US3296524A

US3296524A - Capacitance probe for particulate materials having a pair of plates connected in the manner of a keel

Info

Publication number: US3296524A
Application number: US329049A
Authority: US
Inventors: Loy A Updegraff
Original assignee: Bituminous Coal Research Inc
Current assignee: Bituminous Coal Research Inc
Priority date: 1963-12-09
Filing date: 1963-12-09
Publication date: 1967-01-03
Anticipated expiration: 1984-01-03

Description

Jan. 3, 1967 Y L. A. UPDEGRAFF 3,296,524

CAPACITANCE PROBE FOR PARTICULATE MATERIALS HAVING A PAIR OF PLATES CONNECTED IN THE MANNER OF A KEEL Flled Dec. 9, 1963 28heets-Sheet 1 55) 57 Measuring RR R. F. Reference Circuit Detector l Detector Circuit 75 76 77 RF. 05c. Phase Power gf ggg Amphfler Detector Amplifier 51 INVENTOR lo Loy A. Updegroff WW. I w w 5| 4e Ljji A WW,

Jan. 3, 1967 L. A. UPDEGRAFF 3,296,524

CAPACITANCE PROBE FOR PARTICULATE MATERIALS HAVING A PAIR OF PLATES CONNECTED IN THE MANNER OF A KEEL Filed Dec. 9, 1963 I 2 Sheets-Sheet 2 i 21 1 26 3*? .1 3 9 v 33 I I I u a n a n a i n XNVENTOR Loy A. Updegruff United States Patent 3,296,524 I CAPACITANCE PROBE FQR PARTICULATE MA- TERIALS HAVING A PAIR OF PLATES CON- NETED IN THE MANNER OF A KEEL Loy A. Updegraff, Worthington, (Ohio, assignor t0 Bituminous Coal Research, Inc, Monroeville, Pa., a corporation of Delaware Filed Dec. 9, 1963, Ser. No. 329,049 7 Claims. (Ql. 324-61) This invention relates to a combinative capacitance probe for use in a capacitance measuring system to indicate, for example, moisture content of smaller size coal consists. More particularly, this invention pertains to a unitary probe comprising a capacitor with its plates in fixed perpendicular relation such that one plate rides on the surface of the material While the other is buried in such material in keel-like fashion for continuous indication of the measured ingredient.

The practice of washing'coal has become very prevalent and has led tothe common use of thermal dryers to produce washed coal with a moisture content within a selected range. With many bituminous coals, that goal may be more difficult to achieve inasmuch as, in the mining and sizing of bituminous coals particularly, there is a tendency for a considerable quantity of fines and finer ma terial to be produced which will pass through a screen with a one-quarter inch mesh therein. Thus, while larger sizes of coal, say, one-inch and up sizes, present no particular problem in respect to the control of moisture therein, smaller and finer sizes present in most bituminous coal consists do have a moisture control problem. Such smaller and finer coals screening three-quarters of one inch by zero and finer size consists are excellent for many fuel purposes, but if received by a purchaser with too much moisture therein, they may be refused, or a penalty may be imposed upon the producer; if too dry, the handling of such finer sizes may give rise to dust and air pollution problems. Processing and handling of coal to be economic is performed as much as possible with bulk material handling and conveying systems.

It is highly desirable therefore that the measurement of moisture content in coals be continuous, sensitive and prompt so that corrective steps, whether by control of the coal dryers or otherwise, may be taken either manually or automatically to keep the moisture content within specifications. Probe devices of this invention are uniquely advantageous for determining dielectric constants of particulate materials including composite materials to indicate the extent of the presence of a substance or ingredient such as moisture in the case of coals. Moreover, such devices have a tendency to uniformly measure the capacity of particulate material being handled by virtue of their weight and action with the horizontal and vertical plates or blades thereof being always in fixed, undisturbed relation to one another with extensive area for representative capacitance indication, uniformity of results relative to the material being measured, and, a construction which inhibits obstruction and interference with their action. Capacitance probes of this invention preferably are utilized with, for example, commercial Dynalog electronic measuring instruments of the capacitance type to provide the requisite indicating, recording, regulating and/or control response desired corresponding to the sensing capacity signal provided by the probe.

