US1912997A - Conductivity cell - Google Patents

Conductivity cell Download PDF


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US1912997A US39468929A US1912997A US 1912997 A US1912997 A US 1912997A US 39468929 A US39468929 A US 39468929A US 1912997 A US1912997 A US 1912997A
United States
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Henry C Parker
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Leeds and Northrup Co
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Leeds and Northrup Co
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    • G01N27/00Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/04Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
    • G01N27/06Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a liquid
    • G01N27/07Construction of measuring vessels; Electrodes therefor
    • Y10T137/00Fluid handling
    • Y10T137/0318Processes
    • Y10T137/0324With control of flow by a condition or characteristic of a fluid
    • Y10T137/0329Mixing of plural fluids of diverse characteristics or conditions
    • Y10T137/0335Controlled by consistency of mixture


June 6, 1933. H. c. PARKER CONDUCTIVITY CELL Original Filed 001.19, 1928 2 Sheets-Sheet 1 a mm R 'June 6, 1933,. H. c. PARKER CONDUCTIVITY CELL Original Filed Oct. 19, 1928 2 Sheets-Sheet, 2
M1 :8 I fltorwa Patented June 6, 1933 I (UNITED STATES PATENT! OFFICE HENRY G. PARKER, OF WASHINGTON, DISTRICT OF COLUMBIA, ASSIGNOR TO LEEDS & NORTHRUP COMPANY, OF PHILADELPHIA, PENNSYLVANIA, A CORPORATION OF PENNSYLVANIA CONDUCTIVITY CELL Original application filed October 19, 1928, Serial No. 318,418. Divided and this application filed September 23, 1929.
My invention relates to the construction of a cell utilized in the determination of the conductivity of a fluid, particularly of a caustic solution, for example boiler water, under conditions of high temperature and pressure.
In the operation of steam generators, for example, as boilers, it has been customary to remove a certain quantity of water periodically, as by blow-oil, from the boiler in order to prevent the concentration and accumulation of soluble and insoluble mineral and organic matter from exceeding a given value and so reducing the eiiiciency of the heat transfer structure. Since the fresh water feed to the boiler may vary as to its constituents, it will be apparent that the saidv concentration may vary or build up at different rates, depending also on the rate at which steam is drawn oil, and a method of blow-ofl' not taking this possible variation into consideration will onl roughly control the degree of concentration. value of concentration below which it would be uneconomical to control, due to the large loss of heat in the blow-off water, and the most economical and eflicient operation of the generator will therefore result when the concentration is maintained between such a low limit and a higher predetermined limit. In accordance with the invention claimed in my co-pending application Serial No. 31 416, filed October 19, 1928, of which this application is a division, the blow-oil of a steam boiler, orequivalent, is controlled in accordance with the degree of concentration of dissociated matter within the same; further and more specifically, primary blow-oil controlling means is actuated periodically independently of said concentration, and secondary blowofi' controlling means is act-uated only in response to variationsin the concentration, thereby providing periodic blow-off at varying rates.
In accordance with the invention herein claimed, a cell of corrosion and heat-resisting materials .is disposed in directv contact with hot boiler Water for actuating the secondary blow-oil controlling means; more specifically the cell comprises an electrode There is a Serial No. 394,689.
of nickel spaced from other electrode structure by a distance piece of mica, preferably a column or standard built up of laminated mica impregnated with a phenol condensation product.
My invention resides in the features of construction hereinafter described and claimed.
For an illustration ofone of the forms of my invention and for an understanding of its use in the control of boiler blow-down,
reference is to be had to the accompanying drawings, in which:
Fig. 1 is a diagrammatic view of a control system comprising my invention.
Fig. 2 is a view of apparatus used in the control system shown in Fig. 1.
Fig. 3 is a view, partly in section, conductivity cell.
Fig. 4 shows a specific type of boiler struc ture with which my invention may be asso ciated.
Referring to Fig. 1, a boiler drum 1 has heat supplied to it by heat transfer structure (not shown) in a well known manner and also has means for injecting feed water (also not shown) for maintaining the water level within the boiler at .some predetermined level as at 2. It is a well known fact that as water in the form of steam is carried away from the boiler drum, the mineral matter is left behind, and more particularly soluble matter as salts, or, more generally, electrolytes gradually increases in concentration until the efliciency of the boiler is reduced. The electrolytes dissociate in the boiler water so that a conducting solution results, the unit resistance of the solution depending upon the degree of dissociation but principally upon the total ion concentration.
