System for measuring differential pressure in an underwater environment

Abstract

According to one aspect, the present description relates to a system (100) for measuring differential pressure in an underwater environment, comprising a fibre measuring instrument (101) comprising: a rigid first compartment (110) in fluid connection with a rigid first duct (112) and filled with a first liquid; an at least partially rigid second compartment (120) in fluid connection with an at least partially flexible second duct (122) and filled with a second liquid; a valve (140) configured to allow, in the open position, a fluid connection between the first duct and the second duct and, in the closed position, fluidtight separation of the first duct from the second duct; a membrane (150) configured to separate the first compartment and the second compartment; a porthole (162) arranged in a common wall between a third compartment and one of the first or second compartments; a fibre optic head (170) configured to form, with a transmission / reception unit, a fibre interferometric sensor configured to measure, through said porthole, when the valve is in the closed position, a movement of the membrane resulting from a variation in pressure between the first compartment and the second compartment.

Description

[0001] Differential pressure measurement system in an underwater environment This description relates to a system for measuring pressure in an underwater environment, and more specifically to measuring differential pressure, as well as a method for measuring differential pressure in an underwater environment. State of the art Pressure variation measurements are useful in many applications such as medicine, petrochemistry or aerospace and aeronautics, for example in wind tunnels. In particular, pressure variation measurements are used in geosciences and seabed geodesy to measure variations in sea level, in order, for example, to be able to detect variations in height at several points of the Earth's crust to study and anticipate seismic or volcanic events. These measurements can also be used for tsunami detection.To carry out such measurements of pressure variations, it is known to use resonant quartz crystal pressure sensors, i.e. sensors comprising quartz electric oscillators whose resonant frequency varies according to the pressure constraints to which they are subjected. Such resonant quartz crystal pressure sensors are for example described in the document [Ref 1]. However, such sensors are complex sensors which require a source of electrical energy and electronic components which must not undergo oxidation. Beyond the cost they represent, these sensors must therefore be connected to an external power supply or include a battery at the sensor head, i.e. where the interaction between the sensor and the environment to be measured takes place.This is not desirable when the sensors are deployed in hostile or remote environments in which human intervention, for example to change a discharged battery, is extremely difficult or impossible (deep drilling, seabed, irradiating or electromagnetic environment, high temperature). However, in the case of pressure measurements for geoscience applications, it is often preferable to get as close as possible to certain areas of the Earth's crust where significant tectonic events are taking place. Thus, it is sometimes necessary to carry out pressure measurements in points of the Earth's crust submerged at a depth of several kilometers. In this configuration, the use of quartz crystal pressure sensors is therefore not always adequate and other types of sensors would be preferred.Other types of pressure sensors are known as the membrane interferometric pressure sensor, embodiments of which are disclosed, for example, in [Ref. 2]. A membrane interferometric sensor as described in [Ref. 2] comprises a Fabry-Pérot cavity closed by a membrane. An optical fiber projects a laser beam onto a first face of the membrane which reflects the signal towards the fiber. A second face of the membrane is in contact with an environment, external to the cavity, the pressure of which is to be measured. A pressure variation of the environment causes a pressure variation on the second face of the membrane; the latter deforms, modifying the way in which the laser beam is reflected towards the fiber.By analyzing the interferences that occur between the beam reflected by the membrane and a reference beam, for example from a reflection at the interface between the optical fiber and the cavity, the pressure variations of the environment can be determined. In such a sensor, the generation of the laser signal and the pressure measurement via interference analysis can be carried out remotely from the sensor head, by means of an optical fiber. The laser source, the associated power supply and the electronic components can be moved over significant distances. It is possible to maintain good measurement sensitivity provided that there is low dispersion and low absorption in the transport of the optical beams by the optical fiber. However, the membrane interferometric sensor as described in [Ref.2] is only suitable for micro-pressure measurements, i.e. pressure variation measurements less than or equal to around ten kPa. Such a sensor is therefore not suitable for measuring pressure in the seabed where the surrounding pressure may be greater than several hundred bars, i.e. greater than 10. 7Pa. The present description relates to a pressure measurement system applied to measurements of pressure variations under high pressure, in particular in an underwater environment. Summary of the invention In the present description, the term "comprise" means the same as "include" or "contain", and is inclusive or open and does not exclude other elements not described or shown. Furthermore, in the present description, the term "about" or "substantially" is synonymous with (means the same as) has a lower and / or upper margin of 10%, for example 5%, of the respective value. According to a first aspect, the present description relates to a differential pressure measurement system in an underwater environment comprising: - at least a first transmitting / receiving unit comprising a light source,detection means and signal processing means; - at least one first fiber measuring instrument configured to be positioned at a first position in said underwater environment and comprising: - a first rigid compartment in sealed fluid connection with a first rigid conduit, the first compartment being filled with a first liquid; - a second at least partially rigid compartment, in sealed fluid connection with a second at least partially flexible conduit, the second compartment being filled with a second liquid; - a third conduit for taking underwater pressure, said third conduit being open to the underwater environment and in fluid connection with the second conduit; - at least one first valve configured to allow, in the open position, a fluid connection between the first conduit and the second conduit, in order to maintain equal pressure in the first compartment and the second compartment,and configured to separate in a sealed manner, in the closed position, the first conduit from the second conduit; - a membrane configured to separate the first compartment and the second compartment; - a third compartment, sealed, filled with a fluid at atmospheric pressure; - at least one first porthole, transparent in a given wavelength range, arranged in a common wall between said third compartment and one of the first or second compartments; - at least one first fiber optic head configured to form with said first transmission / reception unit, a first fiber interferometric sensor, said first fiber interferometric sensor being configured to measure, in the closed position of the first valve, through said first porthole,at least a first displacement of the membrane resulting from a pressure variation between the first compartment and the second compartment when the first measuring instrument is subjected to a variation in underwater pressure. By "rigid", it is understood in the present description that the first compartment and the first conduit in fluidic connection with the first compartment do not undergo deformations when they are subjected to the pressures of the external environment relative to the pressure of the first liquid, for example at most 5 bars. In the present description, a second conduit that is at least partially flexible is a conduit that comprises at least locally flexible walls, that is to say walls capable of deforming when they are subjected to the pressures of the external environment relative to the pressure of the second liquid, typically pressures greater than approximately 1 bar. In the present description,a sealed separation produced by the membrane between the first compartment and the second compartment means that the membrane does not allow any liquid to pass below a predetermined threshold pressure, for example a threshold pressure equal to approximately 1 bar, advantageously 5 bars. In the present description, a sealed separation produced by the first valve in the closed position between the first conduit and the second conduit means that the first valve does not allow any liquid to pass below a predetermined threshold pressure, for example a threshold pressure equal to approximately 10 bars,advantageously 200 bars. The first fiber interferometric sensor is a non-contact sensor configured to measure a displacement of the membrane as a function of an interferometric signal resulting from interference between a reference optical beam and a signal optical beam resulting from the reflection by the membrane of an incident optical beam