US2886868A

US2886868A - End point moisture content control apparatus for sand

Info

Publication number: US2886868A
Authority: US
Inventors: Harry W Dietert; Randolph L Dietert
Original assignee: Harry W Dietert Co
Current assignee: Harry W Dietert Co
Priority date: 1956-10-01
Filing date: 1957-07-29
Publication date: 1959-05-19
Anticipated expiration: 1976-05-19

Description

May 19, 1959 H. W.DIETERT ET AL 2,886,868

END POINT MOISTURE'CONTENT CONTROL APPARATUSFOR SAND Filed July 29, 1957 2 Sheets-Sheet l INVENTORS HARRY W. DIETERT ATTORNEYS y 1959 H. w. DIETERT ETAL 2,886,868

END POINT MOISTURE CONTENT CONTROL APPARATUS FOR SAND Filed July 29, 1957 2 Sheets-Sheet 2 INVENTORS HARRY w. DIETERT B .DIETERT RANDOLPH g: ATTORNEYS United States Patent Ofi ice 2,886,868 Patented May 19, 1959 END POINT MOISTURE CONTENT CONTROL APPARATUS FOR SAND Harry W. Dietert and Randolph L. Dietert, Detroit, Mich., asslgnors to Harry W. Dietert Company, Detroit, Mich., a corporation of Michigan Application July 29, 1957, Serial No. 674,822 6 Claims. (CI. 22-89) The present invention relates to end point moisture content control for sand.

It is an object of the present invention to provide an automatic system for controlling the addition of water to foundry sand during the milling in accordance with the increasing moisture content due to addition of water and preferably also the temperature of the mixture, characterized in that the measurement of moisture content and/or temperature and the transmission of these measurements into the control system is accomplished without any moving parts.

It is a further object of the present invention to provide a system as described in the preceding paragraph in which both moisture content and/or temperature measurements are taken as variable capacitances which are transmitted to a control circuit for a vacuum tube.

It is a further object of the present invention to provide an automatic system for adding water to sand in a mill, mixing the sand and water, sensing the moisture content of the sand, and preventing termination of the addition of water which might result from sensing of a moist sample of sand in the mill while the average moisture content is below that required.

More specifically, it is an object of the present invention to provide a system for adding water to sand in a mill, the system including a moisture sensitive element adapted to measure moisture content of sand at a particular point in the mill, and including means for preventing accidental termination of addition of water which might result from passage of a small volume of moist sand across the moisture sensing element.

Still more specifically, it is an object of the present invention to provide apparatus for sensing moisture content of sand in the mill as water is added thereto, means for continuing the addition of water until the moisture content reaches a predetermined fraction of the required final moisture content, means for reducing the rate of flow of water into the mill until a second higher value of moisture content is sensed, and means for thereafter repeatedly sensing moisture content at intervals to provide for addition of water if any of the subsequent moisture sensing operations results in a sensing of less than required moisture content.

It is a further object of the present invention to provide end point control for sand moisture in a mill including a moisture probe located in a wall of the mill, mixing members in the mill movable over the moisture probe and adapted to compact a specimen of sand against the probe, at scraper movable over the probe following each compacting operation effective to remove compacted sand from the surface of the probe, said probe including a metal button having a surface exposed at a surface of the Wall of the mill, and a surrounding bushing formed of tetrafluoroethylene available under the trade name Teflon.

It is a further object of the present invention to provide a moisture probe comprising a tube terminating at one end in a conical seat, an annular bushing received in said one end of said seat and having an outwardly enlarged internally and externally conical portion engaged in said seat, a probe button formed of metal received in said bushing having an outwardly enlarged conical end portion seated in the internal conical portion of said bushing.

It is a further object of the present invention to provide a probe of the character described in the preceding paragraph including resilient means for urging said probe button firmly against the seat.

Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings, illustrating a preferred embodiment of the invention, wherein:

Figure 1 is a combined schematic view and wiring diagram of the end point moisture content control for sand.

Figure 2 is a longitudinal sectional view through the moisture probe.

