US2898088A - Earth borehole logging system - Google Patents

Description

Aug. 4, 1959 R. L. ALDER 2,898,088

EARTH BOREHOLE LOGGING SYSTEM Original Filed March l, 1954 3 Sheets-Sheet 2 Aug. 4, 1959 R. L. ALDER EARTH BoREHoLE LOGGING SYSTEM 3 Sheets-Sheet 3 Original Filed March 1, 1954 4 7 m mmm N I d l L`\v\\n\ zu. 2 s b w .I -Ilif d lll. FHI] Ill-.D

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INVENTOR, @05597 f@ United States Patent EARTH BOREHOLE LOGGING SYSTEM Robert L. Alder, La Canada, Calif., assignor, by mesne assignments, to Dresser Industries, Inc., Dallas, Tex., a `corporation of Delaware Continuation of application Serial No. 413,405, March 1, 1954. This application February 110, 1958, Serial No. 714,433

12 Claims. (Cl. Z55-24) The invention hereinafter disclosed relates to earth borehole logging systems of the type wherein a signal comprising or representing desired information, such as various physical quantity measurements obtained by one or more devices in a well borehole, is produced within the borehole and transmitted to a point exteriorly of the borehole for recording, analysis or other purposes.

More particularly, the invention relates to improvements in the beforementioned type of borehole logging systems whereby the stream of drilling Huid normally pumped downwardly under pressure through the interior or bore of a hollow drill string is, in addition to being employed as the transmission medium serving to convey the signal to the top of the borehole, also employed in a novel manner to energize the signal-producing means within the borehole.

An example of the general type of system to which the invention 4thus more particularly is directed is disclosed in the copending application of Alder, Serial No. 368,042, tiled July 15, 1953.

The present invention provides an improvement upon a system disclosed in a copending application of Peterson, Serial No. 413,397 filed March l, 1954, and provides a system in which disadvantages attendant upon using the somewhat abrasive drilling fluid itself as a medium for actuating moving metallic parts of a signal-producing means are obviated.

The present invention is a continuation of copending application Serial No. 413,405 to Alder, led March 1, 1954, now abandoned.

Borehole logging systems adapted for logging while drilling and which have utilized energy derived from the moving drilling fluid as a source of power at the bottom of the borehole have, because of the relatively large power requirements of the kind of apparatus heretofore used, required for such purpose impellers, turbines, and like devices, the rotors of which were situated in the drilling lluid stream and coupled to drive either an electric generator or a complicated arrangement of mechanical parts which were in turn utilized to furnish power to the measurement and signal-producing devices. These systems suffered the obvious disadvantages of requiring at least one rotating shaft or member extending from within a relatively protected enclosure through a seal and into the stream of abrasive drilling fluid, and from having a relatively large number of somewhat complicated moving parts located in an environment of severe mechanical vibration and shock. Also, large drilling cuttings or chips or other solid material carried by the drilling fluid stream were apt to clog or damage such turbine and impeller devices and thereby interrupt the proper operation of the apparatus.

With the previously mentioned and other disadvantages of prior art earth borehole logging systems in view, it is an important object of the present invention to provide a borehole logging system comprising a simple means utilizing the drilling uid stream to provide directly both the energy for producing signals within the borehole for 2,898,088 Patented Aug. 4, 1959 transmission to the earths surface, and the transmission channel or means for conveying the signals to the earths surface.

Another object of the invention is to provide in a borehole logging system a simple means for utilizing the natural pressure gradient in the drilling fluid stream owing in the drill string to move a drilling iluid stream flow-changing means into flow-changing relationship with the drilling fluid stream, whereby to produce a signaling pressure change in the drilling lluid stream.

Another object of the invention is to provide an earth borehole logging system free of the aforementioned disadvantages and defects of prior art earth borehole logging systems.

Another object of the invention is to provide an earth borehole logging system capable of operating during drilling operations as well as during periods of suspension of drilling.

Another object of the invention is to provide in a borehole logging system a simple drilling iluid stream pressure-change signal-producing apparatus in which drilling fluid pressures at iirst and second separated points in the drilling fluid stream, between which exists a pressure differential, are alternately translated through a captive or confined body of control fluid to a resilient member forming a portion of the boundary of the drilling luid stream at a point between the rst and second points to cause the resilient member to alternately move to and from a. drilling fluid stream ilow-changing attitude to cause propagation of one or more pressure-change signals in the drilling iluid stream.

Broadly, the present invention resides in a novel apparatus by which useful information obtained by suitable instrumentalities within a borehole may be transmitted or translated to the surface of the earth by one or more signals in the form of changes in character of flow of the drilling iluid, such changes preferably being in the form of a plurality of repetitive pressure changes in the drilling iluid stream in the drill string, either during drilling or between drilling periods, or both, utilizing the normal pressure gradient in a portion of the drilling iluid stream to supply substantially al1 of the energy required for initiating and producing such pressure changes. As in the aforementioned application of Peterson, the pressure in the drilling uid stream is, by suitable means, picked up or sensed at a lirst point and transmitted or translated to a exible member positioned to vary the resistance to iiow of the drilling uid stream at another point to cause the flexible member to be flexed into the stream to increase the resistance to flow thereof, or ilexed out of the stream to decrease the resistance to flow thereof, thereby producing ow or pressure-change signals in the stream. Since substantially all of the energy required to produce the drilling iluid pressure change signals may thus be derived directly from the drilling iluid stream, and may be utilized without conversion to another form of energy, the turbogenerator or rotary means heretofore employed may be dispensed with; with the remaining requirement only that an actuating or triggering means of small size and requiring only an extremely small amount of energy for its operation be employed for initiating and controlling the action of the signaling apparatus.

The hereinbefore mentioned and other objects, advantages and features of novelty will become evident hereinafter wherein a preferred embodiment of apparatus and best mode contemplated for carrying out the invention are described in conjunction with the drawings, in which like reference characters designate the same or similar parts throughout several views, and in which:

Figure l is a view, partly in elevation and partly in vertical section, illustrating a typical earth borehole drilling rig and borehole produced thereby, and illustrating a 3 typical general arrangement and location of the apparatus of the invention;

Figure 2a is an enlarged vertical sectional view, partly diagrammatic, of a part of the apparatus shown at the lower portion of Figure 1;

Figure 2b is an enlarged vertical sectional view, partly diagrammatic, of a part of the apparatus shown at the lower part of Figure 1 and forming an extension of the structure depicted in Figure 2a;

Figure 3 is a hydraulic circuit diagram, greatly exaggerated in scale in parts, partly in longitudinal section and partly in plan, illustrating the functional relationships of apparatus elements and hydraulic connections depicted in Figures 2a and 2b, the several portions of the apparatus being diagrammatically drawn to different and exaggerated scales for purposes of clarity of illustration; and

Figure 4 is a fragmentary sectional view similar to a portion of Figure 2a, but depicting certain parts in an operating position or attitude.

