PRESSURE SENSOR

Abstract

The present invention relates to a diaphragm for a pressure sensor, the diaphragm comprising a hybrid sol-gel film. The invention also extends to a process for the manufacture of said diaphragm for a pressure sensor and to a pressure sensor comprising said diaphragm.

Claims

1. A diaphragm for a pressure sensor, the diaphragm comprising a hybrid sol-gel film.

2. A diaphragm according to claim 1, wherein the hybrid sol-gel film is formed from a hybrid sol-gel composition formed from at least one metal alkoxide precursor and at least one organosilane precursor.

3. A diaphragm according to claim 2, wherein the metal alkoxide precursor is according to formula M(OR).sub.x and/or formula M(OR).sub.x and/or formula M(OR).sub.X-n(R).sub.n, wherein M is a metal atom, each R is independently an alkyl group, each R is independently an organic group, X is the valence of the metal atom, M, and n is from 1 to X1 (X minus 1).

4. A diaphragm according to claim 3, wherein the metal, M, is zirconium (Zr).

5. A diaphragm according to claim 4, the metal alkoxide precursor comprises zirconium butoxide (Zr(OBu).sub.4) and/or zirconium propoxide (Zr(OPr).sub.4).

6. A diaphragm according to claim 3, wherein R is derived from pivalic acid, acetic acid, (alk)acrylic acid, alkyl(alk)acylate or combinations thereof.

7. A diaphragm according to claim 6, wherein R is derived from (alk)acrylic acid, alkyl(alk)acylate or combinations thereof.

8. A diaphragm according to claim 1, wherein the organosilane precursor is according to formula R.sup.1.sub.4-xSi(OR.sup.2).sub.x, wherein each R.sup.1 is independently a substituted or unsubstituted alkyl group, each R.sup.2 is independently an alkyl group and X is 1-4.

9. A diaphragm according to claim 8, wherein the organosilane precursor comprises methacryloxypropyltrimethoxysilane (MAPTMS).

10. A diaphragm according to claim 2, wherein the hybrid sol-gel composition is prepared by a method comprising the steps of: (a) providing a first precursor composition comprising an organosilane precursor and an aqueous solution of a strong acid; (b) providing a second precursor composition comprising a metal alkoxide precursor; (c) contacting the first precursor composition and the second precursor composition to form a reaction mixture; and (d) causing the reaction mixture to undergo hydrolysis for a period of time, T, to form the sol-gel composition.

11. A diaphragm according to claim 1, wherein the hybrid sol-gel film has a film thickness from 2 to 100 m.

12. A process for the manufacture of a diaphragm for a pressure sensor according to claim 1, the method comprising the steps of: (a) providing a hybrid sol-gel composition formed by a process comprising the steps of: (i) providing a first precursor composition comprising an organosilane precursor and an aqueous solution of a strong acid; (ii) providing a second precursor composition comprising a metal alkoxide precursor; (iii) contacting the first precursor composition and the second precursor composition to form a reaction mixture; and (iv) causing the reaction mixture to undergo hydrolysis for a period of time, T, to form the sol-gel composition; (b) coating an end portion of a capillary with the hybrid sol-gel composition produced in step (a); and (c) curing the hybrid sol-gel composition to form the hybrid sol-gel film.

13. The process according to claim 12, wherein the capillary is a glass capillary.

14. A pressure sensor comprising an optical fibre, a diaphragm according to claim 1, and a wall defining a cavity between an end face of the optical fibre and the diaphragm, wherein the diaphragm is configured to deflect under a difference between a pressure in the cavity and a pressure external to the sensor.

15. A pressure sensor comprising an optical fibre, a diaphragm manufactured according to the process of claim 12, and a wall defining a cavity between an end face of the optical fibre and the diaphragm, wherein the diaphragm is configured to deflect under a difference between a pressure in the cavity and a pressure external to the sensor.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0112] For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example only, to the following experimental data and accompanying drawings, in which:

[0113] FIG. 1 is a cross-sectional view of an example pressure sensor according to the present invention.

DESCRIPTION OF EMBODIMENTS

[0114] An EFPI sensor 10 according to the present invention, shown in FIG. 1, comprises an optical fibre 12, a diaphragm 14 and a capillary 16, for example a glass capillary. The diaphragm 14 is provided at a first end of the capillary 16, whilst the optical fibre 12 is provided in a second end of the glass capillary, thereby defining a cavity 18 within the capillary between an end face of the optical fibre 12 and the diaphragm 14.