emitted by said light source. Interferometric sensors known from the state of the art can be implemented in a differential pressure measurement system according to the first aspect. For example, an optical displacement sensor comprising a Fabry-Pérot type interferometer is known and described for example in [ref. 3]. According to one or more exemplary embodiments, said at least one first fiber optical head comprises: - a first optical fiber for transporting a light beam emitted by the light source to the membrane,one end of said first optical fiber being arranged in the third compartment; - a first collimator for collimating said light beam emitted by the source and coming from said first optical fiber, in which said end of the first optical fiber is located in an object focal plane of the collimator. The reference optical beam is then for example an optical beam resulting from the reflection by the end of said first optical fiber of the incident optical beam emitted by said light source. The differential pressure measuring system according to the first aspect thus comprises at least said first fiber measuring instrument which, in operation, is intended to be positioned at a first given position in the underwater environment whose pressure variations are to be measured and said first transmission / reception unit which can be moved away from the measurement location,thanks to fiber technology. The first transmission / reception unit comprises the light source, the signal detection and processing means and may also comprise electrical power sources, data recording and / or transmission means. Data processing can thus be carried out at great distances, up to a few tens of kilometers, making it possible to envisage underwater installations of measuring instruments at great depth. Furthermore, in a differential pressure measurement system according to the first aspect, the first and second compartments remain in equal pressure when, in an initial state, the first measuring instrument is lowered into the underwater environment to the seabed and the first valve remains open. To initiate the differential pressure measurement, the first valve is closed. The first compartment, rigid,is isolated from the external pressure and maintains the initial pressure. The second compartment, in fluid connection with an at least partially flexible conduit itself in connection with the underwater environment, is subjected to variations in external pressure and transmits the external pressure. A force is then created on the membrane producing a displacement proportional to the pressure variation. The applicants have noted that the original arrangement of the first measuring instrument with two compartments separated by a first valve according to the first aspect allows differential pressure measurements in an underwater environment at high pressure, typically up to 5 bars without risk of deterioration of the membrane,since the membrane only undergoes pressure variations and not the pressure of the underwater environment. It is thus possible to carry out a descent to the deep seabed in complete safety for the instrument and to avoid any high-pressure resistant protective structure which is complex, expensive and which reduces the reliability of the instrument. According to one or more exemplary embodiments, said first measuring instrument further comprises a mechanical protection enclosure which is not sealed with the underwater environment, inside which the first compartment, the second compartment and the third compartment are arranged. According to one or more exemplary embodiments, the fluid which fills the third compartment is a gas, for example dry air or nitrogen. According to one or more exemplary embodiments, the membrane comprises a thin film, for example a silicon, stainless steel or titanium film,for example in the form of a disc. A thickness of the thin film is for example between approximately 90 µm and approximately 500 µm. Advantageously, the membrane may further comprise a thin reflective layer, for example made of gold, to optimize the reflection of the incident beam. A thickness of the reflective layer is for example between approximately 2 µm and approximately 10 µm. The reflective layer is for example located in a central region of the membrane. According to one or more exemplary embodiments, the first liquid which fills the first compartment and / or the second liquid which fills the second compartment is transparent in a wavelength range between approximately 1300 nm and approximately 1600 nm. This wavelength range corresponds to wavelengths of light beams advantageously used in a fiber interferometric sensor, for example a sensor described in [Ref. 3]. By transparent,it is understood that the liquid has an absorption of less than approximately 20% in the entire wavelength range considered. According to one or more exemplary embodiments, the first liquid which fills the first compartment and / or the second liquid which fills the second compartment has a density substantially equal to that of water, or greater than that of water, advantageously greater than or substantially equal to twice that of water. The first liquid and / or the second liquid is preferably immiscible with water to limit the risks of leakage. A liquid denser than water is easier to handle. In exemplary embodiments, the first liquid which fills the first compartment and / or the second liquid which fills the second compartment has a kinematic viscosity coefficient close to that of water,for example between approximately 1 and approximately 15 mm² / s to optimize the instrumental response. For the first liquid and / or the second liquid, it is possible to use, for example, perfluorinated oils having a density equal to approximately 1.9, for example LYF oils from the company Lubrilog® (8 mm² / s at 20°C) or silicone oils with a density approximately equal to 1, for example silicone oils from MerckEurolab® or Lubrilog® (1 mm² / s at 20°C). In exemplary embodiments, the first compartment comprises a wall made of a material whose coefficient of thermal expansion makes it possible to at least partially compensate for the thermal expansion of the first liquid located in the first compartment. The larger the first compartment, the greater the sensitivity of the measurement but the thermal sensitivity also increases. In exemplary embodiments, the first liquid has a coefficient of thermal expansion of between approximately 10, -5 / °C and approximately 1.1.10-3 / °C approximately 9.6.10 -4 / °C and approximately 1.1.10 -3 / °C. In exemplary embodiments, the wall of the first compartment may be made with a material having a high thermal expansion coefficient, for example between 150.10 -6 / °C and 230.10 -6 / °C, for example between 200.10 -6 / °C and 230.10 -6 / °C, for example high-density polyethylene (HDPE) or low-density polyethylene (LDPE). According to one or more exemplary embodiments, the first liquid and the second liquid are identical and the first compartment and the second compartment are thus filled with the same liquid. In other exemplary embodiments, the first liquid and the second liquid are different. For example, the first liquid that fills the first compartment has a coefficient of expansion lower than the coefficient of expansion of the second liquid that fills the second compartment, a ratio between the coefficients of expansion being for example between approximately 1 and approximately 100, advantageously between approximately 10 and approximately 100. The applicant has shown that with a difference between the coefficients of expansion of the first liquid and the second liquid, the impact of temperature variations on the measurements could be reduced.When choosing a different first liquid and a different second liquid, it is advantageous to choose immiscible liquids. According to one or more exemplary embodiments, said first measuring instrument further comprises a fourth compartment in fluid communication with the first compartment, said fourth compartment comprising a wall and at least one first rigid element immersed in the first liquid circulating within said fourth compartment, said at least one first rigid element having a coefficient of thermal expansion lower than the coefficient of thermal expansion of the wall. In cases in particular where the thermal expansion of the material from which the wall of the first compartment is formed does not compensate for that of the first liquid, such an arrangement makes it possible to increase the volume of the assembly consisting of the first and fourth compartments, and to compensate for the expansion of the first liquid.Indeed, it will be possible to generate a lower average thermal expansion of the rigid element / liquid assembly which can be compensated by the thermal expansion of the material from which the wall of the fourth compartment is formed. For example, said at least one first rigid element has a coefficient of thermal expansion of between 10. -6 m / m / °C and 20 x 10 -6m / m / °C. For example, said at least one first rigid element is made of stainless steel, titanium, invar, or pyrex. In exemplary embodiments, said at least one first rigid element is a solid or hollow element. There may be a first main rigid element and additional rigid elements smaller than the main rigid element to refine the compensation for thermal expansion. According to one or more exemplary embodiments, said first measuring instrument further comprises a second fiber optic head arranged in said third compartment and configured to form, with said transmitting / receiving unit, a second fiber interferometric sensor configured to measure a temperature variation of said third compartment. The temperature