Referring now to the drawings, and more specifically to Figure l, there is shown a typical earth borehole 2 formed by a conventional rotary drilling method through successive strata 4, 6, 8, and 12 of the earth, the upper portion of the borehole being lined with a conventional surface casing 14 cemented in as shown at 16 to form a well head. Suspended in the borehole is a generally tubular drill string including a bit 18, a drill collar 20 connecting the bit to one or more sections of drill pipe 22, and a kelly bar 24 arranged to rotate the pipe. The drill string is supported for rotation by a rotary swivel 26 carried by a traveling block 28 of a conventional drilling rig. The rig comprises a derrick 30, a power unit or draw works 32 arranged to rotate the kelly bar by means of Abevel gearing 33, a drilling fluil circulating pump 34 having an intake 35 extending into and receiving drilling fluid from a sump 36, the pump having an air chamber or surge tank 37 and discharging to a pipe 38 through which the drilling fluid is delivered under pressure to the interior of the drill string by way of a flexible hose 39 and the rotary swivel 26. The apparatus thus far enumerated, with the exception of drill collar 20, may be of conventional design, and is shown to illustrate the environment of the instant invention and to aid in a clear explanation of the operation of the system of the invention. The drilling fluid received by pump 34 from sump 36 through intake 35 is forced into pipe 38 past or through a surge tank 37 which acts to greatly decrease fluctuations in pressure in pipe 38, resulting from the inherent delivery characteristics of the pump which customarily is of the positive displacement, constant speed type. The drilling fluid flows under pressure in a stream downwardly Within the drill string and is discharged therefrom through suitable orifices or jets in the drill bit into the borehole, and returns to the top of the borehole through the annular space surrounding the drill string. At the top of the borehole, casing 14 is provided with a side discharge pipe 40 for discharge of the drilling fluid into sump 36, from which the drilling fluid may be withdrawn for re-circulation after screening or settling.

Referring now to Figures 2a and 2b, drill collar 20 extends downwardly from drill pipe 22 to drill bit 18, is attached to those elements by conventional joints 21 and 23, respectively, and is provided with an interior passage through which the drilling fluid stream flows as indicated by the arrows. Snugly fitted within the lower end of the smooth bore 42 of drill collar 20 is an instrument spool 43 (Figure 2b) arranged to support a sealed instrument case in a manner hereinafter more fully described. The spool is preferably held in fixed position in bore 42 by suitable means, such as internal lock rings, or as shown, by set screws 44 threaded through the drill collar and engaging in a suitable annular groove 45 in the exterior face of the instrument spool. Sealing means, such as an elastic ring 46 mounted in a suitable annular recess in the periphery of the lower flangelike end of the instrument spool 43 may be employed to seal off an annular space 47 between the drill collar wall and the instrument spool from entry of drilling fluid in an evident manner. At its upper end spool 43 is provided with a plurality of radially extending, circumferentially spaced apart legs 48 snugly fitting bore 42, and a smooth radial and face provided with an annular groove 49.

Situated immediately above spool 43 in bore 42 of the drill collar is a lower drilling fluid pressure-sensing and translating device comprising an internally recessed barrel-like body member 50 to the inside of which is suitably secured at opposite ends thereof a flexible and preferably resilient sleeve or tube 51. In the embodiment illustrated the sleeve or tube is secured to the body member by means of pinch rings 52 and 53 forming parts of a barrel and secured in turn within respective ends of member 50 by circumferentially disposed series of screws 54 and 55, respectively. The internal configuration of barrel member 50 is such as to provide an annular clearance space or cavity 56 between the internal face of member 50 and the outer face of the intermediate portion of flexible sleeve 51. Space 56 thus forms a fluid-tight cavity. The exposed internal face of sleeve 51 is arranged to form a portion of the boundary of the drilling fluid stream flowing through the drill collar, and accordingly may act to sense the pressure in the drilling fluid stream at that location within the drill collar, and by reason of its flexible nature may translate the sensed pressure to a medium such as a hydraulic liquid contained in space 56. Space 56 is effectively sealed from the drilling fluid stream by the pinching action of rings 52 and 53 on the end portions of sleeve 51. The lower pinch ring 52 of the barrel is provided with a smooth lower end face adapted to coact with a sealing ring 49a situated in the aforementioned annular groove 49. Upper pinch ring 53 of the barrel is likewise provided with an upper smooth radial end face adapted to coact with a sealing ring 57a contained within a circular groove 57 formed in the lower end of an apparatus supporting spool 60 hereinafter described. Barrel body member 50 is preferably concentrically positioned within the bore 42 of the drill collar by means of a series of integral radially extending legs 58 and 59 arranged in circumferentially spaced apart positions around the lower and upper ends, respectively, of the menber, and snugly fitting the interior wall of the drill col ar.

Situated in the drill collar immediately above the lower pressure-sensing device is an apparatus supporting spool 60 having flanged ends carrying radially extending circumferentially spaced-apart legs 61, 62, by means of which the spool is concentrically and firmly positioned within the bore 42 of the drill collar. The lower flange of spool 60 is provided with a smooth radial end face coacting with the sealing ring 57a contained in the aforementioned circular groove 57 and the upper end of spool 60 is likewise provided with an annular groove 63 in which is seated a sealing ring 63a as indicated.

