FR2558255A1 - DIFFERENTIAL PRESSURE SENSITIVE INDICATOR DEVICE - Google Patents

Abstract

DIFFERENTIAL PRESSURE INDICATOR DEVICE WITH THERMAL COMPENSATION AND AUTOMATIC LOCKING REQUIRING AN INTERNAL RESET AFTER OPERATION; IT INCLUDES A HOUSING 12 WHOSE WALL IS CROSSED BY A CHAMBER 18 IN WHICH A PISTON IS MOVED 26 ELASTICALLY REQUESTED TOWARDS A LOCKED RING 70 FITTED WITH LOCKING MEANS 80, 102 AND TOWARDS THE PISTON. AN INDICATOR ORGAN 110 IS LOCKED IN THE RING 70 AND INCLUDES A CENTRAL MAGNET 114 WHICH COOPERATES WITH A CONCENTRIC MAGNET 74 SOLID WITH THE RING 70; THE AXIAL DISPLACEMENT OF INDICATOR BODY 110 INDICATES THE EXCEEDING OF A DETERMINED DIFFERENTIAL PRESSURE; A THERMOSENSILBLE SYSTEM 84, 80 PROHIBITS THE OPERATION OF THE DEVICE AT LOW TEMPERATURES, FOR EXAMPLE DURING THE STARTING OF THE APPLIANCE WITH WHICH THE DEVICE IS ASSOCIATED; APPLICATION IN PARTICULAR TO SYSTEMS FOR MONITORING FILTER CLOGGING BY CLOUDING.

Description

The present invention relates to indicators sensitive to

  differential pressures and more particularly to sensitive indicators

  temperature-sensitive so as to prevent low-temperature operation

  its temperatures. The present invention further relates to indicators of this type which include an internal reset means. In hydraulic systems through which fluids such as oil or the like are passed through a filter, it is desirable to provide an external mechanism for indicating when the element

  filter has clogged enough to require replacement. At-

  tense as the pressure drop across the filter increases proportionally

  When the fouling material accumulates in this filter, an adequate indication can be obtained by using a differential pressure device adjusted so that it is actuated as soon as the pressure drop in the filter reaches a predetermined value. In the prior art, there are several such devices which constitute external means suitable for indicating the state of clogging of the element.

  filtering and which include compensation for low-

  temperature, in order to avoid the emission of an erroneous clogging signal

at low temperatures.

  A device of this type is known from US Pat. No. 2,942,572

  delivered to David Pall on June 28, 1960. In this device, a first element

  magnetic field is arranged to attract a second magneti-

  as long as the two elements are separated by a distance less than a predetermined spacing. A biasing mechanism propels the second magnetic element to an indication position each time the predetermined spacing is exceeded. The first magnetic element which moves reciprocally with a sensitive piston

  pressure changes is normally requested towards the second element

  magnetic by a predetermined force. The second magnetic element

  can also move reciprocally with a piston. While the se-

  cond magnetic element is retained and biased towards the first element

  magnetic by magnetic attraction as soon as it is sufficiently

  near, it is normally biased in a direction which moves it away from the first magnetic element. This biasing force is able to overcome the magnetic attraction force as soon as the first and second

  magnetic elements are separated by a predetermined distance.

  The magnitude of the force produced by the magnetic attraction, compared to the biasing force (here that of a spring) determines the predetermined differential pressure which triggers the device. To - 2 prevent such a trip from occurring at low temperature, it is

  provided with a bimetallic strip arranged so as to prevent the movement of the second element

  magnetic as long as the temperature is below a predefined threshold

  completed. The bimetal consists of two blades or curved tongues

  welded. The bimetallic strip contracts inward at low temperatures so that the inner blade surmounts a flange, which prevents actuation of the pressure indicator. Thus, the blades of the bimetallic strip undergo strong stresses at low temperatures. This risks leading to excessive fatigue of the bimetallic elements and consequently to a failure of the pressure indicator. Finally, this device

  is complex and expensive to manufacture.

  In another US patent No. 3,117,550 issued on January 14, 1964 to Cole and property of the assignee of this patent application, there is described a pressure indicator device sensitive to temperature,

  which has an elastically unstable element. This elastic element-

  unstable consists of a spherical or conical bimetallic disc, held and sealed along its peripheral edge, so that the resulting force acting on the disc due to pressure

  is oriented in the proper direction to cause the flattening-

  disc. However, as soon as a predetermined combination of pres-

  temperature and temperature is achieved, the disc is inverted with considerable force and remains in this inverted state until it is restored

  either manually or automatically when the pressure is de-

  tense. This device is also complex and requires precise calibration

  of the disc so that it can react to the combination of pressure and time

temperature.

