Concentration measurement of refrigerant/refrigeration oilmixture by refractive index

Mitsuhiro Fukuta*, Tadashi Yanagisawa, Satoshi Miyamura, Yasuhiro Ogi

Department of Mechanical Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, 432-8561, Japan

Received 15 August 2003; received in revised form 20 November 2003; accepted 24 December 2003

Abstract

Refrigeration oil having good miscibility with refrigerant is generally used in refrigeration units. Precise measure-ment of the mixing ratio of refrigerant to refrigeration oil is required for a sufficient understanding of the refrigerationcycle. In this paper, refractive index is chosen as a property which indicates the mixing concentration of the refrigerant/oil mixture. A laser displacement sensor is used to detect a change of optical path which changes according to the

refractive index of test medium. The refractive indices of pure refrigerant, pure refrigeration oil and refrigerant/refrig-eration oil mixture are measured with several combination of refrigerant/oil. It is found that the difference of refractiveindex between the refrigerant and the oil is sufficient for the measurement of the mixing concentration of refrigerant/oil

mixture, and that the refractive index of the refrigerant/oil mixture changes almost linearly according to the mixingconcentration. These data will be utilized for development of an in situ sensor in refrigerant compressors.# 2004 Elsevier Ltd and IIR. All rights reserved.

Keywords:Mesurment; Concentration; Refrigerant; Mixture; Lubricant; Refractive index

Mesures des concentrations en frigorigene/lubrifiant demelanges a l’aide de l’indice de refraction

Mots cles : Mesure ; Concentration ; Frigorigene ; Melange ; Lubrifiant ; Indice de refraction

1. Introduction

In general, refrigeration oil of good solubility withrefrigerant is used in refrigeration compressors. The

solubility changes according to the pressure and thetemperature and it influences the performance or relia-bility of the compressor as well as the operation of the

cycle. For example, when the concentration of refriger-ant in the refrigeration oil stored in the compressor

increases, the viscosity of lubricant decreases sig-

nificantly and it causes changes of lubrication char-acteristics and sealing ones in a compression chamber.In addition, the oil concentration in the refrigerant cir-

culating in the cycle, which is termed the oil circulationratio, affects the pressure drop and the heat transfer inheat exchangers. The measurement of the concentration

of refrigerant in the oil or the oil circulation ratio,therefore, is very important to improve cycle perfor-mance and to ensure the reliability of the system. Themost general way to measure the concentration of

refrigerant/oil mixture is a sampling method [1], but themeasurement is time consuming and reduces the amountof oil in the compressor or cycle. Although one can

0140-7007/$35.00 # 2004 Elsevier Ltd and IIR. All rights reserved.

doi:10.1016/j.ijrefrig.2003.12.007

International Journal of Refrigeration 27 (2004) 346–352

www.elsevier.com/locate/ijrefrig

* Corresponding author. Tel.: +81-53-478-1054; fax: +81-

53-478-1058.

E-mail address: tmmfuku@ipc.shizuoka.ac.jp (M. Fukuta).

estimate the concentration based on the solubility databy measuring the pressure and the temperature, there is

no guarantee that the condition is always saturated orsometimes there is no solubility data from oil manu-facturers. Because of such shortcomings of these meth-

ods, real-time measurements of the concentration ofrefrigerant/oil mixture have been developed.Certain sensors and principles for the real-time

measurement were proposed, which detect viscosity[2,3], acoustic velocity [2,4–8], density [2,8–10], absorp-tion of light [11–13] or dielectric constant [11,14].Refractive index [11,15,16] is one of the properties

which changes according to the concentration of refrig-erant/oil mixture. So far, however, no sufficient data ofthe refractive index of the refrigerant/oil mixture has

been published. In this study, the refractive index ofrefrigerant/oil mixture is measured by utilizing a laserdisplacement sensor. The characteristics of a refractive

index sensor incorporating the displacement sensor, andthe change of refractive index against the concentrationof refrigerant/oil mixture are discussed. In addition, theinfluence of oil degradation on the measurement is dis-

cussed from the view point of practical applications ofthe sensor to the cycle.

