Eaton Crouse-Hinds Z130, Instruction Manual

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12
INM MTL 130-0200 Rev 4
7 TECHNICAL DESCRIPTION OF SENSOR
The zirconia oxygen sensor (see sketch below) is an impervious tube-shaped zirconia (zirconium
oxide) element with a closed end coated externally and internally with porous metal electrodes,
typically platinum. At high temperatures, typically above 400°C, the zirconia becomes an
oxygen ion conductor, which results in a voltage being generated between the electrodes
dependent upon the differences between the partial pressures of the oxygen in the sample
and the oxygen in a reference gas (generally air). The voltage generated is determined by the
Nernst equation:-
Cell output = 2.303RT log P1
4F P2
where:-
R = molar gas constant
T = absolute temperature of cell in °K
F = Faraday constant
P1 = partial pressure of oxygen in the
reference (air in most cases)
P2 = partial pressure of oxygen in the sample.
Thus, with air on both sides of the cell, the output is zero (log1=0).
The reference electrode is negative with respect to the sample electrode for sample
concentrations of oxygen higher than that of air and positive for concentrations less than
that of air. Depending on the application either the internal or external electrode is used as
the reference.
The output voltage is processed electronically to provide signals suitable for display or for
process control purposes.
Cell output
(typically 45mV/decade
Zirconia tube
Electrodes
pO
2
= P2
pO
2
= P1