EP0547122A1 - Irrigation sensor device - Google Patents

Abstract

A plant-like irrigation sensor device (55) which functions to indicate the dynamic irrigation state prevalent in plants growing around the device, includes a root-like base portion (23) and attracting water, and visual indicating means for indicating the amount of water entering the base portion from the earth in its immediate environment.

Description

IRRIGATION SENSOR DEVICE

The present invention relates to an irrigation sensor device.

Efficient irrigation-scheduling strategies have to rely on reliable estimates of the optimum water requirements of a growing crop. Current practice relies on indirect methods for assessing this requirement and these are based on static estimates of the available water in the root zone.

The best indicators of the suitability of the soil moisture regime in a given field to a particular crop are the plants in that community. In theory, it should be possible for plant- derived signals to be usefully employed in providing the input into computer-controlled irrigation-scheduling systems. However, developing satisfactory instruments to sense the required information off living plants has proved difficult and it is an object of the present invention to provide an alternative system. In its broadest aspect, the present invention comprises a plant analogue Irrigation sensor device which Is operative in a similar manner to a living plant to give an indication of water status, e.g. the water content of the soil in the neighbourhood of the device or the dynamic water status likely to be prevalent in plants growing in the neighbourhood of the device, i.e. the flow rate of water being taken in by these plants from the surrounding soi 1.

Conveniently, the device includes a water-attracting base portion and indicator means operative to indicate the amount of water entering the base portion of the device from soil in the immediate vicinity of the base portion.

Conveniently, the base portion comprises a water-attractive material e.g. a sugar or like solution, housed in a water-permeable container e.g. formed of a semi-permeable membrane.

Conveniently, the device includes control means for preserving the water-attracting properties of the base portion during use of the device. Where the base portion includes a sugar or like solution, the control means conveniently comprises a pump for extracting over- diluted solution from the root portion, passing the extracted solution through a supply (e.g. crystals) of the dissolved material to increase the concentration of the solution, and thereafter returning the solution to the root portion.

Conveniently, the indicator means comprises a flow eter operated by the flow of liquid from the sensor device to indicate the dynamic water status likely to be prevalent in plants growing in the vicinity of the device.

Conveniently, the flowmeter comprises a peristaltic motor.

Conveniently, in this case, the peristaltic motor comprises a roller member engaging a small-bore soft PVC lay-flat tube, the roller member being rotated in operation of the motor as the roller-induced depression in the tube is moved along the tube by a flow of liquid therethrough.

As an alternative to the peristaltic motor, the flowmeter may instead comprise a passageway, an electric contact which in operation is moved from one end of the passageway to the other by a flow of liquid through the passageway, and a control system which is operated by the contact on its arrival at either end of the passageway to reverse the flow of liquid through the passageway and thereby move the contact back in the opposite direction. Conveniently, in this case, the passageway is provided by a fine bore tube and the contact comprises a mercury pellet moving along the bore of the tube.

As a further alternative, the flowmeter may comprise a cylinder, a piston which is moved along the cylinder by a flow of liquid through the cylinder, and a control system which is operated by the piston on completion of its displacement in either direction to reverse the flow of liquid through the cylinder and thereby move the piston back in the opposite direction. Conveniently, in this case, the piston comprises a composite piston presenting to the liquid within the cylinder, piston heads of different area to one another.

Conveniently, in one such arrangement, the piston comprises a three-headed composite piston, the areas presented to the liquid in the cylinder by the two end heads being equal to one another but less than the areas presented by the central head thereby to enhance the displacement of the piston in the cylinder 1n response to a given flow of liquid therethrough. Conveniently, in this arrangement, the passage of liquid past the piston heads is prevented by rolling seals fitted between these heads and the inner wall of the cylinder.

Conveniently, instead of comprising a flowmeter, the indicator means may instead comprise a pressure gauge responsive to the pressure within the sensor to indicate the water content of the soil in the neighbourhood of the device.

