The zirconia oxygen analyzer is suitable for measurements of ppm to % levels of oxygen in a gas or mixture of gases. The zirconia cell is an electrochemical galvanic cell employing a high temperature ceramic sensor containing stabilised zirconium oxide.
Within an instrument the zirconia cell is mounted in a temperature controlled furnace with the necessary electronics to process the signal from the detection cell. Typically measurements are displayed directly via a digital display as oxygen concentration over the range 0.01ppm to 100%.
The theory behind Systech’s zirconia oxygen analyzer
The zirconia cell is a high 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 shown schematically in Figure 1 below.
The tube is completely gas tight and made of a ceramic (stabilised zirconium oxide) which, at the temperature of operation, conducts electricity by means of oxygen ions. (Note: In sensors of this type, the temperature has to be above 450°C before they become active as an electrolyte conductor). The potential difference across the cell is given by the Nernst equation.
Where:
E is the potential difference (volts)
R is the gas constant (8.314 J mol-1 K-1)
T is the absolute temperature (K)
F is the Faraday constant (96484 coulomb mol-1)
P1 & P2 are the partial pressures of the oxygen on either side of the zirconia tube
Thus, if the oxygen partial pressure at one of the electrodes is known and the temperature of the sensor is controlled, then oxygen measurement of the potential difference between the two electrodes enables the unknown partial pressure to be calculated.
Note
The partial pressure of the gas is equal to the molar concentration of the component in a gas mixture times the total pressure of the gas mixture.
PO2 = CO2 P2
where:
PO2 = Oxygen partial pressure
CO2 = Molar concentration of oxygen
P2 = Total pressure
Example
For atmospheric air:
CO2 = 20.9%
P2 = 1 atmosphere
PO2 = (0.209/100) x 1
PO2 = 0.209 atmospheres