Other objects, features and advantages of this invention will be apparent from the following description and the' accompanying drawings, which are illustrative only, in which FIGURE 1 is a view in side elevation of one capacitance probe embodiment of this invention in operatively mounted form on a relatively moving particulate material Patented Jan. 3, 1967 for desired measurement of a particular substance therein having a distinguishable dielectric response, such as moisture in coal;

FIGURE 2 is a plan view of the embodiment shown in FIGURE 1;

FIGURE 3 is a somewhat larger view in side elevation of that combinative capacitance probe embodiment;

FIGURE 4 is a top plan view of the embodiment shown in FIGURE 3;.

FIGURE 5 is a rear end view of the embodiment shown in FIGURE 3;

FIGURE 6 is a bottom plan view of the embodiment shown in FIGURE 3;

FIGURE 7 is a cross sectional detail view, somewhat further enlarged, taken along line VIIVII of FIG- URE 4;

FIGURE 8 is a transverse cross sectional view, with some parts removed, of a modified embodiment of this invention taken along line VIIIVIII of FIGURE 9;

FIGURE 9 is a bottom plan view, somewhat reduced, of a major portion of the bottom of the modification shown in FIGURE 8; and

FIGURE 10 is a schematic view of one form of complete measuring system employing the invention.

Referring to the drawings, FIGURES 1, 2 and 10 illustrate a capacitance measuring system utilizing one embodiment of a capacitance probe 10 of this invention. In the depicted system, a transversely dished rubber-fabric con- ,veyor belt 11 is traveling either horizontally or on an incline in the direction of arrow 12 relative to probe 10. Belt 11 carries thereon a relatively finer size consist of coal 13 having a moisture content which it is desired to have the illustrated system measure continuously. Belt 11 is supported at appropriate intervals along its length and travels along above the ground supported by a conveyor frame having, among other members, an arch comprising two upstanding angles 14 to each side of belt 11 and a cross pipe 15 fixed at its ends to the upper ends of angles 14. An inverted U-shaped bracket 16 is provided with holes 17 in the upper portions of the depending legs of bracket 16 for passage therethrough of pipe 15. Bracket 16 is positioned by collars 18 on pipe 15 above coal 13 and between the frame members 14. The lower portions of each of the depending legs of bracket 16 are provided with transversely aligned pivot pins 19 to which the upper ends of a Y-shaped pull or guide link 20 are respectively pivotally connected. The outer ends of link 20 are in spaced parallel relation to the respective inner sides of the depending legs of bracket 16 so that link 20 may pivot about the axis of the pins 19. The lower end of link 20 is provided with a split clamping block 21 to receive and hold fast to the upper end of a tension or pull rod 22 to tow probe 10 in the sense that as the particulate material 13 travels past, flat portion of probe 10 rides on the flat crown of such material with the blade of the probe submerged therein to sense the capacitance thereof continuously and transmit the sensed signal, whether constant or changing, to the remainder of the capacitance measuring system.

Rod 22 is provided with a cross tube 23 fixed to the lower end thereof with its ends close to a selected pair of laterally aligned holes 24 in the upper portion of two parallel Walls 25 fixed to the opposite sides of probe 10. A small round bar 23a passes through tube 23 and selected holes 24 and is long enough so that its ends project outwardly beyond such holes. The ends of the bar 23a are drilled for cotter pins to hold the bar in place as a pivot between rod 22 and probe 10. Consequently, substantially irrespective of the depth of material 13 on belt 11, or its distance below pipe 15, probe 10 will remain in operative position relative to and downwardly from the top of such material 13 and in alignment with the direction of relative movement between probe and the particulate material 13 being measured.

