Method and apparatus for optical measurement of concentration and temperature of liquids

• US 6,549,276 B1
• Filed: 07/09/2001
• Issued: 04/15/2003
• Est. Priority Date: 10/30/1998
• Status: Expired due to Fees

• Current Assignee: The Research Foundation for The State University of New York (State University of New York)
• Original Assignee: The Research Foundation for The State University of New York (State University of New York)
• Inventors: Longtin, Jon P.
• Primary Examiner(s): STAFIRA, MICHAEL PATRICK
• Application Number:
  US09/830,735
• Time in Patent Office: 645 Days
• Field of Search: 356/128, 356/134, 356/335, 356/336, 356/337, 356/338, 356/341, 356/343, 356/388, 356/394
• US Class Current: 356/128
• CPC Class Codes: G01N 2021/414 Correcting temperature effect in refractometers
• CPC Class Codes: G01N 2021/4153 Measuring the deflection of light in refractometers
• CPC Class Codes: G01N 21/4133 Refractometers, e.g. differential

Abstract

Disclosed are a method and an apparatus for precision optical measurement of solute concentration and temperature of a transparent liquid. The apparatus includes a vessel (12) which contains a sample of liquid (14), the refractive index of which is to be determined, a light source (22) and a beam position sensor (30). The vessel (12) has an entrance side and an exit side, with a known angular relationship therebetween, and is situated in a known ambient The light source (22) is located so as to cause a beam of light to impinge on the entrance side at an angle with respect to a normal to an outer surface of the entrance side, to pass through the liquid (14), and to then pass through the exit side, from which it exits at an angle with respect to a normal to an outer surface of the exit side. The beam position sensor (30) provides data for determining the exit angle thereby permitting the refractive index of the liquid to be determined, based on the angles.

Claims

1. A method for determining the concentration of a given component in a sample of a multi-component liquid mixture, said method comprising the steps of:

(a) determining the refractive index of the sample of the multi-component liquid mixture by;

(a-1) causing the sample of the liquid mixture to be contained in a vessel having a planar entrance side and a planar exit side, with a known angular relationship therebetween, said vessel being immersed in known surroundings;

(a-2) causing a beam of light to impinge on said entrance side at an angle θ

_i with respect to normal to an outer surface of said entrance side, to pass through the sample of the multi-component liquid mixture, and to then pass through said exit side, from which said beam exits at an angle θ

_e with respect to a normal to an outer surface of said exit side; and

(a-3) calculating the refractive index of the sample of the multi-component liquid mixture, based on said angles θ

_i and θ

_e, by applying Snell'"'"'s law at;

an interface between the surroundings and said entrance side;

an interface between said entrance side and the sample of the multi-component liquid mixture;

an interface between the sample of the multi-component liquid mixture and said exit side; and

an interface between said exit side and the surroundings; and

(b) comparing the refractive index of the sample of the multi-component liquid mixture determined in step (a) to predetermined data relating different concentrations of the given component of the multi-component liquid mixture to corresponding values of the refractive index of the multi-component liquid mixture, so as to determine the concentration of the given component in the sample of the multi-component liquid mixture corresponding to the refractive index of the sample determined in step (a).

2. The method of claim 1, wherein:

step (a) further comprises the additional step of;

(a-4) providing a beam position sensor which senses a position of said beam of light after said beam exits said exit side, and wherein step (a-3) uses said sensed position of said beam in calculating the refractive index of the sample of the multi-component liquid mixture.

3. The method of claim 2, wherein:

said entrance and exit sides of said vessel have a substantially uniform thickness and a substantially uniform refractive index; and

step (a-3) comprises determining the refractive index of the given sample of the multi-component liquid mixture according to a first equation;

4. The method of claim 3, further comprising the additional step of performing calibration and measurement to determine W₀, θ

_e,0 D, said calibration and measurement including;

causing a first reference liquid of known refractive index to be contained in said vessel;

causing a relative rotation between said vessel and said beam position sensor such that, for the first reference liquid, said beam of light exits said exit side in a direction substantially perpendicular to said beam position sensor, whereby said beam of light impinges on the beam position sensor at the reference position;

measuring W₀ and recording θ

_e,0;

emptying said vessel of the first reference liquid;

causing a second reference liquid of known refractive index to be contained in said vessel, the refractive index of the second reference liquid being different than that of the first reference liquid;

determining values of d,W and θ

_e corresponding to the second liquid, the values of W and θ

_e being respectively calculated from said second and third equations; and

using the values of d, W and θ

_e in said first equation to determine D.