  In another US patent 3,812,816 issued to Juhasz it is

  writes a differential pressure indicator equipped with a thermo element

  sensitive. This device has a pressure sensitive mechanism as well as an indicator which cooperates with this mechanism. The mechanism sensitive to

  the pressure moves under the influence of a differential pressure

  gendré in a fluid treatment system to provide an indi-

  appropriate cation when a certain differential pressure exists between system components. The device further comprises a locking mechanism which has the role of maintaining the indication until the causes having generated the differential pressure have been remedied. The device also includes a temperature-sensitive element in the form of a spiral bimetal which is fixed at one end to a piston body and at the other end to a rod provided with a head, so -3 -

  that the bimetal can rotate this head. As the temperature

  ture of the fluid increases, the head moves counterclockwise while

  due to the expansion of the bimetal, until notches or surfa-

  these cams move to a position behind locking balls. These locking balls can then be removed

  a partial engagement in a groove to allow the piss

  tone to move freely under the influence of any different pressure

  fluid, as long as the fluid temperature is maintained

  held at a preselected level. This design is also complex and requires precision machining of parts to achieve the locking mechanism. Other devices, such as that described in US Pat. No. 3,974,795, use bimetallic elements and aim to pocket

  the displacement of a piston when it is subjected to strong pressures

  differential sions due to the low temperature of the fluid

  of. This results in excessive stress on the components, and risks

  to cause the bimetallic elements to rupture.

  All of the prior art devices mentioned above are

  complicated. Several of them require the use of cod-

  teux, such as bimetallic elements. In all these devices of the prior art, the heat-sensitive element is isolated from the fluid

of work.

  It is also sometimes desirable to provide means for resetting the indicator device from the inside when the filter has been replaced. However, it is preferable to avoid resetting the

  device from the outside, in order to have a certain

  degree of certainty that the filter has actually been cleaned or replaced

  placed when the indicator is reset.

  Among the existing devices there are some which aim to obtain these

  results but, unfortunately, they have functional limitations-

  which result either in premature actuation or in

  no actuation, depending on the conditions in which you are

Fri

  Therefore, what you have to realize is an indi-

  differential pressure sensitive sensor that combines the characteristic

  than a temperature-sensitive device, in order to pocket the function

  operation at low temperature, with a reset feature

  internally. Such a device should be reliable enough to

  fishing for accidental operation when in fact there is no clogging

255825S

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  of the filter, since the reclosing is internal. However, such a dis-

  positive must indicate the actual state of clogging of the filter in order to avoid deterioration of other components, which could otherwise result

clogging of the filter.

  The object of the present invention is to provide an indicator device sensitive to differential pressure, which comprises an internal reset means as well as a device sensitive to temperature.

  to prevent operation at low temperatures.

  The differential pressure sensitive indicating device according to the present invention comprises a housing or casing having a first end as well as a second end opposite the first and a chamber disposed between these ends. A piston can move in a sealed manner in the internal chamber and divides it into a first compartment adjacent to the first end and a second compartment adjacent to the opposite end. First and second passages are provided for respectively connecting an inlet pressure of the first

  compartment and an outlet pressure from the second compartment. A pre-

  mier biasing device is provided between the piston and the case to bias the piston towards the first end. The piston can

  consequently move in the internal chamber under the effect of the pressure

  differential pressure produced between the inlet pressure and the outlet pressure.

  A ring controlled by the piston is housed in the first compartment.

  ment and can actually come up against the piston. A second available

  solicitation signal is provided between the shoemaker and the ring

  piston in order to urge this ring towards the opposite end or two-

  th end of the shoemaker. A temperature sensitive stop element is arranged between the ring controlled by the piston and the case. The purpose of this temperature-sensitive stop element is to prevent displacement

  from the piston slave ring to the opposite end when the temperature

  erasure in the internal chamber is less than a predetermined value, and allow the displacement of the ring controlled by the piston when the

  temperature in the internal chamber is higher than this predefined value

  completed, so that above this predetermined value of

  temperature the piston slave ring can move under the stress

  citation of said first biasing device between its initial position

  tial and a position moved towards the piston. A second stop element is provided for locking (unlockably) the ring controlled by the piston in a displaced position when this ring has moved

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  a predetermined amount toward the opposite end of the shoemaker. A gold-

  resetting gane protrudes from the outer end of the piston and passes through the opposite end of the case, which allows this resetting member to be actuated to return the slave ring to the piston in its initial position. An indicator means is movably mounted in the case and can move selectively through the first end of the case as a result of the movement of the ring controlled by the piston over the predetermined distance mentioned above, in the direction of the opposite end.

of the bootmaker.

  The first object of the present invention is to provide a device

  positive differential pressure sensitive indicator which is reliable in service and which includes an internal reset means as well as a temperature sensitive device in order to prevent the operation of the differential pressure sensitive indicator device at low temperatures.

  Another object of the present invention is to provide a dis-

  positive capable of indicating the existence of a differential temperature

  excessive between entry and exit. This device also constitutes

  a thermal stop means of the indicator, which directly measures the actual temperature of the fluid and which, however, when subjected to

  large pressure drops due to the presence of low temperature fluids

  peratures, allows the piston to move to a second position while

  leaving the indicating mechanism in a non-indicating position.

  Furthermore, the present invention aims to provide a dis-

  positive capable of indicating the existence of a differential pressure ex-

  separation between an input and an output, this device comprising an element

  thermosensitive in order to compensate for extreme pressure drops due to the temperature of cold fluids, a device in which erroneous indications can be detected due to shocks or vibrations when

  the pressure difference is close to a predetermined level.

