Michell Instruments XZR200, User Manual

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XZR200 User’s Manual
22 97337 Issue 4, July 2018
APPENDIX B
Sensor Function:
ZrO
2
Square
Al
2
O
3
Disc
Sensing
Chamber
Platinum
Ring
At the core of the XZR200 oxygen analyzer is a cell consisting of two Zirconium Dioxide
(ZrO
2
) squares coated with a thin porous layer of platinum which serve as electrodes.
The platinum electrodes provide the catalyst necessary for the measured oxygen to
dissociate, allowing the oxygen ions to be transported through the ZrO
2
.
The two ZrO
2
squares are separated by a platinum ring which forms a hermetically
sealed sensing chamber. At the outer surfaces there are two further platinum rings
which, along with a center platinum ring, provide the electrical connections to the cell.
Two outer alumina (Al
2
O
3
) discs fi lter and prevent any particulate matter from entering
the sensor and also remove any unburnt gases. This prevents contamination of the cell
which may lead to unstable measurement readings. The above fi gure shows a cross-
section of the sensing cell with all the major components highlighted.
A heater coil which produces the necessary 700°C (1292°F) required for operation,
surrounds the sample cell. Both are housed within a porous stainless steel cap to
fi lter larger particles and dust. The fi lter cap also protects the sensor from mechanical
damage.
Pumping Plate:
The fi rst ZrO
2
square works as an electrochemical oxygen pump, evacuating or re-
pressurising the hermetically sealed chamber. Depending on the direction of the
reversible DC constant current source the oxygen ions move through the plate from one
electrode to the other. This in turn changes the oxygen concentration and therefore the
pressure (P
2
) inside the chamber. As we only evacuate then re-pressurize the chamber
using the gas around the sensor, the pressure inside the chamber is always less than
the ambient pressure outside the chamber. The electrical connections to the cell are
shown in the fi gure below.