| 摘要 |
A capacitance measurement circuit 1 has a pair of level indicating LEDs 2,3. Via one input 4, the circuit is connected to the metallic wall W of a container C of water. This connection can be direct or, as indicated by broken lines, via earth. Via a second input 5, the circuit is connected to a probe 6. This has a stem 7 with a transversely arranged disc 8 at its lower end. Spaced above the lower end, a second, larger, transverse disc 9 is secured to the stem. The stem and the discs are of conductive metal, typically copper, and electrically connected and common. The entire probe is insulated, except for its point of contact with the input 5.When the liquid is just below the level of the lower disc 8, the capacitance between it and earth, the liquid being conductive, will be determined by the area of the disc, its separation from the liquid and the dielectric constant of the intervening gap. The capacitance varies in proportion to the gap. Hence when the liquid is well below the disc the capacitance will be small. As soon as the liquid covers the disc, the capacitance will reach a maximum-for this disc. The maximum will be dictated by the thickness of the insulation material separating the disc as such from the liquid and the fact that the area is in effect doubled by the liquid covering both surfaces.
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