5. The method of claim 1, further comprising the additional step of recording the temperature of the sample of the multi-component liquid mixture, wherein step (b) comprises:

(b-1) comparing the refractive index of the given sample of the multi-component liquid mixture determined in step (a) to values of the predetermined data relating the different concentrations of the given component of the multi-component liquid mixture to corresponding values of the refractive index of the multi-component liquid mixture, for at least two temperatures near the recorded temperature of the sample of the multi-component liquid mixture; and

(b-2) interpolating between the values of the predetermined data at the at least two temperatures near the recorded temperature of the sample of the multi-component liquid mixture so as to obtain an interpolated concentration of the sample of the multi-component liquid mixture.

6. The method of claim 1, wherein the predetermined data relating different concentrations of the given component of the multi-component liquid mixture to corresponding values of the refractive index of the multi-component liquid mixture are double valued, such that individual values of the refractive index in at least a portion of a given range of the corresponding values of the refractive index correspond to two different values of concentration, said method further comprising the additional steps of:

(c) storing previously measured values of concentration for the given sample of the multi-component liquid mixture taken at prior points in time, said previously measured values of concentration being indicative of the expected range of values of the concentration being determined; and

(d) using the previous values of the concentration stored in step (c) to select which of the two different values of the concentration corresponding to the value of the refractive index determined in step (a) is correct.

7. A method for determining a change in concentration of a given component in a sample of a multi-component liquid mixture from an initial concentration of the given component corresponding to an initial refractive index of the sample at an initial temperature, said method comprising the steps of:

(a) determining the refractive index of the sample of the multi-component liquid mixture by;

(a-1) causing the sample of the liquid mixture to be contained in a vessel having a planar entrance side and a planar exit side, with a known angular relationship therebetween, said vessel being immersed in known surroundings;

(a-2) causing a beam of light to impinge on said entrance side at an angle θ

_i with respect to a normal to an outer surface of said entrance side, to pass through the sample of the multi-component liquid mixture, and to then pass through said exit side, from which it exits at an angle θ

_e with respect to a normal to an outer surface of said exit side; and

(a-3) calculating the refractive index of the sample of the multi-component liquid mixture, based on said angles θ

_i and θ

_e, by applying Snell'"'"'s law at;

an interface between the surroundings and said entrance side;

an interface between said entrance side and the given sample of the multi-component liquid mixture;

an interface between the sample of the multi-component liquid mixture and said exit side; and

an interface between said exit side and the surroundings;

(b) determining a change in refractive index, Δ

n, by subtracting from the initial refractive index the refractive index determined in step (a-3);

(c) measuring the temperature of the sample of the multi-component liquid mixture; and

(d) determining the change, Δ

C, in the concentration of the given component in the sample of the multi-component liquid mixture from the initial concentration according to the approximate formula;

8. A method for determining the temperature of a sample of a liquid, the liquid being one of a pure liquid and a multi-component liquid mixture having a substantially constant concentration of its components, said method comprising the steps of:

(a) determining the refractive index of the sample of the liquid by;

(a-1) causing the given sample of the liquid to be contained in a vessel having a planar entrance side and a planar exit side, with a known angular relationship between said entrance side and said exit side, said vessel being immersed in known surroundings;

(a-2) causing a beam of light to impinge on said entrance side at an angle θ

_i with respect to a normal to an outer surface of said entrance side, to pass through the sample of the liquid, and to then pass through said exit side, from which it exits at an angle θ

_e with respect to a normal to an outer surface of said exit side; and

(a-3) calculating the refractive index of the given sample of the liquid, based on said angles θ

_i and θ

_e, by applying Snell'"'"'s law at;

an interface between the surroundings and said entrance side;

an interface between said entrance side and the sample of the liquid;

an interface between the sample of the liquid and said exit side; and

an interface between said exit side and the surroundings; and

(b) comparing the refractive index of the given sample of the liquid determined in step (a) to predetermined data relating different temperatures of the liquid to corresponding values of the refractive index of the liquid, so as to determine the temperature of the sample of the liquid corresponding to the refractive index of the sample determined in step (a).

