US2897671A - Apparatus for determining the evaporation end point of volatile liquids - Google Patents

Description

4, 1959 J M PHELAN ETAL 2,897,671

APPARATUS FbR I'JETERMINING THE EVAPORATION END POINT OF VOLATILE LIQUIDS Filed Nov. 21, 1955 4 Sheets-Sheet 1 LIGHT METER FIG-I James M Phelan James A. Wilson Inventors,

WW Afforney 4, 1959 J. M. PHELAN ETAL 2,397,671

APPARATUS FOR DETERMINING THE EVAPORATION END POINT OF VOLATILE LIQUIDS Filed Nov. 21, 1955 4 Sheets-Sheet 2 MICROAMMETER COOLING 28 COILS LIGHT souRcE James M. Phelan James A. Wilson Attorney- Aug. 4, I 5 J. M. PHELAN ET AL 2,897,671

APPARATUS FOR DETERMINING THE EVAPORATION END POINT OF VOLATILE LIQUIDS Filed Nov. 21, 1955 4 Sheets-Sheet 5 James M. Phelan James A Wilson Inventors By/r Attorney Aug. 4, 1959 J. M. PHELAN ETEAL 2,897,671

APPARATUS FOR DETERMINING THE EVAPORATION END POINT OF VOLATILE LIQUIDS 4 Sheets-Sheet 4 Filed NOV 21, 1955 N4 v oI N EN mvw o a e a a L 4 EN m A I o o 1 o 0 iv mvw AI wvw vvw 02m I? km a I new i .B 2N mmm EN EN EN NNN 2N vwm mum A I v A O O l I 5% vm v 0mm 92w N- 5m 2a 8a Em m 2a Inventors James M. Phelan James A. Wilson Patented Aug. 4, 1959 APPARATUS FORDETERMINING THE EVAPORA- TION END POINT 40F VOLATILE LIQUIDS James M. Phelan, tCranford, and James A. Wilson, Stanhope, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware Application November 21, 1955, Serial No. 548,050

7 Claims. (Cl. 73-53) The present invention relates to a photo-evaporimeter apparatus adapted for the determination of the evaporation rates of volatile liquids. More particularly the in vention relates to an apparatus for the photo-electric determination of evaporation rates. Specifically, the invention contemplates a means whereby a non-transparent, light-transmissive, wettable material is made more translucent by wetting with the volatile liquid to be tested, and the intensity of light transmitted by the material is sensed and measured during evaporation of the liquid from the material. The evaporation end point of the liquid is determined at that point at which no further change occurs in intensity of the light transmitted. U11- der certain conditions, the determination may be based on a measurement with reference to the light transmission-characte1istic of the material before Wetting. This basis is only feasible when the volatile liquid employed is substantially colorless and does not effect a permanent change in the light transmissive character of the wettable material. The invention particularly contemplates employment of a wettable material which is an absorbent material.

?It is an object of the invention to provide a means whereby the evaporation end point of a liquid may be determined easily and in a relatively short period of time, as compared with conventional procedures, and without complicated or delicate equipment and techniques. Another object contemplated is to provide a convenient and simple apparatus and'operating procedure for evaporation end point determination wherein not only the overall 'rate of evaporation is indicated, but also the rate of evaporation for any time interval less than required for-total evaporation or end point determination. It is a further object of the invention to permit such determinations to be 'made from extremely smallzsample :portions of the volatile liquid. It is .also an object-of the invention to provide :a system .and procedure for determination of-evaporation end points and/or evaporation rates, adapted for employment in conjunction with suitable systems for zproce'ss'and product :control in the refining or blendingof fluid materials such as in the refining of hydrocarbon materials, orin the blending of refined products,-wherein the evaporation characteristics of-such materials are criticallfactors.

The invention audits-objects may be more fully=understood from the following description, when it-is read in conjunction withthe'accompanyingdrawings wherein:

Fig. l is an exploded view, in perspective of .a simple form of the apparatus contemplated;

Fig. 2 is a schematic showing in vertical section, of another form of the apparatus according to the present invention;

Fig. 3 is a diagrammatic showing representative of an apparatus accordingto the invention, as it may be applied to a substantially continuous determination of the evaporative characteristics "of a process flow :stre'am, andrifor the purpose of controlling tsuch characteristics inv any desired fashiomand Fig. 4 is adia-gramm-atic showing of a portienof the apparatus as illustrated by Figure 2 wherein an'alternate arrangement for passing a strip of absorbent material through the device is illustrated.