2. Experiment

2.1. Principle of measurement

The methods to detect the refractive index are classi-

fied mainly into three methods. One is to detect achange of light intensity through a sensor [17,18], thesecond is to detect a change of optical path [15,19,20]and the third is to detect the critical angle [11,16,21].

The purpose of this study is to provide fundamentaldata which is useful in the development of the sensor.The method by detecting the optical path, therefore, is

used in this study because the measurement can be donerelatively easily with high accuracy although it is notsuitable for miniaturization of the sensor.Fig. 1 shows the principle of the measurement. The

test medium, refrigerant/oil mixture, is stored in thepressurized chamber having a glass window. The inci-dent light enters the test medium through the glass and

reflects off the surface of a base plate. When the incidentbeam passes through interfaces between air andthe glass, and between the glass and the test medium,

the refraction occurs because of different refractiveindices of air, glass and test medium. The optical pathchanges according to the refractive index of the test

medium as shown in Fig. 1. In order to detect thechange of optical path, a laser displacement sensor isused in this study. The sensor is the diffusion reflectiontype, which detects the diffusion light at the reflection

surface by collecting the light on a CCD with a com-pound lens. However, when the incident light passesthrough the liquid test medium, the diffusion reflection

also occurs in the test medium and the diffusion reflec-tion on the surface is hardly detected. The sensor,therefore, is used as a regular reflection type with an

inclined position, which detects the regular reflectionbeam as shown in Fig. 2. The regular reflection lightreflects with the same angle, �, as the incident light onthe base surface, then it is collected on the CCD by thelens. The sensor output shows the distance of d in Fig. 2.

Fig. 1. Principle of measurement.

Fig. 2. Detection of regular reflection.

Nomenclature

d sensor output, mh depth of test medium, m

‘ distance from sensor to glass, mn refractive indext glass thicknessx distance, m

� angle of optical path, rad

Subscript

1 air2 glass3 test medium

M. Fukuta et al. / International Journal of Refrigeration 27 (2004) 346–352 347

Fig. 3 illustrates geometrically the actual optical pathand an imaginary one for the output of the sensor. The

refraction angles follow Snell’s law as expressed by Eq. (1).

n1sin�1 ¼ n2sin�2 ¼ n3sin�3 ð1Þ

where, n1, n2 and n3 are the refractive indices of air, glassand test medium, i.e. refrigerant/oil mixture, and �1, �2and �3 are the angles of light in each substance, respec-tively. In Fig. 3, the distance x is expressed as follows.

x ¼ 2 ‘tan�1 þ ttan�2 þ htan�3ð Þ ð2Þ

where ‘ is distance from the sensor to the glass surface, tis the glass thickness and h is depth of the test medium.The laser displacement sensor detects a reflection sur-face at an imaginary point (A) in Fig. 3, then it outputs

the distance d. The relationship between d and x isexpressed by Eq. (3).

x ¼ 2dsin�1 ð3Þ

With reducing Eqs. (1)–(3), the relationship betweenthe refractive index of test medium, n3, and the sensoroutput, d, is obtained.

d ¼

‘tan�1þttan sin�1 n1n2

sin�1

� �� �þhtan sin�1

n1n3

sin�1

� �� �

sin�1ð4Þ

2.2. Experimental setup

Fig. 4 shows the schematic diagram of experimental

setup. The laser displacement sensor used in this studyhas the measuring range from 25 to 35 mm with resolu-tion of 1 mm. The laser is a semiconductor type and thewave length is 670 nm. A refractive index sensor unit

