VOLUMETRIC MOISTURE CONTENT SENSOR

An apparatus for measuring volumetric moisture content using an affixing method for sensing probes that reduces bending and breaking stress on the probes. The volumetric moisture content sensor has sensing probes secured against compressive axial movement during insertion into a test media, while allowing limited radial movement. Radial movement is limited via the use of a compressed elastomeric sleeve surrounding the portion of the rod supported by the sensor housing. When in the uncompressed state, there is a clearance fit allowing free movement for removal and replacement of the probes. The compressed elastomeric sleeve is used in conjunction with a rod support mechanism such that the sleeve seals out moisture, dust, and other contaminants from an interior portion of the sensor. The compressed elastomeric sleeve provides a force that assures a proper electrical connection between the sensor probes and an electronic circuit of the sensor.

FIELD OF THE INVENTION

The present invention is in the technical field of moisture content measurement. More particularly, the present invention is in the technical field of volumetric moisture content of soil, growing media, and other homogeneous or nonhomogeneous agricultural products. More particularly, the present invention is in the technical field of a method for attachment of sensing probes to such a volumetric moisture content meter.

BACKGROUND OF THE INVENTION

Water content or moisture content is important to the growth of plants as well as the safe storage of raw and refined agricultural products. There are many techniques to measure the moisture content of a substance. Most generally these can be broken down into gravimetric (mass based) or volumetric measurements. Volumetric measurements have the advantage of accounting for air or pore space in a media like soil, and therefore more readily relate to understanding the quantity of water available within the root zone of the plant. It is desirable to make such volumetric moisture content readings in the field without disturbing the growth of any roots or plants within or near the test area.

Many electronic instruments have been developed to measure volumetric moisture content based on the high ratio of the dielectric constant of water (approx. 80) to that of dry soil and air (approx. 1-3). Such instruments employ circuits that measure this dielectric constant via capacitance, wave propagation through a transmission line, or wave propagation reflected from the end of an open transmission line. In all cases, it is desirable to place the electrodes that make up the transmission line or the plates of the capacitor directly into the media being measured. The use of rods protruding from the sensor head has proven to be a highly effective way to produce such a sensor, which is easily inserted into the test media.

Therefore, it is important to anyone who uses such a device that the rods be abrasion resistant, corrosion resistant, and break resistant. To function correctly, they must also be electrically conductive. Such rods are typically manufactured from stainless steel to meet all these requirements. Since small diameter rods require less force to insert into the media being tested and disturb the media less, they are preferred. The drawback to smaller diameter rods is that they are more prone to bending. The base of the sensor rods is typically attached or fixed to a ridged support on the sensor. As a result, any flexing of the rods imparts a large concentration of bending stress at this attachment point.

Bending therefore occurs at this high stress point and can result in breaking of the rods after repeated bending and straightening cycles as the user attempts to correct distortion of the rods when rocks or other discontinuities on the media cause rod flexing during insertion.

It would therefore be desirable to have a portable apparatus for measuring volumetric moisture content by electronic means requiring protruding rods that are not as prone to bending when objects are struck during use. Embodiments of the present invention provide such a means to limit bending and/or breaking of these sensor rods compared to the current methods employed.

BRIEF SUMMARY OF THE INVENTION

In one aspect, embodiments of the present invention provide a volumetric moisture content sensor that includes a sensor head with a circuit for determining the water content of a media. The circuit is disposed in a sensor housing with a plurality of sensor probes that are removably attached to the sensor housing and protrude outward from the sensor housing. In a particular embodiment, each of the plurality of sensor probes is configured for insertion into the media. Additionally, at least one of the plurality of sensor probes has an elastomeric sleeve arranged to absorb stress caused by movement of a respective sensor probe.

In a particular embodiment, a function of the sensor head is activated via an electronic display, an electronic controller, a smartphone, a smartwatch, or a computer. In a further embodiment, the electronic display is electrically connected to the sensor head through a support pole. In a further embodiment, the support pole includes bonding wires that electrically connect the sensor head to the electronic display. The support pole is connected at its proximal end to the electronic display and its distal end to the sensor head. In an alternative embodiment, the electronic display is attached to the distal end of the support pole. In a further embodiment, a wireless transmitter is configured to transmit data from the volumetric moisture content sensor to the display for one of a smartphone, a smartwatch, an electronic controller, or a computer.

In a particular embodiment, the sensor housing includes an upper sensor housing and a lower sensor housing. Additionally, the upper sensor housing and the lower sensor housing are configured to form a single continuous body.

In a particular embodiment, the circuit is mounted within the upper sensor housing and electrically connected to the plurality of sensor probes, and a removable retaining cap is mounted to the lower sensor housing.

