Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For oxygen or oxygen containing compound
Reexamination Certificate
1998-12-31
2001-03-13
Warden, Sr., Robert J. (Department: 1744)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For oxygen or oxygen containing compound
C205S784500, C204S406000, C204S424000, C204S425000
Reexamination Certificate
active
06200458
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for detecting the oxygen content of a gas by means of at least one concentration cell having a measuring electrode. The concentration cell operates in accordance with the Nernst principle and the measuring electrode communicates with the gas to be measured. The concentration cell also includes a reference electrode connected to the measurement electrode via a solid electrolyte. Periodic pump current pulses are preferably applied across the measurement and reference electrodes. The invention also relates to an arrangement for detecting the oxygen content in a gas.
BACKGROUND OF THE INVENTION
A method of the kind described above is disclosed, for example, in U.S. Pat. No. 5,632,883. The application of periodic pump current pulses to the measurement and reference electrodes affords the advantage that, in this way, two switching states occur. In the first state, the current source is coupled to the concentration cell and, in the second state, the current source is decoupled from the concentration cell so that the voltage, which serves as the measurement signal, between the measurement and reference electrode is not influenced by the current source in the decoupled state. In this way, a disturbance of the measurement signal because of the in-coupling of the current is eliminated. This in-coupling of the current leads to an additive voltage component of the measurement signal which, in turn, defines a disturbance of the measurement signal because of the temperature dependency of the internal resistance of the solid electrolyte.
In such a method and arrangement for detecting the oxygen content in a gas, it is problematic that, for a clocked pump reference, the reference pump current must be selected higher than for a non-clocked pump reference in order to obtain, over a time average, an adequate oxygen partial pressure in the concentration cell.
This leads to the situation that intense polarization effects occur on the concentration cell because of the higher pump current especially in the cold state of the concentration cell. Therefore, the actual sensor signal is made incorrect because of the polarization effects. Additive voltage components on the actual measurement signal occur because of the polarization effects. These voltage components lead to a signal shift which is present even after the pump current pulses.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the invention to provide a method for detecting the oxygen content in a gas which is so improved that errors of the measurement signal are minimized or completely eliminated. These errors are caused by polarization effects in the measurement probe.
The method of the invention is for detecting the oxygen content in a gas to be measured. The method includes the steps of: providing at least one concentration cell operating in accordance with the Nernst principle with the concentration cell including: a measuring electrode communicating with the gas; a solid electrolyte; and, a reference electrode connected to the measuring electrode through the solid electrolyte; applying a pump-current pulse between the reference electrode and the measuring electrode; and, directly after the end of each pump-current pulse, reversing the current flow between the reference electrode and the measuring electrode in a pulse-like manner.
A very rapid decay of the polarizations in the concentration cell is made possible by this pulse-like reversal of the current which flows between the reference electrode and the measurement electrode
In principle, the most different possibilities exist for reversing the current flow between the reference electrode and the measurement electrode.
An advantageous embodiment provides that a current pulse of a pregiven duration and of a pregiven current intensity is applied between the reference electrode and the measurement electrode. The current pulse flows in the reverse direction. With this counter-current pulse, a shift of the probe voltage is most significantly reduced. This shift is caused by the polarization effects.
Advantageously, the duration and the current strength of the counter-current pulses are so selected that the charge, which is generated by the counter-current pulses, is significantly less than the charge generated by the pump current pulses.
Another embodiment provides that a short circuit of a short duration (short-circuit pulse) is effected between the reference electrode and the measurement electrode. This embodiment is especially simple to realize. Shifts of the probe voltage are almost entirely reduced even with such a short-circuit pulse. These shifts are caused by the polarization effects in the concentration cell.
The invention also relates to an arrangement for detecting the oxygen content in a gas. The arrangement includes at least a concentration cell, which operates in accordance with the Nernst principle. The concentration cell has a measurement electrode communicating with the gas to be measured and a reference electrode which is connected to the measurement electrode via a solid electrolyte and is in contact with a reference gas volume. The concentration cell also includes a pump current source via which preferably periodic pump current pulses can be coupled between the reference and measurement electrodes.
With reference to the above, it is a further object of the invention to improve an arrangement of the kind described above for detecting the oxygen content in a gas so that a very rapid reduction of the changes of the measurement voltage can be realized. These changes are caused by the polarization effects in the concentration cell.
According to another feature of the invention, a switch device is provided with which, after each pump current pulse, a short circuit or a low-ohmage connection is provided over a pregiven time span (short-circuit pulse) between the measurement electrode and the reference electrode.
Up to now, no detailed description has been provided as to the form of the counter-current pulse and of the short-circuit pulse. Advantageously, the counter-current pulse or the short-circuit pulse are shorter than the pump current pulse. In this way, it is avoided that there is a pumping off of the oxygen from the reference chamber of the concentration cell after a reduction of the polarization effects.
Preferably, the duration of the counter-current pulse or of the short-circuit pulse is approximately one twentieth to one tenth of the duration of the pump current pulse.
Furthermore, the duration and the current intensity of the counter-current pulses is so pregiven that the charge, which is generated by the counter-current pulses, is significantly less than the charge generated by the pump current pulses.
REFERENCES:
patent: 4384935 (1983-05-01), De Jong
patent: 4902401 (1990-02-01), Lin et al.
patent: 4988418 (1991-01-01), Beck et al.
patent: 5080775 (1992-01-01), Yamauchi et al.
patent: 5389225 (1995-02-01), Aagard et al.
patent: 5611909 (1997-03-01), Studer
patent: 5632883 (1997-05-01), Hoetzel
patent: 5974857 (1999-11-01), Yamashita et al.
patent: 52-071293 (1977-06-01), None
Brida Peter
Graser Theodor
Hoetzel Gerhard
Olsen Kaj K.
Ottesen Walter
Robert & Bosch GmbH
Warden, Sr. Robert J.
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