Apparatus and method for fluid analysis

A method and apparatus for fluid analysis includes a sensor for determining the index of refraction of a fluid. Advanced methods of fluid analysis relate index of refraction and other measured physical characteristics of the fluid.

TECHNICAL FIELD 
The present invention relates to measuring instruments, and more 
particularly to devices for analyzing fluids using measurements of index 
of refraction and other physical characteristics. 
BACKGROUND OF THE INVENTION 
Refrigerant gases containing chlorine, for example difluorodichloromethane 
(known as "R12"), have been phased out of use in new refrigeration systems 
for their harmful effects on the environment. Tight regulatory controls 
have been imposed governing the reuse and reclamation of such 
refrigerants. 
As a result of the new regulatory standards, it has become common to find 
refrigerant systems that are contaminated by additions of other more 
readily-available refrigerants. A common "shade-tree" mechanic technique 
for repairing an automotive refrigeration system is to add R134a or R2 to 
an R12 system. R134a is now a commonly available non-chlorinated 
refrigerant, whereas R12 and R22 are not available. Many cars in the 
southern and western parts of the United States now contain unoriginal 
mixtures like two-thirds R12 and one-third R22. Similarly, in the past, it 
was known that an old refrigerator could be "juiced" to make it work a 
little while longer. The result in either case is an adulterated mixture 
of refrigerants, which plays havoc with the current requirements for 
reclaiming and recycling all chlorinated refrigerants. Currently, 
regulations permit contamination of only 0.5% in reusable, reclaimed 
refrigerant. 
Thus, there presently exists a need for an instrument that HVAC and MVAC 
technicians can use to analyze refrigerants for identity of refrigerants 
and their purity. 
One prior attempted solution to the problem is disclosed in U.S. Pat. No. 
5,498,873. That solution, based on infrared absorption, was incomplete in 
a number of respects. The technique is similar to Near Infra-red 
Spectroscopic (NIR) techniques that have been used on contamination 
detection on critical space hardware. In practice, all species of 
refrigerant containing a hydrogen, such as the HCFC's, are easily 
distinguishable. The major difficulty is that if the two refrigerants were 
different in Cl or F, but contained the same number of H's on each carbon, 
then the NIR spectra are not easily distinguishable. The refrigerants R20, 
TCA, and TCE, each containing one or more hydrogens, are easily 
distinguishable. In contrast, both R113 and R11 contain no hydrogens and 
show no spectra in the NIR. Further, TCA and R141b, each containing three 
hydrogens on one carbon, are very nearly indistinguishable in the NIR. 
Thus there are significant drawbacks in using NIR for identification of a 
specific refrigerant. 
In contrast, it has been found that the indices of refraction of the 
refrigerants in the liquid state may be readily used to separate out the 
species by that property. Index of refraction is defined as the angular 
change in a beam of light passing through the interface of two different 
substances. The technique of using index of refraction is based on the 
fact that each refrigerant has a different atomic composition and 
therefore a different index of refraction. Since the index of refraction 
is linear with respect to any two components, fairly accurate estimates of 
two component mixtures can be made. 
A difficulty arises, however, in obtaining a reliable measurement of the 
more volatile refrigerants and comparing that to known values. Good 
control of temperature and pressure are required. Commercial index of 
refraction measuring instruments, such as the Abbe refractometer, already 
exist for liquids; but there are no instruments available which are 
capable of handling volatile species, such as R12, R22, etc. Hence, an 
instrument which can be used to perform index of refraction measurements 
on the volatile refrigerants in standard containers and refrigerating 
systems is required. 
Thus a need presently exists for an instrument which can measure the index 
of refraction of all the comnmonly-available refrigerants and can be 
mounted on any standard connection to an AC system or bottle. The 
instrument must be temperature controlled to establish the index 
measurement for each specific refrigerant. These temperatures will be 
established beforehand, making the measurement very straight-forward for 
the technician making the measurement. 
