METHOD AND SYSTEM FOR MEASURING PRESSURE OF FLUIDS CONTAINED IN SEALED VESSELS

FIELD OF THE INVENTION

The present invention relates in general to sensing internal pressure within sealed vessels. In more particular the present invention relates to a system for remote sensing of a pressure of a fluid contained in a sealed vessel and to optical sensors.

BACKGROUND OF THE INVENTION

Systems for externally measuring a pressure of a fluid contained in sealed vessels are known. For example, US Patent 6,520,006 discloses magnetically coupled pressure gauge providing for remote sensing and measuring a pressure of fluid contained in a sealed vessel. However such pressure indicators comprises a member to be installed inside the vessels prior to pressurizing them with fluids.

Optical sensors such as pressure and/or strain gauges are common in the marketplace. For example fiber Bragg grating (FBG), which are optical fibers having one or more segments in which the refractive index of their core is modulated to form a grating, provide for sensing a strain induced across them. Reference is now made to Fig. 1 - 2C. In Fig. 1 a segment of FBG 10 is schematically shown. Grating 12 is written on core 14, which is enfolded with cladding 18. The segment of fiber core 14 including grating 12 consists of a plurality of intervals, such as the interval whose end points are 16 and 17, successively disposed along grating 12, across which the refractive index ε

varies by a magnitude of δε. In Figs 2A-2C spectra, namely the intensity of light as a function of the wavelength, of the illuminating, transmitted and reflected light by grating 12 is respectively shown. The spectrum of the transmitted portion of light propagating along FBG 10 whose exemplary spectrum is given by the graph shown in Fig. 2A is shown in Fig. 2B. The spectrum of the reflected portion of this light is respectively described by plot 24 shown in Fig. 2C. Plot 24 has a peak at a wavelength λe, which is the same wavelength in which a minimum occurs in the spectrum of the transmitted light. By distorting the FBG, such as by stretching it, this peak is displaced by δλ (which is indicated by the double arrows 26) such that the respective spectrum of the reflected portion is presented by plot 28.

Monitoring vibrations and/or strains occurring over time in a structure, such as of a bridge, can be accomplished by means of a FBG. For such purposes both ends of a stretched FBG are attached to the structure such that strains occurring in the structure are induced across the FBG. An initial zero setting is accomplished by measuring the initial reflective wavelength of the FBG for given environmental and structural conditions immediately following its installation. By repeatedly measuring the reflective wavelength, strains occurring in, and/or over, time are estimated. In a US patent application US20020154860A1 a pressure gage consisting of a FBG housed in a capillary tube is disclosed. A pressure applied onto the tube's walls induces strains across the FBG. The level of the pressure is derived from the measured values of the reflected wavelengths which are shifted from the wavelength of the unstressed FBG. The known fiber optic extrinsic Fabry-Perot interferometer (EFPI) sensors are commonly used as temperature and/or pressure sensors. For measuring a pressure of a fluid, a diaphragm attached to a mirror of the EFPI is exposed to the fluid. The pressure applied onto the diaphragm causes a change in the phase of the output fringes of light reflected from the EFPI. These changes are translated to pressure levels. However employing such

pressure sensors either comprising FBG or EFPI requires their exposure to the contained fluid.

In US Patent 5,452,087 a method and system for measuring pressure in a pressure-containing vessel, such as a combustion compartment of an internal combustion engine, is disclosed. In accordance with the disclosed method an EFPI connected to a single mode fiber is embedded in a metal part disposed within a wall of the vessel. The pressure exerted on the wall induces longitudinal strain on the embedded EFPI thereby exposing the EFPI to the fluid is avoided. However incorporating the metal part enclosing the EFPI with a wall of the vessel breaches the wall's integrity.

In a European patent application EP0678887A1 an interferometer having a mesh of fiber optics in each of its both optical paths is disclosed. One of these meshes is attached to the surface of a vessel whose internal pressure is different than the pressure external to the vessel, whereas the other mesh is free. The changes in phase between both optical paths provide in accordance with the disclosed method for estimating the internal pressure within the vessel. Therefore the integrity of the walls of a vessel containing the fluid whose pressure is such measured is retained intact. However both meshes of fiber- optics have to be retained at the same temperature otherwise one is not able to distinguish between the changes measured in the phase of the transmitted light originated by a temperature difference and the changes originated by the strain induced by the difference between the external and internal pressures. Furthermore, since the phase varies in the range [0, 2π] the absolute value of the pressure cannot be determined unless the pressure is continuously increased from zero to its desired value. In addition a detailed calibration process has to be carried out for each temperature of the vessel given an ambient temperature of the free mesh of fiber optics independently, as well.

