Electricity: measuring and testing – Conductor identification or location – Inaccessible
Patent
1988-09-12
1989-12-12
Eisenzopf, Reinhard J.
Electricity: measuring and testing
Conductor identification or location
Inaccessible
324 61P, 324439, 324448, 73 611R, 73 612, G01R 2700, G01N 2700
Patent
active
048870234
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an apparatus for detecting contamination of a capacitive sensor.
Capacitive sensors are used in many applications to identify materials placed between electrodes of the sensor. One example of the use of a capacitive sensor to determine the water content of oil is described in British Patent Specification No. 2,087,084. The circuit described in the above mentioned patent specification works extremely well but there is a tendency for the sensor to become contaminated with debris carried in the oil and water mixture the characteristics of which are to be determined and this can result in a considerable drift or instability in the sensor output.
In an attempt to overcome the problem of debris build up on a capacitive sensor it is known to shield exposed portions of the sensor with a guard electrode. The guard electrode is fed with an excitation signal which is identical in amplitude and phase to the signal supplied to the sensor and this has the effect of desensitizing those areas of the sensor which are surrounded by the guard. As debris builds up on the guard electrode there is no measurable effect on the output of the sensor until the build up of debris becomes so large as to neutralize the effect of the guard and introduce drift in the sensor output.
When dealing with fluid such as unrefined petroleum oil it is very difficult to estimate the amount of time taken for a particular level of contamination to build up on the guard electrode and therefore it is necessary to strip down the sensor structure to check for excessive contamination at regular intervals. On many occasions these inspections reveal no significant contamination but they must nevertheless be conducted to take account of worst case situations.
It is an object of the present invention to provide means for monitoring the build up of contamination on a capacitive sensor incorporating a guard electrode.
According to the present invention there is provided an apparatus for detecting contamination of a capacitive sensor, comprising a guard electrode surrounding a portion of the sensor, and a circuit for supplying an excitation signal to the sensor and an identical excitation signal to the guard electrode, wherein the circuit comprises means for periodically isolating the guard electrode from the excitation signal, and means for detecting the difference between the sensor output when the guard electrode is supplied with the excitation signal and when the guard electrode is isolated from the excitation signal, the detected difference being a function of the contamination of the guard electrode.
Preferably, the variation of the detected difference is monitored automatically and an output signal is generated as soon as a predetermined change in the detected difference occurs.
An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 is an illustration of a conventional capacitive sensor incorporating a guard electrode;
FIG. 2 is a schematic illustration of a circuit incorporated in the present invention for connection to the sensor of FIG. 1; and
FIG. 3 is a detail circuit diagram of a circuit operating in accordance with British Patent Specification No. 2,087,084 but incorporating modifications in accordance with the present invention.
Referring to FIG. 1, the illustrated sensor is used to monitor the water content of oil supplied via an inlet 1 to a pressure vessel 2. The pressure vessel supports a flange 3 defining holes 4 to receive fixing bolts. A housing 5 is provided to receive circuitry which will be described below.
A cylindrical inner electrode 6 is supported on a rod 7 which extends through a boss 8 into the housing 5. A guard electrode 9 is arranged around the rod 7. Identical excitation signals are fed to the inner electrode 6 and the guard electrode 9 from the circuitry within the housing 5. The pressure vessel 2 is earthed and thus the structure defines a capacitor the plates of which are separted by an annular space tha
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patent: 4626774 (1986-12-01), Regtien
Eisenzopf Reinhard J.
Harvey Jack B.
Mestra A.G.
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