Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For oxygen or oxygen containing compound
Reexamination Certificate
1998-06-12
2001-01-23
Warden, Sr., Robert J. (Department: 1744)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For oxygen or oxygen containing compound
C205S784500, C204S427000, C073S001060
Reexamination Certificate
active
06177001
ABSTRACT:
TECHNICAL FIELD
The present invention relates to sensors, and in particular, to oxygen sensors used for detecting the level of oxygen present in a gaseous environment.
These sensors employ a thin layer of oxide material, typically zirconia, which is electroded on opposed sides of the layer and, in a typical mode of operation, acts as a Nernst cell. The sensors are operated at an elevated temperature of at least approximately 700° C. which provides a suitably increased level of oxygen ion mobility, via a mechanism analogous to that of the mobile lattice “holes” that allow electron flow in semiconductor materials, and the electrochemical oxygen ion flow is used to determine the oxygen gradient across the layer. A difference in partial pressures of oxygen across the wall induces a flow of oxygen ions between the electrodes, and the cell produces a voltage across the layer from which the ratio of the two partial pressures is determined.
By applying a reference gas of known oxygen content to one side of the sensor and admitting a sample gas from a process environment to the other side, the level of oxygen in the process environment may be derived from the magnitude of this voltage. However, owing to manufacturing and other variations in layer thickness and surface effects, various corrections in the form of bias offsets or factors of proportionality may be necessary, both initially and as the sensor ages, in order to convert this signal to an effective measurement of the prevailing oxygen level. Moreover, such sensors are generally not amenable to usage in dirty environments or in a gas environment that evolves quickly, such as a flue gas measurement environment.
Certain constructions have been proposed for generating an internal reference, so that the sensor is able to calibrate itself using the available gas rather than requiring a known reference standard to be separately supplied. Sensor systems of this type have been described by C. Franx, in Sensors and Actuators, Vol. 7, No. 4 (August, 1985), pp. 263-270, and by D. M. Haaland in Analytical Chemistry, Vol. 49, No. 12 (October, 1977), pp. 1813-1817. A typical such system employs two identical ZrO
2
disks, each with platinum electrode faces on both sides, and attaches the disks to opposite ends of a platinum or ceramic cylinder or spacer ring such that the disks form opposed end walls of a sealed cannister-like chamber. One of the two disks is operated as a standard ZrO
2
oxygen sensor, which, according to the Nernst equation, provides a signal which is a function of the O
2
partial pressures within and outside of the chamber. The other is connected with a reverse bias to operate as an electrochemical oxygen ion pump and to pump the interior of the chamber down to zero oxygen pressure between measurements. Assuming that the actual volume of the chamber is first determined, and preferably after first performing a factory reference calibration to develop a meter factor and determine a Nernst offset, the sensor may be subsequently re-zeroed in the field by a pumpdown procedure without requiring a separate span gas to be provided for reference. The internally referenced sensor operation of these devices may be useful for measuring oxygen levels provided that the sensor response time is sufficiently short compared to the time scale on which the sample gas fluctuates.
The foregoing construction has the disadvantage, however, that the process for assembly of the chamber bounded by two zirconia disks requires passage of two wires from the interior to the outside of the chamber, and of course also requires sealing the spacer ring in a leak-proof manner around the entire periphery of its top and bottom to the two disks which constitute the major active transport and measurement surfaces of the device. The overall construction presents a relatively large surface area, but this is achieved in a construction that requires two long narrow sealing junctions, about the peripheries of both disks, and is therefore vulnerable to small leaks which can shift measurement values during or between readings. These factors can impair the achievable levels of manufacturing quality, or can lead to increased development of micro-leaks once the sensor is put into service, and thus may require frequent recalibration.
It is therefore desirable to provide an improved oxygen sensor and method of determining oxygen level with an internal reference.
It is further desirable to provide such a sensor having decreased susceptibility to leakage.
It is also desirable to provide such a sensor having improved response characteristics.
SUMMARY OF THE INVENTION
These and other desirable features are provided in an oxygen sensor in accordance with the present invention, wherein a zirconium oxide body with a continuous inside face and a continuous outside face forms a thin shell enclosing a chamber of fixed volume. In discrete or separate time intervals, a controller in one time interval applies a signal across the two faces so that the sensor acts as an electrochemical oxygen ion pump, and in another time interval, alternately monitors the voltage produced by the oxygen ion current across the shell to determine the oxygen level. At each stage, the integrated pump current yields a measure of the oxygen added to or removed from the chamber. By pumping so that the chamber has been emptied, or has attained equilibrium with the outside, an internal calibration is achieved.
Preferably the shell is a tube with inner and outer surfaces electroded by respective layers of porous platinum, and the tube has length which is substantially greater than its diameter, and it is dependably sealed with an end plug through which a single electrode wire passes for connecting to the inner platinumized electrode surface. A suitable material for forming the sensor is a mixture of about 91% zirconium oxide and 9% yttrium oxide, and this material is formed or shaped as a paste or slurry into a single body to the final shell shape, e.g., a tube. The body is sintered at about 1400° C. to strengthen and set its shape, and the sintered body is then platinum coated to form porous electrodes on each surface. A lead is attached to the inner electrode surface and brought out through the open end of the tube, which is then sealed with a plug.
In a preferred mode of operation, a controller self-calibrates the sensor by energizing the electrode surfaces with a controlled flow of current at a first polarity to pump oxygen out of the chamber. Following a brief relaxation interval after pumping, a processor reads the cell output, and repeats the pumping and reading steps until it determines that the chamber is, as a practical matter, effectively empty. That is, the Nernst voltage indicates that the internal oxygen concentration is negligible with respect to the required measurement accuracy. This level may be, for example, about 200 mV for a typical flue gas application. The controller then switches polarity, and operates as an electrochemical oxygen ion pump in the reverse direction to fill the chamber to the prevailing oxygen concentration. In general, the amount of oxygen transported across the wall is directly proportional to the charge provided to the pump, so this procedure, when carried out in a gas of known oxygen concentration, calibrates the sensor. To operate as a pump and null the pressure difference across the sensor wall, a controller preferably reads the Nernst voltage, applies an amount of charge calculated to substantially achieve equilibrium, and then, after a relaxation time, again reads the Nernst voltage, repeating further pumping if necessary. The pumping current is preferably applied as a sequence of current pulses, and the &Dgr;P Nernst cell output is read until the signal across the electrodes drops below noise threshold, indicating that &Dgr;P
oxygen
is effectively nulled. Preferably, the controller integrates the applied charge. For example, when applying fixed duration pulses of constant current, this is done by simply counting pulses and multiplying by a meter or conversion facto
Nutter & McClennen & Fish LLP
Olsen Kaj K.
Panametrics, Inc.
Warden, Sr. Robert J.
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