FR2743146A1 - Refractive index measuring of fluid or viscous body - Google Patents

Abstract

The method uses a luminous source (5) consisting of selectable lasers and optical components able to project a thin ray of parallel light into a prismatic tank (3) via a deviating unit (54). The prismatic tank contains two parallel sided plates arranged in V-formation at an angle between 58 and 62 deg. The prismatic tank (3) containing the test sample may be rotated on a plate (15) attached to an arm (17) which also supports the detection unit (7). The detection unit uses two photosensitive half discs whose output is fed to a comparator (65). A calculator (9) with memory and display (12,11) enables a refractive index to be found from successive measurements with and without the sample in the prismatic tank.

Description

 The invention relates to a refractometer and to a method for measuring, at a given wavelength and at a given temperature, the refractive index or a refractive index variation of a viscous fluid or body. isotropic and transparent at said wavelength.

 The invention finds particular application in the development of an optical fiber network associated with gas distribution pipes and for detecting any leaks.

 It is particularly in this context to measure the sensitivity of the refractive index of polymers to the absorption of various hydrocarbons comprising natural gas, and in particular to methane, these polymers can therefore be used, at least locally. , as fiber optic sheaths.

It is known that there are already refractometers comprising
a light source for emitting at least one incident light beam,
a prismatic tank arranged on the path of the light beam and comprising two walls adapted to be traversed by said beam and arranged in "V" (inclined with respect to each other),
mobile detection means around said tank, for detecting (at least) the beam coming from the incident beam and refracted by the tank,
means for measuring the angle (s) of the position of said detection means around the tank, and
calculating means for providing, as a function of the angle measurement obtained, the refractive index or a variation of the refractive index of the sample of said fluid or flowable body which has been introduced into tank.

 Such a refractometer is illustrated in the article "Laser light refractometer" pages 3010 to 3013 of the review "Applied optics / vol 23 No. 17/1 September 1984". But, in this article, the measurement of the index of the sample is a function of the index n of refraction of a parallelepiped block in which the prismatic vessel has been shaped.

 However, an essential interest of the invention is to make it possible to determine the absolute value of the refractive index of the sample previously placed in the tank (this after "initialization" of the refractometer), from the sole measurement of the deflection of the incident beam, without reference to an index standard.

 Note that this way of proceeding is also contrary to the traditional methods of the type "limit refraction" or total repair "which, like the previous refractometer, give a relative index compared to that of a block (usually a block of glass ) which is necessary to know the index at the wavelength used for the incident light beam and the temperature of the experiment.

 It will also be noted that the invention is also different from the so-called "minimum deflection" method through a prism, since this method requires several successive adjustments with rotation of the vessel.

The solution of the invention proposed to the disadvantages noted in the prior art consists, on the refractometer already presented
in that each wall of the "V" of the prismatic vessel (fixed) has two faces (strictly) parallel to each other,
in that said angle measuring means comprise means for measuring (a priori in a so-called "initialization" phase) a first angle "a" between the emitted beam reflected by said first wall of the prism and the direct beam, (not deviated, neither by the first nor by the second wall of the prism), a second angle "b" between said beam reflected by the first wall and that reflected by the second wall, and a third angle "D "corresponding to the deflection angle between the beam reflected by said first wall and the beam deflected by the sample disposed in the tank, after measuring said first and second angles,
and in that said calculating means comprise means for measuring the refractive index of the sample, from the formula

 being the index of refraction of the air at a temperature (T) and for a given wavelength (A).

 It will thus be possible in particular to perform measurements at several wavelengths, with a precision better than 10-4 on the value of the refractive index n of the sample and of 104 on its variation. These measures will in particular make it possible to build a database of polymers that are particularly sensitive to certain gases, and in particular to methane.

 Another problem that the invention has also attempted to solve relates to the practical realization of the prismatic vessel and its immediate structural environment, taking into account the precision of the measurements that it is desired to obtain, of a desired compact industrial production of the refractometer, associated with a reliability of the latter.

