IL103683A - Device and method for measuring characteristics of fluids - Google Patents

Description

A DEVICE AND METHOD FOR MEASURING CHARACTERISTICS OF FLUIDS The present invention relates to optical devices for measuring various characteristics of fluids, and particularly the level of a liquid, It also relates to a light guide and method for measuring liquid levels.

Optical devices which measure various characteristics of a fluid are useful in a wide variety of applications, such as sensing the level of a liquid. Optical liquid level sensing devices generally include a light guide which is dipped into a liquid-containing vessel. The light guide is in optical communication with a light source and a photodetector which measures the light transmitted through the light guide. Light guides are also employed for sensing other fluid parameters, such as index of refraction arid temperatures of a liquid or a gas.

Total internal reflection is a well-known phenomenon in optics. If the angle of incidence of the light exceeds the critical angle, the light is totally reflected into the denser medium, and does not · enter the less dense medium, except for an evanescent wave which penetrates a short distance.

Intensity-modulated light-guide liquid level sensors are well-known. Such liquid level sensors are described, for example, in U.S. Patent 4,311,048.

In prior-art intensity-modulated light-guide liquid level sensors, total internal reflection takes place preferentially at the light guide-air interface.

At the light guide-liquid interface, total internal reflection does not occur and most of the flux escapes the light guide through its surface, thus reducing considerably the light signal intensity at the output which is detected by a photodetector . However, this type of sensor has a highly nonlinear characteristic (signal vs. liquid level) and in practice, for liquid level measurement, is most useful only as a binary on-off sensor. There are very many types of optical light-guide sensors which operate on the principle of changes in conditions for total internal reflection.

It is one of the objects of the present invention to overcome the drawbacks and deficiencies of the prior art devices and to provide a sensor that has a substantially linear characteristic, greatly extending its useful range and that prevents deleterious signal attenuation due to the escape of light through the light-guide surface.

According to the invention, this is achieved by providing a device for measuring characteristics of fluids, in particular the level of liquids in containers, comprising a light guide in the shape of an elongate body made of a light-transmissive material and adapted to be immersed in said liquid; a light source producing a substantially monochromatic light beam directable into said light guide; a photodetector disposed so as to detect said light beam after multiple passages thereof across and along said light guide and to provide a signal indicative of the intensity of said light beam after said multiple passages means to receive said signal and to interpret it in terms of level height of said liquid in said container, characterized in that at least some portions of said light-guide body are coated with a relatively thin, light-transmissive film, the index of refraction of which differs from the index of refraction of said light-guide body.

The invention further provides a light guide in the shape of an elongate body made of a light-transmissive material, characterized in that at least some portions of said elongate body are coated with a relatively thin, light-transmissive film, the index of refraction of which differs from the index of refraction of said light-guide body.

The invention still further provides a method for measuring characteristics of fluids, in particular the level of liquids in a container, comprising the steps of providing a light guide in the shape of an elongate body made of a light-transmissive material, at least some portions thereof being coated with a relatively thin, light-transmissive film, the index of refraction of which differs from the index of refraction of said light- guide body; providing a light source producing a substantially monochromatic light beam directable into said light guide; providing a photodetector adapted to detect said light beam after multiple refractions and reflections across and along said film-coated light guide, and to produce output signals to be fed to indicator means; at least partly immersing said light-guide body in a substantially vertical position in a liquid the level of which is to be measured, thus producing a film-ambient medium interface comprised of a film-liquid interface and a film-air interface; directing said light beam against the interface between said light-guide surface and said film, causing said beam to be partly reflected into said light guide and partly refracted into said film, said refracted light beam in said film striking said ambient-medium interface at an angle of incidence larger than . the critical angle at that interface, rendering the latter totally reflective and causing a thus reflected beam to be refracted into said light guide; measuring, after said multiple refractions and reflections, the output of said photodetector, said output being indicative of the level of liquid in said container.