In a particular embodiment, the circuit is mounted to the upper sensor housing using one or more screws. Additionally, each of the one or more screws is configured to provide an electrical connection between one of the plurality of sensor probes and the circuit. In a further embodiment, the circuit is configured to provide a signal that is transmitted through the plurality of sensor probes in the media.

In a particular embodiment, the retaining cap includes a threaded ring and a fixed end plate. The end plate fits into the threaded ring and has holes therein to accommodate the plurality of sensor probes. In some embodiments, a perimeter portion of the end plate has a lip or edge geometry configured to retain the end plate inside the threaded ring. Additionally, the threaded ring rotates relative to the end plate. In a particular embodiment, the threaded ring has an annular convex surface, and the edge geometry is a concave surface that extends around the perimeter portion, and seats around the annular convex surface.

In some embodiment, the volumetric moisture content sensor includes a temperature sensor that protrudes from inside of the sensor housing through the retaining cap and can determine a media temperature.

In a particular embodiment, each of the plurality of sensor probes has a cylindrical shaft with a pointed tip at a first end and a radially-extending head at a second end opposite the first end. The pointed tip is configured for insertion into the media.

In a particular embodiment, the elastomeric sleeve is assembled onto the cylindrical shaft. In a further embodiment, the radially-extending head has a diameter greater than that of the cylindrical shaft to provide a surface for the elastomeric sleeve to push against.

In a particular embodiment, the sensor housing includes a plurality of tubular linings, each disposed within a hole in the lower sensor housing. In a further embodiment, each of the holes is sized to accommodate the radially-extending head of one of the plurality of sensor probes.

In a particular embodiment, each of the plurality of tubular linings comprises Teflon®. However, in alternative embodiments, the plurality of tubular linings may also comprise a low coefficient of friction material.

In a particular embodiment, the elastomeric sleeve is configured to deform when compressed along an axial direction. In a further embodiment, the elastomeric sleeve is capable of absorbing stress from the deflection of one of the plurality of sensor probes in a direction perpendicular to a longitudinal axis of the cylindrical shaft. This arrangement permits the elastomeric sleeve to seal out moisture, dust, and other contaminants from an interior portion of the volumetric moisture content sensor and, more specifically, from an interior portion of the lower sensor housing. In the embodiments shown, the elastomeric sleeve may be a synthetic rubber material or a natural rubber material.

In a particular embodiment, the volumetric water content sensor provides a means to limit and/or prevent bending or breaking of protruding sensor rods that form either a transmission line or capacitor as part of the electronic moisture sensing method. The invention comprises metal sensor rods affixed to the sensor/meter via a material that allows them to flex in any lateral or radial direction without causing permanent deflection of the metal rods or fixing material. With this manner of construction, the rods are allowed to flex when encountering a rock or other anomaly in the test media during their insertion. Once withdrawn, the rods return to their true position as the flexible affixing media returns to its neutral state.

Embodiments of the volumetric water content sensor also use the flexible affixing media to assert an axial force on the sensor rods to assure electrical contact between the rods and the electronic sensing circuit. This feature of the invention assures electrical contact is maintained throughout the flexing and subsequent return to neutral of the sensor rods.

While the invention will be described in conjunction with certain preferred embodiments, there is no intent to limit it to these embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as indicated within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

A volumetric water content sensor100for determining volumetric moisture content, constructed in accordance with an embodiment of the invention, is shown inFIGS.1-3. This preferred embodiment comprises an electronic display portion20, a support pole80, and a moisture sensor portion30, as shown inFIGS.1-3.

A particular embodiment of the volumetric water content sensor100functions in the following manner. The user first activates the electronic display20, which then starts the electronic function of the sensor head30, which in the case of the illustrated embodiment is attached to the display20via support pole80that contains connection wires. The support pole is connected at its proximal end to the electronic display and its distal end to the sensor head. However, in an alternative embodiment, the electronic display is attached to the distal end of the support pole. In a further embodiment, the user may activate the sensor head30via an electronic controller, a smartwatch, a smartphone, or a computer, such as a tablet, laptop, or desktop computer. The user then inserts probe rods40,45into the media being tested, such as soil, growing media, or other moisture containing media, such as agricultural products, for example.

An electronic circuit on circuit board61in sensor head30then detects the volumetric moisture content by sending electronic signals to and/or from probe rods40,45. This moisture dependent sensed signal is then transmitted back to the display portion20via wires inside support pole80. This moisture content information is then displayed to the user directly, and/or stored in the unit's memory, and/or transmitted by some means to an external display or storage device like a computer, an electronic controller, or smartphone, or smartwatch. As used herein, the term “computer” includes tablet computers, laptop computers, desktop computers, and notebook computers.

