GB2134254A - Refractometer for fluids - Google Patents

Abstract

There is provided an apparatus for measuring the refractive index of fluids. The apparatus comprises a substantially U-shaped sensor rod 2 made of a transparent material and is submersible in the fluid 4. The bent portion of the sensor rod 2 has a diameter of curvature D at least five times larger than the diameter d of the sensor rod 2. The two ends of the U-shaped sensor rod 2 are connectable to a housing 10 and a pulsed LED light source 14 inside the housing has optical access to the face of one of the sensor-rod ends. A light detector means 24 also disposed inside the housing 10 has optical access to the face of the other one of the sensor rod ends. A reference detector 18 monitors light supplied to the rod 2. <IMAGE>

Description

SPECIFICATION Refractometer for fluids The present invention relates to optical measurements, and more specifically to the continuous measurement of the refractive index (R.l) of fluids by means of light flux transmitted through an optical light guide immersed in the fluid.

The use of refractive index as an indicator for salinity, degree Brix, solvent concentration, specific gravity or other characteristics of a fluid is well known, and is embodied in commercial instruments commonly known as refractometers, most of which use the criticalangle method for determination of R.l.

Some modifications of this method use multiple reflection in a glass rod, e.g., the devices described in U.S. Patents 2,569,127 (Eltenton) and 3,311,014 (Witt).

The feature common to these devices is a transparent optical rod (or a plurality thereof) which acts as a light guide when immersed in a fluid of lower R.l. than its own. Light impinging on one end of a transparent rod is trapped due to the phenomenon of total internal reflection (T.l.R) and escapes via the other end, or via the same end, after multiple reflections within the rod. Since this is the effect used in fiber optics, these devices have been called fiber optic refractometers, although usually a rod, rather than a fiber, is used.

The amount of light traversing the rod in the above-mentioned manner depends, among other factors, upon the R.l. of the fluid in which the rod is immersed. Conversion of the light signal into an electrical signal by means of a photodetector enables continuous measurement of the refractive index, and thereby of the required characteristic property of the fluid. It must be stressed that T.l.R. trapping occurs only if the R.l. of the rod is greater than that of the liquid.

Most of the above-mentioned instruments utilize a special sample chamber containing the rod and the fluid. Thus the characteristics of the sampled region may differ from those of the bulk fluid. This is especially true for temperature-dependent characteristics, since the R.l. is a function of temperature. In those devices in which the rod does extend into the bulk of the fluid, (e.g. Witt) one has to avoid penetration of the fluid into the measurement chamber containing light source, photodetector and other components, by means of a leak-tight seal such as O-rings or adhesives.

Such seals result in optical contact between the rod and the sealant, due to which much of the light trapped in the rod will escape at the contact regions, thus reducing the sensitivity of the device. In some cases the device may be largely insensitive to fluids with an R.l.

smaller than that of the adhesive. Also, the sensitivity of fiber optic refractometers being greatest for an R.l. close to that of the rod, the range of high sensitivity in prior-art instruments is limited to a narrow region predetermined by the rod material.

It is one of the objectives of the present invention to overcome the limitations and drawbacks of the prior-art refractometers and to provide an apparatus for the continuous measuring and recording of the refractive index of a fluid which apparatus is highly sensitive over a relatively wide range determined by the geometry of a light-guide sensor.

This the present invention achieves by providing an apparatus for measuring the refractive index of fluids, comprising a substantially U-shaped sensor rod made of a transparent material and being submersible in said fluid, the bent portion of which sensor rod has a diameter of curvature at least five times larger than the diameter of said sensor rod, a housing to which are connectable the two ends of said U-shaped sensor rod, a light source inside said housing, which light source has optical access to the face of one of said sensor-rod ends, and at least one first light detector means inside said housing, which light detector means has optical access to the face of the other one of said sensor rod ends.

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 for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what it believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no ateempt is made to show structural details of the invention in more detail that 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: Figure 1 is a partly schematic representation, in partial cross section, of a preferred embodiment of the refractometer according to the invention, and Figure 2 is a greatly enlarged view, in cross section, of the fittings and optical components of the embodiment of Fig. 1.