variation within the third compartment will be very substantially identical to the temperature variation within the first liquid and / or second liquid.By measuring possible temperature variations, it will be possible to compensate for possible measurement artifacts linked to thermal expansion, particularly of the first liquid.According to one or more exemplary embodiments, said first measuring instrument further comprises: - a fourth compartment, sealed, filled with a fluid at atmospheric pressure, for example air; - a second porthole arranged in a common wall between said fourth compartment and the other of the first or second compartments; and - a third fiber optic head configured to form with said first transmission / reception unit, a third fiber interferometric sensor, said third fiber interferometric sensor being configured to measure, in the closed position of the first valve, through said second porthole, a second displacement of the membrane resulting from a pressure variation between the first compartment and the second compartment when the system is subjected to a variation in underwater pressure.A double measurement will make it possible to determine a possible drift of the first fiber interferometric sensor, drift resulting for example from a drift in the wavelength of the light source, and to correct the resulting measurement artifacts. According to one or more exemplary embodiments, said first measuring instrument further comprises a fourth fiber optical head arranged in said third compartment and configured to form with said first transmission / reception unit, a fourth fiber interferometric sensor configured to measure a drift of said first fiber optical displacement sensor. Such a fiber interferometric sensor will be able to detect a drift of the first sensor and to correct the resulting measurement artifacts, while eliminating the need for a fourth compartment with a second window.According to one or more exemplary embodiments, the system for measuring differential pressure in an underwater environment according to the first aspect further comprises a second differential pressure measuring instrument configured to be positioned at a second position in said underwater environment. With differential pressure measurements carried out at different positions, it will be possible to interpret more reliably the origin of these underwater pressure variations. In preferred exemplary embodiments, said second measuring instrument is a fiber measuring instrument which cooperates with said first transmitting / receiving unit for measuring the differential pressure. For example, the second measuring instrument is a measuring instrument comprising characteristics similar to those of the first measuring instrument. It is then possible to work with the same source for both measuring instruments.This makes it possible to correct a possible internal drift of the two instruments, by taking the difference in the measurements provided by the two instruments, this drift being identical for the two instruments because they operate with the same light source. It is then possible to analyze, for example, a variation in altitude or a relative vertical displacement between the two instruments, for example when one of them is located at the circumference of an underwater volcanic edifice and the other on a summit area. It is then possible to analyze a variation in sea level between the two instruments, for example a difference in sea level produced by a tsunami. According to a second aspect, the present description relates to a method for measuring differential pressure in an underwater environment implemented by means of a differential pressure measuring system according to the first aspect.According to one or more exemplary embodiments, the method comprises: - opening the first valve to equalize the pressure of the first compartment with the second compartment; - installing, in the underwater environment for which a differential pressure is to be measured, said first measuring instrument; - closing the first valve to separate, in a sealed manner, the first conduit from the second conduit; - measuring, in the closed position of the first valve, through said porthole, a displacement of the membrane resulting from a pressure variation between the first compartment and the second compartment when the first measuring instrument is subjected to a variation in underwater pressure. Brief description of the figures Other advantages and characteristics of the invention will appear on reading the description, illustrated by the following figures: [Fig.1], represents a diagram of an example of a differential pressure measurement system in an underwater environment according to the present description; [Fig. 2], represents a diagram illustrating the displacement of the membrane in a differential pressure measurement system according to the present description, when the system is in use; [Fig. 3A], represents a diagram of a membrane, in a first exemplary embodiment; [Fig. 3B], represents a diagram of a membrane, in a second exemplary embodiment; [Fig. 4], a photo showing an example of a membrane conforming to the membrane illustrated in Fig. 3B; [Fig. 5A], represents a diagram of a membrane in a third exemplary embodiment, in use in an example of a differential pressure measurement system according to the present description, in an initial state; [Fig. 5B], represents a diagram of the membrane described in Fig.5A, in use in an example of a differential pressure measurement system according to the present description, during a differential pressure measurement; [Fig. 6], represents a diagram of another example of a differential pressure measurement system in an underwater environment according to the present description; [Fig. 7], represents a diagram of an example of another example of a differential pressure measurement system in an underwater environment according to the present description; [Fig. 8], represents a diagram of another example of a differential pressure measurement system in an underwater environment according to the present description; [Fig. 9A], represents a diagram of a collimator in an example of a first fiber optic head of a measurement apparatus according to the present description, the collimator being configured for spatial shaping of a light beam from an optical fiber of said first optical fiber and its orientation towards the membrane; [Fig.9B], represents a diagram illustrating in more detail the operation of the collimator shown in Fig. 9A, when said collimator has an alignment defect. The appended figures are schematic and are not necessarily to scale, they are intended above all to illustrate the principles of the invention. In the figures, identical elements (or parts of elements) are identified, when possible, by the same reference signs. Detailed description of the invention Fig. 1 represents a diagram of an example of a differential pressure measurement system 100 in an underwater environment according to the present description. The system 100 comprises at least a first measuring instrument 101 and an optical transmission / reception unit 102.The measuring instrument 101 comprises a first rigid compartment 110 in fluid-tight connection with a first rigid conduit 112 and a second compartment 120 at least partially rigid, in fluid-tight connection with a second conduit 122 at least partially flexible. The first compartment is filled with a first liquid 131 and the second compartment is filled with a second liquid 132. In exemplary embodiments, the first compartment and the second compartment are filled with the same liquid, the first liquid and the second liquid being identical. The second conduit 122 is in fluid connection with a third conduit 123b open to the underwater environment for taking underwater pressure. The second conduit 122 is partially flexible to facilitate the transmission of external pressure, in operation. For example, as illustrated in FIG.1, the second conduit 122 is directly connected to the third conduit 123b via a second valve 141, for example a 3-way T-valve allowing liquid to be added and through which the external pressure P is transmitted. ext The measuring instrument 101 further comprises a first valve 140 configured to allow, in the open position, a fluid connection between the first conduit 112 and the second conduit 122. The first valve 140 thus makes it possible, in the open position, to maintain equal pressure between the first compartment 110 and the second compartment 120. The first valve 140 is furthermore configured to separate in a sealed manner, in the closed position, the first conduit 112 from the second conduit 122. The first valve 140 therefore makes it possible to isolate the compartment 110 and to maintain an initial pressure P iinside the first compartment 110. The differential pressure is then quantified relative to this initial pressure. As illustrated in Fig. 1, the measuring instrument 101 also comprises a membrane 150 configured to separate the first compartment and the second compartment. The measuring instrument 101 further comprises a third, sealed compartment 160, filled with a fluid 161 at atmospheric pressure P atm , for example a gas, for example dry air or nitrogen, and a porthole 162, transparent in a given wavelength range, arranged in the example of Fig. 1 in a common wall between the third compartment 160 and the second compartment 120. In exemplary embodiments, at least the second compartment 120 and the third compartment 160 comprise walls with a coefficient of thermal expansion less than 20 x 10 -6m / m / °C, for example stainless steel or titanium walls. In exemplary embodiments, the first compartment 110 comprises a wall made of