[0115] The optical fibre 12 is a single mode fibre. The optical fibre 12 is fixed in place by a fusion splice 20, such that it extends partially within the capillary.

[0116] Applied pressure causes a deflection of the diaphragm 14, thereby modulating the length of the cavity 18. In some examples, the optical fibre 12 comprises an in-fibre Bragg grating (FBG) 22, which is used as a reference sensor to eliminate temperature cross-sensitivity of the EFPI pressure sensor 10.

[0117] An example method of fabricating the EFPI sensor comprises inserting a single mode optical fibre into a capillary having a 133 m inner diameter and 220 m outer diameter. The capillary has a length of 5 to 10 cm. The capillary is then fused with the optical fibre using a fusion splicer, providing a seal between the optical fibre and capillary. After fusion of the capillary with the optical fibre, the capillary may be cleaved using a manual scriber or cleaver, such that the length between the end of the capillary and the end face of the single mode fibre is 1 to 2 mm.

[0118] The capillary is then polished, manually or using a polishing machine, with different grit-size polishing paper, so that the length between the end of the capillary and the end face of the single mode fibre is 40 to 80 m. The length between the end of the capillary and the end face of the single mode fibre can be monitored using a magnified optical microscope or an optical spectrum analyser.

[0119] The diaphragm 14 is formed from a hybrid sol-gel film according to the present invention.

[0120] The hybrid sol-gel film forms the diaphragm at the end of the capillary, as shown in FIG. 1. In an example, the hybrid sol-gel film wraps around the first end of the capillary, and thereby also extends around the outer cylindrical surface of the capillary along at least part of the length of the capillary, as shown in FIG. 2.

[0121] As illustrated in FIG. 1, incident light is reflected within the EFPI pressure sensor 10 at the end face of the optical fibre 12 and at the inside and outside surfaces of the diaphragm 14. All three reflections propagate back through the optical fibre 12, generating interference fringes which can be expressed by the following equation (1):

[00001]$\begin{array}{cc}I\left(,L\right)=A_1^2+A_2^3+A_3^2-2A_1{A}_2\cos \left(\frac{4L_C}{}\right)+2A_3{A}_1\cos \left(\frac{4\left(L_C+\mathrm{nd}\right)}{}\right)-2A_2{A}_3\cos \left(\frac{4\mathrm{nd}}{}\right)& \left(1\right)\end{array}$

[0122] In equation (1), the first and second cosine terms describes the interference between the end face of the optical fibre 12 and the inner and outer surfaces of the diaphragm 14 respectively. The third cosine term describes the interference between the inner and outer surface of the diaphragm 14. A1, A2 and A3 are the amplitudes of the reflected light at the end face of the optical fibre 12 and the inside and outside surface of the diaphragm 14; n is the refractive index of the diaphragm 14; L.sub.c is the length of the air cavity 18 between the end face of the optical fibre 12 and the inside surface of the diaphragm 14; d is the thickness of the diaphragm 14 and is the optical wavelength. The length L.sub.c of the air cavity 18 changes when the diaphragm 14 deflects as a result of a pressure difference between a pressure inside the cavity 18 and outside the sensor 10.

[0123] The change in pressure can be determined based on the change in the length of the air cavity 18. The change in length of the air cavity L.sub.P due to the static pressure difference LAP at the centre of the diaphragm 14 may be expressed by equation (2):

[00002]$\begin{array}{cc}{L}_P=\frac{3\left(1-{}^2\right)}{16{\mathrm{Eh}}^3}.Math.a^4.Math.P=a_{21}P& \left(2\right)\end{array}$

[0124] In equation (2), h is the radius of the diaphragm, E is Young's Modulus, is the Poisson's ratio. The deflection of the diaphragm 14 thereby depends linearly on the pressure difference, and so the phase of the first and second cosine terms in equation (1) will also depend linearly on the pressure difference.