In Fig. 1, the numeral 1 designates a casing or housing for a light source such as a fluorescent tube indicated in the drawing by the numeral 2. The housing 1 is provided with a cover 3 in which there is providedha port such as is designated by the numeral 4. The port 4 is shown to be located in the cover between twostripe'le, ments 5 and 6 respectively, extending laterally over the cover in substantially right angular relation to the longitudinal axis thereof and between them forming a recess 7, wherein the cover is the bottom wall. If desired, of course, the recess may be a part of the cover being formed integrally therein. This recess is adapted to receive a strip of a non-transparent, light transmissive, wettable material designated in the drawing by the numeral 8. Preferably, the strip of material is an absorbent material and has a longitudinal dimension greater than the width of the cover 3 or casing 1 and is provided with a defined circular area 9 imprinted or otherwise impressed upon the surface of the strip. The circular area 9 is of substantially the same dimension as the port 4 and is spaced from one end of the strip 8 by a distance substantially equal to the distance of the port 4 from either edge of the cover. Also as shown or indicated, the strip 8 has a lateraldimension substantially equal -,to the width of the recess portion 7 and the circular area 9 is located on the strip in spaced relation to the side edges thereof; When the strip is inserted in the recess, the circular area 9 will substantially correspond to, and be disposablein substantial registration with the part4. The depth of the recesses provided by {the thickness of the elements 5 and s -'is gauged so as to be slightly greater than thethiclgness or" the strip of absorbent material 8. i

The elements 5 and 6, or the surface of the'cover 3, when the recessed portion is formed integrally, provides a support for a light meter 10, such as -a Standard 'Vll eston Light Meter -(Model No. 715-) as manufactured and or fereddor sale by the Weston Electric lnstmmentJQorporation of New-ark, NJ. The instrument as employed in the present invention was provided with a gradient scale covering a 50 range with 1 divisions. For the present purpose, the back ofthe meter was removed and replaced with a shield having a-port therein corresponding to the port lt in its dimension, andlocated sothat the shield port could L-be made to coincide with "the light port 4. Preferably, th /port 4 is also coveredwith some translucent material, such as :thin'white paper, frosted glass, or cloth, inlorder :toslightlydiffusethecmitted light. The intensity of :the light-source may be fixed, but-preferably some suitable and conventional means for controlling light-intensity is included inxthe circuit connected thereto.

in the operation of- -the.device .-as illustrated by Fig. =1,

a strip of absorbent :material such as .that indicated by thenumeralS, is wetted with ailiquid for which'theevaporation fpoint is to .be determined. IWettingis accoin-- tervals over .=s11.ch periodrofstimeas required -to-reach a* point of nochange in the. recordedzintensity of. light transmittedihwugh th62fib50l'b6l1t material. The time --take n to reach {this-point iisathe evaporation time for theliquid;

Preferably, the determination is madeunder controlled temperature conditions. This may be accomplished by enclosing the instrument with a shield spaced from the instrument from about one to about two inches, and wherein the shield isprovidedwith suitable openings to permit a substantially uniform flow of air upwardly within the shield. For the purposes intended, the heat derived from the light source, in conjunction with thermal circulation of air over the surface of the strip, is usually sufficient to induce evaporation of the sample applied to the absorbent strip. In the region of the sample employing a 6" double type fluorescent lamp, the temperature has been found to average between 109 and 120 F. Where especially accurate results are required, the determination should be 4 mined in a series of separate determinations for each liquid. In each determination, the ambient atmosphere of the instrument was air, conditioned and substantially maintained at a constant temperature of 72 F. and a relative humidity of 40%. Each liquid was further subjected to an evaporation end point determination by means of a Modified Jolly Balance, employing conventional techniques for the operation thereof, to provide a basis of comparison for the results obtained by the presently disclosed apparatus and method.

The nature of the liquids employed in these determinations and the results obtained are indicated from the following tables:

Table I Photo-Evaporimeter Modified Jolly Balance Boiling Liquid Range, Evaporation Time for Individual F. Runs in Seconds Average Percent Relative Evap. Relative Time in Devia- Time Time in Time Seconds tion (To1uol=1) Minutes (Toluol=1) Run#l Run#2 Run#3 Run#4 152-158 40 50 45 5:11 0. 39 0. 229-233 110 110 120 115 $4 1. 00 60 1. 00 251289 100 170 105 5:3 1. 43 7s 1. 278-290 250 260 260 257 :112 2. 23 130 2. 10 317-350 660 720 390 :l:4 e. 00 360 6. 00 316-385 960 1,080 1, 090 1,040 $5 9.04 600 10.00

carried out in an atmosphere of air maintained at a sub- Table II stantially constant temperature and humidity.