Embodiments of the invention will now be described, by way of example only, with reference to Figures 1 to 6 of the accompanying drawings in which: Figure 1 1s a vastly simplified schematic vertical section of the base portion of a typical plant;

Figure 2 1s a diagrammatic vertical section of a plant analogue irrigation sensor device in accordance with the present invention; Figures 3a, 3b and 3c, collectively referred to as Figure 3, are, respectively, a diagrammatic vertical section of a preferred version of the same device, a cross-section of the root analogue component used, and a schematic vertical section of a modification to this version; Figure 4 shows an exploded perspective view of various parts of the device shown in Figure 3;

Figure 5 is a perspective view of a practical embodiment of the device; Figures 6a and 6b, collectively referred to as Figure 6, diagrammatically illustrate, at two different stages of operation, an alternative design of flowmeter to the peristaltic pumps used in the arrangements of Figures 3, 4 and 5; Figure 7 shows, in exploded view, a practical embodiment of the Figure 6 design;

Figures 8a and 8b, collectively referred to as Figure 8, diagrammatically Illustrate a further design of flowmeter and its principle of operation; and Figure 9 is a part elevation, part vertical section of a static version of the device for use as a moisture meter.

The manner in which a plant responds to the presence of water in the soil surrounding the plant, can best be understood with reference to Figure 1 in which the water-absorbing cells 1 in the root cortex of a typical plant 2 have large vacuoles filled with a fluid rich in sugars. Water in the surrounding soil 3 is moved by osmosis into these cells, particularly in a region close to the root tip 4. Water can also be channelled into the xylem vessel 5 which transports the water ("W") to the leaves 6 where part of 1t evaporates into the atmosphere through the adjustable apertures of stomata 7 in the leaf. The sugars "s" synthesised in the leaf cells 8 are transported via the phloem vessels 9 back to the root where they are utilised to re-concentrate the fluid in the vacuoles in the root cells 1. Without this osmoregulation system, the root cells 1 would otherwise lose their osmotic potential as a result of dilution caused by the inflow of soil water. Zig-zag Hne 10 represents radiant energy from the sun received by the leaf cells 8.

Turning now to Figure 2 of the drawings, this shows a plant analogue irrigation sensor device 12 which is designed in accordance with the teachings of the present invention so as to provide a rough mechanical equivalent of those parts of the plant's structure that enable it to effect, water transference from the soil, through the plant's xylem vessels, into the atmosphere. Thus in the device of Figure 2, the water-absorbing cells 1 of the plant's root cortex are replaced by a cylinder of semi-permeable membrane 13 containing a solution 14 of sugar. The entire cylinder is buried in the soil 15. The osmotic influx of water "w" from the moist soil into the cylinder 13 flows out of the root via the tube 16 which in the device of Figure 2 replaces the xylem vessels 5 of the plant 2 in Figure 1.

As the influx of water into the root analogue cylinder 13 would, in time, dilute its contents and osmotic flow would cease, there is a need to compensate for this effect and maintain the osmotic absorption capability of the physical model (as in the case of the real plant) by re-concentrating (or "osmoregulating") the cylinder sugar concentration. This is achieved in the Illustrated embodiment, by the pump 17 which extracts some of the diluted fluid in the xylem vessel analogue tube 16 and passes it over sugar crystals 18 contained in a sealed compartment 19. Concentrated sugar solution "s" now returns via a phloem vessel analogue tube 20 back into the root analogue cylinder 13.

The mesophyll tissue in the leaf from which water evaporates into the atmosphere in a real plant is modelled in the sensor device 12 by the porous pot 21 which is connected by tube 22 to the xylem vessel analogue tube 16 as shown.

It is to be noted that the phloem analogue circuit composed of items 16,17,18,19,20 and 13 comprises a closed circuit and hence the osmotic influx of water into the root analogue cylinder 13 must pass out of this system through tube 22 to the leaf analogue porous pot 21. As the negative suction potential generated by the evaporation of water from the leaf analogue porous pot 21 is applied directly to the root analogue cylinder 13, this enhances the available potential for the movement of water into the cylinder 13.

It is possible to draw a close analogy between the physical system depicted in Figure 2 with the system operating in a real plant as set out in greatly simplified form in Figure 1. The missing element is the energy input into the system required to synthesise the sugars. In the plant of Figure 1, this energy, which is utilised 1n photosynthesis, comes from the radiant energy of the sun Intercepted by the leaf 6. In the mechanical analogue device 12 of Figure 2, however, this radiant energy is replaced by the energy consumed by the prime mover driving the pump 17 together with the external energy expended in producing the sugar crystals 18.

Turning now to the preferred version of irrigation sensor device shown 1n Figures 3 and 3£, It will be seen that the root analogue component 23 of the device 24 consists of a rigid backing tube 25 which has a pointed end 26 to facilitate installation in the soil. This backing tube is provided with a number of external grooves 27 which hold the sugar solution.