The capacitance probe 10 comprises an embodiment having a fiat underside plate 26 which may be made of stainless steel with an upturned leading sled end 27. Plate 26 in the illustrated embodiment may act as the ground plate, although preferably not at actual earth potential, in the measuring capacitance system in which probe 10 is to act as the sensing element for variations in the capacity of the particulate material on which probe 10 and the rest of the system are operating at the time being. Front end 27 may be provided with an eye 28 for a spring 29 to be connected between rod 22 and such eye 28. The presence of spring 29 does not hinder any change in the angle between rod 22 and plate 26 if the level of material 13 should change somewhat and thus maintains a predetermined range of pivotal or angular positions between the tow means or rod 22 and the probe 10. Furthermore, the spring 29 keeps front 27 from hanging down about the axis of cross bar 23a when there is little or no material on belt 11, as at the start or finish of a flow of material fill by means of the conveyor of which belt 11 is a part. Plate 26 also may be provided with upstanding counterweight posts 30 threaded at the upper ends for nuts 31 and use without any counterweight, if desired, or with one or more counterweights 32 (FIGURE 3) to insure the desired weight and balance in the assembled and system connected probe 10. Thereby, plate 26 will ride on the top surface of the material 13 with the flat underside of plate 26 in contact therewith and a vertical depending blade 33, acting the other capacitance plate in probe 10, buried for its full height in such material 13. Such top surface in the case of coal is usually somewhat crowned from side-to-side when it is carried on a conveyor belt and the weight of the probe 10 is preferably made sufficient to press the operative portion of the coal immediately therebeneath into relatively uniform bulk density and full engagement with the respective capacitor plates in probe 10.

Horizontal plate 26 is provided with a longitudinally extending opening 34, symmetrical to each side of the longitudinal center line, which extends entirely through plate 26 in a vertical direction; the terms horizontal and vertical as used in connection with probe 10 being relative terms in the sense that the probe may be used either on a level, or in an inclined position, in the course of its operation. Further, the respective horizontal and

vertical capacitor plates

26 and 33 are perpendicular to one another with plate 33 being T-shaped in cross section to provide a head 45 (FIGURES 7 and 8) in the interest of the stability of the assembly. The superstructure of probe 10 to which plate 33 is rigidly affixed, as is plate 26, comprises a box having a heavy metal cover 37 preferably of a non-magnetic stainless steel, the material out of which

plates

26 and 33 are made. Cover 37 is provided with an opening 37a therethrough above which a moisture proof coaxial cable connector 47 is fastened by bolts passing through a flange 47a and a sealing gasket 47b. A coaxial cable 46 is provided and a lead 470 soldered to a terminal 46a on the head of a machine screw 45a. A succession of screws 45a extend down the longitudinal centerline of head 45 to fasten plate 33 to the remainder of the probe assembly. Lead 47c has enough length Within enclosed space 44 inside the box to enable cover 37 to be lifted when countersunk head bolts 40 are removed from their threaded engagement with plate 26. When the screws 40 are tightened in position as shown in FIGURE 7, the space 44 is sealed against the intrusion of any moisture and to insure remaining such, the edges around the heads of the bolts 40 and around the threads thereof where they show on the underside of plate 26 may be hard-soldered, as with a silver braze.

The underside of the box comprises a block 35 of thermosetting insulation of the same area as cover 37,

but made of amaterial such at Bakelite, Micarta or other resin and which may be strengthened with fiberglass fabric layers therein. Block 35 is drilled for the passage of the bolts 40 and fastening screws 45a, washers preferably being used under the head of screws 45a to enable it to hold plate 33 more securely. The sides of space 44 are bounded by a metal ring 36 which is unbroken and rectangular in plan as are

