass through the lattice at a rate determined by temperature and the difference in the oxygen partial pressures of the sample gas and the reference gas (usually air) on the other side of the lattice.  The passage of oxygen ions through the lattice produces a voltage across the platinum electrodes.  The magnitude of this voltage is a logarithmic function of the ratio of the oxygen partial pressures of the sample and reference gas, and, since the partial pressure of the reference gas is predetermined, the voltage produced by the cell indicates the oxygen content of the sample gas.  The voltage of the cell is temperature-dependent, and therefore the cell must be maintained at a constant temperature.  The voltage increases as the oxygen concentration of the sample decreases.  The cell produces no voltage when air is on both sides of the lattice.

The operation of the high-temperature electrochemical oxygen detector is illustrated in the figure below:

Advantages and Disadvantages

The zirconium oxide oxygen analyzer has excellent response characteristics to changes in oxygen content.  Its major advantage is that it can be easily used as an in situ instrument; the measuring probe can be placed directly into a flue for high-temperature combustion gas analysis, thus eliminating the need for extractive sampling equipment, and the probe can be mounted in virtually any position.  It is an ideal instrument for determining combustion efficiency.

The major disadvantage of this type of analyzer is that the zirconium oxide cell has a relatively short life (typically less than 18 months) and replacement costs are relatively high.  Failures are usually caused by diffusion of one or both of the platinum electrodes through the zirconium oxide layer, eventually shorting the cell.  Another drawback is that as the cell ages, it becomes increasingly more difficult to calibrate.  In applications that require frequent on/off periods for the analyzer, thermal cycling can cause stress cracks that can greatly shorten cell life and overall reliability.

The zirconium oxide oxygen analyzer is not recommended for trace oxygen analysis.  Any reducing gases (hydrogen, hydrocarbons, carbon monoxide) will chemically reduce (consume) oxygen at the high temperatures necessary for operation and will result in lower than actual readings of oxygen content.

Typical Usage

Zirconium oxide oxygen analyzers are used primarily in combustion control applications to determine combustion efficiency.  One major use is the measurement of oxygen concentration in the exhaust gases of steel-producing blast furnaces, which consume great amounts of air during combustion and require continuous monitoring to maintain optimum oxygen levels in the flue gas.  Manufacturers of chemicals and petrochemicals, ceramics, and glass also use zirconium oxide analyzers for stack applications.