Figure 3 is a longitudinal section through the thimble' element of the probe.

Referring now to the drawings, the sand. mill 10 is adapted to receive sand from a plurality of hoppers 12 and water is added to the sand through valve means later to be described, but the addition directlyto the mill is through sprinklers 14 connected to a receptacle 16 which receives water from a discharge pipe 18. The sprinklers 14 are suitably supported as for example by a part 20, for rotation with the rollers 22.

In Figure 1, which is diagrammatic in character, the sprinkler heads are shown as located above the rollers 22 but in practice they are preferably circumferentially spaced therefrom.

The sand mill 10 is of the type including the relatively heavy rollers 22 previously referred to and in addition and rotatable therewith, there is provided a plow 24 having a leading edge spaced only slightly above the bottom wall of the mill and adapted to scrape the sand therefrom after it has been compacted by the rollers 22. In practice the leading edge of the plow 24 may be spaced slightly upwardly from the bottom wall of the mill, as for example approximately A of an inch.

Located in the bottom wall of the mill 10 isa mois ture probe 26, details of which will be presently described. The moisture probe is preferably located directly under the path of the rollers 22 so that as the rollers pass over the moisture probe, they compact the sand against the moisture probe to give a reading the value of which is determined simultaneously by the moisture content of the sand and its state of compression.

Due to the specific nature of the moisture probe, the succeeding passage of the plow 24 thereover is effective to remove all sand previously compacted against the face of the probe. The moisture probe is adapted to sense a capacity which is dependent on the moisture content of the sand.

Means are provided for sensing the temperature of the moist sand in the mill. This means c'omprises'a temperature sensitive ceramic condenser 38 exposed in a wall of the mill to the moist sand. The condenser is available on the open market and no claim to details of this condenser apart from the combination is made here- Opposite sides of the capacitor or condenser 38 are connected to terminals X and Y of the moisture sensitive bridge later to be described.

The moisture probe 26 is connected by lines 50 and 53 to a moisture measuring bridge indicated at 53, which is a measuring instrument identified as Tektor unit #101, manufactured and sold by the Fielding Instrument Divi sion of the Robertshaw-Fulton Control Company. The

line 50 is grounded as indicated at 54, and line 52 is connected through a high capacity capacitor 56 to instrument connector X. Also connected in line 52 is a variable trim capacitor 58 etfective to make the required adjustments in the unit. The variable capacitor 3%, actuated in accordance with the temperature of the mixture in the mill is connected to the instrument connection X and to a second instrument connection Y. The instrument connection X is connected to the grid of a vacuum tube, for example a 6SN7 tube, connected as shown to have its output applied to a relay coil M which actuates switch contact arms M1 and M2. Also connected to the instrument connections are an adjustable first point capacitor 62. and an adjustable end point capacitor 64. The end point capacitor 64 is connected in parallel with a high capacity, as for example 2000 m.m.f., fixed capacitor 66, by normally closed contacts Ria of a relay R7 later to be described.

Described in general terms, the system operates as follows: The operator pushes a button which starts rotation of the mixing apparatus, the sand being dumped in at this time or subsequently. Inasmuch as sand may be dumped in from one of a plurality of hoppers, it is desirable to eifect substantial mixing of the sand before any attempt is made to determine its moisture.

After a predetermined interval of mixing, operation of the moisture control device is initiated. A timer is started which will insure rechecking the moisture content after a predetermined interval even though the instrument may previously have indicated suflicient moisture. This is because of the possibility of a false reading and rechecking after a predetermined interval will insure continued' operation of the instrument until the moisture content is adequate.

During operation of the instrument, the first point valve Va, and the end point valve Vb are opened and water is supplied to the sand while it continues to be mixed. Inasmuch as the mixing operation comprises the passing of the rollers 22 over the moisture probe 26, followed almost immediately by passage of the plow 24, it will be appreciated that even when the sand reaches the required moisture content, this correct moisture content will be indicated only at intervals determined by the passage of the rollers 22 over the probe. After the scraper or plow 24 has passed over the probe, the sensing system will indicate a moisture deficiency until the succeeding sample of moist sand is compressed against the probe.