A semi-permeable membrane 28 fits snugly on the backing tube 25 and is sealed thereto by an O-ring fitted at the top end of the device. During placement of device 24 into the soil it may be necessary to provide a protective outer sleeve 29 (Figure 3£>> which can either be left in place (if suitably perforated) or can be withdrawn after positioning the root analogue component 23 in the soil at site.

Osmotic inflow of water from the soil will take place into the grooves 27 and the solution in these is replenished by fresh sugar solution through a phloem analogue tube 30 which forms part of the closed sugar solution pumping circuit by tubes 31, a glandless recirculating pump 32, sugar crystals 33, sealed compartment 34, tube 35 and feed tube 30. As will be clear from Figure 3a, the tube 30 communicates with grooves 27 via radially disposed drillings at the lower end of the rod 25.

As shown, a leaf analogue porous pot 36 is connected through the xylem vessel analogue tube 37 to a water reservoir 38. The flowrate "r" into the root analogue is monitored by a first flowrate measuring device 39 and the corresponding transpiration rate "t" from the leaf analogue is assessed by a second flowrate measuring device 40. The main difference in the fluid transport arrangement in the apparatus shown in Figure 3 and that described with reference to Figure 2 is the fact that in the Figure 3 version, the suction developed by the porous pot is not applied directly to the root analogue component 23. This change is desirable because the high suction pressure developed in the porous pot 36 cannot generally be sustained reliably in the xylem vessel analogue tube 37 without serious risk of local cavltation problems. Even if cavitatlon problems could be overcome in such circumstances, the high suction pressure could generate such large loads on the surface of the semi-permeable membrane 28 that this would lead to practical problems in providing adequate support by the backing tube 25. However bypassing this direct connection in the manner illustrated in Figure 3, does not introduce any problem in that the osmotic potential of the solution within the root analogue component 23 can be increased by altering the strength of the sugar solution to compensate for the loss of this potential.

Turning now to the mode of operation of the system, as illustrated 1n Figure 3, the osmotic inflow rate "r" of soil water into the root analogue component 23 is measured by a first flowmeter 39 while a second flowmeter 40 measures the full potential transpiration rate "t" through the leaf analogue porous pot 36.

The actual transpiration rate of a plant is not necessarily the full potential rate indicated by flowmeter 40, because in a real plant the stomatal opening in the leaf would normally alter the connection to the free atmosphere, the degree of stomatal opening depending on the water status of the plant. This effect can be simulated mechanically in the device of Figure 3 by fitting a suitable adjustable sliding shield (not shown) to mask the evaporation area of the porous pot 36. This shield will then have to be adjusted to match the surface area exposed to the atmosphere by a real plant. However, this mechanical adjustment is difficult to achieve in practice and in the preferred design, the porous pot is left to sense the maximum evapo-transpiration rate and the signal obtained from the flowmeter 40 is suitably weighted to match stomatal adjustment in response to the physiological needs of the plant. The following discussion assumes weighting of the flowmeter 40 signal in this way. Flow eters 39 and 40 are preferably such as to produce a digital or analogue electric signal which is fed to a microprocessor (not shown). This latter 1s programmed to record the two signals and after comparing them with each other and/or with certain preselected reference values, to output an appropriate signal to a visual indicator (not shown).

Thus, the information provided by the flowmeters 39 and 40 can be utilised so that the onset of moisture stress in the system is signalled by the visual indicator when the output from flowmeter 39 falls below a certain pre-set value (calibrated against the crop being monitored and the soil type in which it is grown).

Similarly, if at any given moment, the flowrate indicated by the transpiration rate flowmeter 40 exceeds that of the root flowmeter 39, then the visual indicator signals a danger of temporary wilting of the crop.

Lastly, if adverse flowrate indications from the root flowmeter 39 are coupled with signals indicating an excessive differential flowrate between the root flow and transpiration flow, then the microprocessor will cause the visual indicator to signal danger of crop failure.

Although it is evident that this type of decision-making is most conveniently carried out by a microprocessor dealing with digital electrical signals as above described, a mechanical system of flowrate measurements can be used instead, if desired. Thus in one alternative embodiment (not shown), for example, flowrate is estimated by measuring the speed of movement of a liquid meniscus in a fine capillary tube and the advance of the meniscus is timed between opto-electronic sensors set at fixed intervals along the tube. Although this method proves to be accurate and reliable, it suffers from the relative disadvantage that the capillary tube has to be reset at regular intervals and it is thus not suitable for a field instrument which should be able to operate without frequent operator intervention.

