Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing
Reexamination Certificate
2000-03-21
2001-10-30
Tung, T. (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
C204S427000, C204S431000, C205S784500
Reexamination Certificate
active
06309534
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to apparatus and method for measuring the composition of gases using ionically conducting electrolytes. In particular, it relates to sensors useful in the detection of gases, such as oxygen in air/fuel mixtures, extending over a range of compositions from those which are fuel-rich, containing minute quantities of oxygen, to those containing several percent of oxygen.
BACKGROUND OF THE INVENTION
The monitoring and control of gases during combustion is of increasing importance. Therefore, it is necessary to develop inexpensive, robust and reliable sensors having high sensitivity and selectivity. The sensors can be used to control the combustion process, and to detect when inefficient combustion is occurring, when toxic gases are being released and when the combustion unit requires tuning.
Two types of solid state sensor have been used for the determination of the oxygen content of gases. The electrolyte that is usually used is stabilised zirconia which comprises a solid solution of zirconium oxide and calcium oxide, magnesium oxide or yttrium oxide. In one mode, the potential is measured across the electrolyte when the gas to be measured is on one side of the electrolyte and a reference gas, of known oxygen partial pressure, is on the other side of the electrolyte. In another mode, the electrolyte is separated from gas by a pinhole or a porous layer, a potential is applied and the current measured.
In the potentiometric mode, the potential is measured across the electrolyte and the gas composition on either side of the electrolyte is given by the well-known Nernst equation:
−ZEF=RT lnp′
x2
/p″
x2
(1)
where Z is the charge carried, E is the measured potential, F is Faraday's constant, R is the gas constant, and P′
x2
and P ″
x2
are the pressures of gas on either side of the electrolyte. It can be appreciated that, if the potential is measured and the gas composition is known on one side of the electrolyte, the gas composition on the other side of the electrolyte can be determined. This approach has been used to great effect in controlling the internal combustion engine and burners, around the stoichiometric ratio, where the gas goes from highly reducing to highly oxidising over a small range of air/fuel ratio. Under these conditions, the oxygen pressure changes dramatically and a large potential change results.
However, for some engines and many combustion systems, it is preferable to operate in a region where excess air is used and the residual gases contain free oxygen. In such a lean-burn system, unlike the circumstances occurring around the stoichiometric region, there is very little change in oxygen partial pressure, which results in very small changes in the Nernst potential, given by equation (1). In many cases, the predicted change in potential is less than the scatter in the reading.
In order to circumvent this problem, an amperometric sensor has been used which comprises an electrolyte plate or cup, with an electrode on each face, separated from the gas source by a pinhole or porous coating. On the application of a voltage across the electrolyte, via the electrodes, oxygen ionises to ions which pass through the electrolyte to where the ions are discharged to oxygen gas at the other electrode. The supply of oxygen to the ionising electrode is controlled by the supply of oxygen via the pinhole or the porous coating and, therefore, the measured current is dependent on the supply of oxygen.
As the concentration at the electrode surface is zero (as all the oxygen is ionised), the current is linearly dependent on the oxygen concentration in the gas external to the pinhole or the porous coating. Furthermore, the measured current is usually found to be independent of the applied potential. However, it is extremely difficult to maintain the size of the pore or the porosity to any degree of reliability. For this reason, these sensors are not widely used, and the development of lean-burn engines and combustion systems has been restricted.
SUMMARY OF THE INVENTION
If two different partial pressures of a gas are separated by an ionic conductor which conducts the ion of the gas and a potential is applied between the two electrodes attached to the surfaces of the electrolyte, a current flows. The present invention is based on the discovery that, if the magnitude of the potential is kept the same but reversed, a different current flows, and that the ratio of the currents is a function of the difference in partial pressures.
The ratio of the current in the forward (I
for
) and the reverse (I
rev
) direction is quantitatively related to the partial pressure of oxygen in the test gas through the following equations:
I
for
/I
rev
=(V
appl
+emf)/(V
appl
−emf) (2)
where,
emf=2.303[RT/4F] log [pO
2
(ref)/pO
2
(test)] (3)
According to one aspect of the invention, apparatus for measuring the composition of oxygen or any other gaseous species, includes
(a) an electrolyte which conducts ions of the gaseous species and which separates two compositions containing the gas species to be measured;
(b) means for applying a potential difference across the electrolyte;
(c) means for reversing the applied voltage; and
(d) means for determining the ratio of the respective currents.
The present invention provides a single sensor which can operate at high sensitivity over the whole range of partial pressures of a gas. Compared to the amperometric sensor, no pinhole or porous layer is required, which makes the apparatus considerably simpler. Further, the measured currents and current ratio are always a function of the applied potential which is unlike the amperometric sensors where the current is independent of the applied voltage. Depending on the magnitude of the potential, the present invention makes it possible to measure the oxygen partial pressure accurately over the whole range. This makes the sensing apparatus far more flexible than either the potentiometric or amperometric sensor.
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patent: 0520528 (1992-12-01), None
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Fray Derek John
Kumar Ramachandran Vasant
Saliwanchik Lloyd & Saliwanchik
Tung T.
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