high-density polyethylene or another material having a coefficient of linear thermal expansion greater than approximately 200 µm / m / °C; this makes it possible to at least partially compensate for the expansion of the first liquid in the first compartment and thus minimize the thermal sensitivity of the instrument. As will be described in more detail with reference to Fig. 8, the first compartment may also be in fluid communication with a fourth compartment within which is immersed in the first liquid a rigid element (for example made of stainless steel, titanium, invar or pyrex) expanding less than the first liquid. It is then possible to adjust the equivalent expansion of the liquid + rigid element assembly with the temperature and thus optimize the thermal compensation. As illustrated in Fig.1, the first compartment 110, the second compartment 120 and the third compartment 160 can be arranged in an enclosure 180 for mechanical protection but not sealed with the outside. The enclosure 180 provides mechanical protection for the different compartments (shocks, handling, etc.) but must not be sealed with the surrounding environment so as not to hinder the measurement of the differential pressure. It is for example in the form of a cylinder with a length for example less than or equal to 1 m and an external diameter for example greater than 10 cm, for example between approximately 10 cm and approximately 20 cm. The first valve 140 is for example placed outside this enclosure so that it can be manipulated by a remotely operated underwater vehicle (or “ROV” according to the abbreviation of the English expression “Remotely Operated Vehicle”). The first valve 140 can also be located inside the protective enclosure 180 if a solenoid valve is chosen.The first valve 140 is advantageously sealed against pressure variations greater than several tens of bar, during the operating time of the instrument which may be greater than several years. The first valve is for example made of stainless steel or titanium. To ensure equal pressure between the first compartment and the second compartment (first valve open), the second compartment 120 at least partially rigid is in sealed fluid connection with the second conduit 122 at least partially flexible. The material forming the flexible conduit 122 may be chosen to be compatible with the external environment and with the second liquid 132, that is to say non-porous and non-chemically reactive. For example, a polyvinyl chloride (PVC) type material may be chosen for the material forming the flexible tube. In the example of Fig. 1, the flexible tube 123 is located inside the protective enclosure 180.Outside the enclosure, the flexible tube 123 of the second conduit 122 may be extended by a section 124 made of a more robust and rigid material to withstand external impacts, for example stainless steel or titanium. The section 124 extends to the first valve 140 separating the first conduit 112 and the second conduit 122. The first rigid compartment 110 is in fluid-tight connection with a first rigid conduit 112. The first rigid conduit 112 comprises, for example, a tube, as illustrated in Fig. 1. It is sufficiently rigid so as not to undergo deformations when subjected to the pressures of the external environment relative to the pressure of the first liquid 131. The objective is to ensure that the first liquid 131 inside the first compartment 110 does not undergo any compression despite the pressure variations of the surrounding external environment.In other words, it is sought to ensure that the volume of the first compartment remains at an initial reference pressure before closing the first valve 140 which isolates the first compartment 110 from the second compartment 120. For example, the first conduit 112 is made of stainless steel or titanium. The second liquid 132 which fills the second compartment 120, the second conduit 122 which is at least partially flexible and the third conduit 123b is preferably non-absorbent in the near infrared, more precisely, preferably transparent in a wavelength range between approximately 1300 nm and approximately 1600 nm. Indeed, light beams at such wavelengths in particular can be transported in very long optical fibers with very low attenuation. For example, wavelengths used today in telecoms, namely 1310 nm and 1550 nm, can be used.For the second liquid 132, water can be used, but since water absorbs in the near infrared, this reduces the distance that the incident optical beam can travel in the liquid, and consequently the measurement range for the differential pressure. Thus, in practice, a liquid that is immiscible with water and denser than water will be chosen to minimize the risk of leakage. An oil could be chosen, for example, an oil with a density at least twice that of water. For example, a perfluorinated oil could be used. The first liquid 131 may be identical to the second liquid 132. However, different liquids, preferably immiscible ones, could be chosen.For example, the first liquid 131 that fills the first compartment may have a coefficient of expansion lower than the coefficient of expansion of the second liquid 132 that fills the second compartment, the difference between the coefficients of expansion being for example between about 1 and about 100, for example between about 10 and about 100. The applicant has shown that with a difference between the coefficients of expansion of the first liquid and the second liquid, the impact of temperature variations on the measurements could be reduced. For example, for the first liquid 131, pure water may be used, and for the second liquid, an oil may be used. In exemplary embodiments, the first measuring instrument further comprises a valve 142 arranged on a conduit 110b in fluid communication with the first compartment 110 and / or a valve 143 arranged on a conduit 120b in fluid communication with the second compartment 120.These valves make it possible to inject the liquid inside the instrument and / or to drain it. This also makes it possible to isolate the liquid from the outside during transport of the instrument, for example before launching it into the water. The first measuring instrument 101 further comprises a fiber optic head 170 configured to form, with the transmission / reception unit, a fiber interferometric sensor. This fiber interferometric sensor is configured to measure, in the closed position of the first valve, through the porthole 162, a displacement of the membrane resulting from a pressure variation between the first compartment 110 and the second compartment 120 when the first measuring instrument is subjected to a variation in underwater pressure. The porthole 162 makes it possible to pass the incident light beam, for example a laser beam having a diameter of between approximately 0.5 mm and approximately 3 mm from the third compartment (160, Fig.1) at atmospheric pressure to one of the first and second compartments separated by the membrane. In the example of Fig. 1, the porthole 162 is arranged in a common wall between the third compartment 160 and the second compartment 120 connected to the at least partially flexible conduit and which transmits the pressure variations of the external environment when the first valve 140 is closed. However, in other configurations, the porthole can quite easily be arranged in a common wall between the third compartment 160 and the first rigid compartment 110 configured to remain at the same pressure P. iof reference when the first valve 140 is closed. The porthole 162 has suitable optical and mechanical properties. It is made of a material that can withstand the external pressure of the seabed, and in particular the deep seabed, for example up to approximately 600 bars. For example, for the porthole, silica, or advantageously sapphire, whose mechanical properties for resistance to high pressures are very good, may be used. The porthole 162 is also designed to allow the incident laser beam and the reflected laser beam to pass through with the least possible absorption and the maximum transmission. To minimize parasitic reflections, the porthole may be coated with an anti-reflection treatment, advantageously on each of its faces. In exemplary embodiments, the porthole has the shape of a blade with flat and parallel faces.It is for example cylindrical, and comprises for example a diameter of between approximately 2 cm and approximately 5 cm and for example a thickness of between approximately 5 mm and approximately 15 mm to optimize resistance to high external pressures. In exemplary embodiments, the first measuring instrument has dimensions d1, d2, d3 with d1 between approximately 5 cm and approximately 15 cm, d2 between approximately 10 cm and approximately 20 cm, d3 between approximately 50 cm and approximately 1 m. As illustrated in Fig. 1, the fiber optic head 170 generally comprises an optical fiber 172a, 172b configured to transport into the third compartment 160 an incident light beam B1 emitted by a light source 177 of the transmission / reception unit, remote from the medium in which the measurement is made. As will be described in more detail later, the incident light beam is directed towards the membrane 150 and is reflected by said membrane.The reflected beam interferes with a reference beam to form an interferometric signal sent to detection means 178 of the transmission / reception unit. The signal thus detected is processed by signal processing means 179 of the transmission / reception unit to determine a displacement of the membrane 150. In operation, a displacement of the membrane causes a modification of the optical path undergone by the reflected beam and therefore a modification of the interference signal. In