  These various aims, advantages and characteristics of the invention, as well as others still, will emerge more clearly for any specialist.

  in art on reading the following detailed description which

  refers to a preferred embodiment shown in the Figures of the accompanying drawing, in which:

  FIGURE 1 is a side elevational view in section of the device.

  positive indicator sensitive to differential pressures according to the pre-

  invention; FIG. 2 shows a partial section made along line 2-2 of FIG. 1 where we see the visible external part of the indicator device sensitive to differential pressures according to the invention; FIGURE 3 is a section similar to Figure 1 but showing the state of the indicating device when a significant differential pressure is indicated in a low temperature condition; FIGURE 4 is a section similar to Figures 1 and 3 where we see the conjunction of a high differential pressure and a high temperature; FIGURE 5 is a section similar to Figures 1, 3 and 4, where one

  sees the indicating device after the establishment of a con-

  joint of high differential pressure and high temperature, when this condition no longer exists, and FIGURE 6 is a partial view similar to Figures 1 to 5 but

showing a variant detail.

  If one refers more particularly to Figure 1 which shows an exemplary embodiment of the differential pressure sensitive indicator device, generally designated by the reference numeral 10, it can be seen that this device comprises a shoemaker or casing 12 which has a first end 14 and an opposite end 16. A complex passage 18 connects these two ends 14 and 16 of the case 12 together, and consists of a bore and a series of counter-bores, which will each be described in detail below, their diameter increasing between the first

  end 14 and the opposite end 16.

  This complex passage 18 in fact comprises a first bore 20 which

  crosses the case 12 from one end 14 to the other end 16. A pre-

  mier counterbore 22, coaxial with the first bore 20, is formed from

  from the opposite end 16 and ends near the first end

  mity 14. An annular shoulder 24 is thus formed between the first bore

  sage 20 and the first counterbore 22.

  A primary piston 26 is slidably mounted in the complex passage

  18. This piston 26 has an enlarged part or head 28 housed with a slide.

  ment and at least partially in the first bore 22, as well as an elongated part or plunger 30 which arises coaxially on this head 28 so as to pass through the first bore 20 and end in

  a so-called resetting end 124. An annular groove 32 is formed

  mée in the external cylindrical surface 34 of the head 28 of the primary piston

  re 26. In this annular groove 32 is housed an O-ring 36 which carries

  against the internal cylindrical surface of the first counterbore 22 of my-

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  to form a tight sliding seal between the head 28 and the bot-

  tier 12. The primary piston 26 therefore divides in a watertight manner

  che, the complex passage 18 into a first sealed compartment 38 formed on the side of the first end 14 of the case and a second compartment 40 formed on the side of the opposite end 16 of this case. A helical compression spring 42 is placed in the first sealed compartment

  38 between the annular shoulder 24 of the boot 12 and an annular shoulder

  similar area 44 formed between the head 28 and the plunger 30 of the primary piston

  mayor 26 towards the opposite end 16 of shoemaker 12.

  A clearance 46 coaxial to the complex passage 18 is formed in the head 28 of the primary piston 26 opposite the plunger 30 for a purpose which will be explained below. The head 28 has an annular abutment surface 48 which concentrically surrounds the clearance 46, also for a purpose

which will be described later.

  The complex passage 18 further includes a second counter bore whose diameter is larger than that of the first counter bore 22

  and which extends from the opposite end 16 of the boot 12 towards the

  annular shoulder 24. A frustoconical surface 52 is formed between the first counterbore 22 and the second counterbore 50. A passage 54 is formed in the case 12 and extends between a source of fluid under high pressure and the second against -bore 50, which makes this source of fluid under high pressure communicate with the second compartment

waterproof 40.

  Finally, a third counterbore 18 is formed in the passage

  complex 18, as well as a fourth counterbore 58, a fifth counterbore

  bore 60 and a frustoconical counter-bore or chamfer 62, these counter-

  bores having increasing diameters between the second counterbore and the opposite end 16. Annular shoulders 64 and 66 are formed respectively between the third counterbore 56 and each of the second and fourth counterbores 50 and 58. An internal thread 68 is tapped in the part of the fourth counterbore 58 which is adjacent

at the fifth counter bore 50.

  A ring 70 slaved to the piston 26, having a cylindrical surface

  72 of smaller dismeter than the second counter bore 50, is lo-

  gée in the second sealed compartment 40. This slave ring 70 is

  fitted with an annular magnet 74 glued against the cylindrical surface

  that internal 76 of the ring. This annular magnet 74 has a north pole and a south pole, as shown in the drawing. The slave ring 70 also has a front annular front surface 78 intended to bear against

  the annular abutment surface 48 of the primary piston 26.