Sealed vessels containing pressurized fluids, such as gases, liquids, or any mixture thereof, are occasionally stored for a relatively long term prior to their usage. Incorporating common pressure gauges with such vessels by penetrating their wall breaches their integrity, which in turn deteriorates their reliability. Therefore measuring the internal pressure within sealed vessels

without breaching their integrity is necessary. A reliable method and system providing for externally measuring the pressure of a fluid contained in a sealed vessel, is beneficial.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic presentation of a fiber Bragg grating (FBG);

Fig. 2A is graph of an exemplary spectrum of light conducted into the FBG shown in Fig. 1 ;

Fig. 2B is a graph of the spectrum of the transmitted portion of the light whose spectrum is shown in Fig. 2A through the FBG shown in graph 1 ;

Fig. 2C shows graphs of the spectra of the reflected portion of the light whose spectrum is shown in Fig. 2A by the FBG shown in Fig. 1 , in two different strain conditions respectively;

Fig. 3 is a segment of a block diagram of a system for externally measuring pressure of fluids contained in sealed vessel according to a preferred embodiment of the present invention;

Fig. 4 is a sectional view schematically sowing a pressure sensing device (PSD) attached according to a preferred embodiment of the method of the present invention, to a vessel for containing pressurized fluid;

Fig. 5 is a scheme of a setup for calibrating a PSD according to a preferred embodiment of the method of the present invention;

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In accordance with the present invention a system and method for externally measuring pressure of fluids contained in sealed vessels is provided. A fluid refers hereinafter to any substance being at a liquidized state, gaseous state, or any mixture thereof. A plurality means hereinafter a number of items including at least one item. The present invention regards vessels withstanding any practical levels of pressure including considerably high levels, such as within the range of a few hundreds of atmospheres. The walls of such vessels are typically made of metal, such as steel or aluminum, and/or composite materials, such as walls having a metallic core made of steel or aluminum, which is further encapsulated with fibers embedded in a matrix. The vessels may have any geometrical shape. Normally such vessels are spherical and/or cylindrical whose one or two bases are either spherical or planar.

The system of the invention

A system for externally measuring pressure according to the present invention has a plurality of pressure sensing devices (PSDs), of which one or more PSDs are mounted onto the external surface of any of a plurality of sealed vessels prior to their being filled with the fluid whose pressure is to be measured. Preferable configurations of the system are those in which each vessel is furnished with only one PSD. Each PSD includes at least one segment of FBG in which a Brag grating having a predefined reflective wavelength is written across its core. The various Brag gratings of a linear array of FBGs differ in their reflective wavelengths, such that a measured value of a reflective wavelength can be associated with the respective grating and therefore with a specific PSD. The pressure of a fluid contained in the sealed vessel is measured in accordance with the present invention by employing a wavelength division multiplexing technique as known.

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Reference is now made to Fig. 3 in which a segment of a block diagram of a system for externally measuring pressure of a fluid contained in sealed vessels according to a preferred embodiment of the present invention is shown. System 30 has one or more PSDs, such as PSDs 32 and 33 each of which is mounted onto its respective vessel, coupler 34 and analyzing unit 36. Analyzing unit 36 includes light source 38 and spectrum analyzer 40. A controller, not shown, is further installed in analyzing unit 36. Such controller provides for operating the system, activating the light source and the analyzer, and transmitting measurements results to, and/or downloading working parameters from, a remote computer. The analyzing unit need not be integrally connected to the coupler. An optional connector installed on coupler 34, not shown, may provide for repeatedly disconnecting and reconnecting the analyzing unit from or to the set of the PSDs. Light source 38, such as a fast sweeping laser, a light emitting diode (LED), a superluminescent solid state or fiber source, is aimed at the PSD. FBGs 42 and 44 are linear arrays of Brag gratings, such as gratings 46, 46A and 48, 48A respectively, each of which has its respective predefined reflective wavelength. The linear arrays need not be integral units consisting of a single fiber along which Brag gratings are successively embedded. Discrete pieces of FBGs each having a single grating associated with its respective reflective wavelength, may be serially connected to each other by means of fiber-optic connectors, as known, to form a linear array.