The absorbent material employed for the strip 8 may TimeIuSeconds be substantially any non-transparent material having the l gi gggigg ability to transmit light under the conditions of operation. Reading A B 0 D E F A variety of materials has been found to aiford satisfactory results when employed in the manner set forth 3.0 7 9.5 10 10 18 above. Such materials include both felted and woven g8 2% g fibrous materials, such as blotting paper, filter paper, and 11. 5 so n 54 54 i natural fiber cloth. Although an absorbent fibrous ma- $3? jg 5,3 22 3 terral is generally preferred for the purposes set forth, 22.0 01 ag 2%? fritted glass, and ground or etched glass may also be 1 507 730 used. When using a ground or etched glass strip, the $3 33 32% liquid sample is applied to the rough surface thereof. Y O L Finely woven metallic cloth has also been. employed with (Total M success. In this instance, however, the light transmission effects are reversed, the intensity of light transmitted by the dry cloth being greater than that transmitted by the wetted cloth. Thus the percent change is indicated by an increasing light value instead of a decreasing value.

In tabulating the results as shown by light meter readings, for uniformity, it is preferred that this be done in terms of percent change in total light meter scale deflection for any given time interval. When using materials such as mentioned above, excluding metallic cloth, total scale deflection may be defined as the maximum meter reading with 100% sample present, that is when the wetted strip of absorbent material is first inserted, minus the final meter reading after complete evaporation of the sample, at that point where no further change in the light meter scaled reading is apparent. When using a metallic cloth, the total scale deflection will be the final reading minus the initial reading, but still at that point at which no further change occurs. The error in meter reading is entirely dependent upon the calibration of the meter itself and the spacing of the scale markings. With the scale marked in 1 intervals it is possible to read it Within 0.5 Under such circumstances, the meter reading error should not be more than about equivalent to the readable scale unitdivided by the total scale deflection.

Employing an apparatus substantially as shown by Fig. 1 of the drawings according to the method set forth with reference thereto, the evaporation rate and end point of a series of sixdifferent hydrocarbon liquids were deter- In Table I, the several hydrocarbon liquids are designated alphabetically, and identified by their typical boiling ranges. The liquid designated B is toluol, and as a relatively pure material is used as a reference in the results tabulated, and particularly for comparison of evaporation times determined according to the present invention with those determined conventionally by use of a Modified lolly Balance. The total evaporation time for each liquid on each run made therewith is also shown. The average of these runs in each instance is shown in comparison with the time for evaporation of the sample required for operation of the Modified lolly Balance. The shorter times of determination as shown over that for the conventional smaller volume (0.002 to 0.05 cubic centimeter as compared with about 2.0 cubic centimeters) may beemployed according to the present invention.

In view of the fact that the differences in the evaporation times of the respective systems is so wide, provision is made in Table I for a proportional comparison. The evaporation time for toluol by either system is taken as a base figure equal to 1. The time for evaporation of each of the liquids in either system is then shown in its proportional relationship to the evaporation time for toluol. When thus related, the substantial uniformity of results by either system is evident.

Table II is derived from data obtained from the evaporation determinations made for each of the liquids designated in Table I, and shows the percent change in light .in Table I.

meter scale deflection indicated from a curve of average value plotted from such data. In this table, the total evaporation time shown'forea'chliquidis as also shown In addition, the equivalent total time in minutes is included'in 'parenthesisfor each liquid.