Returning now to the illustrated embodiment, a peristaltic motor (the inverse of a peristaltic pump) is used for each of flowmeters 39,40, the rotor speed of the motor being directly proportional to the rate of fluid flow through it. The angular position of the rotor is sensed electronically and the resulting signal sent to an appropriate microprocessor as above described. Figure 4 shows the essential elements of one such arrangement 1n which the two (four-lobed) peristaltic motors (41,42) are sited between the phloem pump (43) arm the root analogue component 23 as shown.

Referring first to the phloem pump 43, this is designed as a centrifugal pump with backwardly curved clockwise-rotating blades in the impeller 44 to give appropriate pump characteristics. In addition, it features a glandless electric motor impeller drive 45 using a rotating magnet. This ensures that the phloem circuit will remain sealed at all times. Reference numeral 46 indicates a casing for impeller 44.

Turning now to the upper peristaltic motor 42, this comprises the usual roller member 48 which is rotated anti-clockwise as the roller-induced depression 1n the peristaltic tube 49 Is moved along the tube by a flow of fluid from the root flow inlet tube 50 through to the corresponding outlet tube 51.

The problem of excessive back pressure 1s overcome by the use of a suitable flexible plastics tube for item 49, e.g. a small¬ bore soft PVC lay-flat tube.

The lower peristaltic motor 41 is of similar construction to motor 42 and once again, the roller member is rotated anti¬ clockwise by a flow of fluid from an inlet tube 52 to an outlet tube 53 (leading to the leaf analogue porous pot 36).

The geometric lay-out of parts illustrated in Figure 4 is suitable for miniaturisation and the phloem pump together with the two peristaltic motors can be readily mounted in a compact case. The resulting device is shown in Figure 5 which illustrates a production version of the irrigation sensor 55 which, it is envisaged, could be mass-produced in plastics at fairly low unit cost. Thus referring now to Figure 5, reference numerals 23 and 35 respectively indicate the root analogue component and the leaf analogue porous pot (as before). Reference numeral 57 indicates a casing containing the phloem pump 43, the root flow peristaltic motor 42 and the shoot flow peristaltic motor 41. The casing 57 is also provided with handles 58,59, a computer output socket 60, for the microprocessor (not shown), and a solution level sight gauge 61 for a combined water reservoir and sugar container 62. The arrangement is completed by a differential flow visual indicator 63. In the sensor described above with reference to Figures 2 to 5, the osmotic movement of water from the soil is induced by a sugar solution placed inside the semi-permeable cylinder. Sugar 1s not the only possibility, however, for use as the osmotic solution and any other chemical that can form a suitable solution in water as the solvent will do instead e.g. common salt.

The continuous through-flow peristaltic motor flowmeter of Figures 3 and 4 can be replaced by a reciprocating system. Two possible versions of this alternative type of flowmeter will be described. The basic principle of the first type and its mode of operation are diagrammatically illustrated in Figures 6a and 6b where flowmeter 70 comprises a precision microbore tube 71 containing a mercury pellet 72. In operation, fluid displaced by the root analogue sensor (not shown) enters one side of the tube 73 (Figure 6a) and displaces the mercury pellet to the other end of the tube (marked 74). When the pellet reaches end 74 of the tube 71, the flow is reversed by transferring the fluid output from the sensor from tube end 73 to end 74 and the pellet 72 then reverses its former direction of travel and traverses to the tube end 73 (Figure 6b). The fluid in the unshaded section of tube 71 is exhausted from the tube. The above cycle is repeated so long as there is a low pressure supply of fluid from the sensor. As the distance of the motion of the mercury pellet is known for each traverse, the volume per 'stroke' can be calculated from a knowledge of the bore of the tube and hence if the time taken for the mercury pellet to reciprocate between the ends of the tube is measured, the flowrate can be estimated. The range and resolution of flowrates can be selected by choosing the appropriate length and bore of the tube. An extremely fine bore tube, e.g. one of 1 mm Internal diameter will give a very high resolution at very low flowrates.

An electro-hydraulic system for accomplishing the change in direction of the mercury pellet is also shown in Figure 6ι. Thus the tube 71 Is shown furnished with two sets of electrical contacts 75 and 76. When the mercury pellet bridges the contacts an electrical circuit is completed.