Each of the PSDs of system 30 has a pair of segments of FBG including Brag gratings one of which belong to FBG 42 and the other to FBG 44. Each of the segments of FBG 42, such as the segment including Brag gratings 46 of PSD 32, or grating 46A of PSD 33, are firmly attached to a surface of a sealed vessel. Attaching is such accomplished that the respective segments of fiber are stretched while being fully attached to the surface of the wall. Only one end of each of the respective segments of FBG 44, such as those including Brag gratings 48 and/or 48A, is attached to the respective surface of the vessel, whereas the other end of the segments of fibers is kept

loose. Such attaching provides for good thermal connection between Brag gratings 48 or 48A and the surfaces of the respective vessels. The pressurized fluid forces the sidewalls of a sealed vessel thereby straining them. Therefore FBG 42 provides for measuring a strain induced across walls of the respective vessels, whereas FBG 44, which is isolated from such mechanical strains, provides for measuring the respective temperature of the walls. Brag grating such attached to provide for strain measurements and FBGs including such gratings are referred hereinafter as pressure sensitive Brag grating or FBGs respectively. The other gratings or FBGs which are isolated from mechanical strains induced across the surface of a vessel are referred hereinafter as temperature compensating gratings or FBGs. Coupler 34 couples light emitted from light source 38 to FBG 42 and/or FBG 44. Analyzer 40, which is any common optical spectrum analyzer, a tunable Fabry-Perot filter, a colored glass filter or the like, provides for measuring the wavelengths of the returned signals.

Alternatively, only one linear array of FBG provides both for measuring pressure as well as the temperature of the sealed vessels. In such a case a pair of successively disposed Brag gratings along the single FBG is included in each PSD of the system of the invention. One of the Brag gratings of a pair is pressure sensitive and the other is temperature compensating.

Mounting a PSD onto a vessel

Applying a PSD onto a vessel is is affected such that the respective Brag grating of at least one PSD is fully attached to the surface of a vessel at a predefined location and along a predefined direction. Reference is now made to Fig. 4 in which PSD 60 is shown mounted onto vessel 62. Segment 64 of the FBG including the pressure sensitive Brag grating (which is fully attached to the sidewall of vessel 62) as well as one end of the temperature compensating Brag grating (which is the second Brag grating of this PSD) ₁ not shown. Layer 66 of a specified adhesive material provides for the attaching. Cover 68

provides for securing both segments of FBG from external mechanical shocks and/or an exposure to chemicals. Cover 68 is attached to the sidewall using layer 70, which is optionally of an adhesive material accommodated to withstand the environmental conditions to be faced by the vessel furnished with this PSD that may be different from the, adhesive material of layer 66.

The process of adhering a FBG to a surface of the vessel is similar to the process of attaching common strain gauges to a surface which a strain induced across it is to be measured. A detailed description of such process and the respective adhesives to be employed can be found for example in bulletin B -137, titled "gauge installation and surface preparations" of Vishay Electronic GmbH

(http://www.vishay.com/brands/measurements_group/guide/ib /b137/137d1.htm) . A thorough preparation of the surface including mechanically abrading and chemically cleaning and degreasing are applied prior to the adhering. Following the adhering a curing process is applied at a suitable temperature, which is higher than the anticipated operating and/or storing temperatures of the vessel onto which a PSD has been mounted.

The location and the direction along which a pressure sensitive Brag grating is attached onto a surface of the vessel may impact the functional behavior of a shift caused to the reflected wavelengths of the FBG as a function of pressure. Preferable are locations and directions in which the shift in the reflected wavelength is linear with, or can be fairly approximated by a piecewise linear function of, the pressure. Selecting a location on a surface of the vessel and a direction along which a FBG is attached is derived from the strain analysis, which is typically accomplished during designing the vessel, selecting its geometrical shape and the material from which its sidewall is made of. Normally, one PSD having a single Brag grating dedicated for strain measurements and a second grating dedicated for temperature compensation may provide the desired sensitivity and accuracy of the pressure levels such measured. For example, a direction along a major circle of spherically shaped vessel is preferable for attaching a pressure sensitive grating of a single PSD.

Calibrating a PSD and measuring a pressure of a contained fluid

Prior to the measuring of a pressure of a fluid contained within a sealed vessel in accordance with the method of the present invention, a PSD has to be mounted on the surface of at least one vessel and a calibration scale has to be generated. For measuring the pressure, the analyzing unit is connected to the coupler and further activated. The reflective wavelength of each of the Brag gratings of each PSD are measured and recorded. The difference between the wavelength of the pressure sensitive and the temperature compensating gratings of a pair of Brag gratings of any PSD is computed. The computed difference gives the value of the component of the currently measured reflective wavelength, which is temperature independent, associated with the respective PSD. For estimating the level of the pressure, this component of the reflective wavelength is further compared to a calibration scale associated with the respective PSD.

