FIG. 1A shows the stress acting (σ) on the material, and FIG. 1B shows the responsive strain (ε) in the viscoelastic material. As seen in FIG. 1B, the stress history H1 and H2 is memorized by the material, which is causing bias/delays and gradual decays in the strain exhibited by the material in response to the applied strains Sa,Sb.

FIG. 2 shows a sensor unit 20 (also referred to herein as fluid cell) configured to substantially compensate and attenuate creep/deformation phenomena. The sensor unit 20 in this non-limiting example comprises a base portion 21 carrying a sensing layer 24, and a cover portion 22 having at least one deformable wall section 22d. The base portion 21 is configured to define a measurement chamber 21c, and a fluid passage 21p configured to communicate fluid media with the measurement chamber 21c. The sensing layer 24 comprises at least one deformable (e.g., viscoelatic or flexible) portion/element 24n carrying one or more sensing elements 24s therein (and/or thereon). The sensing layer 24 is configured to attach to the top side of the base portion 21, and sealably cover the opening 21n of the measurement chamber 21c by its deformable portion/element 24n. The cover portion 22 is configured to attach to the top side of the base portion 21 and define a sealed fluid cavity/cell 22c filled with fluid media 22f (e.g., gas, air, or any other suitable compressible media) interacting with the sensing layer 24.

Optionally, but in some embodiments preferably, the cover portion 22 is used to sealably attach the sensing layer 24 to the base portion 21 such that its deformable portion/element 24n is located over the opening 21n of the measurement chamber 21c. In this, and other, embodiments the sensing layer 24 is configured to partition between the fluid cavity/cell 22c and the measurement chamber 21c. In some embodiments the width of the fluid passage 21p substantially equals to the width of the opening 21n of the measurement chamber 21c.

With this configuration, fluid media If flowing into (or out of) the measurement chamber 21c and interacting with the deformable portion/element 24n of the sensing layer 24 covering the opening 21n of the measurement chamber 21c and of the measurement chamber 21c, responsively causes deformations of its deformable portion/element 24n towards (or away from) the sealed fluid cavity/cell 22c. The fluid media 22f enclosed inside the sealed fluid cavity/cell 22c is configured to present a certain level of resistance to the deformations of the deformable portion 24n, and thereby attenuate the pressure P applied by the fluid media If thereover. The attenuated pressure over the deformable portion 24n is sensed by the one or more sensing elements 24s, which generates measurement signals/data indicative thereof.

The sensing layer 24 comprises in some embodiments electrical conductors and/or circuitry 25 electrically connected to the one or more sensing elements 24s, and configured to receive the measurement signals/data therefrom, process, store, and/or transmit the measurement data. The circuitry 25 can be configured to transmit the measurement data to an external device/system (not shown) wirelessly (e.g., using IR, RF—Bluetooth, WiFi, Zigbee, NFC, signaling, or suchlike) and/or over data communication wires/bus (e.g., USB, UART, or suchlike).

Optionally, but in some embodiments preferably, one or more sensor elements 22s are provided in the deformable wall section 22d, for measuring one or more properties/conditions of the fluid media sealed inside the fluid cavity/cell 22c. Alternatively, or additionally, the one or more sensor elements 22s provided in the deformable wall section 22d can be configured for measurement of one or more properties/conditions of the fluid media If affecting the external fluid pressure P. The one or more sensor elements 22s can be similarly coupled to the circuitry 25 for conveying measurement data/signals thereby generated to external devices/systems.

The sensing layer 24 can be implemented by any of the sensor on foil/film embodiments disclosed and illustrated in International Patent Publication Nos. WO 2018/025264, WO 2018/092130, WO 2018/235087, WO 2019/171376, WO 2020/089915, WO 2020/129069, of the same Applicant hereof, the disclosures of which is incorporated herein by reference. The attachment/integration of the sensing layer 24 to the base portion 21 can be carried out using any of the techniques disclosed and illustrated in these International Patent Publications. The cover portion 22 can be attached to the base portion 21 by laser welding, ultrasonic welding, heat welding, gluing, overmolding or suchlike.