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings: Fig. 1 is a schematic illustration of a preferred embodiment of a liquid level sensing device according to the present invention; Fig. 2 shows a film-coated light guide as used in the present invention; Fig. 3 represents the enlarged detail A of Fig. 2 for a film with an index of refraction larger than the index of refraction of the light-guide material; Fig. 4 is an analogous enlarged detail A for a film with an index of refraction lower than the index of refraction of the light-guide material; Fig. 5 is a graph of reflection coefficients r for p and s polarizations as functions of the angle of incidence at the film-ambient medium interface; Fig. 6 is a graph of reflection phase shifts δ for p and s polarizations as functions of the angle of incidence at the film-ambient medium interface, and Fig. 7 is a graphic illustration of the relationship between the liquid level and the output voltage produced by the photodetector, as compared to the same relationship for a conventional liquid-level sensing device.

Reference is now made to Fig. 1, which schematically illustrates a liquid-level sensing device according to a preferred embodiment of the present invention. The device is immersed in the liquid container 10 and is operative to sense the level of the liquid in this container. The device typically comprises a light source 12 which communicates via an optical fiber 14 with an Optical directional coupler 16. The optical directional coupler 16 is in optical communication with a light guide 18 and, via an optical fiber 22, with a photodector 24. The output signal of photodetector 24 is received by an indicator unit 26 which is operative to provide a suitable indication of the liquid level in the container 10. Shown are also the height of the liquid level H and the length L of the light guide 18.

A mirror 28, which may consist of a thin metal film, is arranged in operative association with the light guide 18, such as facing the bottom surface of the light guide 18. The directional coupler 16 is arranged so as to direct a beam provided by the light source 12 toward the mirror 28 via the light guide 18. Mirror 28 is operative to reflect the beam back through the light guide 18, with the reflected beam arriving at the photodetector 24 via the directional coupler 16 and the optical fiber 22.

The directional coupler 16 may comprise two-channel optical wave guides spaced closely enough so that flux is transferred from one to another only for a reflected beam. Alternatively, coupling may be provided by arranging the two optical fibers 14 and 22 at a suitable angle relative to one another such that a beam reflected back from the mirror 28 is transmitted to the photodetector 24 via the optical fiber 22.

It is a particular feature of the present invention that the light guide 18 is coated with a film 30 having a refractive index differing from the refractive index of the light guide material, which is typically glass, with total internal reflection occuring both at the film-air interface and at the film-liquid interface when the angle of incidence of a light beam is larger than the critical angle at the respective interfaces, and that, in dependence on the above, indices as well as on film thickness d, wave length and angle of incidence of the light and the indices of refraction of the two media (air above, and liquid below, the level H) , the beams from the light source 12, partly reflected at the light guide-film interface and partly transmitted- into the film 30 at that interface, . and subsequently reflected at the film-ambient medium interface, and re-transmitted into the light guide 18, form a coherent combination to the effect of producing interference, inside the light guide 18, between the above partly reflected beam and the above re-transmitted beam. This interference, which may be either constructive or destructive, is determined by the relationship between the respective phase shifts incurred, inside the film 30, by a beam reflected by the film-liquid interface and that reflected by the film-air interface. This interference modifies the intensity of the light reaching the photodetector 24, and, thus, its electrical output.

It should be noted that this interference is not limited to those two beams, but is essentially a multiple-beam phenomenon, because of recurring incidences of the beams propagating in the film. However, the phase difference, and thus the nature of the interference between the beams, whether constructive ' or destructive, is substantially that determined by the phase difference between the initial two beams.

The following are examples of materials found suitable for application on a light guide made of glass (N = 1.5), with the liquid being water (N = 1.33) and with d denoting the film thickness, N the index of refraction and the preferred angle of incidence: Al203 : N = 1.6; Yo = 79°; d = 0.2 um Ti Ox: N = 2.3; Optical fibers 14, 22: Fiberglass-glass (GLPC) or plastic clad silica (PCS), commercially available from 3M Specialty Optical Fibers, West Haven, Connecticut, U.S.A. Alternatively, a fiber-optic cable may be used, such as the cable commercially available from Ensign-Bickford Optical Technologies, Inc., Van Nuys, California, U.S.A.

Light guide 18: A glass rod may be used of a cross-section either circular or rectangular and of the required length. Representative cross-sectional dimensions are about 10 mm diameter for the circular cross-section, and 4 mm x 4mm or 6 mm x 3 mm for the rectangular cross-sections. Suitable optical glasses are BK1, K7, or F6, commercially available from Spindler & Hoyer, Goettingen, Germany.