The two refrigerants closest in index of refraction are R12 and R123, which 
have indices of 1.2870 and 1.2754, respectively. Thus, measuring index of 
refraction to 4 significant figures will be a necessity. 
A refrigerant monitor must be small and rugged enough for a technician to 
carry the unit to the equipment requiring a sample to be analyzed to 
determine which refrigerant and to what purity level was currently 
contained in the equipment. 
SUMMARY OF THE INVENTION 
The present invention in its simplest aspect provides a simple and rugged 
instrument for measuring the index of refraction of a fluid. A sample of 
the fluid is introduced into a sample chamber, and a source of light is 
passed through the interface of the fluid and a window in the chamber. An 
angle detector is positioned outside the chamber and adapted to determine 
the angle of the light emerging through the window, which leads to a 
direct determination of the index of refraction. 
In a more specific aspect of the invention, the index of refraction 
measurement is combined with at least one other measured physical 
characteristic of the fluid sample, such as temperature or pressure, and 
then the fluid is identified by reference to known, pre-determined 
relationship data for a plurality of different fluids. In a more advanced 
apparatus, both temperature and pressure are measured and combined with 
the index of refraction measurement, and then the temperature is varied to 
obtain additional data, to identify the constituents of two-part 
("binary") mixtures of certain fluids and measure the percentages of the 
mixtures. 
In one embodiment, this invention was used in a device known as the 
Refrigerant Monitoring System ("RMS") providing instrumentation and 
software for monitoring the purity of CFC's, HCFC's and other 
refrigerants. This embodiment provides identification and purity of a 
specific CFC or HCFC refrigerant based upon the measurement of its index 
of refraction and/or hydrostatic pressure at measured temperatures. Binary 
mixtures, and to some degree higher order mixtures, can be analyzed using 
index of refraction and hydrostatic pressure measurements taken at several 
temperatures. The simple quality monitoring of refrigerants in specific 
classes of HVAC units (including automotive) or refrigerant recovery 
systems can be accomplished using index of refraction measurements alone. 
Extending the scope to include the identification of refrigerants and 
binary mixtures in a broad class of refrigerants requires the use of both 
index of refraction and hydrostatic pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In one specific form, the invention is incorporated in a specific device 
developed to monitor refrigerants. While the Refrigerant Monitoring System 
("RMS") is described in detail herein, applications of the invention are 
not limited to refrigerant analysis, and many other environments will be 
found for beneficial use of the invention. For example, the device in its 
simplest form is a small, inexpensive, and rugged refractometer for making 
primary measurements of indices of refraction. The principles of fluid 
analysis invented in connection with the RMS can be used by those skilled 
in the art for analyzing fluids of any character, such as contaminated 
surface water, blood, beverages, etc. 
The RMS described in detail herein is a portable instrument which provides 
an identification of the Freon.RTM.-type refrigerants used in HVAC and 
MVAC systems in situ and a purity indication, utilizing the measurement of 
index of refraction in the liquid state at measured temperatures and/or 
combined index of refraction measurement with vapor pressure of the 
liquid. The RMS provides an indication when the soluble impurities in the 
refrigerant exceed a given threshold, which preferably is 0.5% according 
to current regulatory requirements. 
The system includes a thermistor to measure fluid temperature, a 
refractometer capable measuring absolute indices of refraction from 1.1500 
to 1.4000, with a resolution of 0.0002 (.+-.0.0001), and a pressure sensor 
to measure the pressure of the fluid. The primary measurement of index of 
refraction is used to provide an initial estimate of refrigerant 
identification and quantification, with the hydrostatic pressure 
measurement used to provide a secondary discrimination between refrigerant 
components with similar indices of refraction and to analyze mixtures. 
The relationship of index of refraction of refrigerants with respect to 
temperature is known from published references and shown in FIG. 11. FIG. 