exemplary embodiments, a prior calibration of the system makes it possible to associate a displacement of the membrane with a pressure variation. In a differential pressure measurement system 100 as illustrated in FIG.1, it is thus possible to distinguish the measuring instrument 101 with all the elements intended to be positioned in the environment in which the measurement is made and the transmission / reception unit 102 with all the elements remote from said environment, arranged for example on a surface platform (port or marine platform). The transmission / reception unit 102 comprises for example all the electrical, electronic, or power supply elements and, in general, all the elements whose maintenance or operation are difficult in an underwater environment. The transmission / reception unit 101 and the measuring instrument 102 are connected by the optical fiber 172a, 172b. In the example of Fig. 1, the optical fiber comprises a first section 172a for transporting the light between the surface platform and the third compartment. This first section comprises for example a reinforced sheath.Its length may be from a few tens of meters to a few tens of kilometers, for example between approximately 10 m and approximately 40 km. The optical fiber also comprises, in the example of Fig. 1, a second section 172b for transporting the light into the third compartment 160. The measuring instrument may comprise a sealed connection element 176 allowing the insertion of the optical fiber into the third compartment 160. As illustrated in Fig. 1, the fiber optic head advantageously comprises a collimator 174 at the output of the fiber section 172b configured to form a collimated incident beam from the light beam coming from the fiber 172b and a plate 175 for optical alignment of the collimator 174 to optimize the reflection of the collimated beam on the membrane 150. The collimator 174 and the plate 175 are arranged within the third compartment 160.The collimator 174 advantageously allows, by collimating the optical beam, to aim the membrane 150 at a distance of between 5 mm and 20 mm from the collimator. This distance of several centimeters firstly allows the beam to exit from the optical fiber into the air. This allows for a fiber / air interface which allows the generation of a reference beam resulting from the reflection (approximately 4%) on said interface. Furthermore, the collimated beam passes through the second liquid 132 of the second compartment 120 over a distance large enough not to be subject to artifacts which could result from too thin a layer of liquid. The centimeter distance of the membrane from the fiber outlet also allows more flexibility in the choice of the membrane (dimension, thickness) in order to optimize the sensitivity and the measurement range of the instrument. In operation, in a differential pressure measurement system as illustrated in FIG.1, during the descent to the seabed, the first valve 140 is open, the first compartment 120 at pressure P. ext and the second compartment 110 with pressure P i are in equal pressure between them (P ext = P i), but also in equal pressure with the external environment, Then, once placed on the seabed, for the differential pressure measurement, the first valve is closed and maintains an initial state of pressure in the isolated compartment 110. Fig. 2 thus illustrates the deformation of the membrane, when the first valve is closed and a pressure variation is exerted by the underwater environment compared to the initial state. In Fig. 2, for the sake of simplification, only the first compartment 110, second compartment 120 and porthole 162 of the third compartment are shown. The first compartment 110, rigid, remains isolated from the external pressure and maintains the initial pressure P1. The second compartment, in fluid connection with an at least partially flexible conduit, is subject to external pressure variations. The pressure of the second compartment differs.A force is then created on the membrane producing a displacement proportional to the pressure variation. Thus, a pressure P. ext of the second compartment greater than the pressure P i of the first compartment, causes a displacement +dx of the membrane and conversely, a pressure P extlower will result in a displacement – ​​dx. A prior calibration of the system can make it possible to determine the differential pressure from the displacement value dx. Fiber interferometric sensors for measuring the displacement dx are known from the state of the art and can be implemented in a differential pressure measurement system according to the present description. For example, an interferometric sensor comprising a Fabry-Pérot type interferometer is known and described for example in [ref. 3]. As illustrated in Fig. 2, an incident optical beam B1 from the optical fiber 172b passes through the porthole 162 and is reflected on the membrane 150 in displacement due to pressure variations. The reflected beam B2 is transported again by the fiber. Interference occurs between the beam reflected by the membrane B2 and a beam reflected at the end of the fiber, not shown in the figure.When the membrane undergoes a displacement, the reflected beam undergoes a variation of the optical path and a modification of the interference pattern follows. The variations of the interference pattern are recorded by the detection means (178, Fig. 1) then the processing unit 179 makes it possible to deduce the differential pressure measurement. As described previously, a collimator 174, advantageously arranged on an adjustment plate, makes it possible to collimate the incident optical beam. The centimeter distance between the end of the fiber 172b and the membrane made possible by the collimator configured to produce a collimated incident beam, also makes it possible to have a large measurement range and thus to have significant dx displacements of several hundred µm and therefore to be able to measure pressure variations close to 5 bars, while maintaining sufficient sensitivity allowing to have a high resolution of the sea level close to 0.1 mm, or ~1 Pa.The membrane 150 is configured to separate the first compartment and the second compartment in a sealed manner, i.e. it does not allow any liquid to pass below a predetermined threshold pressure, for example a threshold pressure equal to approximately 5 bar, advantageously 10 bar. The parameters of the membrane (material, thickness, dimensions) depend on the desired capabilities, and in particular the sensitivity, i.e. the ability to deform as a function of the pressure variation, the desired differential pressure measurement range, the desired mechanical strength threshold. The material for the membrane is chosen to be compatible with the liquid(s) used and with the materials of the first compartment and the second compartment, in order to avoid oxidation-reduction phenomena which can occur even in the presence of insulating joints between the membrane and the two compartments. Fig. 3A and Fig.3B thus illustrate two first examples of membranes arranged in a differential pressure measurement system according to the present description, for the separation of the first compartment and the second compartment. As for Fig. 2, only the first compartment 110, the second compartment 120 and the porthole 162 of the third compartment are shown. Fig. 3A illustrates a first example of a membrane 150 according to the present description. The membrane comprises in this example a thin film 152, for example a silicon film, for example in the form of a disc. Other materials can be used, for example stainless steel or titanium. The thickness is for example between approximately 90 µm and approximately 500 µm. In the case where the membrane has the shape of a disc, the diameter could for example be between 50 mm and approximately 100 mm.In general, the sensitivity of the membrane depends in a known manner on the mechanical properties of the material used, the thickness of the membrane and the maximum dimensions of the membrane, for example a diameter in the case where the membrane has the shape of a disc. Advantageously, the membrane 150 may comprise a thin reflective layer 151 to optimize the quality of reflection of the laser beam on its surface and increase the optical sensitivity of the instrument. The reflective layer may be located in the region of the membrane where the impact with the incident light beam occurs, for example in a central region. The reflective layer is for example a gold layer, with a thickness for example between approximately 2 µm and approximately 10 µm. The reflective layer is located at least on the side of the membrane which receives the incident light beam. As illustrated in FIG.3A, the membrane 150 is arranged so as to separate the first compartment and the second compartment in a sealed manner. The sealing is for example ensured by means of seals 158, 159. In exemplary embodiments, the first compartment 110 and the second compartment 120 form a cylinder and the membrane separates the cylinder into two parts to form each of the compartments. Each compartment has for example an internal dimension measured in the direction of the incident ray of between approximately 3 mm and approximately 5 mm. The volume of each compartment is for example between approximately 10 cm. 3 and about 100 cm 3. Fig. 3B illustrates a second example of a membrane 150 according to the present description. The membrane comprises in this example a thin film 153 (with a thickness of, for example, between approximately 90 µm and approximately 100 µm) with a concentric accordion shape making it possible to increase the linearity range of the instrument. As previously, the membrane 150 illustrated in Fig. 