The closing of contact set 76 actuates the relay coil 77 which changes the position of the two-way switch 78. This switch activates the solenoid 79 which is mechanically linked to a two-way change-over valve 80 which alters position as town in the figure. The fluid entering the metering system from the sensor via the inlet 81, which in the previous position was fed to tube end 73, 1s now transferred to end 74. At the same time, the tube end 73 is connected to an exhaust line 82. When the mercury pellet reaches the contacts 75, a first solenoid 83 1s actuated and the switch 78 transfers power to a second solenoid 84 and the flow is reversed to the initial state. This process continues with the pellet traversing from tube end 74 to tube end 74 and back. A practical detail of the system is that the solenoid system 85 (incorporating coils 77 and 83 and the two-way switch 78) is essentially a latching two-way relay. Thus when either of the two contacts 75 or 76 is closed, this "relay" changes polarity but when that contact opens as the pellet moves away after fluid reversal, the switch position of switch 78 is maintained until the contact at the other end of tube 71 is again activated.

In order to measure the flowrate with this reciprocating arrangement, it is only necessary to count the number of times the switch 78 actuates over a given period. Each resetting of the latching relay contacts in switch 78 implies that a fixed volume of fluid, equal to that contained in the bore of the tube between contacts 75 and 76, has passed through the system.

As discussed earlier, the operator is free to select both the length between contacts 75 and 76 and the bore of the tube 71. This provides a very flexible system for deciding the degree of resolution of the flowrate measurement required. In practice a series of calibrated tubes of differing bore volume can be produced for substitution in the measuring unit in place of tube 71.

One practical layout of the mercury bead reciprocating flowmeter is shown in the exploded view of Figure 7 where it is Identified by reference numeral 90.

As shown, flowmeter 90 includes a metering tube 91 which is sealed into two manifold blocks 92 and 93, into which are plumbed two electrically actuated two-way miniature hydraulic valves 94 and 95. These miniature valves may conveniently be the commercially produced valves which are in regular use in the United Kingdom in applications such as ink-jet printers (manufactured by Lee Co., for example). These valves represent the units labelled V-_| and V₂ 1n Figure 6.

In operation, fluid from the sensor (not shown) enters the metering system through tube 96 and is exhausted out of tube 97 (respectively labelled 81 and 82 in Figure 6b). The sub-manifold 98 simply contains passages which transfer fluid from one end of the metering tube to the other and it also acts as the base to which the other components are fixed.

In Figure 7, the mercury pellet contacts on the metering tube 91 have been omitted for clarity. The trap wells 99 and 100 are designed to catch any mercury that escapes from the metering tube and vent the system to exhaust. To vary the metering quantity per stroke, the tube 91 can be readily changed after first removing the manifold 93.

The second type of reciprocating flowmeter to be described, is flowmeter 109 in Figures 8a and 8b. In this case, the mercury pellet 72 of the previously described version is replaced by a mechanical metering piston 110 using rolling diaphragm seals.

This latter comprises three piston sections 111, 112 and 113 each of which 1s fitted with a rolling diaphragm seal 114. The three piston sections are joined together by a rod 115. The middle piston section 111 divides the metering tube into two chambers A and B, and ports.116 and 117 respectively connect with these two chambers. It will be seen that piston sections 112 and 113 are of equal size (sectional area A₂) but are smaller than piston section 111, which has a greater sectional area of A-_| .

The principle of operation is shown in Figure 7b. Suppose, for example, that fluid from the sensor (not shown) enters chamber A via port 116 and fluid leaves chamber B by port 117.

The pressure rise in chamber A will displace the composite piston 110 to the right by a distance L. This movement takes place because the area of piston section 111 is greater than that of piston section 112 and hence there is a net thrust to the right (assuming chamber B is open to the atmosphere). The volumetric changes taking place as a result of the piston movement L are respectively L (A₁-A₂) for chamber A and -L(A₁-A₂) for chamber B. Together, these two expressions show that the actual volume change taking place in chambers A and B for a given stroke L depends on the area differential (A-_j-A₂) and this quantity can advantageously be made very small by choosing A₂ to be nearly equal to A₁ , enabling the resolution of the metering mechanism to be reduced to fine proportions.