rubber gaskets

38 and 39 of the same size as ring 36, the metal of ring 36 preferably being the same as that of cover 37 and having holes therethrough for the bolts 40. An unbroken ring gasket 35a, rectangular and unbroken in plan, may be provided around the outer underside edge of block 35 between it and plate 26, and a further gasket 35b may be interposed between head 45 of plate 33 and the underside of block 35. The inverted moat comprising the portion of opening 34 not occupied by head 45 is filled with a waterproof epoxy potting compound 43 (FIGURES 6 and 7) which adheres to block 35 and the respective edges of opening 34 and head 45 and hardens in sealing relation to the rest of the .assembly, the underside of the potting compound 43 being finished off flush with the underside of plate 26. From the top of cable connect-or 47, coaxial cable 46 and a flexible shielded conductor connect probe 10 to the remainder of a capacitance measuring system, such as the one illustrated in FIGURE 10.

Vertical plate 33 preferably is provided with a downwardly and rearwardly sloping rounded front or inclined leading edge 48 (FIGURE 6) and the uppermost corner of that front edge extends at least to the bottom surface of plate 26 so that any foreign matter in the material 13 which may pass beneath the sled front 27 will not catch on plate 33 but will tend to be moved downwardly and rearwardly along front edge 48 and pass away from probe 10 beneath bottom edge 49, if it does not earlier slip or pass to one side of plate 33 and from thence away from probe 10. In furtherance of this purpose the bottom edge 49 makes an obtuse angle with the leading edge 48. Further, it will be seen that by such keel-like capacitor construction, probe 10 has

plates

26 and 33 of relatively extensive area in fixed position relative to one another, without detrimental shunting and with a minimal quantity of solid dielectric in the field of the capacitance probe 10 for increased sensitivity. The fixed relation of

plates

26 and 33 also provides a fixed gap therebetween for such field which is but schematically and generally illustrated by trace lines in FIGURE 7. The particulate material,

coal in the instant example, engages all of the underside of probe 10 between the

plates

26 and 33 to result in significant and uniform penetration of the material, a full field effect on the material, and measurement sensing of changes in the dielectric constant of the particulate material in accordance with changes in its moisture content. Irrespective of the depth of particulate material being measured by such a probe of this invention, the sledding thereon of plate 26 promotes full and exact submergence of the vertical plate 33 below head 45 and uniformity of density in the upper portion of the particulate material penetrated by such probe, without retention or troublesome drag respectively in and on such material. Further, because of the weight of probe 10 preferably centered over the keel plate 33, the shape of such plate and the presence of fines in size consists of coal on which such moisture measurements are desirable, void conditions in the particulate material going past the respective capacitor plates are inhibited, thereby contributing to reliable performance.

In the modification of a capacitance probe of this invention shown in FIGURES 8 and 9, parts corresponding generally in structure and function to parts in the first above-described embodiment are provided with the same reference numerals respectively, with the addition of a prime accent thereto. In probe 10', a fused ceramic, generally boat-shaped shield member is used having a vertical slot 71 opening out through the back thereof to accommodate the vertical blade portion of plate 33'. A longitudinally extending recess 72 in the upper portion of member 70 accommodates head 45 of plate 33'. Plate 33' is fastened to the rest of the assembly of probe by screws 45a extending through the insulation sheet 35'. Shield member 70 is provided with a bow portion 73 the center of which is a flush continuation forwardly and upwardly to the underside of plate 26 of the slope of that portion of front edge 48' extending below the bottom edge of member 70. Member 70 may be held in place by waterproof epoxy potting compound, or by direct or adhesive bonding with one or both of the

plates

26, 33, for example to the underside of head 45' and the sides of the vertical blade portion of plate 33' within member 70, in which case the spaces around the edges head 45' and opening 34' are first filled to flush surface relation by such a potting compound. Probe 10 also is used in a capacitance indicating or measuring circuit system such as that illustrated in FIGURE 10. Therein, member 70 tends to promote uniformity and averaging of the capacitance response through a particulate material, such as coal, in that the field lines of force radially outwardly from the intersection between the planes of plate 26' and plate 33 and therefore nearer or within the fringe area of the field tend to be more uniform and provide somewhat more representative response under variations in dielectric values of the material Ibeing measured. To this end the presence of the member 70 has the effect of elongating the average length of the force lines passing through the intervening particulate material.

