GB2369682A - Moisture sensor - Google Patents

Abstract

To measure moisture or humidity, a probe in the form of electrical capacitor means, the capacitance of which varies depending on surrounding moisture content, is connected (via 11a, 11b) to an oscillator circuit (not shown), the frequency of oscillation of which is a function of said capacitance. A counter is connected to the circuit and is operative to count its oscillations in a given time and display these as a measure of the moisture content adjacent said probe. The probe is of particularly simple form, consisting of a cut open loop or a pair (10a, 10b) of conductive wires which are coated or covered in substantially waterproof, electrically insulating material and folded or twisted into two or more helices.

Description

MOISTURE SENSOR This invention concerns a moisture sensor, which is particularly designed for measurement of moisture in soil, for example as part of larger soil moisture replenishment apparatus, but is likely to have much wider application in a variety of technical fields, wherever measurement of humidity or moisture in a medium is required.

Previously known soil moisture sensors have measured the electrical resistance of soil between a pair of spaced apart electrodes which may be inserted a few centimetres into the soil. In this respect, the resistance of wet soil is typically about 10 x 104 ohm, rising to 10 x 10'ohm when it is dry.

This resistance can be detected, e. g. by an electrometer amplifier, and either compared to a threshold value to trigger other apparatus, such as irrigation apparatus, or simply used as a measure of moisture level, after suitable calibration of the sensor.

Such sensors, which are reliant on resistance measurement, have been found to be unreliable owing to the instability associated with high impedance electrometers and the variability of electrical contact between the electrodes and the soil.

Accordingly, an alternative has been sought.

In this respect, it is an object of the invention to provide a moisture sensor, suitable for use in measuring soil moisture, which will provide consistent and accurate results and will not readily break down or wear out, and is inexpensive to produce.

With this object in view, the present invention provides a moisture sensor comprising a probe in the form of electrical capacitor means, the capacitance of which varies depending on surrounding moisture content, an oscillator circuit, to which said capacitor means is connected and the frequency of oscillation of which is a function of said capacitance, and a counter which is connected to said circuit and is operative to count its oscillations in a given time and display these as a measure of the moisture content adjacent said probe. Thus, instead of directly measuring electrical resistance, as in the prior art, the sensor of the invention measures capacitance, but indirectly by measuring the oscillation frequency in the associated circuit. In a preferred embodiment of the invention the probe is formed from a single length of conductive wire which is coated or covered in substantially waterproof, electrically insulating material, which is folded or formed into a loop and which is cut open at the fold or the furthest extent of the loop to provide a pair of closely spaced cut ends. Alternatively the probe may comprise a pair of wires having closely spaced end sections, each wire being similarly coated or covered in a substantially waterproof, electrically insulating material.

Since the probe is fully insulated from the surrounding medium, e. g. soil or moist air, it will not suffer any electrochemical degradation (e. g. rusting) associated with the previously known resistance measuring probes. The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of a preferred embodiment of the moisture sensor of the invention; and Figure 2 is a sketch of a practical embodiment of a probe, which is part of the sensor of the invention.

As shown in Figure 1, a sensor of the invention comprises a probe (10) connected to an electrical oscillator circuit (12), which is connected to a counter (14).

As shown in Figure 2, in a simple, practical embodiment, the probe (10) may consist of a length of wire, about 60cm-100cm long, which is folded in half to provide spaced apart limbs, then twisted into a double helical configuration, then folded in half again to form a pair (loa, 100) of semi-rigid double helices, i. e. 4 strands in all, about 12cm-25cm long. The free ends (lla, llb) of the wire are stripped to allow for connection to the oscillator circuit (12). At about the mid point of the wire, which is the original fold, the wire is cut, as indicated at (13). The cut ends are spaced apart by a gap of the order of a millimetre or a few millimetres and effectively constitute a capacitor.