'The apparatus as illustrated by Fig. Zis basically similar to that as shown by Fig. 2. In Fig. 2, the numerals "11 and *12 respectively designate an -instrument body and'acapp'ing piece therefor. These elements may be'ofany 'desi're'd's'hape, but in the form contemplated by'thedrawingthey are circular. The body 11'is provided with a generally rectangular recessed slotted portionf l3, which extends diametrically across the body, opening upwardly through the upper end of the body and 'radiallythereof 'at'eaeh end of the slotted portion. 1he"body1=1 and piece ll'each'defines a concentric tubular'pa'ssageway, -14'and respectively,opening through their upper and "lower ends. These passageways are of substantially equaldiameter, and'as shown, when the body and capping piece are assembled, are disposed in substantially coaxial-relation. Theupper end of the passageway 14, defines a port which opens through the "bottomof'the recess portion 13. As-in the device as illustrated by Fig. 1, the passageways and thereby the port is intended 'to have a diameter less than the width "Of'thfi recessed portion 13. With the .capping piece 12 applied as shown, it cooperates with the body 11 and recessed portion '13 to form a passageway, rectangular incross section, which intersects the axis of the passageways 14'and 15 at right angles thereto.

The numeral 16 designates a slide adapted-for insertion'in the "recess portion 13, with the lower'surface of the slide 'in slidable surface contact with the bottom wall of the recess portion, and supported thereby. The upper surface of the slide is preferably recessed to receive a strip of a non-transparent light-transmissive material of the character previously described, and desighated in Fig. 2 by the numeral 17. A stop -member 1'8 i's disposed laterally of the slide 16 at its forward end, providing an abutment for the forward'end of the strip '17. The slide '16 is further provided with an orifice or-port 19-opening'through the recessed portion thereof intermediate the ends, and a lateralstop member 20*dependent'from the-lower surface at a distance from the center-of port '19 substantially equal to that from the axis of passageway 14 to the outer wall of thebody I l. By'rneans of stop 21), the port 19 is automatically brought into coaxial registration with the "passageways 14 and 15-when'the slide is fully inserted in the recess '13. In-similarfashion, the stop -18 provides for proper registration with 'port 19 of an indicated spot for application of a liquid sample on the strip 17, such as the circular area 9 on the strip 8 shownand described withreference to Fig. 1. In this instance, such a circular area would have acenter spaced from the forward end of the'strip 17 bya dimension equal to the distance from 'the'stopto'the center of the port 19. Further, as'shown in Fig. 2, the port 19 is preferably provided with a light difiusion means such as a frosted glass insert indicated *bythe numeral 21 in the drawing.

The overall thickness of the slide 16 with strip 17 applied thereto 'is gauged to the-depth'of the recess 13, so as to'be substantially less than such depth and-topro vide a free space above the strip 17 whichopens inwardly fromeach end-of the recess into communication with the passageway 15. The passageway 15 in turn communicates with a conduit passageway 2'2 'extending radially therefrom through the cap piece. A nipple 23 threaded into the outer end of the "passageway provides for-connection to either a suction pump-or a pressure pump, not shown, forcirculation of air over the :upper surface of the :strip 17 In the apparatus as illustrated by Fig.2, the required light source is indicated by the numeral 24. Preferably this source constitutes a projection lamp and condensing in the inlet conduit 103. operating and actuating-means may 'be substituted for those shown -for the purpose-of illustration. 'A-product lens system with means for cooling the lamp and the" light source generally in order to minimize theheating effect thereof. Thebeam of light from such source may be'focused so as topass directly into the lower endof the passageway 14, or "preferably, as shown, it may be focused on an adjustable reflecting means such as a mirror indicated by the-numeral 2 5, and the reflected 'beam dire'etedrintothe passageway 14b'y suitable adjustment of the reflecting mea'ns. In any event, .in order to maintain "a substantially constant temperature .in the duit 'coil "26 shown, including inlet "'27 and outlet thereof.

In Fig. 2, further, the numeral 29 designates "a light sensitive element supported over the upper end of the capping piece 12 so as to'expose 'a light sensitive surface over the upper end of the passageway d5. This "lightsensitive element 'may be "of any conventional "type including a light meter as represented and described with reference to Fig. 1. v In Fig. 2,"thelightsens'itivemeans shown is "representative-of a photo-electric cell unit also manufacturedand 'otfered for sale bythe Weston Electric Instrument Company of Newark, NFL, and designated as their model '856,'Type 1. 'Electric leads '30 'and'3'1 connect the unit 2910 a microammeter 32, adapted to register and indicate the strength of signals produced in the photo cell unit 29 by light transmitted "from the light source 24 through the light-transmissive strip 17.