At this time it is desired to add the water rapidly to bring the moisture content up to approximately but definitely below the desired value. Accordingly, at this time control of the instrument is by the first point capacitor 62 and temperature compensating capacitor 38. Eventually, the instrument senses the proper moisture content for a brief interval and the system is arranged at this time to close the large capacity first point valve Va and to shift control of the instrument to the combination of the three capacitances, the end point capacitance 64-, the temperature compensating capacitance 38, and the modifying capacitance 66. Capacitance 66 is a large value capacity, for example 2000 micro-micro-farads, which is sufficient to stop the 6SN7 tube from oscillating and in turn deenergizes relay M and causeing M2b to make contact at M2c which completes the circuit to contacts R511 to energize relays R7 and R4, thus taking capacitance 66 out of the circuit at contacts R7a for the remaining part of the cycle, leaving the control of the instrument to the sum of the end point capacitance 64 and temperature compensating capacitance 38. Thereafter, water continues to be added to the mill at a reduced rate by the end point valve Vb until the first indication of the ultimate desired moisture is obtained. It is recognized however, that this may be a false indication resulting from sensing the moisture content of a small specimen not indicative of the true average moisture content of the sand. Ac-

cordingly, means are provided at this time to close the end point valve while the mixing of the sand continues. During the following interval moisture readings are taken periodically as the rollers 22 pass over the sand probe. So long as these moisture readings all indicate sufiicient moisture the valve Vb remains closed. A timing means is provided to operate over for example three seconds, which prevents opening of the end point valve during the brief intervals between successive sensing operations. For example, resensing may occur every two seconds and the timer may be set to time out in three seconds. At the end of two seconds if the sensing of moisture indicates suificient moisture, the timer is reset to zero.

Sn the other hand, if during this rechecking interval a moisture sensing operation indicates insufiicient moisture, the moisture valve opens and remains open until a second sensing of adequate moisture. This operation continues for an interval determined by an additional timer which operates to terminate the rechecking operation and to maintain the end point valve Vb closed, thus ending the cycle.

When the system is set in operation, the capacities of the moisture probe 26, the trim capacitor 58, the temperature responsive capacitor 38, and the first point capacitor 62, are all connected to the instrument connections X, Y. When sufiicient moisture has been added to the mill llti to increase the capacity of the moisture probe 26 to a predetermined value, the 6SN7 tube will oscillate and will energize the relay coil M.

Oscillation of die 6SN7 tube is dependent upon the algebraic sum of the capacities connected to the instrument connections X and Y.

The addition of water through the discharge pipe 18 is through a first point valve Va which is air controlled and the supply of air controlling the valve is in turn controlled by a winding Val which will subsequently be described. At the same time an end point valve Vb is provided also controlled by air, which in turn is controlled by solenoid Vbll. The arrangement is such that when the solenoids Val and Vbl are energized the corresponding valves are closed. The valves of course are open when the respective windings are de-energized.

The operation of the complete system will be described in connection with the illustrated circuit, which will be described to the extent necessary to understand the system. A. 1l0-volt power line indicated at '70 is connected to the control circuit through a manual control switch '71. The control circuit includes the transformer Ta which is energized whenever the manual switch 71 is closed and which in turn supplies the primary of a second transformer Tb having the 250-volt, 6.3-volt secondary windings illustrated in the Tektor unit.

In operation sand is dumped into the mill from the hoppers 12 without regard to the moisture content of the sand in the hoppers. For example, the sand in one of the hoppers may be substantially moist and this sand may be dumped into the mill first so that it immediately contacts the moisture probe 26. The drier sand from the other hopper is at the top of the mill and is ineffective on the moisture probe. In order to insure that this condition does not prevent the required addition of moisture, the circuit includes timing means operable to provide a cycling of the control system after a predetermined interval irrespective of whether or not the instrument cut on the supply of water during the first timed interval. This means comprises a timer resistance T1 having a switch arm Tlla associated therewith. The switch arm Tia may for example be in the form of a bi-metallic contact member which is normally openand which closes after the resistance Tl has been energized for a substantial period, as for example 15 seconds. Closure of the manual switch 71 starts heating resistance element Tl through contact R2a, the contact being in the illustrated position when the relay R2 is de-energized.