 The solution of the invention is to propose that this refractometer is such that the detection means are mounted on a turntable rotating around the vessel, along an axis of rotation located inside said prism, close to its edge, said tank being fixed to a frame on which is fixed the source.

 The invention has also endeavored to solve the problems related to the definition of the detection means of the refractometer, so as to meet the already stated requirements of accuracy, reliability and industrial realization.

The solution of the invention is to propose that the means for detecting the refractometer comprise
target receiving means of the beam considered, these means being adapted to convert the position of the impact of said beam on the target into electrical signals,
and a collimation lens interposed between the tank and the detection means.

 Advantageously, said target means will comprise two cells sensitive to said light beam separated from each other by a non-sensitive space.

 With regard now to the angle measuring means and the associated calculating means, it will be noted that the invention has endeavored to allow the measurement of both the refractive index of the sample and the a variation of this index.

In the first case, the calculation means of the refractometer will be designed to provide a calculation from
measurements of said first and second angles, while the vessel is empty, then said third angle (deflection angle), while the sample is in the tank,
- and the mathematical formula already indicated.

 In the second case, the measurement and calculation means will be adapted to take up several angles of deviation and to perform an index difference calculation from the same mathematical formula, respectively.

 To obtain results relating to a "fine" variation of the index of a sample, it should be noted that the refractometer of the invention allows both continuous measurements and point measurements (manual or automated).

In what follows, the present invention will now be described in more detail in the context of a preferred embodiment, this in relation to accompanying drawings in which
FIG. 1 is a general schematic view of a refractometer according to the invention,
- Figure 2 shows the tank with the different rays that come to think or refract,
FIG. 3 shows the pivoting arm carrying the detection means,
FIG. 4 is an enlarged front view of the detection bicell, marked IV in FIG. 3;
FIG. 5 represents an enlarged detail view of the light source,
FIG. 6 illustrates the operating principle of the refractometer of the invention,
- and Figure 7 is the curve of the variation of the voltage from the bicellule (VA - VB), as a function of the deflection angle (D), at a given wavelength.

 In FIG. 1 first of all, it can be noted that the refractometer 1 consists essentially of three parts: a prismatic vessel 3, a light source 5 and a detection system 7. A detection unit 7 is associated with this detection system. calculation 9 which will make it possible to supply on a display unit 11 data relating to either the refractive index of a sample disposed in the vessel 3, or to a variation of this index, as a function of a physical evolution or said consecutive sample, for example, to the penetration of methane in a polymer.

 The tank 3 and the light source 5 are fixed relative to the frame 13 of the apparatus.

 Around the measuring tank is arranged a rotation plate 15 integral with the arm 17 on which is mounted in a fixed position the detection device 7, this opposite the source 5.

 It should be noted that advantageously, the axis of rotation 3a (FIG. 3) of the pivoting arm 17 is located inside the tank, closer to its edge 23 than its end, or base, opposite, 33. In particular, for a prism height H of 12 mm, the distance k between the axis 3a and the edge 23 is advantageously about 3 mm. Also note that this axis of rotation is located near the path followed by the optical beam passing through the prism.

 Concerning this prism, FIGS. 2, 3 and 6 show that it consists of two walls 19 and 21, arranged in "V", that is to say forming, at the location of the edge 23, a aperture angle P between about 58 and 62.

 The two inclined walls toward each other, 19 and 21, are two transparent blades for wavelengths useful for measurements (transparent glass plates at 680 nm, 1.3 μm and 1.55 μm).

 The two faces respectively 19a, 19b and 21a, 21b of each blade are strictly parallel in pairs. And no transparent block surrounds the tank.

 To form this prism, the two blades 19, 21 may be glued together at the edge 23 and be arranged inside on two inclined faces of the angle P, one with respect to the other. another, a cradle of metal 25 (Figure 1).

 In FIG. 5, it can be seen that the source 5 may in particular comprise three collimated laser diodes 27, 29, 31, and a set of optical components 35, 37, 39, making it possible to send through the tank 3 a thin beam of parallel light.