11 also includes the mathematical formulae for the index/temperature 
relationships, expressed in the form n###=C.sub.1 -C.sub.2 *F, where 
C.sub.1 and C.sub.2 are constants, F is temperature and n### is the index 
of refraction for a given refrigerant ###. Most of the pure refrigerants 
can be identified easily by measuring and correlating the index of 
refraction and temperature of a sample. To expand the coverage to include 
binary mixtures and separate out the species with close indices of 
refraction, the ability to make vapor pressure measurement at various 
temperatures is included. FIG. 12 shows the vapor pressure relationships 
to index of refraction at various temperatures. FIG. 13 is a listing of 
the mathematical formulae relating index of refraction, temperature and 
pressure. 
The construction of the refractometer includes a unique design to obtain a 
continuously diverging angular dependence on index of refraction of the 
fluid by optically coupling a transparent positioner of higher index of 
refraction to a transparent sample cell window. Therefore the resolution 
of the index of refraction measurement can be improved merely by extending 
the optical path of the instrument. FIG. 1 illustrates the basic concepts 
of the index of refraction measuring apparatus. 
Referring to FIG. 1, the index of the fraction measuring apparatus 10 
includes a planar base 12 upon which the various components are mounted. 
The components include a diode laser 14 at one end of the base 12, a lens 
16, a sample cell 18, a positioner 20, and an angle detector 22 at the 
other end of base 12. Laser 14 is a source of light positioned exterior of 
sample cell 18 to direct light into the sample chamber 24 through first 
transparent window 26. Light exits sample chamber 24 and sample cell 18 
through second transparent window 28. Light exiting window 28 travels 
through positioner 20, which preferably is a transparent block of acrylic 
plastic. Angle detector 22 is preferably an array of photodiodes adapted 
to determine the angle of light exiting sample cell 18 with respect to the 
sample cell. Sample cell 18 has walls defining sample chamber 24, as well 
as an inlet opening 30 and an outlet opening 32 for introducing a fluid 
sample into the sample chamber 24. 
The angle .sigma..sub.w of refracted rays in the positioner are determined 
by Snell's Law, .sigma..sub.w =SIN.sup.-1 (n.sub.f * sin .sigma..sub.0 
/n.sub.w), in the usual manner. The light beam from laser 14 enters cell 
24 at 85 degrees with respect to second window 28. All refracted rays 
begin at the same point of origin due to the fixed relationship of the 
laser and the sample cell. Optimal dispersion and resolution can be 
achieved by having the refractive indices of each successive element, 
including the window and transparent positioner and any optical coupling 
therebetween, to be always increasing. Intensity distribution over several 
array elements in angle detector 22 is used to interpolate between 
elements, thereby increasing overall resolution. 
Referring to FIGS. 2-5, where like numerals indicate like and corresponding 
elements, apparatus 10 includes a cover 100 having a carrying handle 102. 
Keypad 104 and display 106 are provided on the top face of cover 100. 
Inlet conduit 108 includes a valve 110, and similarly outlet conduit 112 
includes a valve 114. Conduits 108, 112 terminate at connection fittings 
116, 118. 
FIGS. 3-5 illustrate apparatus 10 with cover 100 removed. The components of 
the index of refraction apparatus are fixed to a planar base 120. The 
components include hollow sample cell 122, having interior and exterior 
walls 124, 125. The interior walls 124 define a sample chamber 126 (FIG. 
5). Sample cell 122 further has walls 128, 130 defining inlet and outlet 
openings 132, 134 through the interior and exterior walls 124, 125 for 
introducing a fluid sample into the sample chamber. Sample cell 122 
further has first and second opposed, spaced-apart transparent windows 
136, 138. A laser 142 is positioned exterior of the sample cell 122 
opposite the first window 136. Lens 144 is interposed between the laser 
142 and the first window 136 for transforming the light emitted by the 
laser 142 into a line of light. The laser 142, lens 144, and first and 
second windows 136, 138 are adapted and arranged such that a line of laser 
light is directed into the sample cell 122 through the first window 136, 
through the sample chamber 126, and out of the sample cell 122 through the 
second window 138. 