After the predetermined initial period, as for example 15 seconds, has elapsed, the switch arm Tla closes energizing relay R2 and moving relay arm R2a to its lower position establishing a holding circuit through the relay R2 and de-energizing timer T1. The relay R2 remains energized for the remainder of the complete cycle. In addition, energization of relay R2 shifts relay arm R217 to the left energizing relay R1. Energization of relay R1 shifts relay arm Rla to the left establishing a connection to the lower portion of the circuit through jumper line 8%, around arm R2b, which remains to the left, holding relay R1 in.

During the interval measured by the timer T1 it may be possible for sufficient water to have been added to the sand and mixed therewith, in which case the operation should be terminated. In other cases a false signal may have resulted in closure of the valves Va and Vb. When the timer T1 times out switch arm R2b moves clockwise, thus momentarily breaking the circuit to the lower portion of the system. When the switch arm R2b is in its lowermost or clockwise rotated position it energizes relay R1 which closes a circuit through switch arm Rla, thus re-energizing the lower portion of the circuit. The interval between energization of relay R2 and energization of relay R1 is substantial and all circuits completed through portions of the wiring diagram below relay R1 in the figure are de-energized so that all holding circuits drop out. When the switch arm Rla completes its movement all circuits are again re-energized and checking of the moisture of the sand is resumed. If in fact, the moisture content of the sand is suificient this recheck results in quick cycling of the instrument to close valves Va and Vb and they will remain closed for an interval determined by energization of a timer E52 later to be described, which finally completes the cycle.

The operation of the system during the interval controlled by the timer T1 is exactly the same as it would be if the timer were omitted. The function of the timer is to restart the complete cycle after a predetermined interval so that additional water can be added if the operation of the system was terminated as a result of a false signal during the first timed interval. A second important function of the initial timing period depends upon the following: It may happen that during the initial timing period a first signal is received from the moisture measuring unit which will have the effect of closing the large capacity valve Va and leaving additional water to be supplied through the relatively smaller end point valve Vb. If the false signal was the result of a small quantity of very moist sand happening to contact the moisture probe, a large volume of water may in fact be required to bring the average moisture content of the sand to the required value. During the initial interval timed by the timer T1, water will be added through the small capacity valve Vb. However, when the timer T1 times out the control circuit is completely de-energized and re-energized, thus starting afresh with the large capacity valve Va open and the valve will remain open until the measuring unit makes the first signal indicating adequate moisture, which signal is sometimes referred to herein as a wet signal.

Assuming that insufficient water has been added to the sand, the rollers and plows continue to rotate, and water is now added to the mill through the valves Va and Vb. The solenoid Val of the first water valve Va is energized through lines 72, 74, 76, selector switch SS1, switch arm R312, switch arm Rla, and jumper 80. Energization of the solenoid Via maintains thefirst point valve Va in sufiicient open condition. In like manner, the end point valve Vb and its solenoid valve Vb1 are energized through lines '72, 74, 73, selector switch SS2, switch arm RSb, switch arm ESla, switch arm Rla, and jumper 80. The addition of water and mixing of the sand continues concurrently until the moisture content of the sand approaches a value near to but substantially below the final required value. At this time the value of the capacitance of the moist sand as sensed by the moisture probe 26 is such that the various capacitances connected to the points X, Y, including the first point capacitance 62, operate to cause the 6SN7 tube to oscillate, thereby establishing a current through the relay coil M sufiicient to shift the contacts M1 and M2 to the left from the position shown.

Closure of the switch M1 establishes a current through relay R3, switch arm Rla, and jumper 80. Energization of relay R3 moves switch arm R3a downwardly from the illustrated position, thus breaking the circuit through the solenoid Val and closing the first point valve Va.