9. The method of claim 8, wherein:

step (a) further comprises the additional step of;

(a-4) providing a beam position sensor which senses a position of said beam of light after said beam exits said exit side, and wherein step (a-3) uses said sensed position of the beam in calculating the refractive index of the sample of the multi-component liquid mixture.

10. The method of claim 9, wherein:

said entrance and exit sides of said vessel have a substantially uniform thickness and a substantially uniform refractive index; and

step (a-3) comprises determining the refractive index of the sample of the liquid mixture according to a first equation;

11. The method of claim 10, further comprising the additional step of performing calibration and measurement to determine W₀, θ

_e,0 and D, said calibration and measurement including;

causing a first reference liquid of known refractive index to be contained in said vessel;

causing a relative rotation between said vessel and said position sensor such that, for the first reference liquid, said beam of light exits said exit side in a direction substantially perpendicular to said position sensor, whereby said beam of light impinges on the beam position sensor at the reference position, whereby the value of d for the beam at said reference position may be taken as zero;

measuring W₀ and recording θ

_e,0;

emptying said vessel of the first reference liquid;

causing a second reference liquid of known refractive index to be contained in said vessel, the refractive index of the second reference liquid being different from that of the first reference liquid;

determining values of d,W and θ

_e corresponding to the second liquid, the values of W and θ

_e being respectively calculated from said second and third equations; and

using the values of d, W and θ

_e in said first equation to determine D.

12. An apparatus for determining the concentration of a given component in a sample of a multi-component liquid mixture, said apparatus comprising:

(a) a vessel which contains said sample, said vessel having a planar entrance side and a planar exit side, with a known angular relationship between said entrance side and said exit side, said vessel being immersed in known surroundings;

(b) a light source which is located so as to cause a beam of light to impinge on said entrance side at an angle θ

i with respect to a normal to an outer surface of said entrance side, to pass through said sample, and to then pass through said exit side, from which it exits at an angle θ

e with respect to a normal to an outer surface of said exit side;

(c) a sensor which determines said exit angle θ

e, thereby permitting the refractive index of said sample to be determined, based on said angles θ

i and θ

e, by applying Snell'"'"'s law at;

an interface between the surroundings and said entrance side;

an interface between said entrance side and the sample;

an interface between the sample and said exit side; and

an interface between said exit side and the surroundings; and

(d) a computer programmed to compare a refractive index of said sample determined using said angles θ

i and θ

e with predetermined data relating different concentrations of the given component of the multi-component liquid mixture to corresponding values of the refractive index of the multi-component liquid mixture so as to determine the concentration of the given component in the sample of the multi-component liquid mixture.

13. The apparatus of claim 12, wherein:

said sensor comprises a beam position sensor which senses a relative position of said beam of light after said beam exits said exit side, said beam position sensor having a known geometrical relationship to said vessel such that said sensed beam position can be related to said exit angle θ

_e.

14. The apparatus of claim 12 wherein the computer stores the predetermined data relating different concentrations of the given component of the multi-component liquid mixture to corresponding values of the refractive index of the multi-component liquid mixture.