For generating a calibration scale related to a PSD mounted onto its respective vessel, first a zero setting has to be accomplished. Light coupled from the analyzing unit is conducted to both Brag gratings of this PSD; whilst a contained fluid does not stress the vessel, such as the case in which the vessel is empty and unsealed. The reflective wavelengths of both the pressure sensitive and the temperature compensating gratings are respectively measured. Then the temperature independent component of the reflective wavelength is computed as described above and further recorded. This value is further regarded as the zero setting of this PSD. Due to strains that might be induced across the pressure sensitive Brag grating during its adhering and along the curing process, the zero setting may differ from the nominal reflective wavelength associated with its respective Brag grating. At this stage at least one cycle in which a fluid is pressurized into the vessel at various levels within a predefined range of pressure levels is carried out; the respective reflective wavelengths of each of the Brag gratings are measured regarding each pressure level and the difference of the respective wavelengths are computed to give the temperature independent components related to the respective

pressure levels, which are further recorded. The respective actual pressure levels of the contained fluid are independently measured by means of a calibrated pressure gauge and recorded accordingly. At the end of these cycles, subtracting the value of the zero setting associated with this PSD rescales the recorded differences. These rescaled differences are the respective shifts in the reflective wavelengths corresponding to the respective pressure levels. A calibration curve associated with the specific location and orientation of the pressure sensitive Brag grating relative to the surface of the vessel is generated in the next step. Fitting the shifts of the reflective wavelength with the respective actual pressure levels, such as by means of least squared deviations technique, generates the calibration curve. The zero setting and the calibration curve constitute the calibration scale associated with a specific PSD. From this stage on, a pressure of the contained fluid can be estimated according to the method of the present invention by respectively measuring the reflective wavelengths of the pressure sensitive and the temperature compensating gratings of a PSD; calculating the temperature independent component of the reflective wavelength and comparing it to the associated calibration scale, by first rescaling with the zero setting and matching the derived value of the shift in wavelengths with the respective pressure level by means of the calibration curve. In cases in which more than one PSD is mounted on a single vessel the pressure of the fluid contained is estimated as known such as by a weighted averaging and/or majority voting.

Two exemplary systems described below, may provide for indicating further potential applications suitable for a system of the present invention.

EXAMPLE 1

A system of the invention providing for measuring the internal pressure within sealed vessels containing compressed helium, such as normally used to pneumatically activate fins actuating system of a typical air-to- air missile is hereby described. PSDs of the invention are mounted onto the vessels containing the pressurized helium. The PSDs are serially connected to

a coupler of the invention installed within the missile by means of fiber optic and suitable connectors. Analyzing unit of the invention can be once in a while or periodically connected to the coupler for measuring the pressure of the fluid contained in the sealed vessels whilst the missile is stored and/or prior to its operational usage.

EXAMPLE 2

Similarly PSDs of the invention are attached to the surfaces of sealed vessels containing pressurized fluids including fire retardants, such as of a fire extinguishing system typically installed in aircrafts. The PSDs are further connected to a coupler by one or more fiber optics and connectors. The analyzing unit can be either installed in the aircraft and automatically operated for measuring the respective pressures of the contained fluids by the flight computer, or incorporated into the supportive maintenance system and periodically connected to the coupler for measuring the respective pressures during cycles of maintenance checks. The respective calibration scales for both system described above can be stored in the controller installed in the analyzing unit.

EXAMPLE 3

Reference is now made to Fig. 5 in which a setup for calibrating a PSD and selecting preferred location and orientation for its mounting according to a preferred embodiment of the method of the present invention is schematically shown. System 80 for externally measuring a pressure of a fluid contained in sealed vessels, includes PSD 82 connected to coupler 84 that is further connected to analyzing unit 86. Analyzing unit 86 includes controller 87 in addition to a light source and an analyzer, not shown. PSD 82 is disposed at the sidewall of vessel 88 such that its pressure sensitive Brag grating is

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13 coaxially attached to the external surface of vessel 88. Pressure piping 90 respectively connect between the inlet of vessel 88, pressure gauge 92 disposed between two valves 94 and pump 96. Pump 96 is further connected to a source feeding it with a fluid, not shown. Common strain gage 98 can be optionally mounted on the surface of vessel 88 at a location and orientations, which are mechanically equivalent to the location and orientation in which the pressure sensitive Brag grating of PSD 82 is attached to the surface of vessel 88. Analyzer 100 provides for electrically reading strain levels as measured by means of the optional strain gage. Both the strain gag and analyzer 100 are not essential for the calibration process. However they are introduced only for reference purposes. Controller 87 has a display and a keyboard for respectively displaying the operator with statuses of the system, measurements readings and computed results; and for manually entering working parameters, activating and control commands to the system. A link to a remote computer, not shown, respectively provides for uploading and downloading measurements data and working parameters from and to the controller.