Basic Elements Of The Sensor - Ametek Thermox 2000 ATEX Series User Manual

Basic Elements of the Sensor

1-2 | Thermox Series 2000 WDG-HPII / HPIIC - ATEX
The WDG-HPII analyzer consists of the following basic systems:
The Plumbing
•
All inlet and outlet tubing (cell housing), the sensing cell, the sensing
cell fitting, and the combustibles flow block and detector (for com-
bustibles).
The Measuring System
•
The sensing cell, combustibles detector, interconnecting wiring, and
the control unit.
The Temperature System
•
The electrical cell heater (furnace), sensor case heaters with adjust-
able thermostat, the type "K" thermocouple (maintains cell operating
temperature), and the sensor board containing cold junction com-
pensation. The sensing cell operates at a constant temperature. The
circuit board in the sensor terminal box switches power to the fur-
nace 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 elec-
trodes. At high temperatures (generally above 1200°F/650°C), oxygen mol-
ecules coming in contact with the platinum electrodes near the sensor be-
come 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 mag-
nitude 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 refer-
ence gas, usually air (20.9% O
Since the voltage of the cell is temperature dependent, the cell is main-
tained at a constant temperature. Some newer high temperature insitu
models use the heat from the process to heat the sensor, and the process
temperature is continuously measured and used in the software calcula-
tion. The oxygen content is then determined from the Nernst equation:
), is used for one of the gases.
2
RT O
1
E = ln
4F O
2