The material and diameter of the wire of the probe (10) is not critical to the operation of the probe, but 1mm diameter copper wire has been found to be suitable in a preliminary test. It is important that the wire should be insulated from the environment. Therefore lacquered wire or wire coated in enamel or wire insulated in some other suitable manner should preferably be used. Since the purpose of the insulation is to separate the wire from moisture in the environment the insulating material should not be of a hydroscopic nature.

In a slightly modified form of probe, the pairs of double helices may be further twisted around each other to enhance the rigidity of the structure. In another embodiment, two separate wires may be used to form the probe as an alternative to making a cut in a single loop. However, the ends forming the capacitor must be brought into close proximity without touching.

The oscillator circuit (12) may be any that is commercially available at a reasonably low cost. For example, a known 555 oscillator, is suitable. Alternatively, an integrated circuit acting as an oscillator may be used. A power supply (not shown) for the oscillator circuit is required and may be simply a 6v battery.

The frequency of oscillation of the circuit (12) equals the frequency of discharge of the capacitor provided by the probe (10). This in turn depends on the moisture level in the immediate surrounding of the probe (10), specifically between the two wire end sections.

The output of the circuit (12), i. e. the number of its oscillations, is counted in a predetermined time, probably of the order of a few tens of milliseconds, by the counter (14).

The counter (14) may be a simple commercially available timer, or part of a programmable microcontroller. This can be calibrated as a measure of the moisture content in the vicinity of the probe (10), e. g. 0 in dry air, 100 in water, so that when the probe (10) is inserted into the soil, for example, a reading of 15 indicates low moisture, while 75 indicates wet conditions. The overall arrangement also includes a resistor (18) connected in the circuit between the oscillator (12) and the probe (10).

The resistor valve will be chosen in conjunction with the capacitance of the sensor as the combination determines the period over which the oscillations should be counted. A suitable resistance would probably be in the region of lOOk ohms. The digital nature of the output from the circuit (12) makes it particularly reliable and sensitive. Another advantage is that the wires of the probe (10) are not immediately liable to degradation, i. e. rusting of corrosion, as they are completely insulated from the medium in which moisture is being measured.

The probe may be used in soil to measure moisture content, as described. It could be used in a wide variety of other situations to measure moisture or humidity in solid, liquid or gaseous media. For example, it could be used in a tumble dryer to measure humidity and thus gauge when the contents are sufficiently dry. It could also be used in other processes where an accurate measure of humidity is desirable. In such cases, a hydroscopic material may advantageously be placed around the cut ends of the wire as a means of increasing what will inevitably be a relatively small signal, as detected by the counter (14).

Claims (9)

1. CLAIMS 1. A moisture sensor comprising a probe in the form of electrical capacitor means, the capacitance of which varies depending on surrounding moisture content, an oscillator circuit, to which said capacitor means is connected and the frequency of oscillation of which is a function of said capacitance, and a counter which is connected to said circuit and is operative to count its oscillations in a given time and display these as a measure of the moisture content adjacent said probe.
2. 2. A moisture sensor according to claim 1 wherein the probe comprises a cut open loop of a conductive wire which is coated or covered in substantially waterproof, electrically insulating material.
3. 3. A moisture sensor according to claim 1 wherein the probe comprises a pair of conductive wires which are coated or covered in substantially waterproof, electrically insulating material.
4. 4. A moisture sensor according to claim 2 or 3 wherein the wire or wires is/are enamel or lacquer coated.
5. 5. A moisture sensor according to claim 2 or 3 or 4 wherein the wire or wires are between 100mm and lOOOmm long.
6. 6. A moisture sensor according to any of claims 2 to 5 wherein the wire or wires are folded or twisted.
7. 7. A moisture sensor according to claim 6 wherein the wire or wires are configured as two or more helices.
8. 8. A moisture sensor comprising a probe which forms a capacitive arm of an electrical oscillator circuit, the output of which is counted.
9. 9. A moisture sensor substantially as hereinbefore described with reference to either or both of the accompanying drawings.