(measuring unit) consists of the laser displacement sen-sor, the optical flat glass (BK7), the test medium cham-ber and the reflection base plate, which enables to

measure the refractive index under the pressurized con-dition. The specifications of the measuring unit aredesigned so that the range of the refractive index of the

test medium from 1.0 to 1.5 corresponds to as wide ameasuring range of the displacement sensor as possible.The measuring unit is connected to a mixing chamber

with a stirrer and the test chamber is filled with therefrigerant/oil mixture. The temperature of test mediumis controlled by an electric heater with circulating the

test medium by a gear pump, and is measured by aT-type thermocouple with the precision of 0.1 �C. Theconcentration of refrigerant/oil mixture is checked by

the sampling method. In this paper, the refractive indi-ces of several refrigerants and oils are measured andalso the refractive indices of their mixtures are exam-ined. The refrigerants used in this study are R134a, R32,

R125, R410A and R600a, and the oils are PAG, PVEand a paraffinic mineral oil. The concentration ofrefrigerant ranges from 0 to 100% and the temperature

is from 30 to 50 �C. In addition, the influence of oildegradation on the measurement is checked from theview point of practical applications of the sensor to the

refrigeration cycle.

3. Results and discussion

3.1. Calibration of sensor

At first, the calibration of the sensor output againstthe refractive index of the test medium is carried out byusing substances whose refractive indices are known.

The sensor output data obtained by the experiment andthe calculated one are plotted versus the refractive indexin Fig. 5. The refractive indices of alcohols, ethanol, 1-

propanol and isoamyl alcohol, are derived from aproperty table [22], those of refrigerants, R125, R32 andR22, are obtained from Refs. [19] and [22] and those ofrefrigeration oils, naphthenic mineral oil, PAG and

PVE, are measured by a handy refractive index meter.The refractive indices of the substances except therefrigerants are given by those with sodium yellow D-

Fig. 3. Actual and imaginary optical paths in sensor unit.

Fig. 4. Experimental setup.

348 M. Fukuta et al. / International Journal of Refrigeration 27 (2004) 346–352

line (589.3 nm) at 25 �C, and those of the refrigerantsare values under different conditions depending on eachliterature. In Fig. 5, the sensor output is shown by therelative value to that of air, and it decreases with

increasing the refractive index. The measured sensoroutput shows good agreement with that calculated byEq. (4), although the measured value is slightly smaller

than the calculated one. The deviation is caused by thecompound lens which is not as simple as the one shownin the previous figures, inaccurate angle at which the

displacement sensor is mounted, lack of flatness ofthe base plate, etc. In this study, the correlationcurve obtained by the experiment is used to convert the

sensor output to the refractive index. The slope of thecurve shows sensitivity of the refractive index sensor andabout �0.07/mm and �0.12/mm around the refractiveindices of 1.2 and 1.5, respectively. Since the resolution

of the laser displacement sensor is 1 mm, the uncertaintyof the refractive index sensor is less than 5�10�4 takinginto account the fluctuation of the output and error of

the calibration curve. In the present design of the sensorarrangement, the measuring range of refractive index isfrom 1.0 to 1.5. If the design is changed so that the

measuring range corresponds to the range from therefractive index of refrigerant to that of refrigeration oil,i.e. 1.2–1.5, it will be possible to measure the refractive

index of refrigerant/oil mixture more accurately.

3.2. Refractive indices of refrigerant and refrigeration oil

The concentration of refrigerant/oil mixture can bemeasured accurately as the difference between therefractive indices of refrigerant and oil is larger. The

refractive indices of pure refrigerant and pure oil,therefore, are measured for several combinations ofrefrigerant and oil. Fig. 6 shows the refractive indices

of R134a and PAG oil against the temperature. Therefractive index of R134a is about 1.19, whereas that ofoil is about 1.45 and is much larger than that of therefrigerant. In general, it is easy to measure the refrac-

tive index with the precision of 1�10�3 and the refrac-tive indices of R134a and PAG oil have enoughdifference for the measurement of the mixing concen-

tration. Over the temperature range in this study, therefractive index of R134a slightly decreases linearly withincreasing the temperature. The refractive index of PAG

oil also decreases with the temperature, but it is lesssensitive to the temperature.The refractive indices of a near azeotropic refrigerant,