For determining this degree of ion concentration, and therefore the concentration of electrolytes within the boiler, electrode structure 3 is mounted in a wall of the boiler drum and extends through the same into contact with the water.
For a detailed description of electrode structure 3, reference is to be had to Fig. 3. A metallic plug or supporting member 80,
which is adapted to be mounted in the boiler wall or equivalent structure, comprises a nut portion 81 having a screw-threaded extension 82 which is adapted to be in threaded engagement with the boiler wall. Member is of condiu-ting material as steel, and has mounted thereto in conducting relation terminal structure, as binding post 83. Extending centrally through member 80, a bore 84 has an insulating sleeve 85 within which is mounted a conducting rod 86 of corrosion-resisting material, as nickel, extending therethrough and terminating in binding post structure 87. Rod 80'is further insulated from member 80 by an insulating disc 88 which may comprise a plurality of mica discs or washers. A housing 89 having a detachable cover 90 secured thereto by screws 91 is mounted upon an annular shoulder 92 of portion 81. Conducto'rs may extend through opening 93 in the side of the housing for connection with terminals 83 and 87.
Centrally extending bore 84 has an enlargement 94 which forms a shoulder 95 within member 80. An insulating sleeve 96 seats within bore 94 against shoulder 95 and receives an enlarged cylindrical portion 97 of conducting rod 86. An insulating washer 98 as of mica, is disposed between member 97 and a flange 99 of sleeve 96. Cylindrical portion 97 is in screw-threaded engagement at 100 with another conducting rod 101, also of corrosion-resisting material, as nickle, which comprises part of an electrode. Rod 101 has a contacting portion or cap 102 (likewise of corrosion-resisting material), mounted thereon in screw-threaded engagement at 103. Contacting portion 102 is adapted -to be in direct contact with the boiler water at all times and consequently is composed of metal, as pure nickel for example, which is adapted to resist the corroding action of the hot boiler water. Rod 101 is electrically insulated from the direct contact of the boiler water by an insulating distance piece or sleeve 104 extending between contacting portion 102 and a cylindrical flange 105 forming an extension of screw- ,threaded portion 82.
Sleeve or distance piece 104 comprises thin discs of mica stacked upon each other. These may be impregnated with a phenol condensation product under predetermined conditions of temperature and pressure so that the resulting structure comprises a hard, solid, insulating sleeve which not only resists the corroding action of the boiler water, but is mechanically sound in that it resists fracture under conditions of high temperature and pressure. Sleeve 104 may, however, consist of but a stack of mica discs clamped together under pressure since the electrical resistance will be still high notwithstanding that a certain amount of boiler water will work its way in between the discs. Furthermore, as a matter of fact, I have found that mica is apparently the only insulating material whose insulating properties are unaffected by the action of the highly caustic solutions found inboiler waters. Quartz, glass, porcelain, bakelite, (a phenol condensation product) lava, stoneware, etc., are
readily softened and finally dissolved by such solutions. In the present instance, sleeve 104 is formed integrally with centrally disposed rod 101, although itwill be apparent that it may be slidably mounted thereon if desired. Sleeve 104 has a reduced portion at either end thereof, and a circumferential flange 106 of the cap 102 fits snugly over one. of said portions. A Washer 107 of bakelite, (a phenol condensation product) porcelain or other insulating material is disposed between a thick coating of acid-proof cement 108 applied to the interior of ca 102, and reduced portion 105. The contacting cap 102 is detachably mounted on rod 101 and is adapted to be maintained in tight engagement with sleeve 104.