Calibration of a probe embodiment of this invention is readily performed in respect of any new, or different, operation on particulate material from the same, or a different source. For example, in the case of bituminous coals, those from different seams may have some and/or somewhat different inherent moisture contents but such do not significantly affect capacitance measurements pertaining to surface moisture which latter is of interest in the coal washing and drying control and to the purchaser. At the start of an operation or series of moisture determination operations, in the case of surface moisture in a coal, readings on the recorder of FIGURE 10 are noted at periodical intervals of time to compare with the moisture content of samples taken from the coal at the same times, the moisture content of such samples being ascertainable by any satisfactory moisture testing method, such as A.'S.T.M. Test D-492-48. Thereupon, the correlation between the recorder reading and the sample is noted and provides the calibration for direct reading, either on a scale or other indicator, or on a recording chart, from which the desired correction in the operation, if any, may be made.

The schematic substantially lag-free capacitancesensitive measuring circuit shown in FIGURE 10, in the form of a Dynalog of the capacitance type made by The Foxboro Company, illustrates one mode of utilizing a new capacitance probe of this invention. Therein, the numeral 10 represents the capacitance probe measuring element 10 (or 10') of this invention. It is shown schematically as a variable capacitor whose capacitance varies with the dielectric constant of the material "being measured, a typical example being the variation of the composite dielectric constant readings on coal with variations in moisture content. Measuring element 10 is connected by a suitable coaxial cable 46 to. the measuring head 51 which consists of a housing containing timing and impedance matching circuitry for coupling the measuring element 10 to a transmission line 51a leading to a capacitance measuring instrument 80. Coaxial cable 46 has its center lead, in the illustrated practice, connected to terminal 47 and depending plate electrode 33, while the shield portion of cable 46 is conductively connected to plate electrode 26 which is used as the wire ground for the system, it being preferred that plate 26 not be at the earth potential of frame 1415 unless certain precautions are taken. To insure against the earth potential of such frame becoming that of plate 26, the clamping block 21 is made herein of an insulating material. Thus, the capacitance measuring system consists of a measuring circuit portion 55 containing the capacitance being sensed by probe It) with its associated circuitry and a reference circuit portion 56. A crystal-controlled oscillator 59 actuates both of these circuit portions with a 1.6 megacycle voltage, amplitude-modulated at 60 cycles to cause each to function as a simple voltage divider; that is, the component values of each determines the magnitude of their respective output voltages. Since the component values of the reference circuit portion 56 are fixed, its output voltage is a fixed percentage of its supply voltage. In the measuring circuit portion 55, the only value that changes is the measured capacitance from probe 10 and as it increases, the output voltage of the circuit portion 55 decreases, and vice versa.

The output voltages of the reference and measuring circuit portions respectively pass through identical radio

frequency detector circuits

58 and 57 which eliminate the 1.6 megacycle frequency component. As a result, the outputs of the detector circuits are 60 cycle voltages which are directly proportional to their 1.6 megacycle inputs. Those 60 cycle voltages-a fixed reference voltage and a variable measurement voltagecomprise in effect the respective voltages across the lower two arms 60 of an electrical bridge; the upper two arms consisting of a fixed reference capacitor 61 and a variable capacitor 66.

When the reference and measurement voltages are identical and the two capacitors are equal, the bridge circuit is balanced and there is no output voltage across its terminals. With a change in the measured capacitance, the measurement voltage increases or decreases, unbalancing the bridge circuit and an output voltage results. The magnitude of this output voltage depends on how much the measured capacitance changes; its phase depends on whether the capacitance increases or decreases.