Positioned immediately above spool 60 in bore 42 of the drill collar is a drilling fluid flow-changing device comprising an internally recessed barrel member 64 (Figure 2a) having secured therein a flexible and preferably resilient tubular sleeve 65. The sleeve may be secured in the barrel member by means of pinch rings 66 and 67 forming end pieces of a barrel and in turn secured to the inside of respective ends of the barrel member 64 by means of suitable screws as indicated at 67a. Barrel member 64 is provided with an internal depression, preferably in the form of an annular groove, providing an annular cavity 68 encircled and normally covered by an outer surface of sleeve 65. Thus cavity 68 is a confined space adapted to be filled with suitable hydraulic fluid. The inner and opposite surface of flexible sleeve 65 encircles and is exposed to the stream of drilling fluid and thus forms a portion of the drilling fluid stream boundary within the drill collar. The confined space or cavity 68 is sealed from the drilling fluid stream by the pinching action of pinch rings 66 and 67 on respective ends of sleeve 65. Suitably secured to the internal face of pinch ring 67, as by means of welded and circumferentially spaced-apart radial vanes 69, is a strong metal by-pass core or tube 70 preferably positioned concentrically within and extending throughout the full length of the exposed inner portion of sleeve 65. By-pass tube 70 preferably is of such internal and external diameters as to provide a space 71 between the tube and sleeve 65 through which a considerable portion of the drilling fluid stream will normally flow, the remainder of the stream by-passing through the interior of the tube. Barrel member 64 is concentrically positioned in bore 42 of the drill collar by means of series of radially extending circumferentially spaced-apart legs 72,Y 73, respectively, which may be formed as integral parts of the barrel member and which snugly engage the interior of the bore 42. Upper pinch ring 67 is provided with a smooth radial end face having therein an annular groove 74.

Secured in bore 42 immediately above the drilling fluid flow-changing device is an upper pressure-sensing or translating device comprising an internally recessed barrel body 75 having secured to its inner face a flexible and preferably resilient sleeve 76, the sleeve being secured in the barrel body by means of suitable pinch rings 77 and 78 as indicated. Barrel body '75 is preferably concentrically positioned in bore 42 of the drill collar at its lower end by a series of radially extending circumferentially spaced-apart legs 79 snugly fitting the wall of bore 24 and at its upper end by a suitably shaped flange 80 constructed and arranged to snugly fit within the bore Lower pinch ring 77 is provided with a smooth radial end face adapted to coact in sealing relation with a flexible sealing ring 74a situated in the aforementioned annular groo-ve 74. Flange 80 of barrel body 75 is equipped with a peripheral lgroove in which is mounted an elastic sealing' ring 81 adapted to coact `with the smooth inner wall of bore 42. The upper radial end faces of flange 80 and pinch ring 78 are provided with complementary beveled edges providing a recess or groove in which is seated an elastic sealing ring 82. Sealing ring 82 is adapted to be `compressed as indicated when drill collar is screwed onto drill pipe 22 and together with the previously mentioned sealing rings is adapted to effectively seal the space lying between the several barrels and spool and the interior wall of the drill collar from entry of drilling fluid. Barrel body 75 is provided with a broad, shallow indentation or groove in its inner face forming, with the cooperation of a portion of the exterior surface of flexible sleeve 76, a confined space or cavity 76a which is adapted to be filled with hydraulic fluid. Sleeve 76 has its interior face or surface arranged to form a portion of the boundary of the drilling fluid stream flowing through the drill collar, and accordingly is, by reason of its flexibility, adapted to sense the pressure of the drilling stream at a point therewithin, and to transfer or translate that sensed pressure to a fluid contained in cavity 76a.

The sealed space lying outside the several described spools and barrels but within bore 42 of the drill collar, is utilized to house hydraulic tubing and apparatus including other portions of the flow-change signal producing apparatus as will hereinafter be more fully described, and is preferably filled with a suitable pressure-compensating oil or other fluid or medium, whereby the relatively high pressure of the drilling fluid may be approximately equalized or neutralized regardless of the depth of the borehole. In order that the pressure of the pressure-compensating fluid around the several barrels and spools and apparatus therein may be roughly or approximately equal at all times to that of the drilling fluid within the spool bore, regardless of pressure variations in the drilling fluid stream inthe drill collar, there is provided a sealed, flexible bellows 84 suitably mounted on spool 60 as indicated and having its inside communicating with the drilling fluid stream by means of a tube 85.

In the apparatus illustrated in Figures 2a and 2b drilling fluid pressure sensed at sleeve 76 may be translated or transmitted to the exterior surface of sleeve 65 by a confined body of oil or hydraulic fluid via a hydraulic tubing 90, a preferably electromagnetically operated twoposition, three port valve 91, hydraulic tubing 92, a normally open pressure control valve 93, and hydraulic tubing 94 communicating with space 68 around sleeve 65. This hydraulic connection and its operation may bemore easily traced by reference to the hydraulic circuit diagram of Figure 3 which represents schematically the hydraulic connections and other apparatus depicted in Figures 2a and 2b and from which it will be evident that the noted pressure translation or transmission can occur only when valve 91 is actuated by energization of its magnet or solenoid. When magnet 91m of valve 91 is energized, valve stem 91s is moved to compress valve spring 91r and open a passage between valve chamber 91c and tubing 96, while simultaneously closing a passage between chamber 91e and tubing 95. It will be assumed that the hydraulic system including the valve and tubing, is filled with hydraulic fluid. Application of the pressure thus sensed at sleeve 76 to the fluid-filled cavity 68 encircling sleeve 65 results in inward flexure of the latter sleeve, since the normal pressure in the drilling fluid stream within sleeve 65 is lower than that sensed at sleeve 76 due to the normal friction loss in the drilling fluid stream between the respective sleeves. This inward flexure of sleeve 65 is caused by a migration of hydraulic fluid from cavity 76a through tubing 90, valve 91, tubing 92, valve 93 and tubing 94 into cavity 68, and is accompanied by a corresponding outward flexure of sleeve 76. Inward flexure of sleeve 65, as more fully explained in the aforementioned copending application of Peterson, reduces the cross-sectional area of the flow space 71 within sleeve 65, and causes a flow change in the form of an increase in pressure in the drilling fluid stream above that sleeve. This constriction of the flow passage is depicted in Figure 4. The increase in pressure in the drilling fluid stream is used as a signal for conveying information to the top of the borehole in a manner hereinafter more fully described. Since production of the pressure-rise signal in the drilling fluid stream has been effected only at the expense of migration of hydraulic fluid 'from the cavity 76a to cavity 68, it is evident that in order to repeat or cause propagation of another signal, a return of hydraulic fluid from cavity 68 to cavity 76a and return of sleeves 76 and 65 to normal attitudes, is necessary. Such return of hydraulic fluid and of sleeves 65 and 76 to normal positions is accomplished by apparatus now to be described.