Alternatively, the embodiment may include a wireless transmitter for transmitting the moisture content information or other data from the volumetric water content sensor100to a display of one of the aforementioned external display devices. As described, the probe rods40,45can be withdrawn from one location in the media and inserted in another location to check moisture at that new location as many times as necessary to characterize the media's volumetric moisture content.

Referring toFIGS.2-3, sensor head30is comprised of sensor housing60, electronic circuit61, probe rods40,45, and retaining cap70. Sensor head30is designed to be mounted on the end of support pole80allowing the user to measure moisture contents of media at ground level and have the display portion20with handles21at a convenient working height for the user. The embodiment shown has circuit board61attached to the upper sensor housing63with mounting screws62. Mounting screws62also function to make electrical connections between sensor electrodes40,45, and electrical pads on circuit board61.

Retaining cap70attaches to lower sensor housing64and retains sensor probes40,45such that they cannot be removed from sensor housing60. Retaining cap70also functions to apply a compressive force to elastomeric sleeve50. When this axial compressive force is applied to elastomeric sleeve50, it expands in a radial direction to fill the inside of cavity66thereby semi rigidly fixing electrodes40,45to lower sensor housing64. Lower sensor housing64is attached to upper sensor housing63via screws67. In this way, lower sensor housing64and upper sensor housing63become one body, shown inFIG.2as sensor housing60. The embodiment shown also has a temperature sensor65that protrudes through retaining cap70for measurement of test media temperature in addition to moisture content measurements.

Referring toFIGS.1-3, sensor probes40,45are identical in the preferred embodiment but could be of varying length or size and could be present in different quantities in other embodiments. In a particular embodiment, electrodes40,45each have a cylindrical shaft76with a pointed tip41at a first end of the cylindrical shaft76, and a radially-extending head42at a second end cylindrical shaft76opposite the first end. Pointed tip41helps to lower the insertion force when the sensor electrodes40,45are inserted into the media being tested for moisture content. The flat head42has a diameter greater than that of cylindrical shaft76and provides a surface for elastomeric sleeve50to push against, therefore restricting sensor probes40,45from being able to be drawn through the inside of said sleeve50.

It is understood that other geometries for the ends or middle of sensor probes40,45could accomplish similar functions in alternate embodiments. In a particular embodiment, sensor probes40,45are constructed of stainless steel, which meets the requirements of good electrical conductivity, good corrosion resistance, good abrasion resistance, and resistance to bending. It is understood that other materials will also meet these requirements and could be substituted. In certain embodiments, sensor probes40,45are common stainless-steel nails used in timber construction industry and therefore available at minimal cost compared to custom fabricated probes of similar geometry.

Again, referring toFIGS.2and3, elastomeric sleeve50is described in more detail. Elastomeric sleeve50can be loose fitting around sensor probes40,45, or loose fitting inside hole66, or loose fitting on both sensor probes40and hole66when in an uncompressed state. Compressing elastomeric sleeve50along its axial direction will cause deformation, which expands it in the radial dimension. In a particular embodiment, elastomeric sleeve50absorbs stress from deflection of sensor probes40,45in a direction perpendicular to a longitudinal axis75of cylindrical shaft76. In a particular embodiment, elastomeric sleeve50is configured to deform when compressed along its axial direction. Typically, elastomeric sleeve50has a length slightly greater length than that of lower sensor housing64, and is therefore compressed when removable retaining cap70is assembled to lower sensor housing64.

In this example, as removable retaining cap70is threaded onto lower sensor housing64, elastomeric sleeve50compresses axially and expands radially around cylindrical shaft76. In this context, “axially” refers to a longitudinal axis of elastomeric sleeve50, which is the same as longitudinal axis75of cylindrical shaft76. Sufficient axial compression and radial expansion results in a firm and tight fit of elastomeric sleeve50on both the outer diameter of sensor probes40,45and inner diameter of hole66. This results in a firm mechanical connection between sensor probes40,45and hole66of the lower sensor housing64, as shown inFIG.2, and protects sensor probes40,45from damage and stress that might otherwise occur due to lateral movement of the probes40,45in directions perpendicular to longitudinal axis75. This configuration enables the elastomeric sleeve50to shield the interior portion of the lower sensor housing64from moisture, dust, and other contaminants, thereby ensuring the accuracy and durability of the volumetric water content sensor100. In the embodiment shown, the sensor probes40,45are essential for measuring moisture content, and must maintain a proper electrical connection to the circuit board61. The elastomeric sleeve50acts as a barrier, preserving the integrity of that electrical connection and protecting the interior portion of the lower sensor housing64from external elements that could cause corrosion, damage, or interference with the aforementioned connection.