Photodetector 22: GM3 8 or GMG815, commercially available from Germanium Power Devices Corp., Massachusetts, U.S.A.

Indicator 26: A voltmeter or an optical power meter, such as an ML9001A, commercially available from Anritsu Corporation, Tokyo , Japan .

The operation of the device of Fig. 1 is as follows: The liquid level sensing device 10 is immersed in liquid in a container 10. A beam is provided by the light source 12 and passes through the optical fiber 14 to the optical directional coupler. 16 which directs the beam into the light guide 18, which is operative to guide the beam down the length of the container 10. Via the mirror 28, the beam propagates back through the light guide 18 and arrives at the photodetector 24 via the optical directional coupler 16 and the optical fiber 22. The photodetector 24 provides an electrical signal to the liquid level indicating unit 26, the magnitude of which signal is proportional to the flux arriving at the photodetector 24. The liquid level indicating unit 26 is adapted or calibrated to indicate the liquid level in the container 10, either empirically, or as based on a suitable expression such as: Vout = K · IjLn · Rr,1 · R m (1) where : vout = output voltage of the photodetector 24 K = coefficient depending on attenuation and electrical characteristics of the system Iin = intensity of light input Rx. = total reflected amplitude coefficient for liquid interface RA = total reflected amplitude coefficient for air interface 1 = integer representing number of reflections at the film-liquid interface m = integer representing number of reflections at the film-air interface 1 and m may be computed as follows: 1 = H/(D . tg (fa) m = (L - H)/D- tg^0 where L is the length of the light guide 18; H is the height of the liquid, D is the effective lateral dimension or, in the case of a cylindrical light guide, diameter, of the light guide and (ρΌ is the angle of incidence at the light guide-film interface.

D.tg^0- ln(Vout-/KI±„)-L- lnRA lnRj- - lnRA It will be appreciated that the device of Fig. 1 may be modified in many ways. For example, the optical fibers 14 and 22 and the optical directional coupler 16 and mirror 28 may be dispensed with. instead, the photodetector 24 may be disposed in direct optical communication with the lower end of light guide 18. Also, couplers such as prism couplers or grating couplers may replace the coupler 16 described above.

Instead of a parallel beam divergent beam may be used, with the angle of incidence^-of—eacn ray exceeding the critical angle of the film-air and film-fluid interfaces.

Reference is now made to Fig. 2, which is an optical diagram of a preferred light path through the light guide 18 and film 30 of Fig. 1, with the thickness of the film greatly exaggerated relative to the light guide diameter. Fig. 3, the enlarged detail A of Fig. 2, illustrates a single-incidence oblique reflection and transmission of a plane wave in a parallel-plane boundaried system including the light guide 18, the film 30 of thickness d, and a quasi-infinite layer of an ambient medium 40. All three media are assumed to be homogeneous and optically isotropic and have indices of refraction N0 (light guide 18), ^. (film 30) and N2 (ambient medium 40), respectively, where N0 > N2, with Fig. 3 representing the case in which > N0. In Fig. 3,^, is the angle of incidence within the light guide 18 and ^ is the angle of refraction in the film 30.. 6i must exceed the critical angle of the film-air interface and of the film-liquid interface so that total internal reflection will occur at the interface with the ambient medium, whether air or liquid.

When a plane wave is incident from inside the light guide on the light guide 18-film 30 interface at an angle (fo t one portion of the incident wave is reflected into the light guide 18 and another portion is refracted into the film 30, the ratio between the two portions being determined by the Fresnel reflection and transmission coefficients of the medium-film interface.

If is large enough for total internal reflection to take place at the film-fluid interface, then whenever the refracted wave inside the film 30 strikes the film-ambient medium interface, total internal reflection occurs at that interface, with the reflected wave being refracted into the light guide 18 as wave or beam 44.

The total reflected amplitude coefficient R pertaining to a plane wave propagating in the film-coated light guide 18 along the preferred light path illustrated in Fig. 2 is a well-known quantity, given, for instance, in Born & Wolf, Principles of Optics, Chapter 1.6, equ. (59) (Pergamon Press, London, 3rd Edition 1965) .