The bridge output voltage passes through the

amplifier subcircuit

63, 75, 76, 77 where it is amplified and its phase is determined. The amplifier output voltage energizes a servomotor 64 to position the rotor of the capacitor 66 through connection 65 in a direction to rebalance the bridge circuit. An instrument pen 52 is linked directly to the servomotor shaft and continuously records on chart 53 in the recorder the value and any changes in the measured capacitance.

When the bridge is rebalanced, i.e., the bridge output voltage is reduced to zero, no positioning power is supplied to the servomotor 64- and the capacitor 66 and pen 52 remains at the new value of the measured capacitance so long as there is no further change in either direction (higher or lower moisture content). In such operation, the capacitance measuring system acts continuously, without significant lag and in correspondence with any variation sensed by probe 10, or 10', in the capacity of the particulate material being measured. Moreover, the amplified output signal from a circuit such as that shown in FIGURE 10 may be used to automatically, or semiautomatically, regulate or control a device, such as a coal dryer, to keep within the predetermined range within which, for example, the moisture content of the finished coal is to be held. Or, such output signal, whether indicated on a scale or recorded as shown, may be used as a guide for manual control of such dryer or other equipment where the capacitance sensed by a probe of this invention shows any need for correction.

Although the illustrative example set forth above concerns itself with the measurement of moisture in coals, more particularly surface moisture, the instant invention is susceptible of use on other materials, for other purposes, including measurement, for example, of bulk densities yielding, upon change, a capacitance change effect. Further, although the depending

blades

33 and 33 are T- headed in cross section, they may be made entirely flat for assembly in probes of this invention; and more than one horizontal and/ or depending flat plate may be used together as one of the two electrodes, preferably arranged in laterally spaced generally parallel arrangement, insulated from the other plate electrode, for use in practices of this invention.

Various other changes may be made in the illustrated embodiments and details thereof and other embodiments provided, in cooperation with the illustrated and other capacitance indicating circuits, without departure from the spirit of this invention, or the scope of the appended claims.

I claim:

1. A capacitance probe system for particulate materials comprising, in combination, an elongated substan-.

tially flat plate having an upturned leading edge and otherwise shaped for slidably riding upon a surface of a particulate material to be measured, said elongated plate having a longitudinally extending opening therein, a substantially fiat second plate positioned with its upper edge spacedly within said opening and its body depending below said first-named flat plate in perpendicular relation thereto in the manner of a keel for submergence below said particulate material surface, said second plate having an inclined :leading edge, said leading edge extending from the material-engaging surface of said firstnamed flat plate, a shield box overlying said opening and said plates and containing a sealed space therewithin a sheet block of insulating material comprising the bottom of said box overlying said opening and extending between the edges of said first-named flat plate opening and the upper edge of said second fiat plate, the sides and cover of said box principally comprising metal to minimize the amount of solid dielectric within the field of said plates, means for fastening said second plate upper edge to said sheet block to insulate said plates from one another, said opening being filled with a hard waterproof insulating sealing compound between the edges thereof and said upper edge, upstanding walls secured to the upper surface of said elongated plate and extending longitudinally and respectively adjacent the lateral edges thereof, means for pivotally connecting said walls to a holding device at least in the course of relative movement between said probe and said particulate material, and shielded electrical conductor means respectively connecting said plates to a capacitance indicating circuit.

2. A capacitance probe for particulate materials comprising, in combination, a substantially flat first plate shaped for slidably riding upon a surface of a particulate material to be measured, a substantially flat second plate rigidly connected to said first plate in depending, perpendicular relation thereto in the manner of a keel for submergence below said surface of said particulate material when said first plate slidably rides thereon, said second plate being insulated from said first plate and having an inclined leading edge, pivotally mounted tow means pivotally connected to said probe for towing said probe at varing heights of said surface, and means for connecting said probe to a capacitance indicating circuit.