The differential area arrangement is also necessitated at present by practical considerations which limit the minimum size of commercially available rolling diaphragm seals. Thus, unlike the mercury bead version, it is currently not possible to make the bore of the tube smaller than about 7 mm and the configuration of the diaphragm limits the stroke L to about 5mm. The differential area arrangement described above thus gives a very much wider latitude than would otherwise be possible, in the selection of piston stroke and the volume of fluid metered per stroke.

Reference numeral 118 indicates a microswitch to which the composite piston rod is mechanically connected, e.g. to operate an identical electro-hydraulic system (not shown) to that illustrated in Figure 6a.

In both versions of the reciprocating flowmeter described above, the metering mechanism provides an electrical signal when a carefully measured quantity of fluid passes through the system. Thus each pulse is proportional to a known volume of fluid passing through the mechanism. To estimate the flowrate it is only necessary to count the number of pulses in a given period. The flowrate is simply proportional to the number of pulses per unit time.

In a separate system developed to do this counting and timing, this is accomplished by using a quartz controlled clock and a totalising counter. It is envisaged, however, that this relatively crude version could be easily developed to process the pulses through a programmed microprocessor, if desired.

Another important component of the sensor is the glandless re-circulating pump 32. This centrifugal unit with backwardly-curved blades works well and no great change is envisaged. It is therefore a simple matter to develop this unit in a miniaturised form.

Advantages of at least preferred embodiments of the present invention are that: (1) the proposed sensor is capable of sensing the dynamic movement of water in the root zone of a growing crop. If the sensor is unable to abstract water from the soil in a given field, then plants in its neighbourhood too will be unable to do so; (2) the sensor has the additional property of being able to sense the onset of temporary wilt conditions and any other pre-determined moisture regimes which are damaging to the plants; (3) the sensor is readily adapted to supply electrical signals which can be readily handled by a microprocessor or computer; (4) the proportions of the soil probe, acting as the root analogue, can be adjusted to simulate the rooting pattern of a given crop; (5) the sensor can be calibrated to simulate a large number of crop types and their special needs; (6) the sensor automatically takes account of the presence of dissolved salts in the soil matrix. These tend to reduce the osmotic potential and thus the sensor takes account of reduced intake rates Induced by this effect; (7) the transport of photosynthates can be adjusted to match diurnal cycles. This facility will Improve accuracy but may not prove to be crucial to the operation of the sensor; (8) an electronic method could be developed to match the workings of the stomatal aperture of the simulated plant. If this calibration is made sufficiently accurate, this represents a significant advance on any sensor currently available; and (9) the response of the sensor to soil temperature matches the actual behaviour of water uptake in a real plant insofar as increase in temperature improves osmotic inflow as 1s the case in a real plant.

Although the illustrated embodiments of the irrigation sensor described so far with reference to Figures 1 to 8 have been designed to measure the dynamic water movement towards an artificial root, another very important (static) application of the invention 1s Its use in assessing in-situ soil moisture content in the field. Known methods used for this fall into two main categories: (a) methods using absorbent blocks measuring moisture tension and electrical conductivity; and (b) measurement of related properties of soil such as neutron scattering. Neither of these methods are presently assessed to be particularly accurate nor convenient to use. There is therefore a distinct advantage in developing a direct reading system which can monitor moisture contents at depths in a wide range of soil types and with this in mind, the outlet from the root analogue sensor of the present invention can be closed off so that the entry of water from the soil under the osmotic gradient will increase the pressure inside the sensor. When this pressure reaches the point where it just balances the osmotic potential, flow will cease. Thus the pressure in the system is an indirect Indication of the moisture content of the soil at the sensor. If, say the moisture content were to drop, then the sensor will have a higher osmotic potential and water will flow out into the soil and the pressure in the sensor will drop. Thus the pressure 1n the sensor can be directly calibrated to represent the water content in the soil. This is essentially a static mode of operation of the sensor.

In the irrigation sensor of the present invention described with reference to Figures 1 to 8, the pressure in the sugar solution is kept at near atmospheric. Thus no special precautions are necessary to support the semi-permeable membrane against the pressure within the root sensor chamber. However, in the moisture meter version just referred to, the semi-permeable membrane will be subject to very high internal pressure and as currently commercially available semi-permeable membranes cannot withstand such pressures, they have to be provided with some form of mechanical support.