In order that the electrode structure may withstand the high temperatures and pressures prevalent in ordinary generating systems, provision is made for not only sealing the conducting portions of the electrode from the boiler water, but for preventing escape of water from the boiler in case the sealing means around the electrode structure should fail. Protecting sleeve 104 as previously described may be of mica and a pienol condensation product composition to withstand high temperatures and the corroding action of the boiler water and at its lower end has the electrode cap 102 tightly screwed thereto on conductor 101. Flange 106 therefore snugly engages a co-acting shoulder 106a on the protecting sleeve. The acid-proof cement forms a thin film over these contacting surfaces and prevents moisture from reaching conductor 101. At its opposite end, rod 101 is in threaded engagement with the enlargement 97 of conductor 86 and is screwed into the same so that shoulder 1065 of sleeve 104 fits tightly against the edge of flange 105. This junction is likewise sealed with acid-proof cement. A spacing member 109 of suitable insulating material, such as bakelite (a phenol condensation product) or porcelain, is disposed between'sleeve 104 and member 97. In case of mechanical failure of part of the electrode structure, the boiler water would be prevented from passing through the electrode structure by the valve action of member 97 with shoulder 95. In other words, as the pressure increased upon 97, it would be forced tighter into engagement with member 96 and 98 which abut against shoulder 95 of the screw-threaded support or plug 80, and therefore more effectively prevent escape of water through the electrode structure. It is very essential that there be such safety means incorporated in electrode structure for use in connection with steam generators, since, due to the high pressures in general use, immediate replacement of a broken electrode through which steam and boiling water was escaping would be very diflicult, if not impossible.
Member 80 comprises the other electrode as it is both in contact with the boiler water boiler drum. Contact may therefore be es tablished from terminal 83 of member 80,
the boiler drum, the boiler water whose concentration is to be controlled, electrode cap 122, conducting rods 101 and 86 to terminal 8 a In Fig. 1, a blow-off conduit 4 leading from boiler 1 has valve 5 and 6 disposed in series therein so that their operation is interdependent. Valve 5 is separately controlled by solenoid 7 or other electrically operated valve mechanism, which is energized periodically by a source of power 9 through switch 10 and conductors 8. A constant speed cam member 11 is adapted to periodically close switch 10 fora pre determined length of time, thereby effecting energization of solenoid 7 and opening of valve 5. When solenoid 7 is de-'energized, as by opening switch 10, valve 5 moves to fully closed position. Valve 5 will therefore be either fully open or fully closed, depending upon the position of switch 10.
Valve 6 is connected to a motor 12 through suitable speed changing gears 13 and 14 and is adapted to be moved to different positions in a manner hereinafter described.
A VVheatstone bridge 15 having resistances r1, 12 and T3 of predetermined values in three of its branches respectively is connected to terminals 83 and 87 of electrode structure 3 by conductors 13 so that the fourth branch of the VVheatstone bridge comprises the variable resistance of a unit volume of the boiler water between the fixed electrodes 102 and 80. Bridge 15 is energized in any suitable manner, as by an electro-motive force 17 and includes galvanometer 18 connected between terminal 19 and an adjustable contact 49 co-operating with a slide wire adjusting resistance 21. It will be apparent that as the resistance between the electrodes changes due to changes in ion concentration, bridge 15 will be unbalanced, assuming that it was previously balanced,
, and a new balance may be restored only by adjusting the movable contact 20 along slide wire resistance 21 until galvanometer 18 shows no deflection. The position of adjusting contact 20 with respect to slide wire resistance 21 may therefore be a measure of indication of the ion concentration, hence saltconcentration within the boiler.