Operation of the device as shown in Fig. -'2 is quite comparable with that'o'f the device as shown in Fig. 'l. A drop of a liquid sample is applied "to the strip 17in place on the slide 16. The slide and strip are inserted in the recess 13 and "positioned substantially as shown. The ammeter needle deflection is read at thispoint and at periodic intervals during evaporation. That *point "at which the needle deflection remains'constant is=then taken as the evaporation *end point. The results obtained during the operation and from inter-val to interval are compared with similar data, obtained by the same operation with'a sample of known evap'oration characteristics.

In the apparatus shownby Figure 3, .the method substantially as set *forth with references "to Figs. 1 and 2 is applied to a system for controlling "a product 'side stream from a distillation column. In the drawing, the

numeral 101 designates a distillation column "provided 'witha condensing coil 102, including an inlet conduit 103 and an outlet conduit 104. -A flow control means such as a solenoidflow control valve 105'is 's'hown as' disposed Obviously other equivalent side'stream is drawn from the distillation column as by 'means of the conduit 106. l henumeral1'07 designates a housing or cabinet which provides an evaporation chamber internally thereof. Theehanziber is provided with an inlet 108 for an introduction-of a stream "of-air of 'subs'tantially constant temperature and humidity, and at a substantially constant flow rate. The numeral 109 designates'an outlet for such from the evaporation chamber.

Internallyo'f the housing 1'07, the numerals 1 10, 111, 112, 113 and 114 designate the several "elements of a series of independentlight sources, each adapted to direct a beam'of light toward one of the corresponding series of photo-electric light sensitive elements, designated by the nur'nerals 110A, 111A, 112A, 113A and 114A respectively, whichare opposed in spaced alignment to the corresponding light sources. The numeral 115 designates a control mechanism of substantially conventional :character 'whichmay include a-recording instrument and such --re1-ays and amplification circuits "as may be required to analyze and translate electrical signals transmitted thereto from the photoelectric cells by way of the circuits designated in the drawing by the numerals 110B, 111B, 112B, 113B and 114B. The numeral 116 designates an electrical circuit for energizing the solenoid valve 105.

In the apparatus as shown by Fig. 3, the numeral 117 designates a strip of an absorbent material similar to that described with reference to Fig. 1. In this instance, the material is contained on a drum 118, being unwound therefrom at a constant speed by means of a motor driven drum 119 on which the tape is wound after passage through the cabinet 107 along a path which passes between the respective series of light sources and photoelectric cells by way of the entrance and exit slots 120 and 121. Suitable idler or tension rolls as indicated by the numerals 122, 123, .124 and 125 may be provided. In an alternate arrangement, the strip 117 may be in the form of a continuous belt substantially as described with reference to Fig. 4.

Means for wetting the tape with the volatile liquid to be tested includes a sample draw-off conduit 126, connected at its downstream end to the product side stream conduit 106, pump 127, flow meter 128 and heat exchange coil 129. The conduit 126 terminates Within the cabinet 107 in a discharge outlet or nozzle 130 disposed directly above the absorbent strip 117, and immediately before the series of light sources and photo-electric cells. A collection trough or funnel 131 is provided immediately below the strip of absorbent material substantially in alignment with the outlet 130 and in communication with a liquid discharge conduit 132 extending outwardly through the cabinet wall as shown. Rollers 133 and 134 are disposed above and below the strip 117 in the area of the collection trough 131, and intermediate the outlet 130 and the series of light sources and photo-electric cells. Means not shown are provided to apply pressure on the strip 117 through the rollers 133 and 134.

In operation of the apparatus according to Fig. 3, the strip of absorbent material 117 is passed through the cabinet 107 as indicated by the directional arrows. A measured and controlled quantity of liquid is withdraw from the product side stream 106 by way of the conduit .126 as by means of the pump 127 and the flow meter 128 This sample stream is passed through heat exchange coils 129, in direct heat exchange relation with the air passed through the cabinet 107 by way of the conduits 108 and 109, to be discharged from the outlet 130 against the strip of absorbent material. In order to insure adequate and uniform wetting of the strip by the sample liquid, the amount of liquid discharged against the strip 117 from the outlet 130 is controlled so as to provide an amount in excess of that required. This excess is then removed from the strip by the pressure of the rolls 133 and 134010 be collected and discharged from the I cabinet by way of the trough 131 and the'discharge outlet conduit 132.

