FR2683906A1 - Fibre-optic relative humidity sensor - Google Patents

Abstract

Fibre-optic relative humidity sensor, characterised: - in that it comprises two segments of optical fibre (1, 2) having a core index n1 and whose opposite end faces are solidly attached with one another by a polymer adhesive (10), one of the said faces having a cut (6) such that reflection from this face is not recoupled into the said optical fibre, - in that the said polymer adhesive is transparent in the wavelength range lying between 0.4 mu m and 1.6 mu m, and that its index n, close to n1 and lying between 1.45 and 1.55, varies reversibly and reproducibly with the relative humidity for a given temperature, - and in that it is combined with means for emitting an optical signal into the segments and means for processing the corresponding back-scattered signal.

Description

Fiber optic relative humidity sensor
The present invention relates to a relative fiber optic humidity sensor.

 There is a known humidity sensor incorporating an optical fiber whose sheath is porous and whose attenuation varies as a function of the relative humidity. Such a sensor does not give satisfactory results because it is not reversible.

 The aim of the present invention is to produce a relative humidity sensor which is reversible, with all the advantages of sensors based on optical fiber: galvanic isolation between the source to be measured and the measuring device, reduced weight and size, immunity vis -to electromagnetic disturbances of the environment.

 The subject of the present invention is a relative fiber optic humidity sensor, characterized in that - it comprises two sections of optical fiber having a core index nl and the opposite end faces of which are secured to each other by a polymer adhesive, one of said faces having a section such that the reflection on this face is not recoupled in said optical fiber, - that said polymer adhesive is transparent in the range of wavelengths between 0.4 gm and 1.6 rm, and that its index n, close to nl and between 1.45 and 1.55, varies in a reversible and reproducible manner with the relative humidity for a given temperature, - and that it is associated with means for transmitting an optical signal in said sections and means for processing the corresponding backscattered signal.

 According to a preferred embodiment, several relative humidity sensors are used along the same measurement fiber, in other words several sensitive points corresponding to the connection according to the invention of two sections of this fiber by said polymer.

 Said optical signal is preferably generated by a transmitting laser coupled to the measurement fiber.

The backscattered signals are processed by known techniques of optical reflectometry also called "Optical Time Domain
Reflectometry ".

 The processing of said signal must take into account the value of the temperature of said sensor. It is also advantageous to couple a sensor according to the invention to a second sensor of the same type as it, but protected from humidity, and which is then capable of emitting a backscatter signal representative only of the temperature.

 Said polymer can be an olygoacrylate having in particular the following characteristics - Viscosity at ambient temperature between 200 and 6000 mPa.s, - Tensile strength between 5 and 20 N / mm2, - Shore D hardness between 60 and 100.

 Other characteristics and advantages of the present invention will appear during the following description of embodiments of a relative humidity sensor according to the invention. In the accompanying drawing - Figure 1 shows very schematically in section the sensitive element of a sensor according to the invention.

- Figure 2 shows the light energy E backscattered by a series of sensors according to the invention located on the same optical fiber.

- Figure 3 shows response curves of a sensor according to the invention as a function of the relative humidity for various temperatures.

- Figure 4 is a very schematic partial view of a measuring fiber comprising sensors according to the invention associated by two.

- Figure 5 shows a response curve as a function of the temperature of a sensor according to the invention protected from humidity.

 The sensitive element of a relative humidity sensor according to the invention (FIG. 1) comprises two sections 1 and 2 of the same fiber for which the core 3 and the sheath have been referenced. The sheath 4 of undoped silica has an outside diameter of 140 mm and an index of 1.452 for a wavelength of 904 nm; the core 3 of doped silica has a diameter of 100 μm and an index gradient with a maximum value of 1.485 for a wavelength of 904 nm; the numerical aperture of the fiber is 0.3.