  A split ring 80 is interposed between the slave ring 70 of the

  piston 26 and the shoemaker 12. This split ring 80 is made of aluminum wire.

  shape memory binding. The split ring 80 is housed in a groove

  annular ge 82 formed in the cylindrical surface 72 of the ring 70 asserting to the piston 26. The external diameter of the annular groove 82 is chosen so that it is equal to or greater than the internal diameter of the split ring 80. That -this is heat treated to give it a predetermined internal diameter, then it is formed again at low temperature to give it a slightly larger diameter so that the reformed external diameter of the split ring is greater

  the internal diameter of the second counterbore 50. Thus, at low temperatures

  ture the split ring 80 cooperates with the annular shoulder 64 so

  to prevent the displacement of the slave ring 70 from the piston towards the

  first end 14 of the case 12. However, there is a particular characteristic of the shape memory alloy, according to which, when this alloy is heated to a predetermined temperature, this material returns to its original shape. Consequently, by heating the fluid which is in the second sealed compartment 40 to a predetermined temperature, the split ring 80 contracts in the annular groove 82 to an external diameter smaller than the internal diameter of the

  second counterbore 50 and thus allows the displacement of the ring as-

  served 70 by the piston 26 towards the first end 14 of the boot case 12. The split ring 80 has proportions such that it projects out of its annular groove 82 only to a maximum extent corresponding to half its diameter, which l prevents jamming when it is urged to contract in the annular groove. So the ring

  servo 70 of the piston only accompanies the movement of the primary piston

  mayor 26 when a temperature higher than the predetermined value prevails

  in the second sealed compartment 40.

  The differential pressure sensitive indicator device 10 according to the present invention further comprises an end cap 86 formed of a head 88 and of a threaded annular skirt 90 coaxial with said cap. The threaded skirt 90 is screwed into the internal thread 68 of the fourth counter-bore 58, so as to be housed in the annular space existing between this fourth counter-bore 58 and the external cylindrical surface

  72 of the slave ring 70 of the piston. An O-ring 92 surrounds the

  tie of the annular skirt 90 which is adjacent to the head 88, and further

  it cooperates with the fifth counterbore 60 as well as with the counter

  9 - frustoconical bore or bevel 62 in order to seal between these

  different parts and seal the second compartment 40.

  The end cap 86 also has a hollow central rod 94, concentric with the threaded annular skirt 90, which passes through the center of the slave ring 70 of the piston and partially the clearance bore 46 formed in the primary piston 26. A annular cavity 96 is thus formed

  between the central rod 94 and the threaded annular skirt 90 of the ex- cap

  end 86, the slave ring 70 of the piston being housed in this annular cavity 96. Stressing means, for example a spring 98, are provided between the base 100 of the annular cavity 96 and the slave ring 70 of the piston, in order to stress this ring 70 towards the first end

14 of bootmaker 12.

  A toroidal coil spring 102 in the shape of a toroid surrounds the cy-

  indric 72 of the slave ring 70 of the piston. This toric spring 102 is essentially trapped between an annular front surface 104 of

  the threaded annular skirt 90 and the annular shoulder 66 of the shoemaker 12.

  The slave ring 70 of the piston 26 is provided with a cylindrical surface

  recessed 106 which is adjacent to the end cap 86. Thus, when

  that the slave ring 70 of the piston is moved towards the first end

  miter 14 of the bootmaker 12 beyond a predetermined distance, the spring

  O-ring 102 meets and is confined to the recessed cylindrical surface 106

  pulls. In this position, the toroidal resistor 102 prohibits the return movement of the slave ring 70 of the piston towards the second end or

  opposite end 16 of shoemaker 12.

  The end cap 86 finally has a bore 108 coaxial with the threaded annular skirt 90 as well as with the hollow central rod 94. This bore 108 extends partially in the central rod 94, starting from the head 88. A piston secondary 110 is mounted axially sliding in this bore 108. This secondary piston 110 comprises an indicator button 112 towards the outside and a cylindrical magnet 114 coaxial with the button 112 towards the inside. The magnet 114 has a north pole and a south pole , as

  indicates the drawing. The indicator button 112 is provided with an annular stop

  laire 118 intended to cooperate with a first stop 120 constituted by a ring crimped in the external face of the head 88 of the cap 86. A second stop 122 is force-fitted into the bottom of the bore 108. This second stop 122 is made preferably by a hard hub and serves to improve the impact resistance of the indicator button, as it is

  described in US Patent 3,128,743.

  It can therefore be seen that the secondary piston 110 can move

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  cer axially in the bore 108 between a first extreme position and

  a second extreme position. In the first extreme position, that my-

  be in Figure 1 in solid lines, the indicator button 112 abuts against the second stop 122. In the other extreme position, shown in broken lines in the same Figure 1, the stop 118 formed on the indicator button 112 meets the first stop annular 120. In the first extreme position, the indicator button 112 is fully depressed in the bore 108, while in the second extreme position, part of the

  indicator button emerges from this bore.

  The device described above operates as follows:

  The initial position of all the components of the device indicates

  sensor 10 sensitive to differential pressure according to the present invention

  tion is shown in Figure 1. The primary piston 26 is biased by the spring 42 against the slave ring 70, which is biased

  in turn against the base 100 of the annular cavity 96 of the end cap

  86, spring 42 being stronger than spring 98.