Referring to Figure 3, as soon as the electromagnet of valve 91 becomes de-energized the valve stem 911sis returned by its spring 91r to a normal position, closing the passage between hydraulic tubing and chamber 91e and simultaneously opening a passage between chamber 91e and an exhaust hydraulic tubing 95 connected to the aforementioned cavity 56 encircling lower pressure-sensing sleeve 51. The pressure sensed by sleeve 51 and transferred to the hydraulic fluid in cavity 56 being lower than the pressure in the drilling fluid stream within sleeve 65, there results a migration of hydraulic fluid from cavity 68 through tubing 94, through the normally open upper passages of valve 93, tubing 92, chamber 91C of valve 91, and tubing 95 to cavity 56 encircling sleeve 51. This migration or movement of fluid from cavity 68 permits the return to normal attitude of sleeve 65, and causes some inward flexure of sleeve 51. It is evident from a consideration of the above description in connection with Figure 3 that, While the system thus far described is capable of propagating at least one complete flow-change or pressurel rise signal, a gradual draining away of hydraulic fluid from spaces 76a and 68 into cavity 56 encircling sleeve 51 would eventually preclude further proper functioning of the apparatus thus far described. Hence, a means is provided for causing return flow of the hydraulic uid, that is, from the space surrounding sleeve 51 to the space surrounding sleeve 76, during normal operation of the apparatus of the system. This means in its preferred form comprises a check valve 102 and a pump 104 (Figure 3) arranged as indicated, the pump having an actuating piston 105 and a one-way delivery piston 106 interconnected by a piston rod 107 slideable through a sealed barrier or partition 108 mounted in the bore of the pump cylinder and held in fixed position therein by suitable means such as a screw 109 as indicated. Pistons 105 and 106 are normally urged upwardly, as viewed in Figure 3, by a compression spring 110, but are arranged to move downwardly and compress or energize the spring upon application of an increase in hydraulic pressure to the upper face of piston 105 through tubing 111 and 114 communicating with the outside of sleeve 76, during which downward movement hydraulic iluid may move past the flexible rim part of piston 106 and into a space 112 provided between piston 106 and partition 108. Fluid may also ow from below piston 105 through a bleed port and tubing 113 to cavity 56 encircling sleeve 51, causing inward ilexure of that sleeve. Check valve 102 inserted in tubing 115 prevents direct passage of hydraulic fluid from tubing 114 through tubing 115 into space 112 between piston 106 and partition 108, but permits flo-w in the opposite direction, as indicated by the arrow adjacent the check valve 102.

Referring again to Figure 3, operation of the pressurechange signal producing apparatus through successive pressure-change signaling cycles will be explained, it being noted that ow of drilling fluid through collar 20 is from top to bottom in that gure, as indicated by the arrow adjacent sleeve 76, and that this flow results in a pressure gradient between any pair of sleeves 76, 65 and 51. The indicated hydraulic tubing and the spaces communicating therewith are filled with a confined body of hydraulic uid. Upon receipt of a pulse of electric current by the magnet of control valve 91 the valve is actuated or opened to permit flow of hydraulic iluid away from sleeve 76 through tubing 90 and valve 91 to sleeve 65 via tubing 92, Valve 93 and tubing 94 under the influence of the pressure differential existing between sleeves 76 and 65. This pressure differential may be caused by natural friction losses, and may be accentuated, if desired, by flow-restrictors such as are diagrammatically illustrated at R and R' in Figure 3. Flow of hydraulic fluid then ensues in the manner indicated, sleeve 76 flexing outwardly and sleeve 65 correspondingly being exed inwardly into the drilling fluid stream. Inward ilexure of sleeve 65 may continue until that sleeve is prevented from further inward movement by tube 70, and causes, due to the resultant increase in resistance to flow of drilling fluid through sleeve 65, the propagation of a ow or pressure-change signal in the form of a pressure rise in the uid stream above sleeve 65. This increase in drilling uid stream pressure is sensed at upstream sleeve 76 and translated to the upper face of piston 105 of pump 104, causing downward movement of the piston and compression of spring 110 and movement of hydraulic fluid from below piston 106 past the periphery of that piston into space 112. Also, hydraulic uid flows from Vbelow piston 105 through the bleed port and tubing 113 to cavity 56 encircling sleeve 51, flexing the latter inwardly to some extent. Upon cessation of the pulse of electric current through the magnet 91m, valve stem 91s returns to normal position, as aforementioned, cutting off translation of pressure from sleeve 76 to sleeve 65 and hydraulically connecting sleeve 65 to sleeve 51, at which latter sleeve the pressure, already below that at sleeve 76 or sleeve 65, may have been further lowered to some extent by the increased resistance to ow of the drilling lluid through the drill collar and past sleeve 65. Flow of hydraulic fluid then ensues from sleeve 65 via tubing 94, valve 93, tubing 92, chamber 91C and tubing 95 to sleeve 51 as aforedescribed, followed by return or outward flexure of sleeve 65 to normal position. Return of sleeve 65 to normal position again allows free flow of drilling fluid therethrough, whereupon the pressure rise above sleeve 65 disappears and the pressure at sleeve 76 drops to normal. Concurrently, the pressure sensed at sleeve 51 remains at, or rises to, normal. This drop in pressure at upper sleeve 76, alone or aided as it may be by the pressure rise at sleeve 51, permits compressed spring 110 to move pistons and 106 upwardly, forcing hydraulic lluid from space 112 through tubing 115 and check valve 102 into the space surrounding the sleeve 76; and simlarly, forcing hydraulic tluid into the space above piston 105 through tubing 111 into the space surrounding sleeve 76. As the pistons move upwardly they draw excess fluid through tubing 113 from around sleeve 51 permitting or causing exure of that sleeve outwardly to normal position at the same time sleeve 76 is flexed inwardly to normal position. Thus, a complete, repeatable signaling cycle is performed. Once the apparatus is placed in operation, pistons 105 and 106 do not reach extremes of their possible stroke, but only make moderate excursions to and fro from a somewhat variable intermediate position at which the force of the partly compressed spring balances the algebraic sum of the forces exerted by the hydraulic fluid on the pistons. Thus, the normal movement of hydraulic iluid from sleeve 76 through valve 91 to and from sleeve 65 and on to sleeve 51 by way of tubing 95 during propagation of a pressure-rise signal, is thereafter compensated by an equal volumetric movement of hydraulic fluid from sleeve 51 to sleeve 76 by way of and by action of pump 104 following completion of the signal propagation. Lengthy and continuous operation of the pressure rise producing apparatus is thus made possible.

