XZR400 Series User’s Manual
Michell Instruments
3
INTRODUCTION
1.2
The MSRS Technology
The zirconium oxide sensors are often referred to as the ‘high temperature’
electrochemical sensors. The principle is based on the Nernst principle [W. H. Nernst
(1864-1941)]. Zirconium oxide sensors use a solid state electrolyte and are stabilized
with yttrium oxide. The zirconium oxide probe is plated on opposing sides with platinum
which serves as the sensor electrodes. For a zirconium oxide sensor to operate properly,
it must be heated to approximately 600°C. At this temperature, on a molecular basis,
the zirconia lattice becomes porous, allowing the movement of oxygen ions from a
higher concentration of oxygen to a lower one, based on the partial pressure of oxygen.
The movement of oxygen ions across the zirconium oxide produces a voltage between
the two electrodes, the magnitude of which is based on the oxygen partial pressure
differential created by the reference and sample gas.
(
T
)
Zirconia
P
P
ref.
P
P
meas.
E
Figure 1
Zirconia Sensor Operating Principle
Within the oven the zirconium oxide MSRS is maintained at a temperature of 634°C.
The MSRS generates a signal that is proportional to the natural logarithm of the partial
pressure of oxygen p (O
2
).
E =
RT
4F
p (O
2
ref)
p (O
2
meas)
ln
With a known reference electrode and a constant temperature it is possible to de
fi
ne
the partial pressure of oxygen using the Nernst-equation (see above).
The conductivity of zirconium oxide increases exponentially with temperature. The oxide
ion conductivity is optimized at temperatures above 600°C.
The MSRS technology allows the design of miniaturized zirconium oxide-based oxygen
sensors. The low mass and volume of the MSRS sensors have a positive effect on the
response time, which is one of the best available on the market.