R410A, and its components, R32 and R125, as well asthat of PVE oil are shown against the temperature inFig. 7. The refractive indices of R32, R125 and R410Aare slightly smaller than R134a’s shown in Fig. 6. The

refractive index of R32 is about 0.01 greater than that ofR125, but the difference is negligible as compared withthat from the refractive index of PVE oil. Therefore,

even if a selective dissolution of R32 and R125 into therefrigeration oil occurs, i.e. the concentrations of R32and R125 in the oil are different, the error of the refrig-

erant concentration in the oil is less than 1% when it isconsidered as the overall refrigerant concentration.In recent years, R600a (isobutane) is used as a refrig-

erant in domestic refrigerators. Fig. 8 shows the refrac-tive indices of R600a and the paraffinic mineral oilwhich is used in the refrigerator with R600a on themarket. The refractive index of R600a is around 1.3 and

much larger than those of R134a and R410A. Therefractive index of the paraffinic oil is around 1.47 andalso larger than PAG’s and PVE’s. Although the differ-

ence between the refractive indices of R600a and theparaffinic oil is smaller than that of the other com-bination, it is enough to detect the concentration of

Fig. 5. Calibration curve. *: Ref. 19, **: Ref. 22.

Fig. 6. Refractive indices of R134a and PAG oil.

M. Fukuta et al. / International Journal of Refrigeration 27 (2004) 346–352 349

refrigerant/oil mixture. The refractive indices of R600aand the paraffinic oil have the same tendency against thetemperature as the other combinations.The typical refractive indices of refrigerants and

refrigeration oils measured in this study are summarizedin Table 1 as a reference.

3.3. Refractive indices of refrigerant/oil mixture

Refractive indices of a few kinds of refrigerant/oil

mixtures were measured with varying the mass concen-tration of refrigerant in the oil. Fig. 9 shows the rela-tionship between the concentration of R134a in the

PAG oil and the refractive index with temperature as aparameter. The refractive index of the mixture decreasesalmost linearly with increasing the refrigerant concen-tration. It means that the refractive index is a suitable

property to detect the concentration of the refrigerant/oil mixture in the whole range of refrigerant concen-tration, in other words the measurement of the refrac-

tive index is useful for both measurements of therefrigerant concentration in the oil and the oil concen-tration in the refrigerant. The difference between R134a

and PAG refractive indices is about 0.25 and it corre-sponds to about 3000 in the resolution of the sensor.Therefore, the precision of the refrigerant concentration

of 2�10�3 is obtained considering the fluctuation of thesensor output and the error of the calibration curve.However, since the accuracy of the sampling method inthis study is not sufficient, more measurements are

needed to get an accurate correlation between the mixingconcentration and the refractive index of the mixture.The refractive index of the R410A/PVE oil mixture

versus refrigerant concentration is shown in Fig. 10 withtemperature as the parameter. It shows almost the sametendency as that described for the previous figure. When

Fig. 8. Refractive indices of R600a and Paraffinic oil.

Fig. 7. Refractive indices of R410A, R32, R125 and PVE oil.

Table 1

Refractive indices of refrigerants and oils

Substance

30 �C 40 �C 50 �C

R134a

1.213 1.206 1.197

R410A

1.186 1.175 1.163

R32

1.191 1.182 1.171

R125

1.181 1.168 1.156

R600a

1.314 1.306 1.297

PAGa

1.453 1.450 1.448

PVEa

1.443 1.441 1.438

Paraffinic oila

1.472 1.466 1.464

a Refractive indices of oils depend on each oils; additives,

viscosity grade and molecular structure of the oil.

Fig. 9. Relationship between refractive index and refrigerant

concentration (R134a/PAG oil).

350 M. Fukuta et al. / International Journal of Refrigeration 27 (2004) 346–352

the temperature is higher than 40 �C and the refrigerantconcentration is from 0.7 to 0.9, there is no data in the

figure. In this region, since R410A and the PVE oil usedin this study become immiscible, the mixture gets cloudylike emulsion, and the sensor does not work under such

conditions.