As the strip of absorbent material moves through the cabinet 107, the circulation of air through the cabinet, by way of conduits 108 and 109, evaporates the liquid sample. The rate of movement of the strip of absorbent material is adjusted to such speed as will insure substantially complete evaporation of the sample prior to the time that it passes the last of the light sources and photoelectric cells in the series. The rate of evaporation of the liquid from the absorbent material strip is analyzed by the photo-electric cells by sensing the amount of light transmitted by the tape. The amount of light which the tape will transmit at any point in its path between the sensing means is related directly to the quantity of liquid which remains uncvaporated. Therefore, the rate at which the liquid evaporates can be measured by noting the rate of decrease in light transmission of the absorbent strip at its moves from the wetting point toward the exit 121 from the cabinet 107. The point at which there is 8 no further decrease in transmitted light corresponds to the point at which the sample is completely evaporated, and the time required for the strip to reach this point is the total evaporation time. Intermediate points can be located-corresponding to any specific degree of sample evaporation.

Light intensity data transmitted from the photo-electric cells are analyzed electrically in the control mechanism 15 providing the evaporation characteristic of the liquid applied to the absorbent strip. One or more detection points can be established as control points and the signals from the corresponding photo-electric cells employed to control the distillation column operating characteristics, as for example by regulating flow of a heat exchange fluid through the condenser coil 102. The signals derived from the several photo-electric cells may furthermore be employed in such fashion as to provide control for a number of process operating variables and product characteristics simultaneously. Likewise, several evaporimeter devices of the character shown in Figure 3 may be combined in an integrated system in such a way as to control several product streams at the same time and with relation one to another.

In Fig. 4, the strip of absorbent material, corresponding to that designated by the numeral 117 in Figure 3, is indicated by the numeral 217. In this modification of the apparatus, the absorbent strip is a continuous strip which instead of being wound on drums such as the drums 118 and 119 of Fig. 3, passes over the idler drum 218 and a motor driven drum 219 in such fashion that the strip is continuously passed into and out of a housing or cabinet 207. In the drawing, the numerals 222, 223, 224 and 225 designate idler rollers similar to the rollers 122, 123, 124 and 125 of Fig. 3. Also the parts designated in Fig. 4 by the numerals 208, 210 to 214- inclusive, 210A to 214A inclusive, 210B to 2143 inclusive, 220, 221, and 230 to 234 inclusive are the counterparts of elements as shown in Fig. 3 which bear similar numbers in a 100 series. In the modified form of the apparatus as shown in Fig. 4, provision is made for cleaning and drying the strip of absorbent material, substantially in the manner shown, wherein the strip 217, passing over the drum 219, is lead into a wash tank 240 containing a body of solvent material, passed therethrough over a series of rollers as indicated by the numeral 241, and then between a pair of pressure rolls 245 and 246 to remove excess solvent prior to passing the absorbent strip through a drying chamber 242 wherein the strip is subjected to the drying action of a stream of heated air passed through the chamber 242 as by means of inlet and outlet conduits 243 and 244- respectively.

As previously indicated, an apparatus of the character illustrated by either Fig. 3 or Fig. 4 may also be employed to control liquid blending operations. For example, by arranging the series location or spacing of the individual light sources and light sensitive means so as to correspond to time intervals between these cooperating elements required to produce a predetermined series of percent change characteristics such as indicated by Table II, and when using a sample of known or desired evaporation characteristics, the signals derived from any or all of the light sensitive means may be utilized so as to provide for automatic control of pumps or valves in a series of flow lines connected in a blending system and in any well known and accepted fashion.

What is claimed is:

1. An apparatus for measuring the evaporation rate of a volatile liquid, comprising means for emission of a linear series of individual light beams of substantially equal and constant intensity, a corresponding series of light sensitive elements wherein each element is interposed in one of said beams and spaced from said emission means along the path of said beam, means for passing an clongated strip of a translucent wettable material between said series of light sensitive and light emission means in a path intercepting said beams, means for at least partially wetting said strip of wettable material with a controlled portion of said volatile material in advance of the first light beam and light sensitive element in said series, means for substantially completely drying said wetted strip in advance of the last beam and light sensitive means in said series, and means for electrically exhibiting the difference in intensity of light transmitted by said strip to the first light sensitive means in said series, and that transmitted to any selected light sensitive means subsequent thereto in said series.