Apparatus for automatically bringing bridge 15 to a. new balance upon variations in the degree of ion concentration in boiler 1 is shown in Fig. 2. Galvanometer 18, which is'of the moving coil type, has secured thereto a deflecting member or pointer 19 adapted to actuate a disengageable clutch mechanism in a manner hereinafter described. A U- shaped bracket member 20 pivoted at 21 has secured to its edge a plate member 22 having edges sloping gradually upwardly from its center, and terminating'in shoulder portions at either end. Bracket 20 has also mounted thereon arm 23 which co-operates with cam 23a, to effect a periodic rocking movement of bracket20. Arms 24 and 25, having extensions 26 and 27 extending toward each other, are pivoted at 28 independently of bracket 20, and at their lower ends have off-set portions 29 and 30 for engaging pins 31 mounted on plate 32. Plate 32 is pivotally carried on the end of arm 33, which is pivoted at a point not shown, and which is also adapted to be engaged by a cam 34. Plate 32 has mounted thereon a normally transversely extending member 35 having off-set portions 36 and 37. Arm 33 is normally urged toward a disc or clutch member 38, and when in this position member 35 is in frictional engagement therewith through friction blocks 39 carried by the extremities of mem ber 35.- Shaft 40, which is driven by constant speed motor 41 through suitable gearing 42 has mounted thereon cams 43 and 44 and the aforesaid cams 23a and 34. Clutch disc 38 is connected to the rotatable shaft 22 upon which are mounted marker actuating disc 57a, slide wire resistance disc 45 and a control disc 46 which will be presently described. An idler disc 47 is loosely carried by shaft 22, being free to rotate thereon within certain limits, and is' engaged by a friction element 48 for preventing excessive freedom of movement. Disc 45 has the slide wire resistance 21 mounted on its periphery with which cooperates contact 49. Idler disc 47 has a rod 50 mounted thereon extending substantially perpendicular to the plane of said disc through a curved slot 51 in control disc 46, and atits outer end has mount- 7 ed thereon a triangular shaped member 52 for co-operation with switches 53 and 54. Mounted also on disc 46 are mercury switches 55 and 56 each having a pair of electrodes or contacts 551) and 56b adapted to be bridged by bodies of mercury 550 and 560 respectively, as best illustrated in Fig. 1.
The operation of the disengageable clutch mechanism is as follows:
Assuming deflection of galvanometer pointer 19 to the right, for example, in response to change in ion concentration, pointer 19, which overlies the edge of plate 22, will move underneath extension 27 of arm 25 and upon upward rocking movement of bracket 20 due to cam 23a, arm 25 will be moved by pointer 19 in a clockwise direction about its pivot 28, and will also tend to rotate plate 32 and member 35 in a clock wise direction about pivot 33a. During the above described movement, cam 34 will also have engaged arm 33 which carries member 35 to move the same away from frictional engagement with clutch disc 38. Arm 25 will therefore be free to rotate member 35 out of its normal horizontal position through an angle dependent on the degree of deflection of. pointer 19. Upon return of arm 33 and member 35 to clutch engaging position by resilient means (not shown), off-set portion 36, due to its lifted position is engaged by cam 34, and since now member 35 is in frictional engagement with clutch disc 38, cam 44 will effect counter-clockwise movement of both member 35 and disc 38, thereby returning member 35 to its normal horizontal position.
Rotation of clutch disc 38 in successive steps effects corresponding rotation of shaft 22 as well as discs 57a, 45 and 46. Disc 57a co-operates with a cord or flexible member 57 carried by pulleys 58, and has secured thereto a marker or recording element 59 for tracing a record upon a movable chart 60. Chart 60 is carried on a drum 61 rotatable on shaft 62 through suitable gearing 63 and 64 interposed between the same and shaft 40. A cam element 11 is mounted on the countershaft 65 for co-acting with switch 10. Slide wire resistance 21 mounted upon disc 45.
moves relative to fixed contact 49 to effect balance of the bridge.
Referring again to Fig. 1, control disc 46 having mercury switches 55 and 56, and switches 53 and 54 mounted thereon co-acts with member 52 attached to the idler disc 47 for controlling actuation of valve 6.
Switches 53 and 54 comprise resilient conducting arms mounted on insulating blocks 53a and 54a secured to disc 46 and have at their free ends co-acting contacts 531) and 54?). Each switch has an angular portion 66 adapted to engage member 52 and spaced but a close distance therefrom. Switches 55 and 56 are secured to the disc by mounting means, as straps 55a and 56a, which are adjustable so that the switches may be inclined at different angles.
Relays 67 and 68 are adapted to be energized from source E1 through the switches on control disc 46. The circuit through relay 67 is completed through conductor 69 connected to one terminal of source E1, relay 67, conductor 70, mercury switch 56, conductor 71, contacts 54?) of switch 54 and'conductor 72 to the other terminal of sources E1. Relay 68 may be energized in like manner through mercury switch 55 and switch 53. It will be apparent therefore that since switches 55 and 53 and switches 56 and 54 are in series with each other each of a co-acting pair must be closed in order to energize the respective relay. Also de-energization of a relay is accomplished by opening but one of the switches in series therewith.