If it is desired to limit the magnitude of the pressure rise signal to a value less than that which would otherwise be produced by the apparatus, this may be accomplished by inclusion of a pressure control valve 93 and a pressure relief valve 116 in the hydraulic uid circuit, as indicated in Figure 3. The presence of these valves is not necessary to satisfactory basic operation of the system, as is obvious from the before-described operation of the hydraulic circuit. Inclusion of valves 93 and 116 permits, however, the amplitude of the pressure rise signals to be limited to a predetermined value. As hydraulic fluid passes from sleeve 76 to sleeve 65 via valve 91, the pressure in tubings 90, 92 and 94 increases. The spring of valve 93, which is a conventional pressure control valve, may be adjusted so that its piston will close the valve at any desired pressure in its operating range, thus stopping flow of hydraulic fluid to sleeve 65 and limiting drilling fluid ilowrestricting inward ilexure of the latter. As soon thereafter as valve 91 operates to connect tubing 92 with cavity 56, valve 93 automatically reopens so hydraulic Huid may pass from cavity 68 to cavity 56. During the period of closure of valve 93, any excessive pressure that may be produced in the hydraulic fluid around sleeve 65 may be relieved by automatic operation of pressure relief valve 116 which is arranged to pass lluid in the direction of its adjacent arrow whenever any such excessive pressure is produced. As is thus made evident, the extent of restriction of drilling fluid ow may be regulated to allow the apparatus to propagate a pressure-rise signal therein of a predetermined amplitude. Also evident is the fact that the duration of a pressure-rise signal may be regulated by corresponding regulation of the lengths or durations of the electric current pulses passed through the magnet portion of valve 91. Accordingly, the apparatus may be employed to transmit information via pressure-rise signals to the earths surface from within the borehole, using any suitable code employing amplitude and/ or pulse-duration variations as well as pulse-time modulation.

From the above it is clear that a ow or pressure-change signal in the form of a pressure-rise may be initiated and propagated in the drilling iluid stream in the drill collar by supplying a pulse of electric current to the solenoid operated valve 91, the result being the temporary hydraulic connection of cavity 76a to space 68 and in migration of lluid from space 76a to space 68, resulting in inward exure of sleeve 65; followed by deenengization of the solenoid and return of 'valve 91 to the normal position illustrated to connect space or cavity `68 to lower space 56 and the return of sleeve 65 to normal condition or attitude. While there may thus be propagated a pressure-change signal comprising a complete pressure-change signal comprising la complete pressure rise and fall to normal, it is sufficient for signaling purposes for only one or the other of the pressure changes to be propagated, and hence in this specification and the claims to follow the terms pressure-rise and pressure-change and owchange are intended to relate to and comprehend either type of pressure-change signal propagation. That is, either a pressure rise, a pressure drop or a pressure rise followed by a pressure drop, or vice versa. As indicated in Figures 2a, 2b and 3, electric current pulses may be supplied to the solenoid of valve 91 by 'way of insulated conductors 98 and 99 which extend through sealing grommets in the case of the magnet portion of valve 91 and extend on to information-obtaining apparatus contained on spool 43 and indicated generally by ordinal 100 in Figure 2b.

Referring to Figures 2a and 2b, it will be noted that the hydraulic auxiliaries such as the valve 91 and pump 104 depicted in Figure 3 are mounted on and around the exterior surface of spool 60 in suitable fashion. For example, binding bands 120 and 121 may pass about the several parts of the apparatus and hold it securely to the spool. The hydraulic tubing or conduits 90, 94,'95,

113 and 114 serving to connect the exible pressure-V sensing devices with the uid flow restricting device and other hydraulic units, may readily extend between those several units between the radially extending legs on the various spools and barrels, as indicated in the mentioned figures.

Referring again to Figures l, 2a and 2b, the exterior of drill collar 20 has applied thereto, as by cementing or vulcanization, a jacket 122 of rubber or other suitable material arranged to insulate electrically a considerable length of the drill collar. Mounted under tension upon jacket 122 and surrounding the drill collar is a ring electrode 123 (Figure 2b) to which is secured an electric conductor 124 passing through an insulation gromrnet 125 and connecting electrode 123 to an internal contact 126 mounted flush in bore 42 of the drill collar. Contact 126 is arranged for electrical coaction with a spring brush 127 suitably mounted on an insulating grommet 128 carried upon and leading to the interior of a sealed, fluidtight case 130 arranged to house the instrumentalities and apparatus of the information-obtaining device. Case 130 also carries at its lower end a grounding contact 131 electrically connected by means of a conductor 131a to a grounding screw 131b secured in a suitable hole in a lower ange 'of instrument spool 43. Thus, it will be noted that bit 18 and the lower exposed portion of drill collar 20, 'which are electrically connected together at their threaded juncture and electrically connected to instrument spool 43 by metallic contact therewith, are arranged to act as a lower electrode of the informationobtaining apparatus. Ring electrode 123 forms the other electrode of the information-obtaining apparatus. The latter apparatus, which per se forms no part of the present invention and may be of any conventional or suitable type, a8, for example, that disclosed in the aforementioned application of Alder, is indicated as comprising in# strumentation contained in a case at in Figure 2b. The information-obtaining apparatus has output leads 132a and 132b extending to and electrically connected with a conventional two-conductor socket 133 mounted on case and into which the plugged ends of the aforementioned conductors 98 and 99 are secured. By these means electrical output pulses produced by the information-obtaining apparatus and representing desired information are transmitted or conveyed to the solenoid of valve 91.

It will be apparent from the preceding description that only such electrical energy as is used by the information-obtaining device or devices and the relatively small amount used by the solenoid of valve 91 (of the order of a fraction of a watt) need be supplied by electric power means contained within the drill collar. Since the information-obtaining means requires but a small amount of electric power and the power consumption of the valve is intermittent and of very low value, only a small and inexpensive battery or electric power source need be carried in the drill collar. To further reduce the amount of electric power required, there may be provided a pressure-operated switch in the main power supply circuit and arranged to close that circuit only when sufficient drilling fluid ow exists to permit signaling operations to be effected. Thus, a pressure-difference responsive switch 156 (Figure 3) of conventional design may have its ports 157 and 158 connected by respective tubings 159 and 160` to respective sleeves 76 and 51 and may be selected to have such operating characteristics that, as soon as suicient pressure gradient is produced by drilling fluid flow between sleeves 76 and 51 to permit pressure-rise signals to be initiated, the piston 161 of the switch will be moved under the inuence of that pressure gradient to cause conducting plunger 162 to complete the main power supply circuit between switch contacts 163 and 164. If and when the drilling uid llow decreases to a value insuicient to permit the pressure change signalproducing device to properly function to produce signals, and while a drill string is being lowered into or raised from the borehole, spring 165 of the switch will move the piston 161 to the position depicted in Figure 3, opening the main power supply circuit. The power supply circuit is indicated at Figure 3 as including a battery 166 contained in the information-obtaining apparatus indicated schematically at 100 (see Figure 2b), but the type of power source used is not material to the present invention. Thus it will be noted that while the drill string is being lowered into the borehole, or being removed therefrom, or while the drilling operation is suspended, and pump 34 is not operating, electric current will not be wasted; and at any time it is desired to obtain information and transmit indications thereof to the surface, 'whether during drilling or during a period of suspension of drilling, it is only necessary to operate pump 34 to produce a normal flow of drilling fluid through the drill string.