The deformable wall section 22d of the cover portion 22 is configured to provide creep/deformation (viscoelasticity) compensation to the sensor unit 20. In order for it to provide creep/deformation compensation properties, the deformable wall section 22d is made in some embodiments from the same material used to make the sensing layer 24, or from a different material having same/similar creep/deformation behavior/properties. The deformable wall section 22d can be attached to the cover portion 22 by laser welding or ultrasonic welding, or gluing, or suchlike. Other parts of the cover portion 22, and also of the base portion 21, can be manufactured by injection molding, CNC, 3D printing or suchlike.

Optionally, but in some embodiment preferably, the deformable wall section 22d is made from the same material of the cover portion 22, and it can be manufactured as an integral part of the cover portion 22.

The geometrical dimensions/structure of the fluid passage 21p can be configured to protect the measuring components associated with the measurement chamber 21c, to thereby prevent abrupt pressure changes thereinside. In some embodiments the fluid passage 21p is configured trap air inside the measurement chamber 21c, to thereby protect the deformable portion/element 24n e.g., from hard water, by creating a small air pillow thereinside. In possible embodiments the sectional area of the fluid passage 21p is about 0.01 mm²to 1000 mm².

In some embodiments the cavity of the measurement chamber 21c, and optionally at least some portion of the fluid passage 21p, can be filled with air acting as an intermediating component (shown at 1f) conveying the fluid pressure P at the opening of the fluid passage 21p into the measurement chamber 21c i.e., the deformable portion/element 24n interacts with the intermediating component instead of interacting directly with the fluid media causing the fluid pressure P. In some possible embodiments the cavity of the measurement chamber 21c, and optionally at least some portion of the fluid passage 21p, are filled with a flexible substance (shown at If e.g., silicone), or any suitable type of a soft material (e.g., gel), capable of conveying the pressure P at the entrance/opening of the fluid passage 21p to the deformable portion/element 24n. In such embodiments the flexible substance (1f) can be configured to convey the pressure P to the deformable portion/element 24n, and also to protect the deformable portion/element 24n from high pressure conditions

If needed, the deformable wall section 22d can be configured to include one or more cavities 24v for accommodating the electric components/circuitry 25 provided in, or on, the sensing layer 24. Optionally, but in some embodiment preferably, the cavity 24v is exploited to accommodate additional sensing elements e.g., temperature, pH, conductivity, optical measurement (not shown), configured to sense fluid properties/conditions inside the fluid cavity/cell 22c, and/or of the outside environment. In possible embodiments the sensor unit 20 is configured as a completely passive unit, that is powered, and communicate data, by means of an external device (e.g., by NFC), for example, as demonstrated in FIG. 8. Alternatively, or additionally, electrical conductors/wires (not shown) connected to the sensor unit 20 can be used for powering and communicating with it, without compromising the sealing of the fluid cavity/cell 22c.

FIG. 3 shows a sensor device 30 having a fluid inlet 30i configured to communicate fluid pressure P with a hollow housing 30h equipped with the sensor unit 20. The sensor unit 20 can be installed inside the hollow housing 30h by a plurality of support members 30s extending inwardly from its internal walls and configured to position the sensor unit 20 substantially centered inside the hollow housing 30h. The support members 30s are configured to define fluid passages 30p for providing substantially free flow of the fluid substance introduced into the hollow housing 30h via the fluid inlet 30i, around and surrounding the sensor unit 20, and thereby cause application of the fluid pressure P over the components of the sensor unit 20 substantially from all directions.

This way, the fluid pressure P applied via the inlet 30i propagates into the hollow housing 30h and into the measurement chamber 21c of the sensor unit 20 centered thereinside, and into the volume surrounding the sensor unit 20. Thus, the same fluid pressure P over the deformable portion/element 24n of the sensing layer 24 is also applied over the deformable wall section 22d of the cover portion 22. Accordingly, when fluid pressure P is applied, the sensing layer 24, and the deformable wall section 22d, are both deflected inwardly i.e., into the fluid cavity/cell 22c, and responsively the volume enclosed by the fluid cavity/cell 22c is reduced. The process inside the fluid cavity/cell 22c can be isothermal or adiabatic. In both cases the pressure inside the fluid cavity/cell 22c is increased in response to increased pressure P operating over the sensor unit 20, because the volume of the fluid cavity/cell 22c is reduced.