Relays 67 and 68 control switches 73 and 74 for connecting reversible motor 12 to a source of power E. Relay 67 therefore is adapted to actuate motor 12 in one direction, and relay 68 in the reverse direction, thereby effecting opening and closing movement of valve 6. v
The operation of the entire system will now be described. Assumin too high concentration of salts within boiler 1, it will be apparent that the desired concentration may be obtained by blowing down a sufficient quantity of water through conduit 4'so that the concentration within the boiler will be reduced by the addition of fresh feed water to compensate for the amount of blow-off. Valve 5 is fully open at predetermined intervals for permitting blow-off, but the rate of flow therethrough will be regulated by the position of valve 6, which may be in any position between its fully open and closed limits. Blow-off therefore takes place through conduit 4 during a fixed predetermined interval but at variable rates. Due to the assumed high concentration of salts within boiler 1, the resistance between the electrodes 80 and 102 will vary from the normal predetermined value and lVheatstone bridge 15 will be unbalanced, causing deflection of galvanometer 18 and its pointer 19. Upon deflection of pointer 19, shaft22 is rotated by clutch disc 38 through its disengageable connection with motor 41, in such direction that movement of the slide wire resistance 21 with respect to its cooperating contact tends to restore the balance of the bridge. For purposes of illustration the slide wire resistance in Fig. 1 has been shown fixed instead of movable, it being immaterial which of the members is fixed with respect to the other.
Simultaneous with the balancing operation of the bridge, control disc 46 controls the operation of motor 12. In the present instance, assuming switch 73 effects control of opening movement of valve 6, disc 46 will be rotated by shaft 22 in a clockwise direction which will cause the angular portion 66 of switch 54 to engage member 52, thereby closing contact 54?) and upon continued movement of disc 46 in a clockwise direction the contacts of switch 56 will be bridged by its body of mercury, thereby completing the circuit through relay 67. Upon energization of relay 67 switch 73 is closed and motor 12 effects opening movement of valve 6. Upon subsequent opening of valve 5, the rate of blow-off through conduit 4 will be accordingly increased and the tendency of such rate of increase will be to decrease the degree of concentration within the boiler.
Control disc 46 will continue to move in a clockwise direction as long as the degree of concentration within the boiler is increasing, but upon attainment of maximum departure from its normal condition, and upon decrease in degree of. concentration, control disc 46 will reverse its direction of rotation and will start rotating in a counter-clockwise direction. i
As discs 46 reverses its direction of rotation, switch 54 will move away from member 52, thereby permitting contacts 54b to open and de-energize relay 67. Motor 12 will therefore be de-energized and valve 6 will remain in its newly adjusted position until either relay 67 or 68 is again energized.
It will therefore be apparent that valve 6 does not continue its opening movement during the entire time that the concentration within the boiler is above its normal value. In other Words, valve 6 is gradually opening only during the period while the concentration is increasing to a maximum departure from its normal value, that is during approximately one-half the time of its departure from normal. By such control it is apparent that overshooting of the con trol is prevented to a large extent as contrasted with previous types of on-off control whereby the control means do not anticipate the return of the controlled condition to normal, but instead apply the control during the entire period of departure of the condition from its normal value. I In the same manner when the concentration is too low, bridge 15 is unbalanced to effect rotation of clutch shaft 22 and control disc 46 in counter-clockwise direction to likewise efi'ect closing of switches 53 and 55, thereby energizing relay 68 and closing switch 74. This energizes motor 12 in such direction that valve 6 will continue to slowly close until the concentration starts to increase, thereby causing reversal of disc 46 and opening of contacts 53b of switch 53. Relay 68 is consequently deenergized and switch 74 returns to its normal open position, thereby de-energizing motor 12. It should be borne in mind that idler disc 47, whose periphery is frictionally engaged by a friction shoe 48 mounted upon a resilient member 48a, is moved only by engagement of portion 66 of switches 58 and 54 with member 52 which is mounted on red 50. Rod 50 has a limited degree of movement within slot 51 of disc 46 thereby permitting member 52 to remain stationary while disc 46 reverses its direction of rotation. During such reversal of rotation, the contacts of one switch are permitted to open as the pressure between said switch and member 52 is relieved, and the opposite switch contacts are closed by its portion 66 as it moves into value. In either case, however, switches 53 and 54 will open upon reversal of rotation of the control disc, such reversal indicating that the concentration is returning towards its normal value. 7
Cam 11, instead of being driven by motor 41 through interposed gearing, may have independent actuating means for driving the same at any desired speed.