In a particular embodiment, elastomeric sleeve50is press-fit onto sensor probes40,45and has a clearance or loose fit into hole66, as shown by clearance gap47inFIG.2. This loose fit into hole66allows for easy insertion and removal of probes40,45from sensor housing60when changing probes40,45is necessary. Probes40,45can be changed to accommodate probes of different lengths or when existing probes become worn or damaged. Elastomeric sleeve50can be constructed of any resilient material, which returns to its original shape after being deformed by displacement. This is typically a synthetic or natural rubber material. In certain embodiments, elastomeric sleeve50is constructed of Viton® or urethane rubber.

With reference toFIG.3, hole66ofFIG.2is described in more detail. As previously mentioned, hole66is meant to receive elastomeric sleeve50when sensor probes40,45is assembled to sensor head30. Elastomeric sleeve50is then axially compressed to form a firm radial connection around sensor probes40,45. During the compression, an extra protruding length46ofFIG.2of elastomeric sleeve50is forced into hole66until flush with the end of sensor housing60, as shown inFIG.3. As this compression occurs elastomeric sleeve50slides along the inner wall of hole66to allow compression of the sleeve50along its entire length.

This axial sliding of elastomeric sleeve50along the wall of hole66is enhanced by reduced friction between the interface of these two sliding materials. It is therefore helpful to have a low coefficient of friction on the surface of one or both of these materials. Coatings or lubricants can accomplish the goal of a low coefficient of friction between these materials. Selecting one of the materials to have a low coefficient of friction will also accomplish this goal. Elastomeric materials typically do not have a low coefficient of friction and materials with low coefficient of friction for the sensor body are typically of higher relative cost and do not provide as high of physical strength. Some preferred embodiments facilitate this low coefficient of friction by selecting a high-strength plastic or metal material for the lower sensor housing64with a tubular lining68, as shown inFIG.3. In certain embodiments, elastomeric sleeve50is press-fit into hole66such that the inside diameter of hole66is virtually the same as the outside diameter of tubular lining68. In a particular embodiment, this tubular lining68is made from of Teflon® or a similar low-coefficient-of-friction material.

Now referring toFIGS.2and3, retaining cap70is described in more detail. Retaining cap70is comprised of threaded ring71and end plate72in the embodiment shown. End plate72includes holes73for sensor probes40,45to pass through. The diameter of holes73is selected to be larger than the outside diameter of sensor probes40,45, so that radial movement of said probes40,45is possible without causing the sensor probes40,45to come into contact with holes73of end plate72. It is understood that a sufficient amount of radial movement of sensor probes40,45will always allow contact between the probes40,45and edges of the holes73.

The design of volumetric water content sensor100, and specifically of sensor housing60is such that, as the radial displacement of sensor probes40,45increases, the deformation of elastomeric sleeve50allows for sufficient movement of sensor probes40,45during insertion into the test media. This design allows for displacement of sensor probes40, due to embedded rocks or obstructions without causing permanent damage or deformation in sensor probes40,45.

In a particular embodiment, threaded ring71screws onto a mating thread on lower sensor housing64. End plate72fits into threaded ring71with enough clearance to allow the threaded ring71to be screwed into place while spinning around end plate72. Preferably, end plate72is configured to remain inside the threaded ring71, allowing the threaded ring71to rotate relative to end plate72. Threaded ring71overlaps a portion of end plate72such that as threaded ring71is screwed onto lower sensor housing64end plate72is forced against elastomeric sleeve50and causes it to compress in the axial direction.

A further feature of end plate72is that, in some embodiments, a perimeter portion of end plate72has a lip or edge geometry74, which retains this plate inside threaded ring71, so that when retaining cap70is removed from lower sensor housing64, end plate72does not separate from threaded ring71, thereby limiting the number of parts that can be misplaced or dropped while the user is changing sensor probes40,45. In a particular embodiment, threaded ring71has an annular convex surface79. End plate72has edge geometry74and concave surface78that extends around the perimeter portion, and seats around the annular convex surface79. It is understood that retaining cap70may be comprised of components, such as threaded ring71and end plate72or more parts, which may be of plastic or metal.

In a particular embodiment, all components of end cap70are plastic with low moisture absorption characteristics. It is further understood the end cap70could be affixed to sensor housing60with latches, screws, ramps, or cams to perform the functions as mentioned above. While a particular embodiment of the invention has been described in detail above, it should also be understood that alternate embodiments of volumetric water content sensor100may have more than two sensor probes40,45. In a further embodiment of the invention, there may be only one sensor probes40or more than two probes40,45, with any number or combination of them affixed in the method described by this invention. It is also recognized that the method of compressing the elastomeric support material could be by other means of fastener, latch, or cam.