3B may comprise a thin reflective layer 151 to optimize the quality of reflection of the laser beam at its surface and increase the optical sensitivity of the instrument. Fig. 4 represents, by way of illustration, a photo of an example of such a Copper Berilium (CuBe) membrane arranged in a support 155. For such an accordion-shaped membrane, each undulation can deform independently and produce a displacement of the center of the membrane. The sum of these displacements allows a measurement in a larger linear domain than for a smooth membrane as shown in Fig.3A which can only deform in a single plane. As in the example of Fig. 3A, the seal is for example ensured by means of seals 158, 159. As in the example of Fig. 3A, the first compartment 110 and the second compartment 120 can form a cylinder and the membrane separates the cylinder into two parts, for example two similar parts, to form each of the compartments. Fig. 5A and Fig. 5B illustrate a third example of a membrane 150 according to the present description arranged in a differential pressure measurement system according to the present description. In this example, several membranes are associated with each other to form one or more bellows filled with the first liquid 131. The bellows form the second compartment 120 at least partially flexible, in fluid communication with the second conduit 122. As illustrated in Fig. 5A and Fig.5B, the porthole 162 is in this example arranged in a common wall with the first rigid compartment 110 and the third compartment 160 (not shown). In operation, the incident beam B1 can, as previously, be reflected by a central reflective region 151. Fig. 5A illustrates an initial state when the pressure in the compartment 120 is equal to the initial pressure P. i . Fig.5B illustrates a measurement state when the pressure P ext in compartment 120 is for example greater than the initial pressure P i . An increase in pressure P extis transmitted through the second conduit 122 in fluid communication with the third conduit (not shown in Fig.5A, Fig.5B) open to the underwater environment and causes an enlargement of the bellows(s), which in turn causes a displacement of the membrane. As illustrated by way of example in Fig.5B, the displacement of the membrane 150 results in this example from the enlargement of the two bellows. This results in a displacement equal to dx 1,f – dx 1,i + dx 2,f - dx 2,i, where dx 1,i , dx 2,i are the thicknesses of the bellows measured in the direction of the incident beam, in the initial state, and dx 1,f , dx 2,fare the thicknesses of the bellows in the measuring state. The measurement sensitivity can thus be increased with the number of bellows. In advantageous embodiments, other measures can be provided to measure and at least partially compensate for sensor drifts and / or to measure and at least partially compensate for the effects of temperature variations within the measuring system 100. Fig. 6, Fig. 7 and Fig. 8 thus illustrate different embodiments of differential pressure measuring systems in an underwater environment according to the present description. For the sake of simplification, only the measuring instrument is shown in these figures. Fig.6 thus represents a diagram of an example of a differential pressure measurement system in an underwater environment according to the present description, comprising within the third compartment 160, a second fiber optic head 610 configured to form, with the transmission / reception unit, a second fiber interferometric sensor configured to measure a temperature variation within the third compartment. The second fiber optic head 610 comprises in this example a tube 618 configured to expand with temperature. At the end of the tube is a reflective target 612, for example a gold pellet. A light beam emitted by the same source as that of the first sensor and coming from an optical fiber 172c makes it possible to illuminate the target 612. As for the first optical head 170, a collimator 615 can be used to collimate the beam at the fiber output.A measurement of the displacement of the target 612 resulting from the variation in the length of the tube is carried out as for the measurement of the displacement of the membrane by means of the first sensor. It is possible to deduce therefrom a temperature variation in the third compartment 160 representative of the temperature of the instrument 101. This measurement of the temperature variation makes it possible to correct any measurement artifacts, for example by means of a prior calibration of the system. The calibration comprises for example an estimation of a displacement of the membrane as a function of a temperature variation. Furthermore, the measuring instrument illustrated in Fig. 6 further comprises a fourth sealed compartment 660, filled with a fluid 661 at atmospheric pressure, for example a gas, for example air or nitrogen, a second porthole 662 arranged in a common wall between said fourth compartment 660 and one of the first or second compartments 110, 120.The measuring instrument further comprises a third fiber optic head 670 configured to form, with the transmission / reception unit (not shown in Fig. 6), a third fiber interferometric sensor configured to measure, in the closed position of the first valve, through the second porthole, a second displacement of the membrane. In this example, the first porthole 162 is arranged in a common wall between the third compartment 160 and the second compartment 120 which is at least partially flexible and the second porthole 662 is arranged in a common wall between the fourth compartment 660 and the first rigid compartment 110 but the reverse is also possible.This particular arrangement makes it possible to make two measurements on either side of the membrane 150 and thus to control internal drifts of the sensor associated for example with aging of the laser source and a variation in the associated wavelength which can introduce an artifact on the measurement of the displacement of the membrane and therefore of the differential pressure. In this exemplary embodiment, reflective central gold layers may be located on either side of the membrane. A light beam from the same laser source as that of the first sensor is transmitted by means of an optical fiber to the second porthole 662 for example by means of a sealed connection element 676 allowing the insertion of the optical fiber into the fourth compartment 660.As in the example of the first fiber optic head 170, the fiber optic head 670 advantageously comprises a collimator 674 at the fiber output configured to form a collimated incident beam and a plate 675 for optical alignment of the collimator 674 to optimize the reflection of the collimated beam on the membrane 150. The collimator 674 and the plate 675 are arranged within the fourth compartment 660. The third fiber optic displacement sensor operates in the same way as the first sensor and therefore measures a displacement which should be of the same amplitude but in the opposite direction to that measured by the first sensor. In the event of drift of the source, the sum of the measured displacements is not zero and the correction to be applied can be deduced therefrom. Of course, the second fiber optic displacement sensor illustrated in FIG.6 for temperature measurement and the fourth compartment 660 equipped with the second porthole 662 for drift compensation can be provided in the differential pressure measurement system independently of each other. Fig. 7 represents a diagram of another example of a differential pressure measurement system in an underwater environment according to the present description. The system comprises for example a fiber optic head 610 configured to form a temperature sensor as described previously. In this exemplary embodiment, the system further comprises, within the third compartment 160, a fourth fiber optic head 710 configured to form, with the transmission / reception unit, a fourth fiber optic displacement sensor (reference sensor) configured to measure a drift of the first fiber optic displacement sensor.It is thus possible to control internal drifts of the first sensor, due for example to aging of the source and a variation in the associated wavelength. Compared to the system described in Fig. 6, the fourth sensor makes it possible to avoid an embodiment with two windows and using two sealed passages 176 and 676, which is more bulky and reduces reliability. The fiber optic head 710 comprises in this example a tube 718 made of a material that does not expand with temperature, for example a vitroceramic glass such as zérodur®. At the end of the tube is a reflective target 712, for example a gold mirror. A light beam from the same source as that of the first sensor, coming from an optical fiber 172d, makes it possible to illuminate the target 712. A measurement of the displacement of the target 712 necessarily results from a drift of the source since the tube does not expand.A difference between the displacement signal obtained with the first sensor and the signal obtained with the reference sensor makes it possible to obtain a corrected signal. Fig. 8 represents a diagram of another example of a system for measuring differential pressure in an underwater environment according to the present description. The measuring instrument comprises in this example (optionally) a temperature sensor 610 as described previously and a sensor 710 for measuring the drift of the source, as described previously.The measuring instrument further comprises a fourth compartment 810 in fluid communication with the first compartment 110, said fourth compartment comprising a wall 812 and, according to examples, at least one first rigid element 814 