2. In an apparatus for producing volatile liquids, wherein the evaporation rate of the liquid produced is a critical characteristic thereof, and wherein said apparatus includes control means operable to determine said critical characteristic, including means to actuate said control means, the improvement which comp-rises means for emitting light of substantially constant intensity and for directing said light along a predetermined path, light sensitive means disposed in said light path, in opposed relation to said light emission means and spaced therefrom longitudinally of said light path, said light sensitive means being adapted to produce electrical signals having an proportional to the intensity of the light in said path, means for interposing an elongated strip of a translucent, wettable material in said light path intermediate said light emission and light sensitive means, means for applying a controlled portion of the volatile liquid from said producing apparatus to said strip of wettable material disposed in advance of said light emission and light sensitive means thereby initially wetting said strip, means for drying said strip while it is interposed in said light path, means to receive and exhibit electrical signals produced by said light sensi tive means responsive to light transmitted through said initially wetted strip during and after drying thereof, said means including means to energize said control actuating means in response to any predetermined characteristic of signals received from said light sensitive means.

3. An apparatus according to claim 1 wherein said means for at least partially wetting said strip of wettable material comprises a liquid receiver trough disposed in substantially parallel co-extensive relation to one surface of said strip, a conduit having a discharge outlet directed toward the other surface of said strip and toward said trough, and a pair of liquid extraction rolls adapted to pass said strip between them while applying pressure thereto, said rolls disposed substantially within an area corresponding to that of said trough and spaced from said discharge outlet in the direction of travel of said strip.

4. An apparatus according to claim 1 wherein said elongated strip of wettable material comprises a substantially continuous belt of said material.

5. An apparatus for measuring the evaporation rate of a volatile liquid, comprising a housing defining an elongated evaporation chamber; means for passing an elongated strip of a translucent, wettable material through an extended travel-path including said chamber; means within said chamber for directing individual light beams of substantially equal intensity against one surface of said strip, said means being disposed as a linearly spaced series along said travel-path; a corresponding series of light-sensitive elements disposed in said chamber along said strip travel-path opposite the other surface of said strip, with each element in said series in opposed, paired relation to a corresponding light beam directing means in said series of such means; means disposed within said chamber, in advance of the first of said paired light beam directing means and light-sensitive elements, for

at least partially wetting said strip with a controlled portion of said volatile material; means for passing air of controlled temperature and humidity through said chamber, whereby to evaporate said volatile material from said strip in advance of the last of said paired light beam directing'means and light sensitive elements; and means for electrically exhibiting the difference in intensity of light transmitted by said strip to the first light-sensitive element in said series of elements and that transmitted to any selected light-sensitive element subsequent to the first in said series.

6. An apparatus according to claim 5 wherein said means for at least partially wetting said strip of wettable material comprises a liquid receiver trough disposed within said chamber substantially in parallel, co-extensive relation to one surface of said strip, a conduit extended into said chamber having a discharge outlet directed toward the other surface of said strip and toward said trough, said conduit including a heat exchange section upstream from said outlet and in heat exchange relation to said air passed through said chamber, and a pair of liquid extraction rolls adapted to pass said strip between them while applying pressure thereto, said rolls disposed substantially within an area corresponding and coexten sive with that of said trough and spaced from said dis charge outlet in the direction of travel of said strip.

7. An apparatus for measuring the evaporation rate of a volatile liquid, comprising a housing defining an elongated evaporation chamber; means for passing a continuous strip of a translucent, wettable material through an extended travel-path including said chamber; means within said chamber for directing individual light beams of substantially equal intensity against one surface of said strip, said means being disposed as a linearly spaced series along said travel-path; a corresponding series of light-sensitive elements disposed in said chamber along said strip travel-path opposite the other surface of said strip, with each element in said series in opposed, paired relation to a corresponding light beam directing means in said series of such means; means disposed within said chamber in advance of the first of said paired light beam directing means and light-sensitive elements, for at least partially wetting said strip with a controlled portion of said volatile material; means for passing air of controlled temperature and humidity through said chamber, whereby to evaporate said volatile material from said strip in advance of the last of said paired light beam directing means and light-sensitive elements; means for electrically exhibiting the diiference in intensity of light transmitted by said strip to the first light-sensitive element in said series of elements, and that transmitted to any selected light-sensitive element subsequent to the first in said series; and means for reconditioning said strip of wettable material after passage through said evaporation chamber, comprising in sequence along said strip travel path a wash tank, adapted to contain a cleaning liquid, including means for passing said strip through said tank and for submerging said strip within said tank, means for expressing liquid from said strip, and a dryer chamber, including means for passing drying air therethrough.

References Cited in the file of this patent UNITED STATES PATENTS

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