Basic Elements Of The Sensor; The Oxygen Measuring Cell - Ametek Thermox WDG Insitu User Manual

Sensor

Basic Elements of the Sensor

The WDG-IV series analyzer consists of the following basic systems:
The Plumbing

All inlet and outlet tubing (cell housing), the oxygen cell, the oxygen
cell fitting, and the combustibles flow block and detector (for combus-
tibles). Also includes the calibration gas inlet and aspirator assembly,
with the aspirator used to pull the sample into the sensor.
The Measuring System

The oxygen cell, combustibles detector (RTD or hot-wire type), meth-
ane detector, interconnecting wiring, and control unit.
The Temperature System

The electrical cell heater (furnace), sensor case heaters, the type "K"
thermocouple (maintains cell operating temperature), and the sensor
board containing cold junction compensation. The sensing cell oper-
ates at a constant temperature. The circuit board in the sensor terminal
box switches power to the furnace from the AC mains connected to
the sensor. This board also provides cold junction compensation to the
thermocouple circuit.

The Oxygen Measuring Cell

The sensing element itself is a closed-end tube or disk made from ceramic
zirconium oxide stabilized with an oxide of yttrium or calcium. Porous
platinum coatings on the inside and outside serve as a catalyst and as
electrodes. At high temperatures (generally above 1200°F/650°C), oxygen
molecules coming in contact with the platinum electrodes near the sensor
become ionic. As long as the oxygen partial pressures on either side of the
cell are equal, the movement is random and no net flow of ions occurs.
If, however, gases having different oxygen partial pressures are on either
side of the cell, a potentiometric voltage is produced (See Figure 1-1).
The magnitude of this voltage is a function of the ratio of the two oxygen
partial pressures. If the oxygen partial pressure of one gas is known, the
voltage produced by the cell indicates the oxygen content of the other gas.
A reference gas, usually air (20.9% O ), is used for one of the gases.
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