  As shown in Figure 3, during a cold start, a high differential pressure between the first sealed compartment 38 and the second sealed compartment 40 first moves the primary piston 26 against the resistance of the spring 42 towards the first end 14 of the shoemaker 12. However, the split ring 80 prevents the slave ring 70 from

  piston 26 to follow or accompany this movement of the primary piston 26.

  As the assembly heats up, if the filter element is not contaminated, the differential pressure decreases and the primary piston

  returns to the second end 16 of the boot 12.

  However, as shown in Figure 4, if the pressure differs

  tielle does not decrease sufficiently, once the set begins to-

  heating up, the split ring 80 is deformed due to the change in temperature

  temperature which occurs in the second sealed compartment 40 and, in con-

  sequence, this split ring 80 contracts in the annular groove 82.

  Once the split ring 80 has thus contracted, the slave ring can freely accompany the primary piston 26. If the pressure differs

  is sufficient to move the slave ring 70 of the piston 26

  beyond a predetermined distance to the first end 14 of the bowl

  tier 12, the O-spring 102 meets the recessed cylindrical surface 106 of the slave ring 70 of the piston. It should be emphasized that if the slave ring 70 is not displaced to an extent sufficient to allow engagement of the O-ring spring 102 in the recessed cylindrical surface 106, the slave ring 70 of the piston may return freely

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  in its initial rest position as soon as the differential pressure is relaxed. However, as soon as the toric spring 102 bears against the recessed cylindrical surface 106, the slave ring 70 of the piston cannot

  no longer return to its original position without the need to proceed

  der to a partial disassembly of the device. As the slave ring 70 of the piston moves, the magnetic poles of the annular magnet 74 cooperate with the magnetic poles of the cylindrical magnet 114 in order to move the secondary piston 110, as shown in Figures 4 and 5, in order to indicate the existence of a critical differential pressure condition. If an erroneous reading has occurred, i.e. if the slave ring 70 of the piston has not been

  blocked against any return movement by the O-ring spring 102, the

  positive indicator 10 sensitive to differential pressure can be re-

  engaged by manually pressing the indicator button 112. However,

  as soon as the O-spring '02 bears against the cylindrical surface in re-

  line 106, the slave ring 70 of the piston will be locked in a position which ensures constant stress on the indicator button 112 in the projecting or indication position, even after the hydraulic system

will have been put to rest.

  Thus, the indicating device sensitive to the differential pressure

  according to the present invention will not indicate the need to change

  manage the filter due to temporary differential pressure, for example

  example during a start-up operation. However, if the clogging is

  effective of a filter has been detected with precision, the device can only be reset by accessing the reset end 124 of the rod 30 of the primary piston 26. If a sufficiently energetic force is applied to this end of resetting 124 of the primary piston 26, we can overcome the resistance of the O-ring spring 102 so that the piston

  primary 26 and the asszrvie ring 70 of this piston return to their posi-

  initial tion. The force necessary to avoid automatic reclosing

  tick depends on the tension of the toric spring 102, sliding friction between the components and also the angles formed by the steps between the different surfaces which meet this toric spring 102. As soon as the primary piston 26 has been returned to its initial position, the piston

  secondary 110 can be reset from the outside.

  Figure 6 of the drawing shows a modification to the layout.

* indicator 10 sensitive to the differential pressure according to the invention

  tion, in the case where one must operate under temperature conditions located between close limits. An auxiliary spring 130 is housed

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  in the bottom of the annular groove 82 ', deeper than the annular groove

  area 82 described above. This auxiliary spring 130 assists the ring

  split 80 at low temperatures. At high temperatures, the ring fen-

  due 80 overcomes the biasing force of the auxiliary spring 130 and contracts in the annular groove 82 '.

  The above detailed description of the preferred embodiment

  of the invention constitutes the best mode envisaged by the Applicant at the time

  deposit. However, those skilled in the art will appreciate that variations and modifications can be made to the structure described

  above as well as its mode of operation, without departing from the main

  basic pipes of the invention. For example, these principles can also apply to a device comprising a diaphragm or a membrane or a

  bellows, instead of a piston-operated device. Likewise, the an-

  temperature sensitive water can be placed in either a throat

  ne, either external. These modifications and variants, as well as many other

  very, therefore remain within the scope of the invention.

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Claims (19)