 The end faces 5 and 6 of the two sections are joined together by a polymer adhesive 10 of the olygoacrylate type which can be polymerized under the action of ultraviolet radiation (wavelength between 320 and 380 nm); its refractive index after polymerization is 1.505 for a temperature of 200C and a relative humidity of 50%.

 While the face 5 is orthogonal to the optical axis common to the two sections 1 and 2, the face 6 is inclined relative to this axis.

 A single measurement fiber is provided with several sensors of the type of that of FIG. 1, and a light pulse, for example coming from a laser, is sent into this fiber, at the wavelength of 904 nm.

The backscatter signal E is shown diagrammatically in FIG. 2 as a function of time t; for a given temperature, the amplitude of the echoes Epl, Ep2, Ep is a function of the relative humidity rate of each sensor. The time separating two echoes corresponds to the round trip propagation time of the reflected pulse between two successive sensors.

 The optical characteristics of the fiber sections 1 and 2 and of the adhesive 10 are such that the reflection of a light pulse of width 12 nanoseconds on the face 5 results in an echo whose amplitude is equal to ten times the level of backscatter for a temperature of 200C and a relative humidity of 50%.

pour chaque capteur.
Comme cela apparat dans la figure 2, la valeur E d correspond à l'amplitude du signal précédant l'apparition d'un écho d'amplitude E p
E est le niveau de référence.In order to improve the accuracy, a differential measurement is made and the signal is analyzed.

for each sensor.
As it appears in figure 2, the value E d corresponds to the amplitude of the signal preceding the appearance of an echo of amplitude E p
E is the reference level.

o
FIG. 3 shows the variations in the relative humidity rate (in%) as a function of the previously defined ratio R, and this for various temperatures (200C; 4006; 600C; 700C; 800C; 1000C).

Thus at 800C, a variation of 100 grams of water per kilogram of dry air causes a variation of the ratio R of 1.5.

 As it is essential to take into account the temperature of the sensor according to the invention, it is very advantageous to associate with it a sensor of the same type as it, but isolated from moisture by virtue of a coating of impermeable resin, as shown schematically in the figure 4.

Each sensitive point of the measurement fiber 100 then comprises - a sensor (20 or 30) similar to the sensor of FIG. 1, provided with a coating (21 or 31) porous to moisture - a sensor (22 or 32) analogous to the sensor of FIG. 1, provided with a coating (23 or 33) completely isolating it from humidity, for example made of polyurethane resin no 13 521 sold by the company GEC ALSTHOM.

 FIG. 6 represents the ratio R of a sensor (22, 32) as a function of the temperature.

 The measurement line can include any number of sensitive points 100.

 A "multipoint" measurement line according to the invention can be used in disturbed places, far from the centralization and information processing point.

 Of course, the invention is not limited to the embodiments which have just been described. Without departing from the scope of the invention, any means can be replaced by equivalent means.

Claims (4)

1 / Optical fiber relative humidity sensor characterized by the fact - that it comprises two sections of optical fiber having a core index nl and the facing end faces of which are joined together by a polymer adhesive, one of said faces having a section such that the reflection on this face is not recoupled in said optical fiber, - that said polymer adhesive is transparent in the wavelength range between 0.4 Mm and 1.6 Mm, and that its index n, close to nl and between 1.45 and 1.55, varies in a reversible and reproducible manner with the relative humidity for a given temperature, - and that it is associated with means for emitting an optical signal in said sections and means for processing the corresponding backscattered signal. 2 / relative humidity sensor according to claim 1, characterized in that it is coupled to a second sensor of the same type as it, but protected from humidity, and capable of emitting a backscatter signal representative only of temperature. 3 / Relative humidity sensor according to one of claims 1 and 2, characterized in that said polymer is an olygoacrylate polymer polymerizable under the action of ultraviolet radiation. 4 / Relative humidity sensor according to claim 3, characterized in that said polymer has the following characteristics - Viscosity at room temperature between 200 and 6000 m Pa.s - Tensile strength between 5 and 20 N / mm2 - Shore D hardness between 60 and 100.