Fig. 4, analogously to Fig. 3, represents the case in which Reference is now made to Fig. 5, which is a graphic illustration representing variation in internal reflection intensity as a function of angle of incidence at the interface between the film 30 and the ambient medium 40, for p and s polarization functions, curves 50 and 52, respectively.

Reference is now made to Fig. 6, which is a graphic illustration representing variation in internal-reflection phase shift dr as a function of the angle of incidence at the interface between the film 30 and the ambient medium 40, for p and s polarization functions, curves 60 and 62, respectively, where the medium has a lower index of refraction than the film. When light is reflected from a medium having a higher index of refraction, the phase shift is π. for all angles of incidence larger than Brewster's angle for both polarizations, and zero below that angle for p polarization.

The phase remains unchanged for all angles of incidence when light crosses an interface. In other words, transmission occurs without phase shift.

Interference between the first wave (beam 42) of Fig. 3 and the second wave (beam 44) takes place in the light guide 18. When, for example, Na.>N0>N2, the phase of the first beam 42 is 518 + n, where δ1β is the phase of the incident wave.

The phase of the second beam 44 is δ44 = δ1β + δ30,-ιο + 2B, where δ30,-_ο is the phase shift of the wave at the film-ambient medium interface and B is the phase shift experienced by the wave inside the film on a single passage between the boundaries of the film 30, given by: B = 2R(dA¾)(Nl2 - No2sin2 >o)°-5 (3) where : d = thickness of film 30, typical values of which have been mentioned earlier ^ = free-space wavelength.

Changes in the index of refraction of the ambient medium 40, i.e., at the transition across the air-liquid interface, result in changes in the phase shift δ30,40/ and thus in the total reflected amplitude coefficient, and in observable changes in the output intensity.

As the level of fluid in container 11 rises, so rises the line of transition between the indices of refraction of the liquid and the air on the film-ambient medium interface. The phase shifts for the p and s components are different above and below this line, as described by equations (4) and (5), respectively: arctg 2ΝΧΝ2 (N12-N02sin¾0 ) ° * s (NG2sin2f0-N22 ) ° · 3 (4) N2* (N12-N02sin2^0 ) -Na.4 (N02sin2 0-N02 ) This means that phase shifts δ1^^** for a wave reflected from the film-liguid interface differ from the phase shifts δ £ for a wave reflected from the film-air interface.

This leads to a change in the conditions of interference, whether constructive or destructive, between the first wave (beam 42) and the second wave (beam 44), which will cause an increase or decrease of the output intensity. The behaviour of the output signal, i.e., whether rising or falling with rising liquid level, and the sensitivity of the sensor depend on the angle of incidence, the refractive indices of the light guide 18, the film 30 and the ambient medium 40, and the film thickness d.

Reference is now made to Fig. 7, which is a graphic illustration of the relationship between liquid level and voltage as read by the indicator 26, indicated by a first curve 70, (falling with rising liquid level) or 72 (rising with rising liquid level), compared to the same relationship for a conventional liquid-level sensing device, indicated by a second curve 74.

As shown in Fig. 7, a typical curve 74 for a conventional liquid-level sensing device is very far from linear. Because curve 74 reaches the horizontal axis very fast, the measurement range of a conventional device is very narrow. In contrast, the curves 70 or 72 of the device of Fig. 1 are substantially linear and evidence a wide measurement range which is not limited by the shape of the curve.

While the various mathematical expressions presented in the aforegoing constitute the theoretical basis of the device according to the invention, it is obviously necessary to perform calibration in order to correlate the liquid level with the readings of the indicator 26, i.e., to convert voltages into millimeters of liquid level. Calibration is best carried out by empirical methods, by filling the container to certain level heights and marking the read-out element (e.g., a scale) accordingly. As the indications of the device according to the invention are substantially linear (see Fig. 7), interpolation between only a very few verified scale points is both easy and reliable. Recalibration is obviously necessary when the device is going to be used with a different liquid having a different index of refraction.

It should be noted that the invention is not necessarily limited to a light guide configuration with a plurality of incidences, but is applicable also to a body wherein a single incidence of the beam occurs.