R E V E N D I C A T I O N S   1. A device (10) for indicating a differential pressure   predetermined between an inlet pressure and an outlet pressure, ie   characterized by the fact that it comprises: a) a shoemaker (12) having a first end (14) and an opposite end (16);   b) an internal chamber (18) formed in this case and pre-   feeling a central axis; c) a piston (26) disposed in the center of this internal chamber o10 (18) and able to move in both directions in this chamber along said central axis, this piston (26) further having a part forming an enlarged head ( 28) which bears hermetically against the case so that the piston (26) divides and hermetically separates said chamber (18) into a first sealed compartment (38) located near the first end (14) and a second sealed compartment (40) located near the opposite end (16); d) a first passage (20) provided in the case (12) and which communicates with the outlet pressure, this passage (20) opening into the first compartment (38) of the internal chamber (18); e) a second passage (54) provided in the case (12) and which communicates with the inlet pressure, this passage (54) opening into the second compartment (40) -from the internal chamber (18);   f) a first biasing means (42) housed in the first   mier compartment (38) and interposed between the boot (12) and the piston (26), this first biasing means (42) exerting a first force   predetermined on the piston (26) so as to urge it towards said se-   end or opposite end (16) of the boot (12), so   that when a predetermined differential pressure exists between the pre-   first and second compartments (38, 40), said differential pressure pre-   determined overcomes said first predetermined force of said first biasing means (42), to move the piston (26) in said chamber (18) toward the first end (14);   g) a ring (70) slaved to the piston (26), movable in the se-   cond compartment (40) and having an end portion (78) intended to be actuated by the piston (26), this controlled ring (70) being able to move axially in the second compartment (40) along said central axis while being stressed by the first biasing means (42) and the piston (26) to an initial position located near said second end or opposite end (16); - 14 -   h) a second biasing means (98) arranged in the se-   cond compartment (40) and interposed between the case and the ring (70) as-   served to the piston (26), this second biasing means urging the slave ring (70) towards said first end (14) of the case (12) and against the piston (26) with a second predetermined force which is less than said first force predetermined; i) a stop means (80) sensitive to the temperature interposed between the case (12) and the slave ring (70), this stop means being sensitive to the temperature prevailing in the second compartment (40) and pocketing the displacement from the ring (70) slaved to the piston (26) towards the first end of the case (14) when the temperature in the second compartment (40) is lower than a predetermined value, but allows said displacement when the temperature in the second compartment ( 40) is greater than said predetermined value, and   j) indicator means (110) for indicating that the ring has   served (70) of the piston (26) has been moved to a predetermined extent towards said first end (14) of the case, this indicating means is   then moving between a retracted position of no indication and a po- salient indication.   2. Indicating device according to Claim 1, characterized by   the fact that the indicator means proper (110) is housed in said internal chamber (18) and that, in addition, part of this indicator means (110) projects through said opposite end (16) of the case   in said position of non-indication.   3. Indicating device according to Claim 1, characterized by   the fact that it comprises a locking means (102) which makes it possible to   unlockably rusting said slave ring (70) of the piston   (26), when the slave ring (70) has been moved by a predetermined amount   finished towards the first end (14) of the shoemaker (12).   4. Indicating device according to Claim 3, characterized in that it further comprises a resetting means (124) integral   of said piston (26) and which extends from this piston through an or-   groove (20) provided for this purpose in the first end (14) of the shoemaker   (12), so that the ring (70) slaved to the piston can be re-   engaged in its initial position by the application of ex-   prior to said reclosing means, so as to overcome the resistance   tance of said locking means (102).   5. Indicating device according to Claim 4, characterized in that the said second passage (54) has an opening (20) in c358255 - 15 - said first end (i4).   6. Indicating device according to Claim 3, characterized in that said locking means comprises: a) a first cavity (58) formed in the case (12) and which opens into the second compartment (40) of said internal chamber ( 18); b) a second stop (102) housed in the first cavity (58) and urged elastically against the slave ring (70) of the piston (26), and c) a second cavity (106) formed in the slave ring (70)   piston (26), this second cavity (106) automatically receiving the   end stop (102) when the slave ring (70) is moved on a device   predetermined tance towards said first end (14) of the boot (12).   7. Indicating device according to Claim 1, characterized by   the fact that the biasing means is constituted by a helical spring compression cord (42).   8. Indicating device according to Claim 1, characterized in that the second biasing means is constituted by a spring helical compression (98).   9. Indicating device according to Claim 1, characterized in that it further comprises: a) a first magnet (74) integral with the ring (70) slaved to the piston (26) and having a first pole of a first magnetic charge   which is turned towards the first end (14) of the shoemaker (12) and a se-   cond pole of a second magnetic charge which is turned towards the opposite end (16) of the case (12), these first and second poles generating first lines of magnetic flux, and b) a second magnet (114) movable relative to the shoemaker (12) and placed near the first magnet (74), this second magnet (114) having a third magnetic pole of a third magnetic charge, which is turned towards the first end (14) of the shoemaker (12) and a fourth magnetic pole of a fourth magnetic charge, which is turned towards the opposite end (16) of the case (12), these third and fourth magnetic poles generating second lines of magnetic flux, so that when the first magnet (74) is moved to the first end (14) of the case (12), the first lines of magnetic flux   of this first magnet (74) repel the magnetic flux lines of the   cond magnet (114) so as to move this second magnet (114) to indi-   that the slave ring (70) of the piston (26) has moved from said   predetermined quantity towards said first end (14) of the shoemaker (12). - 16 -   10. Indicating device according to Claim 1, characterized   by the fact that the abovementioned abutment means sensitive to temperature comprises   further takes: a) a stop (64) fixed in said case, and b) a thermo-sensitive stop element (80) integral with the slave ring (70) of the piston (26), this thermo-sensitive stop means   being mobile, depending on the temperature prevailing in the second com-   partiment (40), so as to approach or move away from its stop (64) fixed in the case (12).   