[0125] The thermal cross-sensitivity of the EFPI pressure sensor 10 is caused by the thermal expansion of all components, the diaphragm 14 and the air within the EFPI cavity 18. The change in length based on the change in pressure can be expressed by equation (3):

[00003]$\begin{array}{cc}{L}_T=\left(L_S\left({}_C-0_{\mathrm{SM}}\right)+L_C{}_{\mathrm{SM}}+\frac{P_S}{T_S}S\right)T=a_{22}T& \left(3\right)\end{array}$

[0126] In equation (3), Ls is the distance between the diaphragm 14 and the optical fibre/capillary fusion splice 20, .sub.c and .sub.SM are the coefficients of thermal expansion (CTE) of the capillary 16 and the optical fibre 12 respectively. P.sub.s and T.sub.s are the pressure and temperature during sealing and S is the pressure sensitivity of the EFPI pressure sensor 10.

[0127] FBGs are simple, intrinsic sensing elements which have been extensively used for strain, temperature and pressure sensing. The standard FBG grating is formed as a regular variation in the refractive index of the core of a single-mode (SM) optical fibre. Chirped FBGs may also be used in certain examples where a particular response is desired. Chirped FBGs are typically characterised by a non-uniform modulation of the refractive index within the core of an optical figure. In normal operation a standard FBG causes light propagating in the optical SM fibre core of a particular wavelength (the Bragg wavelength .sub.B) to be reflected back. The Bragg wavelength is defined by equation (4):

.sub.B=2n.sub.eff(4)

[0128] n.sub.eff is the refractive index of the core material and is the period of the grating. All other wavelengths are transmitted in the normal manner through the fibre. The sensing function of a FBG derives from the sensitivity of the refractive index and grating period to externally applied mechanical or thermal perturbation. It is experienced by the FBG through altering the reflected Bragg wavelength .sub.B.

[0129] In the EFPI pressure sensor 10, the FBG 22 is fully encapsulated in the capillary 16. This keeps the FBG 22 strain and pressure free and hence it is only sensitive to temperature. The temperature sensitivity occurs through the effect on the induced refractive index change and on the thermal expansion coefficient of the SM fibre. The Bragg wavelength shift .sub.B,T due to temperature change T is expressed by equation (5):

[00004]$\begin{array}{cc}{}_{B,T}={}_B\left(+\frac{1}{n_{\mathrm{eff}}}{\mathrm{dn}}_{\mathrm{effdT}}\right)T=a_{12}T& \left(5\right)\end{array}$

[0130] In equation (5), dn.sub.eff/dT is the thermo optic coefficient.

[0131] Using the temperature and pressure coefficients form both sensing units a matrix can be constructed as:

[00005]$\begin{array}{cc}\left[\begin{array}{c}{}_B\\ L\end{array}\right]=\left[\begin{array}{cc}0& a_{12}\\ -\mathrm{a21}& a_{22}\end{array}\right]\left[\begin{array}{c}P\\ T\end{array}\right]& \left(6\right)\end{array}$

[0132] The inversion of the matrix can be used to discriminate between the pressure P and temperature T information from the FBG and EFPI pressure sensor.

[0133] Exemplary hybrid sol-gel films, as used as the diaphragm 14 in the pressure sensor 10 of FIG. 1, are provided below.

EXAMPLES

Example 1

Preparation of Hybrid Sol-Gel Composition Example 1

[0134] A first precursor composition was prepared by mixing 15 grams (g) 3-methacryloxypropyltrimethoxysilane (MAPTMS) (available from Sigma Aldrich) and 0.8 g of 0.1 M HNO.sub.3. The mixture was stirred at room temperature for 45 minutes.

[0135] A second precursor composition was prepared by mixing 7.1 g of a 70 vol % solution of zirconium (IV) n-propoxide (Zr(OPr).sub.4) (available from Sigma Aldrich) in propanol and 1.2 g methacrylic acid (MAA) (available from Sigma Aldrich). The mixture was stirred at room temperature for 45 minutes.

[0136] Then, the first precursor composition was slowly added to the second precursor composition over a time period of 5 minutes. After addition was complete, the reaction was allowed to proceed for 5 minutes before a neutral hydrolysis was performed by adding 1.3 g deionised water dropwise to the reaction mixture. The reaction mixture was then stirred at room temperature for 24 hours. The resultant hybrid sol-gel composition was clear and transparent and had a viscosity of about 20 cP.

