METROTEC
7
3
Introduction
3.1
Measuring Principle
Oxygen measurement units are designed to process signals transmitted from an
oxygen sensor constructed of stabilized zirconium oxide. Zirconium oxide, a ceramic
material that is also spoken of as a solid-state electrolyte, acts as an excellent
oxygen-ion conductor when at a high temperature.
Within certain temperature limits, that depend on the doping of the material
concerned, such ion conductors are able to fill empty spaces in their crystal lattice
with oxygen ions. The oxygen ions occur against an electrically conductive surface
that is generally of platinum.
The concentration of oxygen in a measurement gas is thus decisive for the extent of
oxygen activity, and thus for the number of oxygen ions.
An oxygen sensor consists essentially of a solid-state electrolyte with a contact
surface on both sides.
One side of the electrolyte is in contact with a reference gas such as air, and the
other with the gas whose oxygen content is to be measured. The mechanical
construction of the sensor prevents contact between the two gases so that there is
no risk of their being intermixed.
Depending on the application concerned, heated or unheated sensors are used.
Unheated sensors are generally used in furnaces while heated sensors are used for
applications where the gas to be measured is at a temperature of less than around
600 degrees Celsius (the measurement principle necessitates the sensor being
maintained at a temperature of not less than 500 - 650 degrees Celsius).
Heated sensors are maintained at a set temperature by an electronic temperature
regulator that forms part of the electronic control unit. The temperature of both
heated and unheated sensors as measured by the electronic control is an important
parameter for inclusion in the calculation of the oxygen content (oxygen partial
pressure) in accordance with the following equation:
EMF
R T
F
P
P
4
1
2
ln(
)
whereby:
R = 8.31J/mol K
T = Temperature in Kelvin
F = 96493 As/mol
P1 = Oxygen partial pressure on the reference side with
0.20946 bar
P2 = Oxygen partial pressure on the measurement gas
side
EMF = Electromotive forcein Volts
