PHYSICAL BACKGROUND
Partial pressure
Definition:
The partial pressure is defined as the pressure of a single gas component in a mixture of gases. It corresponds to the total pressure which the single gas component would exert if it alone occupied the whole volume.
Dalton’s law:
The total pressure (ptotal) of a mixture of ideal gases is equal to the sum of the partial pressures (pi) of the individual gases in that mixture.
Nernst voltage
Two different ion concentrations on either side of an electrolyte generate an electrical potential known as the Nernst voltage. This voltage is proportional to the natural logarithm of the ratio of the two different ion concentrations.
Zirconium dioxide (ZrO2)
At high temperatures >650 °C zirconium dioxide (ZrO2) exhibits two mechanisms:
1. ZrO2 partly dissociates to produce oxygen ions which can be transported through the material when a voltage is applied.
2. ZrO2 behaves like a solid electrolyte for oxygen. If two different oxygen pressures exist on either side of an ZrO2 element a voltage (Nernst voltage) can be measured across that element.
CROSS SENSITIVITY
The Zirconium dioxide (ZrO2) oxygen sensors measure partial oxygen pressure as explained above and show some cross sensitivity with other gases. Described below are gases or chemicals that will have an influence on the life of the sensor or on the measuring results.
Combustible gases
Small amounts of combustible gases will be burned at the hot platinum electrode surfaces of the sensor. In general combustion will be stoichiometric as long as enough oxygen is available.
The sensor will measure the residual oxygen pressure which leads to a measurement error. The sensor is not recommended for use in applications where there are large amounts of combustible gases present and an accurate O2 measurement is required.
Investigated gases (stoichiometric combustion) are:
• H2 (hydrogen) up to 2%;
• CO (carbon monoxide) up to 2%;
• CH4 (methane) up to 2.5%;
• NH3 (ammonia) up to 1500ppm.
Heavy metals
Vapours of metals like Zn (zinc), Cd (cadmium), Pb (lead), Bi (bismuth) will have an effect on the catalytic properties of the platinum electrodes. Exposures to these metal vapors has to be avoided.
Halogen and sulphur compounds
Small amounts (< 100ppm) of halogens and/or sulphur compounds have no effect on the performance of the oxygen sensor. Higher amounts of these gases will in time cause readout problems or, especially in condensing environments, corrosion of sensor parts.
Investigated gases are:
• Halogens, F2 (flourine), Cl2 (chlorine)
• HCL (hydrogen chloride), HF (hydrogen fluoride)
• SO2 (sulphur dioxide)
• H2S (hydrogen sulphide)
• CFC’s, HCFC’s (Freons) • CS2 (carbon disulfide)