FIG. 4 graphically illustrates pressure vs volume curves of isothermal process (41) at temperature T1 and isothermal process (42) at temperature T2, and the adiabatic process (43) (transition between two isothermal curves) obtained in some embodiments. For example, an abrupt pressure change of the pressure P applied to the sensor device 30 when the sensor device 30 is on the isothermal curve (41) at temperature T1 can cause an adiabatic process (43) and sensor device 30 will jump on isothermal curve (42) at temperature T2, for which the relationship between the applied pressure P and the volume V of the fluid cavity/cell 22c is P·V^γ=K, where K is a constant and the exponent γ>1. This means that the internal pressure inside the measurement chamber 22c increases faster than in simple isothermal process, and thus can be used to protect the system from burst pressure changes at the fluid inlet 30i.

The creep/deformation compensation obtained by the sensor device 30 is demonstrated in the following example for isothermal processes (41) or (42), for which P·V=K′(costant). Assuming the initial volume V1 of the fluid cavity/cell 22c is filled with air and sealed at atmospheric (absolute) pressure P^cell₁=1 Bar, if fluid pressure of 2.5 Bar (2.5 Bar relative to the atmospheric pressure i.e., the absolute pressure applied is P=3.5 Bar) is applied via the inlet 30i, a reduction to about ⅓ in the volume of the fluid cavity/cell 22c responsive to the inward deformations of the deformable portion/element 24n and the deformable wall section 22d will cause the pressure inside the fluid cavity/cell 22c to change into the absolute pressure value of p^cell₂=3 Bar i.e.., ΔP^cell=(P^cell₁−P^cell₂)=3−1=2 Bar. In this case the pressure P′ over the deformable portion 24n equals to the difference between the applied absolute pressure P=3.5 Bar and the new absolute pressure of P^cell₂=3 Bar inside the fluid cavity/cell 22c i.e., the pressure measured by the sensing element 24s is P′=P−P^cell₂=3.5−3.0=0.5 bar).

Similarly, in another example, if fluid pressure of 5 Bar (5.0 Bar relative to the atmospheric pressure i.e., the absolute pressure applied is P=6.0 Bar) is applied via the inlet 30i, a responsive reduction to about ⅕ in the volume of the fluid cavity/cell 22c will cause the pressure inside the fluid cavity/cell 22c to change into the absolute pressure value of P^cell₂=5 Bar i.e., ΔP^cell=(P^cell₂−P^cell₁)=5−1=4 Bar, and the pressure P′ over the deformable portion 24n equals to the difference between the absolute applied pressure P-6 Bar and the new absolute pressure of p^cell₂=5 Bar inside the fluid cavity/cell 22c i.e., the pressure measured by the sensing element 24s is P−P^cell₂=6−5=1 Bar).

As these examples demonstrate, the pressure sensed by the deformable portion/element 24n is the differential pressure that is responsive to the changes in the volume V of the fluid cavity/cell 22c caused by the deformation of the deformable wall section 22d and the deformation of the deformable portion/element 24n itself. Thus, this mechanism attenuates the pressure P′ over the deformable portion 24n of the sensing layer 24, which is also sensed by the sensing element 24s, and correspondingly compensate the creep/deformation phenomena of the viscoelastic materials from which the sensing layer 24 is made. In addition, in this non-limiting example, if the deformable elements of the sensor unit 20 (e.g., the deformable wall section 22d and the deformable portion 24n) are creeping, they deform inwardly into the cavity 22c, and thereby increase the internal pressure inside the cavity 22c, and this increase of the internal pressure oppose the creep deformation and stops it.

This configuration, wherein two deformable elements are on the fluid cavity/cell 22c, can be advantageously used for:

- (i) creep compensation; and/or
- (ii) high pressure measurements (pressure divider)—for example, the sensor can be designed to work with an external pressure of 100 Bar while the differential pressure sensed by the sensing element 24s is actually attenuated to about 20 Bar i.e., the materials of the sensor device experience substantially lower stress/loads (e.g., pressures can be reduced by a factor of 1.5 to 1000, optionally about 10), such that simpler and more cost effective materials can be used.

In case of an adiabatic process (e.g., a step pressure burst P is applied) the internal pressure of the fluid cavity/cell 22c will rise faster and will thereby protect the deformable element 24n (e.g., they will not be damaged by going into plastic deformation region, or become detached from their supports elements). This configuration of the sensor device 30 thus results is a strain sensor that can withstand higher pressure bursts, or over pressure events.