Switch arm R3a completes a circuit through the relay R3 and through the solenoid 82 of a switch having contacts indicated generally at 82a. Energization of solenoid 82 moves switch contacts 82a upwardly thus: disconnecting the first point capacitance 62 and connecting the end point capacitance 64 and the bias capacitance 66 into the circuit. It will be observed that the circuit through relay R3 is held closed by the relay R312 and. hence from this time to the end of the cycle, relays R1, R2 and R3 remain closed.

In addition to the foregoing, energizat'ion of the relay R3 shifts the switch arm R3b downwardly, thus preparing a circuit for subsequent energization of relay R5. This circuit extends from the switch arm R4b to contact M2a, contact M2b, switch arm R3b, switch arm Rla, and jumper 80.

Since this first indication of adequate moisture was based upon control of the first point capacitance 62 and the temperature compensating capacitance 38, subsequent passages of the rollers over the moisture probe will not result in indications of adequate moisture until a substantial additional quantity of water has been added. Ordinarily, it is preferred to add approximately of the water while the first point valve Va remains open, the additional 20% being added at a much slower rate through the smaller end point valve Vb.

As soon as the scraper has removed the moist sand from the moisture probe following this first indication, relay coil M is de-energized and contacts M1 and M2 again return to the illustrated position to the right. At this time a circuit is completed through the normally closed switch arm RSa, contact M2a, contact M2b, switch arm R3b, switch arm Rla, and jumper 80. Energization of the relay R4 closes switch R4a establishing a holding circuit for the relay R4 which keeps the relays R4 and R7 throughout the balance of the cycle. Energization of the relay R7 opens normally closed contacts R7a, thus disconnecting bias capacitance 66 from the circuit and leaving the end point capacitance 64 in control. The operations are continuous without further change until there is a second indication of adequate moisture which as before, energizes the relay coil M and shifts the contacts M1 and M2 to the left. The contact M1 has no further function since it has already energized relay R3 which remains energized through a holding circuit for the balance of the cycle. However, movement of the switch arm M2 to connect contacts M2a and M2b energizes relay R5 through switch arm R4b, contacts M2a, M2b, switch arm R3b, switch arm Rla, and jumper 80. Energization of the relay R5 shifts switch arm RSb to the lower position, thus breaking the circuitto solenoid Vb1 and closing the end point valve Vb) This would normally constitute the end of the cycle but additional provision is made for rechecking the moisture content a number of times to insure against premature termination of the cycle while the average moisture content of the sand is below that required.

The brief interval in which the switch arms M1 and M2 are to the left (before the next succeeding passage of the plow 24) has closed the end point valve Vb, but downward movement of the switch arm RSb has established a circuit through the relay R5 which includes point valve will remain closed.

assases 2 'switch 'arm ESla of a short interval timer E81. Thus, as long as the switch arm ESlla remains closed, the relay R5 will remain energized and the end point valve Vb will remain closed. The motor of the timer E51 is at 'this time energized through switch arm RSa, contacts M20 and M21), switch arm R31), switch arm Rla, and jumper 8h. The timer BS1 may be set for an interval for example of three seconds and after three seconds the switch arm ESila will open if the timer is permitted to run its course. However, during the three seconds in which the timer E51 is timing out, there will be a subsequent sensing of moisture content and if the moisture content of the sand is adequate relay M is momentarily energized and switch arm M2 will interconnect contacts M2a and M21) briefly and then return to interconnect contacts MZb and M20. This will have the effect of breaking the circuit to the motor of timer 1581 at the contact M2c and return of the switch arm M2 to the contact MfZc will restart the timer for timing out the same interval. Thus, so long as the periodic moisture sensing operations sense adequate moisture, the timer will be automatically restarted so that the timer contact arm ESla will never open and the relay R5 will remain energized through switch arm R515, switch arm ESla, switch arm Rlla, and jumper fill. This will interrupt the circuit through solenoid Vbl at switch arm R519 and the end If however, passage of a roller 22 over the moisture probe gives a dry signal, there will be sufficient time for the timer E81 to time out, causing opening of timer switch arm 3381a and breaking the circuit to the relay thus restoring switch arm RSb to its illustrated position. This will complete the circuit through the solenoid V151 and reopen end point valve Vb. The end point valve Vb will remain open until a subsequent sensing of moisture content indicates the correct value thereof at which time the end point control valve will close and rechecking will resume. The timer motor BS1 is restarted and switch arm ESlla closed when relay R5 is next energized, by the next wet signal.

