CAPACITANCE PROBE FOR DETECTING MOISTURE WITH VERY LONG CABLES

• US 3,710,244 A
• Filed: 01/11/1971
• Issued: 01/09/1973
• Est. Priority Date: 01/11/1971
• Status: Expired due to Term

• Current Assignee: George P. Rauchwerger
• Original Assignee: George P. Rauchwerger
• Inventors: Rauchwerger, George P.
• Primary Examiner(s): Krawczewicz, Stanley T.
• Application Number:
  US05/105,254
• Time in Patent Office: 729 Days
• Field of Search: 324/61 317/246
• US Class Current: 324/664
• CPC Class Codes: G01N 27/223 for determining moisture content, e.g. humidity rain detectors on vehicle windows B60S1/0825

Abstract

A probe consists of two spaced elements, each consisting of a piece of foil, sheet, rod or tube, each insulated from the other and from the material whose moisture is to be measured. The elements are connected by two coaxial or one tri-axial cable to a remote piece of electronic equipment including an ultrasonic oscillator which transmits signals to the probes, which thus function as a variable capacitor, the variation being dependent on the moisture content of the said material in which the probes are imbedded. The equipment has a preamplifier (integrated circuit charge amplifier) having high frequency response and very low voltage differential between the two inputs. An integrated circuit peak detector provides a positive D.C. voltage proportional to the input peak voltage from the preamplifier. Such D.C. voltage may be measured by a microammeter to indicate moisture, or the output may be used to control a valve (for irrigation, e.g.) Several probes and valves may be used in an irrigation system, or the like.

Claims

1. Probe means for sensing moisture in a solid or liquid medium by measuring capacitance between and across a first and a second conductive medium comprising a non-conductive, moisture-proof thin plastic sheet base, a first and a second strip of metallic foil separated from each other, a pair of electric lead wires having bare ends, thin metal foil means electrically connecting the bare end of one wire to one strip and the other bare end to the other strip, and a pair of plastic sheets on opposite sides of said probe means securing said strips to said base and insulating said strips from each other and from the exterior of said probe, said sheets comprising homogeneous plastic material completely surrounding said foil means, sheet base and bare ends of wire, said sheets laminated together and hermetically sealing said wires to said probe and insulating said wires and foil from said soil or liquid medium.

2. Probe means according to claim 1 which further comprises an elongated cable having two conductors and grounding means grounding the space between conductors to eliminate capacitance between said conductors, said cable comprising a triaxial cable, the central and outer conductor of said cable connected to said first and second foil and the middle conductor to ground.

3. Probe means according to claim 1 which further comprises elongated first and second coaxial cables, each having its outer conductor grounded and one of said lead wires connected to the central conductor of each said coaxial cable.

4. A probe according to claim 1 in which both said plastic sheets are formed of a material severable along a line between said strips to form two probes, movable relative to each other and each connected to a lead wire.

5. Probe means according to claim 1 in which said base is a cylinder and said strips are on opposite sides of the outside of said cylinder arranged parallel to the axis of said cylinder.

6. In combiNation, probe means according to claim 3 with said probe means imbedded directly in said medium, an oscillator having its output connected to one said cable, a positive peak detector having its input connected to the other said conductor, said detector emitting a positive D.C. voltage proportional to capacitance between said strips and a comparator receiving the output of said detector and comparing said output with an adjustable reference voltage and having a switch opened and closed depending on whether the capacitance between said foils is more or less than a predetermined value as set by said adjustable reference voltage.

7. The combination according to claim 6 which further comprises an amplifier between said other conductor and said detector and a feedback capacitor across said amplifier having a characteristic such that the input voltage sent into one said cable by said oscillator equals the output voltage of said amplifier when said feedback capacitor equals the capacitance of said probe in the material under test and wherein the output voltage of said amplifier varies inversely to the ratio of the feedback capacitor to the probe capacitor.