3.4. Influence of oil degradation

Oil degradation and moisture absorption into the oiloccur in practical refrigeration cycles and compressors.The influence of them on the refractive index is investi-

gated for the practical use of the refractive indexmeasurement. Fig. 11 shows the refractive indices ofPVE oils. The oils used here are PVE oil of VG68 as a

base oil, PVE oil with high moisture contents (500 and1000 ppm) and degraded PVE oil whose total acidnumber (TAN) is 0.25. Note that the vertical axes is somagnified that the difference becomes clear. The refrac-

tive indices in case of the 500 ppm moisture andTAN=0.25 are almost the same as that of the base oil,although the refractive index of the extremely moist oil

of 1000 ppm is slightly higher than the base oil’s. Theresult means that the concentration measurement ofrefrigerant/oil mixture by detecting the refractive index

will be applicable to the practical refrigeration cyclesand the compressors. In Fig. 11, the refractive index ofPVE oil of smaller viscosity grade (VG32) is also shown

as a reference. It shows smaller value than that of thehigher viscosity oil (VG68).Furthermore, the influences of flow and bubble exis-

tence on the measurement were investigated. Although

the output is slightly unstable by fluctuation of localconcentration of refrigerant/oil mixture when the mix-ture is flowing through the sensor section, the flow itself

has no influence on the measurement. On the otherhand, the existence of bubbles does disturb the

measurement and the sensor output becomes unstable.

4. Conclusion

The refractive index of refrigerant/oil mixture wasmeasured under pressurized conditions. Almost the

same output as designed was obtained with a measuringunit utilizing a laser displacement sensor. The uncer-tainty of the refractive index sensor was less than

5�10�4, and that of the mixing concentration of refrig-erant/oil mixture was less than 2�10�3. For the combin-ations of R134a/PAG oil, R410A/PVE oil and R600a/

mineral oil, the difference of refractive index betweenthe refrigerant and the oil was large enough to detect themixing concentration of refrigerant/oil mixture. Therefractive index of refrigerant decreases with increasing

temperature, whereas that of oil is less sensitive totemperature. Since the refractive index of the refriger-ant/oil mixture changes almost linearly according to the

mixing concentration of refrigerant/oil mixture overthe whole concentration range, the measurement ofrefractive index can be applicable to the measurements

of both the refrigerant concentration in the oil and theoil concentration in the refrigerant. In addition, even if aselective dissolution of R32 and R125 occurs in the

mixing of R410A with the refrigeration oil and the oildegradation occurs, the measurement of refractive indexis applicable to practical cycles to measure the mixingconcentration of refrigerant/oil mixture. In the next

phase of this study, we have a plan to investigate therefractive index more precisely in the range where the oilconcentration in the liquid refrigerant is less than 1% by

Fig. 10. Relationship between refractive index and refrigerant

concentration (R410A/PVE oil).

Fig. 11. Influence of oil degradation on refractive index.

M. Fukuta et al. / International Journal of Refrigeration 27 (2004) 346–352 351

improving the sensitivity of the sensor to the refractiveindex. Besides that, the development of a small sensorfor the in-situ measurement is also of interest.

Acknowledgements

The authors would like to thank Daikin Industries,LTD. for providing the refrigerants of R32 and R125,Idemitsu Kosan CO., Ltd. for providing PVE oils and

Mr. T. Sato and Mr. J. Iijima in our laboratory for theirtaking data.

References

[1] ASHRAE Standard 41.4 [Standard method for measure-

ment of proportion of oil in liquid refrigerant]. Atlanta

(GA): ASHRAE; 1984.

[2] Baustian JJ, Pate MB, Bergles AE. Properties of oil–

refrigerant liquid mixtures with applications to oil con-

centration measurement: Part I—Thermophysical and

transport properties. ASHRAE Trans 1986;92(1):55–73.

[3] Baustian JJ, Pate MB, Bergles AE. Measuring the con-

centration of a flowing oil–refrigerant mixture with a

bypass viscometer. ASHRAE Trans 1988;94(2):588–601.