11. A device (10) for indicating a differential pressure   predetermined between an inlet pressure and an outlet pressure, ie   characterized by the fact that it comprises: a) a shoemaker (12) having a first end (14) and an opposite end (16); b) an internal chamber (18) formed in the case (12) and having a central axis; c) a piston (26) disposed in the center of the internal chamber (18) in which it can move along said central axis, this piston further having a part forming an enlarged head (88) which bears hermetically against the case so that the piston (26) divides and hermetically separates said chamber (18) into a first compartment   waterproof (38) located near the first end (14) and a second compar-   seal tight (40) located near the opposite end (16); d) a first passage (20) provided in the case (12) and which communicates with the outlet pressure, this passage (20) opening into the first compartment (38) of the internal chamber (18); e) a second passage (54) provided in the case (12) and which communicates with the inlet pressure, this passage (54) opening into the second compartment (40) of the internal chamber (18); f) a first biasing means (42) housed in the first compartment (38) and interposed between the case (12) and the piston (26), this   first biasing means (42) exerting a first predeter-   mined on the piston (26) so as to urge it towards said second ex-   end or opposite end (16) of the case, such that when a predetermined pressure exists between the first and second compartments (38, 40), said predetermined differential pressure overcomes said first predetermined force of said first biasing means (42) in order to moving the piston (26) in said chamber (18) towards the first end (14); 255.8255 - 17 -   g) a ring (70) slaved to the piston (26), disposed in said   second compartment (40) and which has a part in abutment against the peel   tone, this slave ring (70) being able to move axially in both directions in the second compartment (40) along said central axis, while being stressed by the first biasing means (42) and by the piston (26 ) towards an initial position close to said opposite end (16); h) a second biasing means (98) disposed in the second compartment (40) and interposed between the shoemaker (12) and the slave ring (70) in order to urge this ring towards said first end (14) of the shoemaker (12) and against the piston (26) with a second predetermined force which is less than said first predetermined force; i) a stop means (80) sensitive to temperature, interposed between the case (12) and the slave ring (70), this stop means (80) being sensitive to the temperature prevailing in the second compartment (40) and preventing the displacement of the piston slave ring towards the first   end (14) of the case when the temperature in the second compartment   ment is lower than a predetermined temperature and allowing this de-   placement when the temperature in the second compartment (40) is above   less than said predetermined value; j) a locking means (102) which makes it possible to lock   unlockably said slaved ring (70) to the piston (26) when   that this ring (70) has been moved a predetermined amount in direction   tion of said first end (14), and   k) an indicator means (110) disposed in the center of said chamber   internal bre (18) to indicate that said slave ring (70) to the piston (26)   has been moved so that when the slave ring (70) has been removed   placed by a predetermined amount toward said first end (14) of the   shoemaker (12), the indicating means moves between a position of nonindi-   cation in said internal chamber (18) and an indicating position in which this indicating means protrudes through the opposite end (16) of the shoemaker (12).   12. Indicating device according to Claim 11, characterized   further by the fact that it includes a resetting means (124) so-   lid of the piston (26) and which extends from this piston through a   opening (20) provided for this purpose in the first end (14) of the bot-   tier (12), so that the slave ring (70) can be reset to its initial position by the application of external pressure to said reset means (124), so as to overcome the resistance - 18 - said locking means (102).   13. Indicating device according to Claim 12, characterized in that the second passage (22) has an opening (20) in said first end (14).   14. Indicating device according to Claim 12, characterized in that the locking means further comprises: a) a first cavity (58) formed in the case (12) and which opens into the second compartment (40) of the chamber internal (18); b) a second stop (102) housed in this first cavity (58) and urged against the slave ring (70) of the piston (26), and c) a second cavity (106) formed in the slave ring (70)   of the piston (26) and which automatically receives the second stop (102) when   that the slave ring (70) is moved a predetermined distance   towards said first end (14) of the shoemaker (12).   15. Indicating device according to Claim 11, characterized in that it further comprises: a) a first magnet (74) integral with the slave ring (70) of the piston (26) and having a first -p8le of a first magnetic charge   which is turned towards the first end (14) of the shoemaker (12) and a se-   cond p8le of a second magnetic charge which is turned towards the opposite end (16) of the case (12), these first and second poles generating first lines of magnetic flux, and b) a second magnet (114) movable relative to the shoemaker (12) and placed near the first magnet (74), this second magnet (114) having a third magnetic pole of a third magnetic charge which is turned towards the first end (14) of the shoemaker (12) and a fourth magnetic pole of a fourth magnetic charge which is turned towards the opposite end (16) of the case (12), these third and fourth magnetic poles generating second lines of magnetic flux, so that when the first magnet ( 74) is moved towards the first end (14) of the shoemaker (12), the first lines of magnetic flux   of this first magnet (74) repel the magnetic flux lines of the   cond magnet (114) so as to move this second magnet (114) to indi-   quer that the slave ring (70) to the piston (26) has moved from said   predetermined quantity towards said first end (14) of the shoemaker (12).   