A transparent positioner 146, or "waveguide", is connected to an exterior 
wall 125 of sample cell 122 and has a first face 148 opposite the second 
window 138 and a second face 150 (FIGS. 3 and 4) spaced apart and opposite 
from the first face 148. In the preferred embodiment, the windows are made 
of borosilicate, brand name Borofloat by Edmund Scientific. Preferably, a 
thin film of transparent optical-coupling fluid (not shown) is interposed 
between first face 148 and second window 138, the fluid having an index of 
refraction between that of the window and the positioner. Angle detector 
152 is connected to the second face 150 of the transparent positioner 146. 
In preferred form, angle detector 152 includes two linear photodiode 
arrays 154, 156 staggered as shown in FIGS. 3 and 4. Angle detector 152 is 
adapted to determine the angle of the line of light exiting the sample 
cell 122 with respect to the sample cell, with the linear arrays 154, 156 
being arranged perpendicular to the line of laser light exiting the second 
window 138. The arrays 154, 156 are adapted to generate an electrical 
signal indicative of light-sensitive elements illuminated by the line of 
laser light. Improved resolution in the measurement of refracted angle is 
achieved by scanning over the pixel distribution illuminated and 
calculating the location (even if between pixels) of maximum intensity. 
The laser 142 is shown spaced apart from the sample cell, but it could be 
connected to the sample cell by an additional transparent positioner 
similar to positioner 146. 
In the more complex version of the invention designed for fluid analysis, 
as opposed to simple index of refraction measurement, angle detector 152 
is a first sensor for determining the index of refraction of a fluid 
sample in sample cell 122. A second sensor 158 is provided for determining 
at least one additional physical characteristic of the fluid sample, and a 
third sensor 160 determines at least another additional characteristic of 
the fluid sample. Yet another sensor 162 may be used to determine the 
electroconductivity of the fluid sample, for determining water 
contamination. 
The software algorithms used to determine specific refrigerant purity and 
composition of binary mixtures are illustrated in FIGS. 6 and 7. 
Discrimination between binary mixtures and refrigerants with similar 
characteristics is achieved using multivariate analysis (FIG. 6) or neural 
networks (FIG. 7). 
Referring to FIG. 6, where only index of refraction and temperature are 
measured, the refrigerant sample is placed in sample cell 18 in step 200. 
The temperature and index of refraction are measured in step 202. In step 
204, index of refraction is related to temperature, and in step 206 a 
decision is made as to whether there is a match between the related index 
and temperature with known relationship data. If there is a match, the 
identity of the refrigerant is displayed at step 208. 
If there is not a match, then the deviation is determined at step 210. If 
the deviation is less than 0.5 percent, then that information is displayed 
at step 212. If the deviation is greater than 0.5 percent, then 
calculations are done as shown in step 214 to determine the probable 
binary mixture of refrigerants. The first match for binary mixtures is 
determined in step 216, and displayed in steps 218-20. 
Better discrimination between mixtures and their components is available 
where a third physical characteristic such as pressure is used in the 
analysis algorithm. Pressure can be used to resolve ambiguous or poor 
matching between mixture components and known relationship data. 