The relation between the externally applied pressure P and the internal pressure P^cellinside the fluid cavity/cell 22c can be kept to provide a desired behavior (e.g., substantially linear), by adjusting the geometrical shape of the compensation deformable wall section 22d, and/or by adding support elements to control its deformation.

FIG. 5 schematically illustrates use of deformations regulating elements 22e provided inside the fluid cavity/cell 22c of the sensor unit 50 and mechanically coupled to the deformable wall section 22d to cause a desired relation (e.g., linear, quadratic, quasi-linear, cubic, polynomial, logarithmic, exponential, or the like) between the externally applied pressure P and the internal pressure P^cellinside the fluid cavity/cell 22c. The shape/profile of the support elements 22p is adjusted in some embodiments according to the type of media provided inside the fluid cavity/cell 22c of the sensor unit 50.

In this non-limiting example the fluid cavity/cell 22c comprises support elements 22p used to support and/or limit and/or stop deformations regulating elements 22e (shown as dashed lines) for compensating deformations of the deformable wall section 22d. Optionally, but in some embodiments preferably, the measurement chamber 21c is similarly equipped with deformations regulating elements 21e mechanically coupled to the deformable portion 24n of the sensing layer 24, for respective compensating deformations of the deformable portion 24n, and provide the wanted relation between the externally applied pressure P and the internal pressure P^cellinside the fluid cavity/cell 22c.

The deformations regulating elements 21e,22e can be parts of the cover portion 22 configured to follow/regulate the deformation of the deformable portion 24n, and thereby control the changes of the volume inside the cavity 22c, and the changes of pressure thereinside, correspondingly. The deformations regulating elements 21e,22e can have any suitable shape, form or profile (e.g., arc-shaped, stepped configuration, splined, sinusoidal, linear, or any combination thereof). The dashed lines 22d′ show the deformable wall section 22d as it deformed due to the applied pressure P.

The deformations regulating elements 22e,21e facilitates better control of the internal volume of the fluid cavity/cell 22c, when the respective two deformable elements 22d,24n deforms, thereby providing the desired dependency/relationship between the internal pressure P^cellresiding inside the fluid cavity/cell 22c and the externally applied fluid pressure P acting on the sensor unit 50, which reduces and/or stops creep/deformation phenomena, and support the deformable elements 22d,24n at high pressure to avoid damages or sharp stress points.

As also seen in FIG. 5, in some embodiments the cover portion 22 comprises cavities 55 configured to accommodate components (not shown) of the sensing layer, such as, but not limited to, circuitries, antennas, battery, used to read measurements from the sensing elements 24s.

FIG. 6 schematically illustrates a sensor unit 60 wherein the cover portion 22 is configured to support the sensing layer 24 and the internal cavity 22c of the sensor unit 60. In this non-limiting example, the base portion 21 and the cover portion 22, both have a generally sectional H-shaped components, wherein the horizontal arm 21z of the H-shaped base portion 21 comprises the fluid passage 21p formed therein (bypass through hole). As seen, the sensing layer 24 is sandwiched between the base portion 21 and the cover portion 22, thereby forming the sealed fluid cavity/cell 22c between the horizontal arm 22z and vertical legs of the cover portion 22 and the deformable portion 24n of the sensing layer 24, and the measurement chamber 21c between the horizontal arm 21z and vertical legs of the base portion 21 and the deformable portion 24n. In this embodiment the deformable wall section 22d is implemented by the horizontal arm 22z of the H-shaped cover portion 22 i.e., the horizontal arm 22z, deforms into (or out of) the fluid cavity/cell 22c when the external pressure P is applied. The cover potion 22 can clamp the sensing layer 24 to the base portion 21, as its vertical legs are pressed over the edges of the sensing layer 24 against the vertical legs of the base portion 21.

The shape/profile of the support elements 21z is adjusted in some embodiments according to the type of media interacting with the sensor unit 60 and applying the fluid pressure P.

FIG. 7 schematically illustrates another sensor unit 70 configuration wherein the cover portion 22 is configured to support the sensing layer 24 and form the sealed fluid cavity/cell 22c. In this non-limiting example the base portion 21 have a generally sectional H-shape, wherein the horizontal arm 21z of the H-shaped base portion 21 comprises the fluid passage 21p formed therein e.g., by a pass-through bore, and the cover portion 22 have a sectional n-shape. As seen, the sensing layer