Preparation of Hybrid Sol-Gel Composition Example 2

[0137] A first precursor composition was prepared by mixing 10 grams (g) 3-methacryloxypropyltrimethoxysilane (MAPTMS) (available from Sigma Aldrich), 4.45 g 3-triethoxysilypropylamine (APTES) and 0.8 g of 0.1 M HNO.sub.3. The mixture was stirred at room temperature for 20 minutes

[0138] A second precursor composition was prepared by mixing 3.5 g titanium isopropoxide (available from Sigma Aldrich) and 0.6 g acetic acid (AA) (available from Sigma Aldrich). The mixture was stirred at room temperature for 45 minutes.

[0139] Then, the first precursor composition was slowly added to the second precursor composition over a time period of 10 minutes. After addition was complete, the reaction was allowed to proceed for 5 minutes before a neutral hydrolysis was performed by adding 1.1 g of deionised water dropwise to the reaction mixture. The reaction mixture was then stirred at room temperature for 24 hours. The resultant hybrid sol-gel composition was clear and transparent and had a viscosity of about 30 cP.

Example 2

Preparation of Hybrid Sol-Gel Film Examples 1-9

[0140] The first end of the glass capillary having an optical fibre provided at the second end of the glass capillary, as shown in FIG. 1, was held in a fibre clamp with 50 mm of the first end of the glass capillary extruded. The first end of the glass capillary was then dipped into the hybrid sol-gel composition prepared in example 1 at various dilutions as provided in table 1. The first end of the glass capillary was dipped into the hybrid sol-gel solution to a depth of 20 mm. The glass capillary was held in the hybrid sol-gel composition for 60 seconds. After this time, the glass capillary was withdrawn from the hybrid sol-gel solution at the rate provided in Table 1.

[0141] The coated glass capillaries where then removed from the fibre clamp and suspended in a drying chamber. The drying chamber was then heated from room temperature to 110 C. Once this temperature was reached, the glass capillaries were held in the drying chamber for 30 minutes to thermally cure the hybrid sol-gel compositions. Curing of the hybrid sol-gel compositions formed hybrid sol-gel films which could then be used as a diaphragm in the pressure sensor of FIG. 1.

TABLE-US-00001 TABLE 1 Preparation of hybrid sol-gel film examples 1-9 Rate of withdrawal Hybrid sol-gel film example Dilution (%) (mm/min) 1 20 40 2 40 40 3 50 40 4 80 40 5 95 40 6 0 (pure) 40 7 0 (pure) 40 8 0 (pure) 40 9 0 (pure) 100

Preparation of Hybrid Sol-Gel Film Examples 10-12

[0142] The first end of the glass capillaries having the hybrid sol-gel films of examples 4, 5 and 6 thereon were re-dipped into the undiluted, i.e. pure, hybrid sol-gel composition of example 1 according to the same process as described above for the preparation of hybrid sol-gel film examples 1-9. The rate at which the glass capillaries were withdrawn from the hybrid sol-gel composition are provided in Table 2.

TABLE-US-00002 TABLE 2 Preparation of hybrid sol-gel film examples 10-12 Rate of withdrawal Hybrid sol-gel film example Dilution (%) (mm/min) 10 0 (pure) 40 11 0 (pure) 20 12 0 (pure) 100

[0143] The resultant hybrid sol-gel films were tested according to the following test methods.

[0144] Observation of hybrid sol-gel films: the transverse and axial views of the cured hybrid sol-gel films were observed visually under a microscope. The results are shown in FIG. 2.

[0145] Pressure and temperature responses: the optical properties of the hybrid sol-gel films were tested to determine if they were suitable for use as a diaphragm in an EFPI based on the returned spectral response from the EFPI. The hybrid sol-gel films were tested against a standard silica diaphragm as a reference (i.e. comparative example 1). The results of the pressure and temperature response for hybrid sol-gel example 12 and comparative example 1 are shown in FIGS. 3 to 6, in which FIG. 3 shows the pressure response of hybrid sol-gel example 12, FIG. 4 shows the pressure sensitivity of hybrid sol-gel example 12 versus comparative example 1, FIG. 5 shows the temperature response of hybrid sol-gel example 12 and FIG. 6 shows the temperature response of comparative example 1.

[0146] The results show that the hybrid sol-gel films of the inventive examples are generally of a similar construction and the quality of the hybrid sol-gel films are of a good quality with minimal artefacts. The hybrid sol-gel films also have a relatively uniform surface finish.

[0147] The results also show that the hybrid sol-gel films of the present invention are more sensitive that the comparative silica diaphragm.

[0148] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0149] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

[0150] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0151] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.