 In the example illustrated, the component 35 is a metallized return mirror, the component 37 is a polarizing separator cube and the component 39 is a parallel-faced plate with dielectric multilayer processing. The three components are, in order, associated with laser diodes of 1550, 1330 and 670 nm.

 The rotation of a bar 41 pierced with openings angularly offset 43, 45, 47 (it could also be flats) to select the desired wavelength.

 The detection system 7 is more particularly illustrated partly in FIG. 3 and partly in FIG.

 It comprises, fixed on the arm 17, a focusing lens 51, as well as detection means 53 adapted to receive the refracted beam deflected by the prism 3 and coming from the beam coming from one of the laser diodes 27, 29, 31 , after angular translation at 54 (FIG. 1) and focusing by the lens 51. The detector 53 can in particular consist of two photosensitive cells 55, 57, in the form of half-discs, separated from each other by a zone central intermediate 59, non-sensitive and very narrow. The beam 53a has been schematized in dotted lines.

Of course, its diameter is greater than the width of the space 59. If the impact of the light beam 53a is homogeneous and if it is centered on the zone 59, the electrical signals delivered by the cells 55 and 57 are equal. . These two signals are sent to a differential amplifier 65 which makes the difference and amplifies it. The resulting signal is measured on the voltmeter 63. In this case, the signal is zero. In the case where the impact of the light beam is no longer centered on the zone 59, there is imbalance between the two signals. The electrical signal measured at the output of the differential amplifier is no longer zero.

 In both cases, the computer 9 deduces the direction of the beams.

 The bicellule 53 may be made of InGaAs and have a diameter of approximately 1 mm, with a zone 59 of approximately 0.1 mm in width and for example a maximum of sensitivity around 1400 nm. Note that the sensitive surface of the bicellule is advantageously set at the focus of the lens 51 and that the boundary between the two cells is set perpendicular to the plane of rotation of the plate 15, which, with the arm 17, is here intended to be driven via a stepper motor 56.

 The refractometer is furthermore equipped with an incremental optical encoder in full step 58 (connected to the plate 15), as well as a peripheral microcomputer 60, to which the calculation unit 9, its associated memory 12 and the display unit 11. This microcomputer can in particular automatically control the rotation of the arm 17, using a conventional interface (such as an IEEE interface).

To explain now how the refractometer works, consider first that we want to measure the refractive index of a sample
Either a monochromatic thin beam of SOIO2D parallel rays passing through the prismatic vessel 3 of angle P at its edge (see FIG. 2).

Conventional laws of refraction make it possible to calculate the value of the refractive index nua T of an isotropic sample and transparent to the desired wavelengths, introduced into the tank 3, this as a function of the parameters i, i 'and P; (AT'):

<tb><SEP> m0 <SEP> T, A
<tb> nT, # <SEP> = <SEP> sin <SEP> P <SEP>(<SEP> sin <SEP> i <SEP> + <SEP> sin <SEP> i '<SEP> + <SEP> 2 <SEP> cos <SEP><SEP> P <SEP> sin <SEP> i <SEP> sin <SEP> i ') <SEP>
<Tb>
i and i 'are the angles respectively of incidence and emergence of the probe beam S (or incident beam) coming from one of the laser diodes of the source 5. n0 T is the index of the air; T is the temperature of the sample and the air, # the wavelength of the optical beam, ON and O'N ', the normals at the walls 19 and 21 respectively of the tank 3.

The measurements of the constants of assembly i and
P, as well as the variable i ', will be obtained here using the beams reflected by the walls with parallel faces of the tank. Indeed, the laser beams are today sufficiently powerful (in this case about 2 mW) for a sensitive detector 53, placed in the focal point of the lens 51, to detect the directions of the beams reflected by these walls.

 In FIG. 6, the lens 51 is schematized at 510, 51a, 51D and 51b in the different positions it occupies during the measurements defined below by "o", "a", "D" and "b" respectively .