immersed in the first liquid 131 circulating within the fourth compartment, said at least one first rigid element having a coefficient of thermal expansion lower than the coefficient of thermal expansion of the wall. Such an arrangement makes it possible to compensate for any expansion of the liquid in the first compartment 110. In the first compartment 110, in fact, if the liquid expands at constant pressure, it increases in volume and therefore causes a displacement of the membrane, reflecting a measurement error of negative pressure variation. This expansion of the liquid 131 can be compensated for or minimized thanks to the additional compartment 810 in fluid communication with the first compartment 110.An ideal case would be a compartment having a variation in volume after expansion identical to that of the first liquid. In this case, the membrane 150 will not undergo any displacement error. However, it is difficult to find a material for the compartment having the coefficient of expansion perfectly adapted to compensate for the expansion of the first liquid. It will then be possible to add inside the compartment 810 at least one first rigid element 814 having a lower coefficient of expansion than that of the first liquid, such as stainless steel or titanium, advantageously a very low coefficient of expansion such as invar, zerodur, or silica. The combination of liquid and rigid solid makes it possible to reduce the equivalent expansion of the assembly, and even to adjust it precisely to that of the first compartment by calculating and then placing a proportion of the precise rigid volume.This adjustment can be done by placing additional small rigid elements, for example discs of a certain diameter and a certain thickness. It can be noted that in the examples of the systems illustrated in Fig.6, Fig.7, Fig.8, a single light source is used for a plurality of sensors (170, 670, 610, 710). It is then possible to eliminate or greatly reduce noise sources by differential measurement between the sensors and obtain control of the internal drift of the optical system. In other words, since the drift is identical on the multi-sensor network, it can be eliminated. Furthermore, the cost of the sensors deployed is limited by using a single light source.It is also understood that it is possible to use a single laser source for said first measuring instrument and for at least one second measuring instrument (not shown in the figures), also making it possible to make differential measurements between the 2 instruments and to eliminate any form of internal drift or noise occurring for the 2 instruments. Fig. 9A represents a diagram of a collimator 174 in an example of a first fiber optic displacement sensor according to the present description. The collimator has an optical axis (^) and is configured for the spatial shaping of a light beam coming from the optical fiber 172b of the first sensor (Fig. 1) in order to produce the collimated light beam B1 and to direct it towards the membrane 150, located for example between approximately 5 mm and approximately 20 mm from an output face of the collimator.More specifically, in exemplary embodiments, the collimated light beam is incident substantially perpendicularly on the reflective central portion 151 of the membrane. As illustrated in Fig. 9A, the collimator is mounted on an alignment plate 175 which makes it possible to orient the optical axis of the collimator in a cone with an apex angle, for example between approximately ±5°. The alignment plate comprises, for example, a mobile support 901 in which the collimator is arranged, a fixed support 902 and adjustment screws 903 configured to modify the inclination of the mobile support 901 relative to the fixed support 902 around two axes contained in the plane of the mobile support. This results in a pivoting of the optical axis of the collimator in a cone 905 centered substantially on a direction perpendicular to the plane of the fixed support.The applicants have shown that the first fiber optic displacement sensor thus described is extremely robust in that it operates perfectly well even with a slight misalignment of the collimator of ±1.5° which results in a slight misalignment of the optical axis of the collimator with a direction perpendicular to the membrane 151. Fig. 9B thus represents a diagram illustrating in more detail the operation of the collimator shown in Fig. 9A, when the collimator 174 has an alignment misalignment. In this exemplary embodiment, an object focus of the collimator 174 is substantially located in an output plane P. Fof the optical fiber 172b. As a reminder, in an optical displacement sensor comprising a Fabry-Pérot type interferometer, variations of an interference signal resulting from interference between, on the one hand, a detection light beam (B2, Fig. 2) which results from the reflection of the collimated incident light beam B1 on the membrane and, on the other hand, a reference light beam, which results from the reflection of the incident light at the end of the optical fiber 172b, the reflection resulting from the glass / air interface. The detection beam enters the fiber 172b and combines with the reference beam to produce interference. Any movement or displacement of the reflecting surface 151, or equivalently, any variation in the length of the optical cavity such as induced by a variation in the position of the membrane will result in the generation of dynamic interference fringes.This interference signal is then transmitted by the fiber circuit to the detection means 178 of the sensor (see Fig. 1), remote from the medium in which the measurement is made, then processed in a known manner by the signal processing unit 179 to determine the displacement of the membrane and deduce a differential pressure measurement. As explained previously, measurement artifacts linked for example to variations in wavelength of the emission source, variations in temperature, internal drift, etc. can be corrected. As illustrated in Fig. 9B, part of the incident beam is reflected at the end of the fiber to return to it with a light intensity close to 4% (reference beam). Another part of the beam comes from the end of the core of the optical fiber 172b placed at the optical focus F1 of the collimator 174 (“primary” optical focus) to form an incident beam i1.The incident beam i1 is collimated by the collimator lens 174 and reflected by the membrane into a beam r1 which is no longer parallel to the beam i1 because of the initial misalignment; the beam r1 is transmitted again by the lens which makes it converge towards a point F2 of the focal plane offset from the primary focus ("secondary focus"). The beam r1 is reflected on the plane of the end of the fiber (ferrule) into a beam i2 passing through the lens to be collimated again and thus be parallel to the previous beam r1 between the lens and the target. The beam i2 is reflected like the beam i1 on the target into a beam r2 which is parallel to the 1. er incident beam i1 between the lens and the target. The beam r2 is then perfectly oriented in the direction allowing it to return after passing through the lens to 1 erprimary focus located at the heart of the fiber. It therefore automatically enters the fiber after 2 reflections to interfere with the reference beam. This double reflection as well as the reflection on the secondary focus located on the plane of the end of the fiber with low reflection coefficient allows to have a beam with less light intensity, around 4% of the initial beam, which makes it possible to optimize the interference with the reference beam which itself returned to the fiber at a rate of 4%. With a double reflection, the optical path is 2 times longer, which increases the sensitivity of the measurement by a factor of 2.Thus, as explained above, the use of a collimator arranged such that the optical focus is located in the plane of the end of the tip of the fiber 172b makes it possible to collimate all of the optical beams exiting the end of the fiber 172b and thus target the membrane at a distance several centimeters greater than the state of the art. This large distance is also possible thanks to the use of the alignment plate 175 and the double reflection between the membrane and the end of the fiber located at the focus of the collimator. This double reflection process allows a significant misalignment difference close to ±1.5°, consequently allowing operation without the risk of the optical beam not returning to the fiber. The collimated beam can thus target the membrane through a liquid constantly connected to the external pressure. This makes it possible to have liquid on either side of the membrane to obtain equal pressure.We can then choose a membrane with more sensitive mechanical properties (large diameter and low thickness) to achieve resolutions close to 1 Pa ~ 0.1 mm of water while maintaining a larger displacement domain and therefore a significantly larger measurement range up to 5 bars ~ 50m. The instrument can be moved, transported in equal pressure in an environment going from low to high pressures, for example during a descent to the seabed up to 600 bars or more without risk of deterioration of the elements constituting the instrument, in particular the membrane, which is not the case with the state of the art.Although described through a number of exemplary embodiments, the differential pressure measurement systems according to the present description include various variations, modifications and improvements which will be obvious to those skilled in the art, it being understood that these various variations, modifications and improvements are part of the scope of the invention as defined by the following claims.

