Measuring and testing – Moisture content or absorption characteristic of material
Patent
1996-01-19
1997-12-09
Brock, Michael
Measuring and testing
Moisture content or absorption characteristic of material
7333511, G01N 1910
Patent
active
056948069
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a device for indicating the water content of a medium, such as soil, peat or a synthetic growth medium. The device may, however, be applied more generally to the determination of water contents of solid, liquid or gaseous media.
BACKGROUND
Over-watering or under watering of house plants is frequently a cause of their ill-health or death. There is thus a need for a simple instrument which can indicate the water content of the soil or other plant growing medium.
Devices which measure water content, for example by conductivity, are known. However, these may be complex to construct, fragile and unreliable.
SUMMARY OF THE INVENTION
The present invention provides a device for indicating the water content of a medium, which comprises: that swelling of the hydrogel extends a peripheral portion of the hydrogel, the position of the peripheral portion in conjunction with the indicator means indicating the water content of the medium into which the device is placed.
The use of a water-swellable hydrogel which swells in proportion to the amount of water present, in conjunction with a scale or other indicator means to indicate the water content of a medium, allows the construction of a device which is robust and easy to use and which can be made inexpensively.
In a preferred embodiment, the hydrogel is in the form of a column (e.g. 5 to 20 cm long) and the position of the free end of the column is indicative of water content. Thus, the present device may operate on a similar principal as a thermometer in which liquid in a tube expands as temperature increases; the position of the liquid miniscus being indicative of temperature.
The water content may be read directly from the swelling of the hydrogel over an indicator means in the form of a scale, or the swelling can be used indirectly to drive an indicator means such as a pointer. Swelling of the hydrogel can be arranged to operate a switch or other indicating means when a predetermined water content is reached (indicating the need to add water). In one preferred embodiment the hydrogel is arranged to push against an indicator means in the form of a resiliently biased lever such as to move a pointer across the scale. In another embodiment, the hydrogel is arranged to operate in tension and to pull against a resiliently biased lever and move a pointer.
In a particular embodiment swelling of the hydrogel operates a visual indicator means such as a flag which carries a picture (such as a watering can) which indicates the need to add water or not to add water. A flag indicating the need to add water comes into view when water is required, and a flag indicating that no more water is needed comes into view when the medium contains ample water. Many other such indirect indicating means will be known to the skilled man.
In another embodiment, a strip of hydrogel is laminated to another material of lesser or zero water-swellability (in the manner of a bimetallic strip thermostat) such that as the hydrogel swells it causes the laminate to bend over. The degree of bending indicates the water content.
The scale form of indicator means is analogous to the scale of a conventional thermometer and may be marked with bands, zones or graduations (which may be coloured) indicative of water content. Preferably, the scale is printed or is integrally formed from a thermoplastic material, usually by injection moulding or other known techniques.
Usually the hydrogel will be chosen to swell in a proportional manner (e.g. linearly) relative to water content, in which case the scale will be a linear scale. If the hydrogel swells in a non-linear manner then the scale will be suitably chosen to provide the correct water content reading.
Means are provided for fixing the hydrogel body relative to the scale or other indicator means. Usually, this involves fixing one end of a hydrogel column such that a free end of the column is free to move as the hydrogel swells. For protection the hydrogel column may be enclosed within a tube or located within
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Abstract--SU 966,571, 15 Oct. 1982, for "Soil humidity sensor--has cavity filled with swelling material, monitoring moisture-content of soil".
Abstracts--U.S. Patent 4,511,477, Issued Apr. 16, 1985; and EPO-217791, Issued Mar. 7, 1990--"Absorption of acidic aq. media--by use of a water-swellable polymer contg. alkali metal sulphonate groups".
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Abstract--SU-243090, dated Feb. 2, 1981--"Soil Humidity Sensor".
Graham Neil Bonnett
Martin Charles
Brock Michael
University of Strathclyde
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