 DIR, RB and RH are the directions of the optical beams passing through the tank, reflecting on its first wall 19 and on the second wall 21, respectively, this in the absence of sample, that is to say with an empty vessel 3 (placed here in the air).

 The value of the index "n" will be given by the general formula (A '), in which the constants of the assembly i, P are calculated from the directions DIR, RB and RH.

 The first three measurements below are performed empty tank (that is to say without it contains the sample).

 We will first take directions from the direction of the beam RB reflected by the first face 19 of the tank.

 For this, the direction collimator, that is to say the arm 17, will rotate about the axis 3a to the minimum of the difference of electrical signals detected by the bicellule 55, 57. This corresponds to the position marked "O" in Figure 6.

A calibration measurement (origin) is then performed, with recording via the encoder 58, in the memory unit 12 of the computer which also receives the data from the voltmeter 63.

 The arm is then rotated to measure the angle "a", i.e. the angle between the beam RB and the direct beam DIR. The measurement is stored in memory 12.

 The rotation is then continued to measure the angle "b" between the beams RB and RH.

 This second angle measure is also recorded in unit 12.

 The sample is then introduced into the tank. As already indicated, it is a fluid (gas, liquid) or a viscous material suitable for being poured into the tank.

 The deviation "D", ie the angle between the reflected beam RB and the beam DEV deviated by the sample, is then always measured by rotation of the arm 17.

 By a computer program previously entered in the computing unit 9, it is then easy to express i, P and i 'as a function of the measurements a, b, D performed.

Indeed
i = 90 - a (in degree)
2
b Cen degree)
P = 180 - b
2

<tb> i '<SEP> = <SEP> 90 <SEP> - <SEP>(<SEP>)<SEP> + <SEP> D <SEP>
<tb> (in degree)
Is:

 "a" and "b" are constants of the assembly which depend only on the orientation of the incident beam (S) with respect to the vessel 3 and the edge angle "P thereof.

 To measure the index of the sample, it is therefore sufficient to measure "D" after having "initialized" the device by the recorded measurements of "a" and "b".

 To measure variations of this index, two approaches are possible: a continuous measurement or one-off measurements, whether manual or automated.

 To carry out a continuous measurement, the collimator 51 will first be adjusted in the direction of the beam DEV (angle D), so as to illuminate symmetrically the bicellule 53, that is to say, by balancing the luminous flux on the two active parts 55, 57. In this position, the detected voltage signal is minimum. Leaving the bicell fixed, we follow the position variations of the incident beam by measuring this electrical signal, continuously.

Since the energetic distribution of the flux is not well known (close to a Gaussian close to the emission of the corresponding laser diode, but deformed by the collimation optics), it seems preferable to experimentally measure the function ss = f (VA - VB), rather than performing a calculation of hazardous convolutions; with ss = d / F, d linear displacement of the light spot on the bicellule,
F: focal length of the collimator 51 and VA - V8: voltage difference between the two cells 55, 57, as a function of the decentering of the spot.

In FIG. 7, it can be seen that during a test carried out at the wavelength of 670 nm, the linearity zone extends up to + 30 × 103 degree (angle D) corresponding to a maximum variation of 'index' n 'of the sample of + 4 x
For variations of "n" greater than a few, we will use punctual measurements instead.

 For this, as changes in the refractive index of the sample, for example as the gas enters the polymer, rebalance the flow on the bicellule so that the value of the voltage supplied ( VA - V8) is always at a minimum. And when the equilibrium will be stable over time (in this case characteristic of a saturated polymer), we will note the value of "D". Of course, the deviation for the same sample will have already been calculated before the gas has penetrated.

 The general formula (A) makes it possible to calculate the corresponding variation of "n".

 Note that the value of the ratio signal on to reach variations of "n" inferior to 104.

 Note also that the rebalancing can of course be done manually, but it is preferable then to use a microcomputer to bring the platinum balance by a series of sweeps around the equilibrium position.

 It should also be noted that these measurements made it possible to assess the diffusion rate of a certain number of hydrocarbons used in the polymers studied.