Referring now the the drawings, there is seen in Fig. 1 a substantially U-shaped rod 2 made of a transparent material such as glass.

This rod serves as sensor and, during measurement, is submersed in the liquid 4, the R.l. of which is to be measured. It has been found that, in order to obtain maximum sensitivity, the bent portion of the sensor rod 2 must have a diameter of curvature D that is at least 5 times as large as the diameter d of the sensor rod. Also, with a D/d ratio as large and even larger, the n-range covered by a single sensor rod 2 is wide enough to include the R.l.'s of most liquids of interest. Liquids with particularly high R.l. might require a special sensor rod 2, made of a glass having an R.l. higher than that of the liquid in question.

Via fittings 6, shown in greater detail in Fig.

2 and described further below, the straight, paraliel end portions of the U-shaped bar 2 are introduced into the bottom plate 8 of a hermetically sealable housing 10 provided with a thermally insulating internal lining 1 2 and accommodating the following components: An input-light source in the form of a lightemitting diode (LED) 14 which is electronically pulsed to permit the filtering-out of ambient light. The LED used produces a quasi-monochromatic light of a wavelength of, e.g.

5850 , thus minimizing index dispersion effects (It should be noted that the R.l. is also a function of wavelength). LED's are furthermore known for their iow power consumption and compact size which, as will be shown further below, also facilitates optical coupling to the sensor rod 2.

The LED 14 is mounted in a socket 1 6 which also accommodates a reference photodetector 18 that serves to monitor the input light flux provided by the LED. The signal produced by the photodetector 1 8 is led via a reference signal amplifier 20 to the terminal A.

Light from the LED is "piped" through the sensor rod 2. Depending on the respective angles of incidence at the rod walls, part of the light rays issuing from the LED are totally internally reflected and, disregarding some absorption losses, reach the other end of the rod largely undiminished. Depending on the difference between the respective R.l.'s of the rod material and the medium surrounding the rod 2, another part of the light flux is refracted out of the rod 2 and into the liquid 4.

To establish this "lost" fraction of the flux and, thereby, in conjunction with the reference signal and a calibration curve, the R.l. of the liquid 4, there is further provided in the housing 10: An output-light detector, in the form of a photodetector 22 mounted in a socket 24.

The output signal of this detector is led via an output signal amplifier 26 to the terminal D.

Also accommodated in the housing 10 is a power supply unit 28 for the LED 14. This unit may by itself be a pulser or may be connected via terminal C to an external pulser, and via terminal B, to an external power supply.

Since, as already mentioned, the R.l. is also a function of temperature, the temperatures of both liquid 4 and rod 2 (which, due to the small thermal capacity of the rod are always substantially equal) must be continuously monitored. This is effected by a thermocouple, the hot junction 30 of which is immersed in the liquid 4 and is connectable to an external cold junction and digital voltmeter at terminal E.

A guard strip 31 protects the physical integrity of the sensor rod 2.

The fittings 6 and the associated components are shown greatly enlarged in Fig. 2, the right-hand part of which represents the "input" side and the left-hand part, the "output" side of the device. It is immediately seen that the parallel end portions of the U-shaped sensor rod 2 are not led right up to the LED 14 and the photodetector 22, respectively, but end at a much lower point, in a plane that, in the particular embodiment, passes somewhere through the bottom plate 8. From there to the LED 14 and the detector 22, the light flux is guide through coupling rods 40 and 42 respectively. Intimate coupling between the LED 14 and the upper face of the input coupling rod 40 is established by grinding off the usually convex tip of the LED and cementing the top face of the coupling rod 40 to the flat surface thus obtained, using a transparent epoxy cement 44.Optical coupling between the lower faces of the coupling rods 40 and 42, and the respective upper faces of the sensor rod 2 is effected by carefully pressing these faces one against the other after introducing a drop of a suitable oil.

The provision of this contact force is in fact one of the tasks of the fittings 6.

