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3
Introduction
3.1
Measuring principle
Oxygen measuring devices are designed to process signals transmitted by an oxygen sensor
made from stabilised zirconia. Zirconia, a ceramic material that is also referred to as a solid
state electrolyte, acts as an excellent oxygen ion conductor at high temperatures.
Within certain temperature limits that depend on the doping of the material, ion conductors of
this kind are able to fill empty spaces in their crystal lattice with oxygen ions. The oxygen ions
form on a conductive contact layer that generally consists of platinum.
The concentration of oxygen in a sample gas indicates the degree of oxygen activity or, in other
words, the number of oxygen ions.
A sensor essentially consists 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
sample gas. The mechanical structure of the sensor keeps the two sides separate, preventing the
gases from intermingling.
Heated or unheated sensors are used, depending on the application. Unheated sensors are
predominantly used in furnaces, while heated sensors are used for applications that involve
measuring gas below around 600 degrees Celsius. (The measuring principle dictates a minimum
temperature of 500–650 degrees.)
Heated sensors are maintained at a set temperature by a temperature regulator that forms part
of the processing electronics. The temperature of both heated and unheated sensors is measured
by the electronics and incorporated in the calculation of the oxygen content (partial pressure of
oxygen).
The calculation is based on the following equation:
EMK
R T
F
P
P
=
4
1
2
ln(
)
whereby:
R = 8.31J/mol K
T = temperature in Kelvin
F = 96493 As/mol
P1 = partial pressure of oxygen on the reference side with 0.20946
bar
P2 = partial pressure of oxygen on the sample gas side
EMF = electromotive force in volts
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PZA-MC25_1620-17485-0002-E-0821