16. Indicating device according to Claim 12, characterized in that the said temperature-sensitive stop further comprises: a) a stop (64) fixed in the case (12); b) a heat-sensitive stop element (80) integral with the - 19 -   slave ring (70) of the piston (26), this thermosensitive abutment means being movable, as a function of the temperature prevailing in said internal chamber (18), so as to approach and move away from the abutment (64 ) fixed in the case (12).   17. Indicating device, intended to indicate a different pressure   predetermined differential between an inlet pressure and an outlet pressure   tie, characterized in that it comprises: a) a case (12) having a first end (14) and an opposite end or second end (16);   b) an internal chamber (18) formed in the case and comprising   both a central axis; c) a piston (26) disposed in the center and inside the internal chamber (18), this piston being able to move in both directions   in this chamber along said central axis and having a widened part   gie forming a head (28) which bears hermetically against the case (12) so that the piston (26) hermetically divides the internal chamber (18) into a first compartment (38) adjoining the first end (14)   and a second compartment (40) bordering the second end or end   opposite moth (16); d) a first passage (20) formed in the case (12), which   communicates with the outlet pressure and extends into the first compar-   timent (38) of the internal chamber (18); e) a second passage (54) which communicates in the case (12)   with the inlet pressure and which opens inside the second compar-   timent (40) of the internal chamber (18); f) a first biasing means (42) housed in the first compartment (38) and interposed between the case (12) and the piston (26),   this first biasing means (42) exerting a first predefined force   terminated on the piston (26) so as to urge it towards said second end or opposite end (16) of the case (12), so that   when a predetermined pressure exists between the first and second com   partitions (38, 40) said: predetermined differential ression overcomes said first predetermined force of said first biasing means   (42) in order to move the piston (26) in said chamber (18) towards the first   first end (14); g) a slave ring (70) to the piston (26), disposed in said second compartment (40) and which has a part (82) which abuts against the piston, this ring (70) being able to move axially in both directions in the second compartment (40) along said central axis, while s58255 - 20 -   being requested by the first requesting means (42) and by the   piston (26) towards an initial position close to said oppo- sée (16);   h) a second biasing means (98) arranged in the se-   cond compartment (40) and interposed between the boot (12) and the slave ring (70) in order to urge this ring towards said first end   (14) of the case (12) and against the piston (26) with a second force pre-   determined which is less than said first predetermined force; i) a stop means (80) sensitive to temperature, interposed between the case (12) and the slave ring (70), this stop means (80) being sensitive to the temperature prevailing in the second compartment (40)   and preventing the movement of the piston slave ring towards the pre-   1st end (14) of the case when the temperature in the second   partiment (14) is lower than a predetermined temperature and allowing this movement when the temperature in the second compartment (40) is higher than said predetermined value; j) a locking means (102) which makes it possible to lock the slave ring (70) to the piston (26) in an offset position after the slave ring (70) has been moved in a predetermined measure towards said opposite end (16) , this locking means having the role of maintaining said slave ring in its offset position when said predetermined differential pressure no longer exists between the first and second compartments (38, 40); k) a reset means (124) integral with the piston (26) and which extends from this piston through said second passage (20), this reset means (124) being operable to reset the slave ring ( 70) to the piston (26) in its initial position, and   1) an indicator means (110) disposed in the center of said chamber   internal bre (18) to indicate that said slave ring (70) to the piston (26) has been moved, so that when the slave ring (70) has been moved by a predetermined amount toward said first end (14)   of the bootmaker (12), the indicating means moves between a position of no   indication in said internal chamber (18) and an indication position   in which this indicator means projects through the op- posed (16) of the shoemaker (12).   18. Indicating device according to Claim 17, characterized in that it also includes: a) a first magnet (74) integral with the ring (70) slaved to the piston (26), this first magnet having a first pole d 'a first - 21 -   magnetic charge which is turned towards the first end (14) of the bot-   tier (12) and a second pole of a second magnetic charge which is turned towards the opposite end (16) of the case (12), these first and second poles generating first lines of magnetic flux, and b) a second magnet (114) movable relative to the case (12) and placed near the first magnet (74), this second magnet (114) having a third magnetic pole of a third magnetic charge which is turned towards the first end (14) of the bottom case (12), and a fourth magnetic pole of a fourth magnetic charge which is turned towards the opposite end (16) of the bottom case (12), these third and fourth magnetic posts generating second lines of magnetic flux, such so that when the first magnet (74) is moved to the first end (14) of the case (12), the first lines of magnetic flux   of this first magnet (74) repel the magnetic flux lines of the   cond magnet (114) so as to move this second magnet (114) to indi-   quer that the ring (70) slaved to the piston (26) has moved from said   predetermined quantity towards said first end (14) of the shoemaker (12).   19. Indicating device according to Claim 12, characterized in that the said temperature-sensitive stop further comprises: a) a stop (64) fixed in the case (12); b) a thermosensitive abutment element (80) integral with the slave ring (70) of the piston (26), this thermosensitive abutment means being movable, as a function of the temperature prevailing in said internal chamber (18), so to approach and move away from the stop (64) fixed in the case (12).