Signals in the form of a pressure-change or a plurality thereof propagated in the drilling fluid stream in the drill collar are detected at the top of the borehole by a conventional pressure transducer unit (Figure l) which operates to translate pressure variations within pipe 38 to which the transducer is hydraulically connected as indicated, into variations in electric voltage. The transducer output is transmitted by suitable conductors to a conventional amplifier and filter unit 171 to which power is supplied from a suitable source, such as a battery 172, by conductors as indicated. The output of the amplifier and lter unit is fed to a conventional graphic logging recorder unit 175 via conductors 173, 174. The recorder has a pen indicated at 176 arranged to produce a trace 177 on a strip of graph paper 178, the trace indicating the amplitude, durations and time or depth relationships of the variations in the electric voltage output of unit 171, which in turn are correspondingly proportional to the variations in pressure in pipe 38 and accordingly 1ndicative of the information obtained from within the borehole. The `graph paper is moved past pen 176 by mechanism including gearing housed in the case 179 forming a part of the recorder unit, which gearing is actuated alternatively by a conventional clockwork or, as herein illustrated, by a cable 180 coursing over guide pulleys 181 and 182, the cable having an end secured by siutable means to traveling block 28 of the drilling rig. The thus generally described elements serving to translate and record the pressure variations in pipe 38 correlatively with an indication of time or the position of bit 18 in the borehole, are conventional and well known in the art. They may be similar to those shown and described in the aforementioned application of Alder, and per se are not of the instant application.

From a consideration of the above description of the logging system as exemplified in the embodiment depicted in the drawings, it will be noted that the invention provides a simple, durable means :for extracting and utilizing energy from a moving body of drilling fluid in a drill string in a well borehole to produce one or more signaling flow changes in the drilling fluid stream within or adjacent the bottom of the borehole for transmission through the drilling fluid to the surface of the earth, the flow changes being representative of desired information obtained at or adjacent the lower end of the drill string. Further, it will be noted that all moving parts with the exception of the interior surfaces of the flexible sleeves, which may be and preferably are formed of an extremely tough and wear-resistant elastomer, such as neoprene, are protected from the abrasive effects of the drilling fluid.

While the aforementioned objects of the invention are accomplished by the disclosed specific embodiments of the invention, it is apparent that various modifications and variations of the described system may be made Without departing from the spirit of the invention as defined by the appended claims. For example, information other than that secured by electrode means may be transmitted, it being necessary only that the information be supplied to the valve solenoid intermittently or in the form of one or more pulses of electric current. The relative crosssectional areas and lengths of the various drilling fluid stream passages through the devices of the disclosed apparatus may be Widely varied, and variations in the form and assembly of the pressure-sensing devices may be effected. Accordingly, it is desired not to limit the invention to the specific details of the described embodiments thereof, ibut what is claimed is:

l. In a system rfor simultaneously logging and drilling earth boreholes employing a drill string having a drillin-g fluid flow passage therethrough, down through which passage circulating drilling fluid is forced to flow under pressure, the combination comprising: a drill string; signaling means therein including variable fluid flow resistance means fixed in said drill string in the vicinity of the lower end thereof, actuatable by transfer thereto of a quantity of control fluid under pressure to effect a variation in resistance to flow therethrough of said circulating fluid to produce a pressure rise signal therein; a first separate body of control fluid contained in said drill string; a first sensing means to pick up and transfer to said first body of control fluid, pressure of said circulating fluid in said passage at a point spaced upstream from said flow resistance means; a second separate body of control fluid contained in said drill string; a second sensing means to pick up and transfer to said second body of control fluid, pressure of said circulating fluid in said passage at a point downstream from said flow resistance means; the pressure thereby picked up and transferred to said first body of control fluid being greater than the pressure picked up and transferred to said second body of control fluid; signal control means in said drill string actuatable firstly to supply a quantity of control fluid under pressure from said first body of control fluid to said flow resistance means while removing the pressure of said second body of control fluid therefrom, thereby to actuate said fluid flow resistance means to produce a pressure rise ysignal in said circulating fluid upstream of said flow resistance means, and actuatable secondly to exhaust said thus supplied quantity of said first control fluid from said fluid flow resistance means into said second body of control fluid of lower pressure while removing the pressure of said first body of control fluid therefrom; whereby a quantity of said first body of control fluid of higher pressure is transferred into said second body of control fluid of lower pressure, thereby to actuate said fluid flow resistance means to terminate said pressure rise signal in said circulating fluid; and transfer means energized by the said pressure rise in said first body of control fluid resulting from said pressure rise signal in said circulating fluid to return a quantity of the second body of control fluid to the first body of control lluid sullicient to compensate for the said quantity of control fluid exhausted into said second body of control fluid.