While the present invention deals specifically with level measurement, the film-coated light guide according to the invention can be used to determine other properties of fluids as well. Thus the index of refraction of liquids can be continuously monitored by passing the liquid through a cell, the bottom of which is constituted by a horizontally disposed, film-coated light guide of, say, rectangular cross section, with the monochromatic light beam introduced into it at one end thereof, and the photodector receiving the beam, after multiple reflections, at the other end.

In another application, gas pressures can be measured by providing two communicating liquid-filled vessels, each of which is provided with a film-coated light guide according to the invention. One of the vessels is closed and connectable to the pressurized gas; the other is open to the atmosphere. When connected, the gas under pressure will lower the liquid level in the closed vessel, while the thus-displaced liquid mass will raise the level in the other vessel. Gas pressure is obviously a function of the difference of levels in the two vessels, a difference easily determined by the method discussed. The same principle will obviously also work for negative pressures.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (13)

WHAT IS CLAIMED IS:

1. A device for measuring characteristics of fluids, in particular the level of liquids in containers, comprising: a light guide in the shape of an elongate body made of a light-transmissive material and adapted to be immersed in said liquid;- a light source producing a substantially monochromatic light beam directable into said light guide; a photodetector disposed so as to detect said light beam after multiple passages thereof across and along said light guide and to provide a signal indicative of the intensity of said light beam after said multiple passages; means to receive said signal and to interpret it in terms of level height of said liquid in said container, characterized in that at least some portions of said light guide body are coated with a relatively thin, light-transmissive film, the index of refraction of which differs from the index of refraction of said light guide body.

2. The device as claimed in claim 1, characterized in that the index of refraction of said film is lower than the index of refraction of said light guide body.

3. The device as claimed in claim 1, characterized in that the index of refraction of said film is higher than the index of refraction of said light guide body.

4. The device as claimed in claim 1, further comprising first optical fiber means to lead the light beam from said light source to said light guide, mirror means to reflect said light beam, after said multiple passages, into second optical fiber means communicating with said photodetector .

5. The device as claimed in claim 4, further comprising directional coupler means for facilitating transmission of said light beam from said first optical fiber means to said light guide on the one hand, and transmission of said reflected light beam from said light guide to said photodetector, on the other.

6. A light guide in the shape of an elongate body made of a light-transmissive material, characterized in that at least some portions of said elongate body are coated with a relatively thin, light-transmissive film, the index of refraction of which differs from the index of refraction of said light guide body.

7. The light guide as claimed in claim 6, characterized in that the index of refraction of said film is lower than the index of refraction of said light guide body.

8. The light guide as claimed in claim 6, characterized in that the index of refraction of said film is higher than the index of refraction of said light guide body.

9. A method for measuring characteristics of fluids, in particular the level of liquids in a container, comprising the steps of: providing a light guide in the shape of an elongate body made of a light-transmissive material, at least some portions thereof being coated with a relatively thin, light-transmissive film, the index of refraction of which differs from the index of refraction of said light guide body; providing a light source producing a substantially monochromatic light beam directable into said light guide; providing a photodetector adapted to detect said light beam after multiple refractions and reflections across and along said film-coated light guide, and to produce output signals to be fed to indicator means; at least partly immersing said light guide body in a substantially vertical position in a liquid the level of which is to be measured, thus producing a film-ambient medium interface comprised of a film-liquid interface and a film-air interface; directing said light beam against the interface between said light guide surface and said film, causing said beam to be partly reflected into said light guide and partly refracted into said film, said refracted light beam in said film striking said ambient-medium interface at an angle of incidence larger than the critical angle at that interface, rendering the latter totally-reflective and causing a thus reflected beam to be refracted into said light guide; measuring, after said multiple refractions and reflections, the output of said photodetector , said output being indicative of the level of liquid in said container.

10. The method as claimed in claim 9, further comprising the step of calibrating said indicator means, to correlate the level of said liquid in said container with the output of said photodetector .

11. A device for measuring characteristics of fluids, substantially as hereinbefore described and with reference to the accompanying drawings.

12. A light guide substantially as hereinbefore described and with reference to the accompanying drawings.

13. A method for measuring characteristics of fluids, substantially as hereinbefore described and with reference to the accompanying drawings. for the Applicant: WOLFF, BREGMAN AND GOLLER