A practical static version of the sensor, operating in the moisture meter mode, is shown by way of example in Figure 9. The device (120) Includes a semi-permeable membrane 121 which is supported on a coarse porous plate or disc 122. The porous disc can be a ceramic filter type or made of sintered metal. As its pores are several magnitudes larger then the pores in the semi-permeable membrane it does not act as a barrier to the flow of water from the soil to the semi-permeable membrane and will not itself act as a semi-permeable barrier.

The semi-permeable membrane encloses a chamber 123, which is full of a solution of either sugar or any other salt dissolved in water or any other fluid having an osmotic potential below that of pure water. The chamber is connected by pressure tubing 124 to a pressure gauge 125. When the chamber 123 1s placed in a moist soil, water will enter the porous support 122 and move osmotically into the solution in the chamber 123 through the semi-permeable membrane 121. As items 123, 124 and 125 together constitute a closed system, the pressure in the system will rise as a result of this Influx of water and this pressure 1s measured by the gauge 125. As discussed earlier, this pressure gauge can be calibrated directly in terms of the moisture content of the soil with which 122 and 123 are in contact. The high pressure inside chamber 123 cannot damage the pliable membrane 121 because it is supported on the rigid porous plate 122.

Claims

1. A plant analogue irrigation sensor device which is operative in a similar fashion to a living plant to give an indication of water status.

2. A device as claimed in Claim 1 including a water-attracting base portion and indicator means operative to indicate the amount of water entering a base portion of the device from soil in the immediate vicinity of the base portion.

3. A device as claimed in Claim 1 or Claim 2 in which the base portion comprises a water-attractive material housed in a water-permeable container.

4. A device as claimed 1n Claim 3 1n which the water-attractive material is a sugar or like solution.

5. A device as claimed in Claim 3 or Claim 4 in which the container is formed of a semi-permeable membrane.

6. A device as claimed in any preceding claim including control means for preserving the water-attracting properties of the base portion during use of the device.

7. A device as claimed in any preceding claim when including the limitations of Claim 4 in which the control means comprises a pump for extracting over-diluted solution from the root portion, passing the extracted solution through a supply of the material to increase the concentration of the solution and thereafter returning the solution to the root portion.

8. A device as claimed in any preceding claim when including the limitations of Claim 2, in which the indicator means comprises a flowmeter operated by the flow of liquid from the sensor device to indicate the dynamic water status likely to be prevalent in plants growing in the vicinity of the device.

9. A device as claimed in Claim 8 in which the flowmeter comprises a peristaltic motor.

10. A device as claimed in Claim 9 in which the peristaltic motor comprises a roller member engaging a small-bore soft PVC lay-flat tube, the roller member being rotated in operation of the motor as the roller-induced depression in the tube is moved along the tube by a flow of liquid therethrough.

11. A device as claimed in Claim 8 in which the flowmeter comprises a passageway, an electric contact which in operation is moved from one end of the passageway to the other by a flow of liquid through the passageway, and a control system which is operated by the contact on its arrival at either end of the passageway to reverse the flow of liquid through the passageway and thereby move the contact back in the opposite direction.

12. A device as claimed in Claim 11 in which the passageway 1s provided by a fine bore tube and the contact comprises a mercury pellet moving along the bore of the tube.

13. A device as claimed in Claim 8 in which the flowmeter comprises a cylinder, a piston which is moved along the cylinder by a flow of liquid through the cylinder, and a control system which is operated by the piston on completion of its displacement in either direction to reverse the flow of liquid through the cylinder and thereby move the piston back in the opposite direction.

14. A device as claimed in Claim 13 in which the piston comprises a composite piston presenting to the liquid within the cylinder, piston heads of different area to one another.

15. A device as claimed in Claim 14 in which the piston comprises a three-headed composite piston, the areas presented to the liquid in the cylinder by the two end heads being equal to one another but less than the areas presented by the central head thereby to enhance the displacement of the piston in the cylinder in response to a given flow of liquid therethrough.

16. A device as claimed in Claim 15 in which the passage of liquid past the piston heads is prevented by rolling seals fitted between these heads and the inner wall of the cylinder.

17. A device as claimed in any of Claims 1 to 7 when including the limitations of Claim 2, in which the indicator means comprises a pressure gauge responsive to the pressure within the sensor to indicate the water content of. the soil in the neighbourhood of the device.

18. A device substantially as hereinbefore described with reference to, and/or as illustrated, in Figure 2 or Figure 3 or Figure 4 or Figure 5 or Figure 6 or Figure 7 or Figure 8 or Figure 9 of the accompanying drawings.