2. In a system for .simultaneously logging and drilling earth boreholes employing la drill string having a drilling fluid flow passage therethrough, down through which passage circulating drilling fluid is forced to flow under pressure, the combination comprising: a section of drill string; signaling means therein including variable fluid flow resi-stance means fixed in said section of drill string in the vicinity of the lower end thereof, -actuable by transfer thereto of a quantity of control fluid under pressure to effect -a variation in resistance to flow therethrough of said circulating fluid to produce a pressure rise signal therein; a first separate body of control fluid contained in said section of drill string; a first sensing means to pick up and transfer to said rst body of control fluid, pressure of said circulating fluid in said passage at a point spaced upstream from said flow resistance means; a second separate body of control fluid contained in said :section yof drill string; a second sensing means to pick up and transfer Ito said second body of control fluid, pressure of said circulating fluid in said passage at a point downstream from said flow resistance means; the pressure thereby picked up and transferred to said first body of control fluid being greater than the pressure thereby picked up and transferred to said second body of control l fluid; signal control means in said section of drill string actuatable firstly to supply a quantity of control fluid under pressure from said first body of control fluid to said flow resistance means While removing the pressure of said second body of control fluid therefrom, thereby to actuate said fluid flow resistance means to produce a pressure rise signal in said circulatin-g fluid upstream of said flow resistance means, and actuatable secondly to exhaust said thus supplied quantity of said first control fluid from said fluid flow resistance means into said second body of cont-rol fluid of lower pressure While remov- 'ing the pressure of said first body of control fluid therefrom, whereby a quantity 'of said first body of control fluid of higher pressure is transferred into said second body of control fluid of lower pressure, thereby to actuate said fluid flow resist-ance means to terminate said pressure rise signal in said circulating fluid; and transfer -means energized by the said pressure rise in said first body `of control fluid resulting from said pressure rise signal in said ycirculating fluid to return a quantity of 'the second body of `control fluid to the first body of control fluid following such termination of said pressure rise sufficient to compensate for the quantity of control fluid thus exhausted into said second body of control fluid.

3. In a system for `simultaneously logging and drilling earth boreholes employing a drill string having a drilling fluid flow passage therethrough, down through Which passage circulating drilling fluid is forced to flow under pressure, the combination comprising: a section of drill string; signaling means therein including variable fluid flow resistance means fixed in said section of drill string in the vicinity of the lower end thereof, actuatable by transfer thereto of 'a quantity of control fluid under pressure rto effect fa variation in resistance to flow therethrough of said circulating fluid, to produce a pressure change signal therein; a first separate body of control fluid conytained in said section of string; a first sensing means to pick up and transfer to said rst body of control fluid, pressure :of said circulating fluid in said passage at a point spaced upstream from said flow resist-ance means; a second separate body of control fluid contained in said section of drill string; a second sensing means to pick up and transfer to said second body of control fluid, pressure of said circulating uid in said passage at a point down stream from said flow resistance means; the pressure thereby picked up 'and transferred to said first body of control fluid being greater than the pressure picked up and transferred to said second body of control fluid; signal control means in said section of drill string actuatable firstly to supply a quanti-ty of control fluid under pressure from said first body of control fluid to said flow resistance means while removing the pressure from said second body of control fluid therefrom, thereby to actuate said fluid flow resistance means to produce a pressure change signal in said circulating fluid upstream of said flow resistance means, and actuatable secondly to exhaust said thus supplied quantity of said first control fluid from said fluid ow resist-ance means into said second body of ycontrol fluid of lower pressure while removing the pressure of said first body of control fluid therefrom; whereby a quantity of said first body of control fluid of higher pressure is transferred into said second body of control fluid of lower pressure, thereby to actuate said flu-id flow resistance means to terminate said pressure change signal in said circulating fluid; and transfer means energized by the changes in pressure differential between said first body of control fluid and said second body of control fluid resulting from said pressure change signal in said circulating fluid to return a quantity of the second body of control flu-id to the first body of control fluid sufficient to compensate for the quantity of control fluid exhausted into said second body of control fluid.

4. In a system for simultaneously logging and drilling earth boreholes employing a drill string having a drilling fluid flow passage therethrough, down through which passage circulating drilling fluid is forced to flow under pressure, the combination comprising: a section of drill string; signaling means therein including variable fluid flow resistance means xed in said section of drill string in the vicinity of the lower end thereof, actuatable by transfer thereto of a quantity of control fluid under pressure to eEect a variation in resistance to flow therethrough of said circulating fluid, to produce a pressure change signal therein; a first separate body of control fluid contained in said section of drill string; a first sensing means to pick up and transfer to said first body of control fluid, pressure of said circulating fluid in said passage at a point spaced upstream from said flow resistance means; a second separate body of control fluid contained in said section of drill string; a second sensing means to pick up and transfer to said second body of control fluid, pressure of said circulating fluid in said passage at a point downstream from said flow resistance means; the pressure thereby picked up and transferred to said first body of control fluid being greater than the pressure picked up and transferred to said second body of control fluid; signal control means in said section of drill string actuatable firstly to supply a quantity of control fluid under pressure from said first body of control fluid to said flow resistance means while removing the pressure of said second body of control fluid therefrom, thereby to actuate said ow resistance means to produce a pressure differential signal in said circulating fluid between the upstream and downstream sides of said flow resistance f means, and actuatable secondly to exhaust said thus supplied quantity of said first control fluid from said fluid flow resistance means into said second body of control fluid of lower pressure while removing the pressure of said first body of control fluid therefrom, whereby said quantity of said first body of control fluid of higher pressure is transferred into said second body of control fluid of lower pressure, thereby to actuate said fluid flow resistance means to terminate said pressure differential signal in said circulating fluid; and transfer means energized by the changes in pressure differential between said first body of control fluid and said second body of control iluid resulting from said pressure differential signal in said circulating fluid to return said quantity of the second body of control fluid to the first body of control fluid.

5. In a system for simultaneously logging and drilling earth boreholes employing a drill string having a drilling fluid ow passage therethrough, down through which passage circulating drilling fluid is forced to flow under pressure, the combination comprising: a section of drill string; signaling means therein including variable fluid flow resistance means fixed in said section of drill string in the vicinity of the lower end thereof, actuatable by transfer thereto of a quantity of control fluid under pressure to effect a variation in resistance to flow therethrough of said circulating fluid, to produce a pressure change signal therein; a first separate body of control fluid contained in said section of drill string; a first pressure transfer means in Contact with circulating fluid in said passage upstream from said ow resistance means to pick up and transfer to said first body of control fluid, pressure of said circulating fluid in said passage at a point spaced'upstream from said flow resistance means; a second separate body of control fluid contained in said section of drill string; a second pressure transfer means in Contact with circulating fluid in said passage downstream from said flow resistance means to pick up and transfer to said second body of control fluid, pressure of said circulating fluid in said passage at a point downstream from said flow resistance means; the pressure thereby picked up and transferred to said first body of control fluid being greater than the pressure picked up and transferred to said second body of control fluid; signal control means in said section of drill string actuatable rstly to supply a quantity of control fluid under pressure from said first body of control fluid to said flow resistance means while removing the pressure of said second body of control fluid therefrom, thereby to actuate said fluid flow resistance means to produce a pressure rise signal in said circulating fluid upstream of said flow resistance means, and actuatable secondly to exhaust said thus supplied quantity of said first control fluid from said flow resistance means into said second body of control fluid of lower pressure While removing the pressure of said first body of control fluid therefrom, whereby a quantity of said first body of control fluid of higher pressure is transferred into said second body of control fluid of lower pressure, thereby to actuate said fluid flow resistance means to terminate said pressure rise signal in said circulating fluid; and fluid transfer means energized by the said pressure rise in said first body of control fluid resulting from said pressure rise signal in said circulating fluid to return a quantity of the second body of control fluid to the first body of control fluid sufficient to compensate for the quantity of control fluid thus exhausted into said second body of control fluid.