[4] Baustian JJ, Pate MB, Bergles AE. Measuring the con-

centration of a flowing oil–refrigerant mixture with an

acoustic velocity sensor. ASHRAE Trans 1988;94(2):602–

15.

[5] Meyer JJ, Saiz Jabardo JM. An ultrasonic device for

measuring the oil concentration in flowing liquid refriger-

ant. Int J Refrigeration 1994;17(7):481–6.

[6] Navarro de Andrade JE, Skowron E, Goldschmidt VW,

Groll EA. Oil concentration in liquid refrigerants: in situ

measurement. Int J Refrigeration 1999;22(6):499–508.

[7] Lebreton JM, Vuillame L, Morvan E, Lottin O. Real-time

measurement of the oil concentration in liquid refrigerant

flowing inside a refrigeration machine. Proc of 15th Int

Compr Eng Conf at Purdue 2000: 319–26.

[8] Baustian JJ, Pate MB, Bergles AE. Measuring the con-

centration of a flowing oil–refrigerant mixture: instrument

test facility and initial results. ASHRAE Trans 1988;94(1):

167–77.

[9] Baustian JJ, Pate MB, Bergles AE. Measuring the con-

centration of a flowing oil–refrigerant mixture with a

vibrating u-tube densimeter. ASHRAE Trans 1988;94(2):

571–87.

[10] Bayani A, Thome JR, Favrat D. Online measurement of

oil concentrations of R-134a/oil mixtures with a density

flowmeter. HVAC&R Research 1995;1(3):232–41.

[11] Baustian JJ, Pate MB, Bergles AE. Properties of oil–

refrigerant liquid mixtures with applications to oil con-

centration measurement: Part II—Electrical and optical

properties. ASHRAE Trans 1986;92(1):74–92.

[12] Kutsuna K, Inoue Y, Mizutani T. Real time oil concen-

tration measurement in automotive air conditioning by

ultraviolet light absorption. 1991 [SAE paper no. 910222].

[13] Suzuki S, Fujisawa Y, Nakasawa S, Matsuoka M. Mea-

suring method of oil concentration ratio using light

absorption. ASHRAE Trans 1993;99(1):413–21.

[14] Fukuta M, Yanagisawa T, Ogi Y, Tanaka J. Measure-

ment of concentration of refrigerant in refrigeration oil by

capacitance sensor. Trans of JSRAE 1999;16(3):239–48 [in

Japanese].

[15] Jonsson UJ, Hoglund EB. Determination of viscosities of

oil-refrigerant mixtures at equilibrium by means of film

thickness measurements. ASRAETrans 1993;99(2):1129–36.

[16] Newell TA. Insitu refractometry for concentration mea-

surements in refrigeration systems. HVAC&R Research

1996;2(3):247–55.

[17] Takeo T, Hattori H. Optical fiber sensor for measuring

refractive index. Japan J Applied Physics 1982;21(10):

1509–12.

[18] Hinata S, Himeno N, Sakurai M, Iida H, Yokota N.

Measurement Technique of a refractive index of liquid

using optical fiber sensor. Trans JSME 1995;61(585):318–

1901 [in Japanese].

[19] Yata J, Hori M, Kawakatsu H, Minamiyama T. Mea-

surements of the refractive index of alternative refriger-

ants. Int J Thermophysics 1996;17(1):65–74.

[20] Jaaskelainen A, Silvennoinen R, Peiponen KE, Raty J. On

measurement of complex refractive index of liquids by

diffractive element-based sensor. Optocs Communications

2000;178:53–7.

[21] Shedd TA, Newell TA. Automated optical liquid film

thickness measurement method. Rev Sci Instrum 1998;

69(12):4205–13.

[22] JSME [Data Book: thermophysical properties of fluids].

Tokyo, Japan: JSME; 1983.

352 M. Fukuta et al. / International Journal of Refrigeration 27 (2004) 346–352