 The reader will not be surprised if we indicate that the refractometer is very sensitive to temperature, that is to say that the indices of the samples (and in particular the polymers studied) are sensitive to this parameter. Temperature control means or calibration may therefore be necessary.

Claims (8)

claims  1.- Refractometer comprising  a light source (5) for emitting at least one incident light beam (S),  a prismatic tank (3) arranged on the path of the light beam and comprising two walls (19, 21) adapted to be traversed by said beam and arranged inclined with respect to each other,  detection means (7) movable around said tank, for detecting at least the beam coming from the incident beam (S) and refracted by the tank (3),  means (58, 65, 67) for measuring the angle (s) of the position of said detection means around the tank, and  calculation means (9, 63) for providing, according to the angle measurement (s) obtained, the refractive index or a refractive index variation of a fluid sample or a body flowing or viscous, transparent to the incident beam, isotropic and disposed in said vessel, characterized in that  each inclined wall of the prismatic vessel (3) has two faces (19a, 19b, 21a, 21b) parallel to each other,  said means (58, 65, 67) for measuring angle (s) comprise means (65) for measuring a first angle (a) between the emitted beam reflected by the first wall of the prism and the direct beam, a second angle (b) between said beam reflected by the first wall and that reflected by the second wall of the prism, and a third angle (D) corresponding to the deflection angle between the beam reflected by said first wall and the beam deflected by the sample disposed in the tank, after measuring said first and second angles,  and said calculating means (9, 63) comprise means (9) for measuring the refractive index of the sample, from formula (A)

 being the index of refraction of the air at a temperature (T) and at a wavelength (jet) of said beam.  2. Refractometer according to claim 1, characterized in that the detection means are mounted on a turntable rotating around the vessel, along an axis of rotation located inside said prism, close to its edge, said vessel being fixed to a frame on which is fixed the source.  3. Refractometer according to claim 1 or claim 2, characterized in that the detection means (7) comprise  target means (53) for receiving the beam in question, these target means being adapted to convert the position of the impact on them of said beam into electrical signals,  and a collimation lens (51) interposed between the tank (3) and said target means.  4. Refractometer according to claim 3, characterized in that said target means (53) comprise two cells (55, 57) responsive to said emitted light beam and separated from each other by a non-sensitive space (59).  5. Refractometer according to any one of the preceding claims, characterized in that the means (9) for calculating the refractometer are designed to provide the index from the measurements of said first and second angles (a, b), whereas the tank (3) is empty, then said third angle (D), while the sample is in the tank.  6. Refractometer according to any one of claims 1 to 4, characterized in that  the measuring means are adapted to detect several angles of deflection (D),  and the calculation means (9) are adapted to perform a calculation of different refractive indices from the measurements of said first and second angles, whereas the tank is empty, then of several said third angles, while the sample is in the tank.  7.- Method for measuring, at a determined temperature and wavelength, the absolute value of the refractive index (zero) of a sample placed in the tank (3) occupying a fixed position on the refractometer according to any one of the preceding claims, wherein  illuminating, in a given direction, with a light beam, a first wall (19) of said empty vessel, which is adapted to be traversed by said beam,  measuring the angle (a) between this beam reflected by said first wall (19) and the direct beam, not deviated, neither by the first wall nor by the second wall (21),  the angle (b) between the beam reflected by said first wall (19) and that reflected by said second wall (21) is then measured,  the sample is placed in the tank (3),  measuring the deviation (D) corresponding to the angle between the beam reflected by the first wall (19) and the one having passed through the second wall (21), after being deflected by the sample,  and the index is calculated from said formula

 nOTx is the index of refraction of the air at said temperature (T) and at said wavelength (A) determined.  8. Method according to claim 7, characterized in that the angles are measured by collimation,  using a position detector (7) comprising two photosensitive detection cells (55, 57), separated by a non-sensitive space (59,  and interposing between the detector (7) and the prismatic tank (3) a collimating lens (51).