The basic element of the two identical fittings 6 is seen to be a metal sleeve 32 having two externally threaded ends and a collar 34 which, as can be seen in Fig. 1, is advantageously given a hexagonal shape. This sleeve 32 is inserted into appropriately dimensioned and spaced holes in the bottom plate 8 and, sealing washers 36 having been interposed as shown in Fig. 2, is clamped tight. Into the sleeves 32 fit slidingly lower ferrules 46 and upper ferrules 48, into which ferrules are cemented the straight end portions of the sensor rod 2 and the lower portions of the coupling rods 40, 42, respectively, the faces of the rods-coupling as well as sensor-being substantially flush with the respective ferrule rims 50. At a distance from these rims 50 the ferrules 46, 48 are provided with collars 52 and 54, respectively, which are engageable by union nuts 56 and 58, respectively, that match the threaded ends of the sleeves 32.

By tightening these union nuts 56, 58, the sensor rod 2 and the coupling rods 40, 42 are pressed against one another, as clearly seen in Fig. 2. Conversely, by unscrewing the two lower union nuts 56, the sensor rod 2 is easily withdrawn from the sleeves 32 for cleaning or replacement. An O-ring 60 in the lower, heavier portion of the ferrule 46 seals off any liquid access into the interior of the housing 10, which is of importance in cases where the entire device has to be submerged and not only the sensor rod 2, for instance when monitoring salinity gradients in solar ponds. In such cases, the wiring terminal A-E and the lead-through 62 of the thermocouple 30 (Fig. 1) must obviously also be liquid-tight.

The respective arrangements of the LED 14, the reference photodetector 1 8 and the output photodetector as seen in Fig. 2 are selfexplanatory. However, while the function of the reference detector 18 in R.l. determinations has been explained earlier, it should be noted that in some applications, e.g., process control, the absolute value of the R.l. is of lesser interest than the contancy thereof, in which case the task of the reference detector 1 8 is reduced to monitoring the constancy of the input light flux and, in case of fluctuations, to actuate electronic compensation means. In prior-art devices, this problem could only be solved by much more complex differential-refractometer arrangements.

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 essential attributes thereof, and it is therefore desired that the present embodiments be considered in all respects as illustrative and not restrictive, reference being made to the appended claims, rather than to the foregoing description, and all changes which come with the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (9)

1. An apparatus for measuring the refractive index of fluids, comprising a substantially U-shaped sensor rod made of a transparent material and being submersible in said fluid, the bent portion of which sensor rod has a diameter of curvature at least five times larger than the diameter of said sensor rod, a housing to which are connectable the two ends of said U-shaped sensor rod, a light source inside said housing, which light source has optical access to the face of one of said sensor-rod ends, and at least one first light detector means inside said housing, which light detector means has optical access to the face of the other one of said sensor rod ends.

2. The apparatus as claimed in claim 1, wherein said optical access of said light source is constituted by a substantially straight coupling rod made of a transparent material, one end face of said coupling rod being in intimate contact with the face of one of said sensor-rod ends and the other end face of said coupling rod being in intimate contact with an end face of said light source.

3. The apparatus as claimed in claim 1, wherein said optical access of said light detector means is constituted by a substantially straight coupling rod made of a transparent material, one end face of said coupling rod being in intimate contact with the face of the other one of said sensor-rod ends, and the other end face of said coupling rod having optical access to said first light detector means.

4. The apparatus as claimed in claim 1, further comprising a second light detector means, said second means being located in proximity of, and having optical access to, said light source.

5. The apparatus as claimed in claim 1, wherein said light source is a light-emitting diode.

6. The apparatus as claimed in claim 1, wherein said light source is a pulsating light facilitating the filtering-out of ambient light.

7. The apparatus as claimed in claim 1, wherein said light-detecting means is a photodetector.

8. The apparatus as claimed in claim 1, wherein said housing is hermetically sealable.

9. An apparatus for measuring the refractive index of fluids, substantially as hereinbefore described with reference to the accompanying drawings.