in order to terminate the cycle after a predetermined interval which may be devoted to rechecking, a longer interval timer E52 is provided having contacts 532a in a branch circuit connecting the relay R5 across the lines. Thus, when the switch arm is closed, the relay R5 remains energized, switch arm R55 remains in its lower position, thus interrupting the circuit to the solenoid V111, and finally terminating the cycle.

Initiation or" the longer interval timer which finally terminates the cycle is initiated through normally closed switch arm 1 16a, relay R3, switch arm R521, contact M20, contact M212, switch arm R31), switch arm R10, and jumper 8h. Energization or relay R8 closes switch arm RSa, thus energizing relay R6 which in turn closes switch arm R615, establishing a holding circuit through relay R6 and opening switci'i arm R611. The continued energization of relay R6 and closure of switch arm Rob maintains the motor of timer energized for a predetermined intervar upon the termination of which, timing out of the timer closes switch arm E8241, thus establishing a circuit through relay and moving switch arm R51) downwardly from the illustrated position to break the circuit to solenoid V151. Tlr's finally closes the end point control valve Vb if it was open at this time and marks the of the cycle.

Referring now to Figure 2, the moisture probe is adapted to be mounted in the bottom wall Till] of the tank or mill 1%. More specifically, the bottom wall 11th of the mill is provided with removable wear plates 102 and one of these is recessed as indicated at lt-ll l to receive a flat cup-shaped element 1% which receives aflat plate 108 of a suitable di-electric plastic material, preferably polytetralluoroethylene sold under the trade name Teflon. The plate 1% of plastic material is retained in the cup N96 by means of rivets 1119 having relatively large cylindrical heads 112. Centrally, the plate 108 is provided with a circular opening 114 which receives an assembly comprising a metallic thimble 116, details of which are best seen in Figure 3. The thimble 116 includes a cylindrical wall 118 which is relatively thin, a conical wall portion 120, and a cylindrical recess 122 the side walls of which are provided with openings 124 for the reception of a pin. The lower end of the thimble 116 has a tapped opening 126.

In the assembly, a Teflon button 128 is provided which is shaped to fit within the open end of the thimble 116 and it is retained therein by means of a pin extending through the openings 124 of the thimble and through the lower reduced portion of the button. Surrounding the metal thimble 116 is a plastic sleeve 130 having an upper cylindrical portion to receive the cylindrical portion of the thimble, a conical portion therebetween, and an enlarged cylindrical opening which receives the lower portion of the thimble with clearance. The outer edge of the sleeve 130 fills the opening 114. The upper surfaces of the heads 112 of the rivets, the plastic plate 108, the sleeve 114, the button 120, and the cylindrical portion 118 of the thimble are all coplanar and occupy the plane of the upper surface of the wear plate 102.

Rigidly connected to the thimble is an elongated center post 132, its upper end having a reduced threaded extension received in the tapped hole 126 provided at the lower end of the thimble. In order to retain the assembly in its lowermost position, an abutment ring 134 is provided in a tube 136 the upper end of which is welded to the bottom wall of the cup 196. Above the ring 134 is an indented groove indicated at 137 which forms an abutment against which the lower side of the ring 134 seats. Below the ring 134 is a plastic washer 138 having a downwardly extending reduced extension which receives a metallic abutment washer 140. The lower end of the center post 132 is threaded as indicated at 142 and carries a nut 144 which through a washer 146 supports a plastic washer 148. The plastic washer 148 has a reduced upper extension receiving a brass washer 150. Extending between the metal washers and 150 and surrounding the center post is a compression spring 152. The spring urges the center post and hence the stainless steel thimble, the button and the sleeve downwardly and maintains them in accurately centered and seated relation against the respective conical supporting surfaces. The center post is provided with an elongated insulating sleeve 154 which prevents accidental engagement between the spring and the center post.