6. Apparatus according to claim l and informationobtaining means in the vicinity of the lower end of the drill string operative, in response to the value of a physical quantity within the borehole to be ascertained, to actuate said signal control means in a manner bearing a predetermined relation to such value, thereby to produce said pressure rise signals which are indicative of said value.

7. Apparatus according to claim 5 and informationobtaining means in the vicinity of the lower end of the drill string operative, in response?l to the value of a physical quantity within the borehole to be ascertained, to actuate ysaid signal control means in a manner bearing a predetermined relation to such value, thereby to produce said pressure change signals which are indicative of said value.

8. Apparatus according to claim 6 and means outside of said borehole in communication with and responsive to pressure rise signals in the circulating drilling fluid in said drill string to detect and translate said signals into sense perceptible signals.

9. Apparatus according to claim 7 and means outside of said borehole in communication with and responsive to pressure change signals in the circulating drilling fluid in said drill string to detect and translate said signals into sense perceptible signals.

10. In a system adapted for logging an earth well borehole being drilled by means including a hollow drill string in said borehole and through which a drilling fluid stream flows downwardly from the earths surface under pressure successively past first and second points in said drill string with a pressure gradient therebetween, the combination with said drill string comprising: a rst device for drilling fluid stream pressure-sensing and translating including a barrel member and a resilient tubular sleeve coaXially arranged therein to provide a closed fluid-tight cavity therebetween, and with an inner surface of the resilient tubular sleeve encircling and providing a boundary for the drilling fluid stream at said first point and arranged to sense and translate the drilling fluid stream pressure at said first point; a second device of construction and function like those of said first device and similarly functioning with respect to the drilling fluid stream, at said second point; a third device of construction similar to said first device and encircling and forming a boundary f the drilling fluid stream at a third point between said first and second points, and with its resilient tubular sleeve arranged to inwardly flex upon application of pressure to its fluid-tight cavity to produce a flow restriction and pressure rise in the drilling fluid stream thereabove; control means including a two-position valve means having first, second and third valve ports and an actuating solenoid therefor; first, second and third conduits hydraulically interconnecting the first, second and third ports of said valve with the cavities of said first, second and third devices respectively; a body of control fluid filling said fluid-tight cavities, conduits and valve means; means including an information-obtaining means at least partly contained within said drill string and electrically connected to and operative to supply intermittent information-representing electric current pulses to said actuating solenoid of said valve means, said valve means being adapted to be actuated thereby alternately between first and second positions, and in said first position to interconnect the said first and third ports while disconnecting said second and third ports from one another, thereby to transmit the pressure translated by said first device through said first and third conduits to the cavity and resilient tubular sleeve of said third device to cause the said sleeve thereof to flex inwardly from its normal position into and to produce a pressure rise in the drilling fluid stream thereabove, and in said second position to interconnect the said second and third ports while disconnecting said first and third ports from one another, thereby to transmit the pressure translated by said second device through said second and third conduits to the cavity and resilient tubular sleeve of said third device to cause the said sleeve thereof to flex outwardly to normal position to terminate said pressure rise, thereby to produce in said drilling fluid stream repeatable informationrepresenting pressure-change signals.

11. In a system adapted for logging an earth well borehole being drilled by means including a hollow drill string in said borehole and through which a drilling fluid stream flows downwardly from the earths surface under pressure successively past rst and second points in said drill string with a pressure gradient therebetween, the cornbination with said drill string comprising: a first device for drilling fluid stream pressure-sensing and translating including a body member and a movable member arranged to provide a closed fluid-tight cavity therebetween, and with an inner surface of the movable member providing a boundary for at least a portion of the drilling fluid stream at said rst point and arranged to sense and translate the drilling fluid stream pressure at said first point; a second device of construction and function like those of said first device and similarly functioning with respect to the drilling fluid stream, at said second point; a third device of construction similar to said first device and forming a boundary of at least a portion of the drilling fluid stream at a third point between said first and second points, and with its movable member arranged to inwardly flex upon application of pressure to its fluid-tight cavity to produce a flow restriction and pressure rise in the drilling fluid stream Ithereabove; control means including a two-position valve means having rst, second and third valve ports and an actuating solenoid therefor; first, second and third conduits hydraulically interconnecting the first, second and third ports of said valve with the cavities of said rst, second and third devices respectively; a body of control fluid filling said fluid-tight cavities, conduits and valve means; means including `an information-obtaining means at least partly contained within said drill string and electrically connected to and operative to supply intermittent information-representing electric current pulses to said actuating solenoid of said valve means, said valve means being adapted to be actuated thereby alternately between first and second positions, and in said first position to interconnect the said rst and third ports while disconnecting said second and third ports from one another, thereby to transmit the pressure translated by said first device through said first and third conduits to the cavity and movable member of said third device to cause the said movable member to flex inwardly from its normal position into and to produce a pressure rise in the drilling fluid stream thereabove, and in said second position to interconnect the said second and third ports while disconnecting said first and third ports from one another, thereby to transmit the pressure translated by said second device through said second and third conduits to the cavity and movable member of said third device to cause the said movable member to flex outwardly to normal position to terminate said pressure rise, thereby to produce in said drilling fluid stream repeatable information-representing pressure-change signals.

12. Apparatus according to claim l and means responsive to the pressure differential between said first body and said second body of control fluid and actuatable each time said pressure dierential rises to a predetermined value to terminate the said supply of said control fluid to said flow resistance means thereby to limit the pressure rise of each .of said pressure rise signals substantially to a predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 2,160,766 Thomason May 30, 1939 2,353,143 Bryant July ll, 1944 2,517,820 Aagaard Aug. 8, 1950 2,577,613 Friberg Dec. 4, 1951 2,590,215 Sausa Mar. 25, 1952 2,700,131 Otis et al Jan. 18, 1955 2,786,642 Comb Mar. 26, 1957 FOREIGN PATENTS 182,656 Great Britain July 13, 1922

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