The operation of valve 6 will necessarily be very slow as the variation in degree of concentration of salts takes place at a comparatively slow rate during normal operation of the boiler. The speed at which valve 6 may be most efficiently operated depends upon the time lag'of the control with respect to the actual response to the control. In other words, there is a certain time lag or inertia of the control which results in an appreciable delay between the time that valve 6 is moved to a position for effecting a change in the degree of concentration, and the actual change itself. Furthermore by anticipating this time lag or inertia, the concentration may be so controlled that it tends to return to its normal value after control of valve 6 has been discontinued. This type of anticipatory control is fully disclosed and claimed in my (to-pending application Serial No. 291,103, filed July 7, 1928 for System of control.
Another use to which my electrode structure may be applied is illustrated in Fig. 4. A boiler of the Sterling type, which comprises drums 110 having a mud or blowoff drum 111 interconnected by boiler tubes 112, has in the present instance, elect-rode tructure 3 disposed in one of the tubes 112 through which the boiler water constantly circulates. Electrode structure 3 is connected by conductors 113 in one arm of a l/Vheatstone bridge 15 which co-operates with control mechanism 01' the type previously described to control the blow-oil from drum 111 so as to maintain substantially constant concentration of the boiler water.
It will be understood, however, that electrodes may be disposed within a container which has just received a charge of blowofi' water from the boiler to co-operate with control circuits and apparatus in the manner previously described. With such an arrangement, more simple electrode structure may be utilized as high pressures are not encountered. Between each blow-off it is essential that the container be substantially emptied so that the electrodes may accurately determine the concentration of the succeedin blow-ofi' water.
It will furthermore be understood that my invention is not limited for use in boiler blow down control systems, but may be used to advantage in any system for measuring, recording, or controlling the ion concentration of a fluid under analogous conditions of high temperature, pressure, chemical ac- 10 tivity, etc.
What I claim is:.
1. In a system for determining the degree of ion concentration of a solution, elect-rode structure comprising a supporting member of conducting material forming one electrode, a second electrode insulated from and extending through said first electrode, terminal structure disposed at one end of said electrode structure, corrosion-resisting solutiim-contacting portion at the other end thereof, and a distance piece of insulation comprising a sleeve exposed to said solution and surrounding said second electrode between said contacting portion and said supporting member, said insulating sleeve comprising laminae of mica impregnated with a phenol condensation product.
.3. In a system for determining the degree of ion concentration of a solution, electrode structure comprising a supporting member of conducting material forming one electrode, a terminal connector mounted on said member, a second electrode comprising detachable portions insulated from and extending through said first electrode, one of said portions having a detachable corrosionresisting solution-contacting member secured at one end thereof, and a distance piece in the form of a sleeve of insulation exposed to said solution and comprising stacked disks of mica mounted on said second electrode between said contacting member and said first electrode.
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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2560209A (en) * 1949-01-18 1951-07-10 Economics Lab Conductivity cell
US2593825A (en) * 1948-09-29 1952-04-22 Hall Lab Inc Conductivity controlled dispenser
US2769140A (en) * 1952-03-26 1956-10-30 West Virginia Pulp & Paper Co Means for measuring electrical conductivity of liquids
US2927871A (en) * 1956-03-26 1960-03-08 Bethlehem Steel Corp Control of pickling baths
US3129928A (en) * 1961-06-16 1964-04-21 Butler Bin Company Water feed control system for concrete mixers

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2593825A (en) * 1948-09-29 1952-04-22 Hall Lab Inc Conductivity controlled dispenser
US2560209A (en) * 1949-01-18 1951-07-10 Economics Lab Conductivity cell
US2769140A (en) * 1952-03-26 1956-10-30 West Virginia Pulp & Paper Co Means for measuring electrical conductivity of liquids
US2927871A (en) * 1956-03-26 1960-03-08 Bethlehem Steel Corp Control of pickling baths
US3129928A (en) * 1961-06-16 1964-04-21 Butler Bin Company Water feed control system for concrete mixers

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