15. The apparatus of claim 12, wherein said entrance and exit sides of said vessel have a substantially uniform thickness and a substantially uniform refractive index, said apparatus and said computer receives a representation of said position of said beam of light and which determines the refractive index of the material according to a first equation:

16. The apparatus of claim 12, wherein:

said sensor comprises a photodiode beam position sensor and a position-to-voltage converter which produces an output voltage which is substantially linearly proportional to said relative position of said beam; and

said computer is a digital computer;

said apparatus further comprising;

a first voltmeter which receives said output voltage from said position-to-voltage converter and which supplies a corresponding digital signal to said computer as said representation of said relative position of said beam of light; and

a temperature sensor which is positioned to sense temperature of the sample of the liquid mixture and which is coupled to said computer so as to supply said computer with a digital signal representative of the temperature of the sample of the liquid mixture.

17. The apparatus of claim 16, further comprising a rotation stage which is interposed between said vessel and a mounting surface such that said relative position of said beam of light can be adjusted, during calibration of said apparatus, to be substantially perpendicular to said beam position sensor and impinging on said beam position sensor at a reference position thereof.

18. The apparatus of claim 17, further comprising a scale interposed between said rotation stage and the mounting surface, whereby changes in mass concentration of the liquid may be determined.

21. The apparatus of claim 12, wherein said computer is programmed to perform the comparison of the determined refractive index to values of the predetermined data for at least two temperatures near the temperature of the liquid mixture and to interpolate between the values of the predetermined data at the at least two temperatures so as to correct the value of the concentration of the given component in the sample of the multi-component liquid mixture for the temperature of the sample of the liquid mixture.

22. The apparatus of claim 12, wherein said computer stores previously measured values of concentration of the given component in the sample of the multi-component liquid mixture taken at prior points in time and said computer is programmed to employ the previously measured values to indicate an expected range of concentrations of the given component to permit the selection of a correct value of concentration when concentration is double-valued with respect to index of refraction.

23. The apparatus of claim 12, further comprising:

a temperature sensor which is positioned to sense temperature of the sample of the multi-component liquid mixture and which is coupled to said computer so as to supply said computer with a signal representative of the temperature of the multi-component liquid;

wherein;

said computer is programmed to determine the refractive index of a given sample of the multi-component liquid mixture, based on said angles θ

_i and θ

_e, by applying Snell'"'"'s law at;

an interface between the surroundings and said entrance side;

an interface between said entrance side and the sample of the multi-component liquid mixture;

an interface between the sample of the multi-component liquid mixture and said exit side; and

an interface between said exit side and the surroundings;

and;

said computer is also programmed to determine a change in refractive index of the multi-component liquid mixture, Δ

n, by subtracting, from the refractive index determined by said computer using Snell'"'"'s law, an initial refractive index of the sample of the multi-component liquid mixture at an initial concentration of the given component in the sample and at an initial temperature of the sample measured with said temperature sensor, to receive said signal which represents the temperature of the sample of the multi-component liquid mixture under test, and to determine the change, Δ

C, in the concentration of the given sample of the multi-component liquid mixture from the initial concentration according to the approximate formula;

19. An apparatus for determining the temperature of a sample of liquid, said apparatus comprising:

(a) a vessel which contains said sample, said vessel having an entrance and an exit side, with a known angular relationship, between said entrance side and said exist side, said vessel being immersed in known surroundings;

(b) a light source which is located so as to cause a beam of light to impinge on said entrance side at an angle θ

_i with respect to a normal to an outer surface of said entrance side, to pass through said sample, and to pass through said exit side, from which it exits at an angle θ

_e with respect to a normal to an outer surface of said exit side;

(c) a sensor which determines said exit angle θ

_e thereby permitting the refractive index of said sample to be determined, based on said angle θ

_i and θ

_e, by applying Snell'"'"'s law which;

an interface between the surrounding and said entrance side;

an interface between said entrance side and the sample;

an interface between the sample and said exit side; and

an interface between said exit side and the surroundings;

(d) a computer programmed to compare a refractive index of said sample determined using said angles θ

_i and θ

_e with predetermined data relating different temperatures of the liquid to corresponding values of the refractive index of the liquid so as to determine the temperature of the sample of the liquid.

20. The apparatus of claim 19 wherein the computer stores the predetermined data relating different temperatures of the liquid to corresponding values of the refractive index of the liquid.