FIG. 7 illustrates a software algorithm that utilizes index of refraction, 
temperature, and pressure and their known relationship data, to solve for 
purity and mixture component using neural networks. Calibration data is 
loaded into the apparatus at step 300. Step 302 begins the analysis of any 
given sample, where the physical characteristics of index of refraction, 
temperature, and pressure are measured. The identification of a single 
refrigerant with its associated purity is analyzed at step 306, similar to 
the steps employed in FIG. 6. If a single refrigerant with a purity of 
99.5% or greater is detected, then a report to that effect is generated 
via branch 308 at step 310. In the event an impure refrigerant mixture is 
detected via branch 312, then the solutions in step 314 are applied. Step 
314, entitled Classification Neural Network Dispatch Matrix, is a vector 
of nine methods, eight of which are neural nets, each with six inputs and 
nine outputs. The method to be used is based on a major refrigerant 
component. Given two sets of data (pressure, temperature, index of 
refraction), generated by measuring the data at one temperature and then 
heating or cooling the sample to get the second set of data, the 
appropriate method classifies the secondary refrigerant component. The 
data is passed to a function which solves for the percentages of each 
constituent. The difference between the non-neural network method 
described in FIG. 6 is that the method of FIG. 6 merely moves on the 
percentage function, assuming there is only one reasonable refrigerant 
mixture with any given primary component. In Step 316, the purity is 
solved, with the variable names indicated as follows: 
______________________________________ 
Na index of refraction of A 
Nb index of refraction of B 
Nc index of refraction of the mixture of A and B 
Ga concentration of A 
Gb concentration of B, equal to (1.00-Ga) 
MWa molecular weight of A 
Mwb molecular weight of B 
______________________________________ 
A report is then generated via branch 318. 
Software code listings have been submitted with this application as an 
Appendices. Software Appendix 1 applies to FIG. 6, and Software Appendix 2 
applies to FIG. 7. 
A computer and display hardware used in connection with the algorithms just 
described is illustrated in FIGS. 8-10. The hardware includes a 
microprocessor board 400, which in the preferred embodiment is a Micro 
Genius 2 board manufactured by Z-World Engineering, Inc. Keyboard 104, 
display 106 and thermal printer 402 are the user input/outputs. A 
provision is made so that an external computer 404 may be connected for 
calibration and setup purposes. The photodiode arrays 152 and 154 are 
connected to the microprocessor board 400. In the preferred embodiment, 
arrays 152 and 154 are each a model TSL218 array manufactured by Texas 
Instruments. Temperature sensor 158 in the preferred embodiment is a Radio 
Shack Model No. 271-110. Pressure sensor 160 in the preferred embodiment 
is a model 19C500AM made by Sensym, and is connected to microprocessor 
board 400 by way of an instrument op-amp circuit 406. Conductivity sensor 
162 is connected to microprocessor board 400 by way of a conductivity 
amplifier 408. Circuits 406 and 408 are described below in connection with 
FIG. 10. Thermo-electric heater 410 and thermo-electric cooler 412 are 
provided to vary the temperature of the sample to collect additional sets 
of data, for use with the algorithm of FIG. 7, to more accurately 
determine the percentages and constituents of binary mixtures. 
Referring now to FIG. 9, the pin connections for microprocessor board 400 
include the connections to arrays 152, 154 and the sensors 158, 160, and 
162. LCD display 106 and keypad 104 are connected as shown. 
Referring now to FIG. 10, the support circuits for the apparatus include 
the laser voltage regulator circuit 500 which drops the twelve volt power 
supply at 502 to the required voltage for the laser at 504, via integrated 
circuit 506. In the preferred embodiment, laser 142 is a five mW 670 nm 
semiconductor diode laser manufactured by Meredith Instruments, although 
different types and wavelengths of lasers may be employed. In circuit 406, 
the input from pressure sensor 160 is placed on lines 508, 510, and 
converted to an output voltage at line 512 by way of op amps 514, 516. In 
circuit 408, the input from conductivity probe 162 is placed on line 518. 
The amplified output is sent to the microprocessor board via line 520, 
after amplification in integrated circuits in 522 and 524. In circuit 526, 
the voltage for the printer is dropped from 12 to 5 volts. 
Whereas, the present invention has been described with the respect to a 
specific embodiment thereof, it will be understood that various changes 
and modifications will be suggested to one skilled in the art, and it is 
intended to encompass such changes and modifications as fall within the 
scope of the appended claims.