In order tomaintain the metallic elements of the probe at a temperature at least as high as that of the sand within the mill, suitable electric heating means is associated with the structure, this means being diagrammatically indicated at 156.

At its lower end the tube 136 is connected to an angular conduit which receives a coaxial cable diagram matically indicated at 162. The lower end of the center post 132 is connected to the conductor of the coaxial cable through a resistor 164-.

With the foregoing construction it will be observed that the space between the exposed edge of the stainless steel thimble and the nearest adjacent grounded metallic elements is determined by the spacing of the closest rivets 110 to the thimble. When this space is bridged by a quantity of moist sand, the exact capacitance of the moist sand may be measured by the connection between the center post of the moisture probe and ground.

Inasmuch as the probe is exposed to sand in a mill which is subjected to a mixing action, the abrasive efiect of the sand is to wear away the material exposed thereto. By

employing cylindrical heads 112 on the rivets 110 and by forming the upper end of the thimble in the form of a cylindrical shell, it will be observed that a wearing away of the surface of the elements of the moisture probe by the continued passage of sand thereover will not aifect the capacitance measurement dependent upon the moist sand surrounding the exposed end of the moisture probe.

It is desirable in the present probe to insure that a specimen of sand, having been compacted against the probe to give a reading of moisture content, may be removed in its entirety by the next passage of a scraping element in proximity to the surface of the probe. This is accomplished by employing tetrafiuoroethylene as the resin from which the plate 108, button 128, and sleeve 130 are formed. Preferably, the plastic compound contains approximately 20% coke flour or graphite fill to produce surface characteristics facilitating separation of a compacted specimen of sand therefrom.

The drawings and the foregoing specification constitute a description of the improved end point moisture content control for sand in such full, clear, concise and exact terms as to enable any person skilled in the art to practice the invention, the scope of which is indicated by the appended claims.

What we claim as our invention is:

1. A mill for granular material, means including a valve for adding water to granular material in said mill during mixing, a solenoid connected to operate said valve, a relay connected to control said solenoid, a control circuit for said relay, a first condenser in a Wall of said mill exposed to moist granular material in said mill the capacitance of which is a function of the moisture content of the moist granular material, a second condenser carried by said mill the capacitance of which is a function of its temperature, current control means in said control circuit sensitive to outside capacitance connected thereto, and means connecting said condensers to said current control means.

2. Structure as defined in claim 1 in which said current control means is a vacuum tube.

3. Apparatus for tempering granular material which comprises a mixer for mixing granular material and water, means for adding water to the granular material including a shut-off valve, a variable moisture responsive electrical impedance exposed to moist granular material in said mixer, a variable temperature responsive electrical impedance responsive to temperature of the granular material in said mixer, means electrically contnected to said moisture and temperature responsive impedances operable to establish a resultant electrical impedance dependent upon the value of the moisture content and temperature of the granular material, a solenoid connected to said shut-01f valve for actuating said valve, and a control circuit including said resultant impedance connected to said solenoid and operable to actuate said solenoid to close said valve upon attainment of a predetermined resultant electrical impedance.

4. Apparatus as defined in claim 3 in which said moisture and temperature responsive impedances are variable capacitances.

5. Apparatus as defined in claim 3 in which the control circuit comprises a relay in control of said solenoid, a vacuum tube connected to said relay, and means connecting said resultant impedance to said vacuum tube to control said tube.

6. Apparatus as defined in claim 3 in which said temperature responsive impedance is positioned to be continuously responsive to temperature of the granular mateial in said mixer.

References Cited in the file of this patent UNITED STATES PATENTS 2,709,843 Hartley June 7, 1955 2,825,946 Dietert et al. Mar. 11, 1958 2,848,008 Dietert et al. Aug. 19, 1958 2,856,948 Mortin Oct. 21, 1958