[0002] References [Ref.1] POLSTER, André, FABIAN, Marcus, VILLINGER, Heinrich, “Effective resolution and drift of Paroscientific pressure sensors derived from long-term seafloor measurement”; Geochemistry, geophysics, geosystems, Vol.10, Number 8 (2009). [Ref.2]: TIAN, Bian, ZHAN, Feng, HAN, Feng, et al. “An optical fiber Fabry–Pérot micro-pressure sensor based on beam-membrane structure”; Measurement Science and Technology, 2018, vol.29, no 12, p.125104. [Ref.3]: FR2963421

Claims

CLAIMS 1. System (100) for measuring differential pressure in an underwater environment comprising: - at least a first transmission / reception unit (102) comprising a light source (177), detection means (178) and signal processing means (179); - at least a first fiber measuring instrument (101) configured to be positioned at a first position in said underwater environment and comprising: - a first rigid compartment (110) in sealed fluidic connection with a first rigid conduit (112), the first compartment being filled with a first liquid (131); - a second compartment (120) at least partially rigid, in sealed fluidic connection with a second conduit (122) at least partially flexible, the second compartment being filled with a second liquid (132);- a third conduit (123b) for taking underwater pressure, said third conduit being open to the underwater environment and in fluid connection with the second conduit; - at least one first valve (140) configured to allow, in the open position, a fluid connection between the first conduit and the second conduit, in order to maintain equal pressure between the first compartment and the second compartment, and configured to separate in a sealed manner, in the closed position, the first conduit from the second conduit; - a membrane (150) configured to separate the first compartment and the second compartment; - a third compartment (160), sealed, filled with a fluid (161) at atmospheric pressure; - at least one first porthole (162), transparent in a given wavelength range, arranged in a common wall between said third compartment and one of the first or second compartments;- at least one first fiber optic head (170) configured to form with said first transmission / reception unit, a first fiber interferometric sensor, said first fiber interferometric sensor being configured to measure, in the closed position of the first valve, through said first porthole, at least one first displacement of the membrane resulting from a pressure variation between the first compartment and the second compartment when the first measuring instrument is subjected to a pressure variation; 2. System (100) for measuring differential pressure in an underwater environment according to claim 1, in which the first liquid and the second liquid are identical, the first compartment and the second compartment being filled with the same liquid.

3. System (100) for measuring differential pressure in an underwater environment according to claim 1, in which the first liquid and the second liquid are different, immiscible. 4.System (100) for measuring differential pressure in an underwater environment according to any one of the preceding claims, wherein said at least one first fiber optic head (170) comprises: - a first optical fiber (172b) for transporting a light beam emitted by the light source towards the membrane, one end of said first optical fiber being arranged in the third compartment (160); - a first collimator (174) for collimating said light beam emitted by the source and coming from said first optical fiber, wherein said end of the first optical fiber is located in a focal plane object of the collimator. 5.System (100) for measuring differential pressure in an underwater environment according to any one of the preceding claims, wherein said first measuring instrument further comprises: - a second fiber optic head (610) arranged in said third compartment (160) and configured to form with said transmission / reception unit, a second fiber interferometric sensor configured to measure a temperature variation of said third compartment. 6.System (100) for measuring differential pressure in an underwater environment according to any one of the preceding claims, wherein said first measuring instrument further comprises: - a fourth compartment (660), sealed, filled with a fluid (661) at atmospheric pressure; - a second porthole (662) arranged in a common wall between said fourth compartment and the other of the first or second compartments; and - a third fiber optic head (670) configured to form with said first transmission / reception unit, a third fiber interferometric sensor, said third fiber interferometric sensor being configured to measure, in the closed position of the first valve, through said second porthole, a second displacement of the membrane. resulting from a pressure variation between the first compartment and the second compartment when the system is subjected to a variation in underwater pressure.

7. System (100) for measuring differential pressure in an underwater environment according to any one of claims 1 to 5, wherein said first measuring instrument further comprises: - a fourth fiber optic head (710) arranged in said third compartment (160) and configured to form with said first transmission / reception unit, a fourth fiber interferometric sensor configured to measure a drift of said first fiber optic displacement sensor (170). 8.

9. System (100) for measuring differential pressure in an underwater environment according to any one of the preceding claims, wherein said first measuring instrument further comprises: - a fourth compartment (810) in fluid communication with the first compartment (110), said fourth compartment comprising a wall (812) and at least one first rigid element (814) immersed in the first liquid circulating within said fourth compartment, said at least one first rigid element having a coefficient of thermal expansion lower than the coefficient of thermal expansion of the wall (812).

10. System (100) for measuring differential pressure in an underwater environment according to any one of the preceding claims, wherein said first measuring instrument further comprises a mechanical protection enclosure (180) not sealed with the underwater environment, inside which the first compartment (110), the second compartment (120) and the third compartment (160) are arranged.

11. System (100) for measuring differential pressure in an underwater environment according to claim 10, wherein said second measuring instrument is a fiber measuring instrument which cooperates with said first transmitting / receiving unit for measuring the differential pressure.

12. Method for measuring differential pressure in an underwater environment implemented by means of a differential pressure measuring system according to any one of the preceding claims comprising: - opening the first valve (140) to equalize the pressure of the first. compartment (110) with the second compartment (120); - the installation, in the underwater environment of which it is sought to measure a differential pressure, of said first measuring instrument (101); - the closing of the first valve (140) to separate, in a sealed manner, the first conduit from the second conduit; - the measurement, in the closed position of the first valve, through said porthole, of a displacement of the membrane resulting from a variation in pressure between the first compartment and the second compartment when the first measuring instrument is subjected to a variation in underwater pressure.

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