3. A capacitance probe system for particulate materials comprising, in combination, an elongated substantially flat plate having an upturned leading edge and otherwise shaped for slidably riding upon a surface of a particulate material to be measured, said elongated plate having a longitudinally extending opening therein, a substantially flat second plate positioned with its upper edge spacedly within said opening and its body depending below said first-named flat plate in perpendicular relation thereto in the manner of a keel for submergence below said particulate material surface, said second plate having an inclined leading edge, said second plate leading edge extending from the material-engaging surface of said first named flat plate, shield means overying said opening and said plates and including an insulating member disposed adjacent said opening, means for fastening said second plate upper edge to said insulating member for insulating said plates from one another, said opening being filled with a hard waterproof insulating sealing compound between the edges thereof and said upper edge, upstanding walls secured to said elongated plate and extending longitudinally and spacedly along the upper surface of said first-named flat plate, means for pivotally connecting said walls to a holding device at least in the course of relative movement between said probe and said particulate material, and shielded electrical conductor means respectively connecting said plates to a capacitance indicating circuit.

4. A capacitance probe system for particulate materials comprising, in combination, an elongated substantially flat plate having an upturned leading edge and otherwise shaped for slidably riding upon a surface of a particulate material to be measured, said elongated plate having a longitudinally extending opening therein, a substantially flat secondplate positioned with its upper edge spacedly within said opening and its body depending below said first-named fiat plate in perpendicular relation thereto in the manner of a keelto submergence below said particulate material surface, said second plate having an inclined leading edge and a bottom edge comprising an obtuse-angular continuation of said front edge, shield means overlying said opening and said plates and including an insulating member disposed adjacent said opening, means for fastening the upper edge of said second plate to said insulating member for insulating said plates from one another, means for pivotally connecting said probe to a holding device at least in the course of relative movement between said probe and said particulate material, and means for electrically connecting said plates to a capacitance indicating circuit.

5. A capacitance probe system as set forth in claim 2, in which there are resilient means extending between said elongated plate leading edge and said tow means for maintaining a predetermined range of pivotal positions of said probe relative to said holding device.

6. A capacitance probe system as set forth in claim 2, in which upright counterweight post means are mounted on said first-named flat plate for receiving a counterweight to provide a predetermined weight and center of gravity for said probe in connection with its use on a given particulate material.

7. A capacitance pro be system as set forth in claim 2, in which the upper edge portion of said second plate is substantially enveloped by a generally boat-shaped ceramic shield secured to at least one of said plates and disposed adjacent the intersection between the planes of said respective plates, whereby the average lengths of lines of force through the intervening particulate material is elongated.

References Cited by the Examiner UNITED STATES PATENTS 1,318,365 10/1919 Erway l1l-81 2,895,093 7/1959 Kodama 31726l 3,149,650 9/1964 Horst 324-61 WALTER L. CARLSON, Primary Examiner.

W. H. BUCKLER, E. E. KUBASIEWICZ,

Assistant Examiners.

Claims

2. A CAPACITANCE PROBE FOR PARTICULATE MATERIALS COMPRISING, IN COMBINATION, A SUBSTANTIALLY FLAT FIRST PLATE SHAPED FOR SLIDABLY RIDING UPON A SURFACE OF A PARTICULATE MATERIAL TO BE MEASURED, A SUBSTANTIALLY FLAT SECOND PLATE RIGIDLY CONNECTED TO SAID FIRST PLATE IN DEPENDING, PERPENDICULAR RELATION THERETO IN THE MANNER OF A KEEL FOR SUBMERGENCE BELOW SAID SURFACE OF SAID PARTICULATE MATERIAL WHEN SAID FIRST PLATE SLIDABLY RIDES THEREON, SAID SECOND PLATE BEING INSULATED FROM SAID FIRST PLATE AND HAVING AN INCLINED LEADING EDGE, PIVOTALLY MOUNTED TOW MEANS PIVOTALLY CONNECTED TO SAID PROBE FOR TOWING SAID PROBE AT VARYING HEIGHTS OF SAID SURFACE, AND MEANS FOR CONNECTING SAID PROBE TO A CAPACITANCE INDICATING CIRCUIT.