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Nov. 12, -1946'. F. KAHN 2,411,041 RELATIVE HUMIDIT-Y METER Filed‘Aug. 7, 1942 2 Sheets-Sheet l x 3l 6.9 6'/ 57 ’t ü 7l il :s ü $6 b ¿ç 50 ' l 40 3l u ß mlllmlnulmllnnl llnullmhm y f 45’ 4+’ Fay. .5f Figi INVENTOR 5 la ' ls n ï f- ` _a l M "'açfärfgfymWzzzïfmf ‘ or » ' 3J Nov. l2, w46. F. KAHN 2,411,041 RELATIVE HUMIDITY METER Filed Aug. '7. 1942 2 Sheets-Sheet 2 INVENTOR I» Patented Nov. -l2, 1946l Y 2,411,041 UNITED STATES PATENT. OFFICE _ A2,411,041 K ‘ RELATIVE HUMmi'rY METER Frank Kahn, Philadelphia, Pa. - Application August 7, 1942, serial N6. 454,009 12 claims. (ci '1s-33s) 2 The present invention is a continuation-impart of my patent application, Serial No. 229,281, ñled September 10, 1938, now Patent No. 2,293,064, reading relative humidity >indicator of good - accuracy . Another object oi my invention is to provide . dated‘August 18, 1942, relating to relative humid ity meters.` an‘indicating instrument having an expansible ' scale. The invention relates to hygròmetry, and, more particularly, relates to a direct reading relative> humidity meter. v ' Still another object of my invention :is to pro vide a bimetal wet-air temperature responsive . unit. ' The most convenient and most generally em of space, requires very littlewater, and needs ‘l With these and other objects in view, which will become apparent as the description proceeds, my invention is embodied in a relative .humidity observing system which consists essentially of a wet-air temperature responsive'unit in a stream of the airbeing metered, a dry-air temperature 2,128,462, dated August '30, 1938, issued jointly to Edward R. Wayne and myself, and relating to thermocouple hygrometers, and with my bimetal responsive unit in the same air, a meter having a movable ‘element arranged in cooperative rela tion with both said Wet- and dry-air units and adapted to be deiiected proportionally tothe dif 25 ference in response of said units,vand means re sponsive tothe dry-air temperature of the air being measured to cause the indication of the meter to vary as a function of the dry-air tem Weather Bureau Psychrometric Tables, or _the .equivalent standard formula or chart.A This method of determining relative humidity, perature. p although of high accuracy, is inconvenient and 30 because it is not direct readingÁis unsuited for - recording or for control of humidity conditioning ’ apparatus. On the other hand`reasonably priced direct reading devices, which have- heretofore' been based upon relative tension of- hy'groscopic materials, have been highly unreliable, and have 35 Y - required much too frequent calibrations and ad ' . 'I‘he invention, both as to details of construc tion and combination of parts, will best be under stood from the following description of a speciiic embodiment which I illustrate as an example, when read in conjunction with the accompany ing drawings, in which: . » Fig. 1 is a diagrammatic representation of a relative humidity responsive single thermocouple justments to render their use practical where any _ ' practically no attention or servicing. wet-air thermocoup‘le described in Patent No. the relative humidity may be obtained from the -` vide an automatic direct-reading relative humid ity meter which is accurate, occupies a minimum " with the sling or „whirled psychrometer, with the wet-air 'temperature responsive unit hereinafter described in detail. With the difference between the dry-air and wet-air temperatures determined, v Still a further object of my invention is to pro- ` ployed method for measuring the degree of satu ration ofthe atmosphere with moisture is to observe the temperature of evaporation, that is, the diñ'erence between the dry-air and AWet-air temperatures. The dry-air or ambient tempera ture is read with the ordinary thermometer. The Wet-air temperature may be accurately observed circuit powering the movable element of the meter degree of accuracy is required. ' > of -my invention. 'The increasing importance of air conditioning emphasizes the need for a simple, compact, in 40. Fig. 2 is a' diagrammatic representation of a - relative humidity responsive thermopile circuit ' expensive and reliable device for measuring rela powering the movable element of my meter. Fig. 3 is a diagram showing the relation of tive humidity. Apparatus for` this purpose here tofore available has been extremely large and expensive, ,very inconvenient and laborious to relative humidity to the dry-air temperatures and the diiîerence fbetween the dry- and wet-air operate, or highly unreliable. f The primary object of my invention is to pro temperatures.V Fig. 4 is a diagrammatic view of -my invention , , showing the meter with resilient scale. 5 is a diagrammatic view of a modification An object of'my invention is to produce a direct 50 of Fig. the resilient scale. . ' ’ vide a, direct reading relative humidity meter in ` which> these defects are remedied. ` reading relative humidity meter of high accuracy. Another object of my invention is to provide a reliable direct reading relative humidity recorder. A further object of -my invention is to produce an inexpensive, simple, durable and reliable direct Fig. 6 is a front elevation of the self-contained - direct-reading relative humidity meter of my v invention. y Fig. 7 isa fragmentary front elevation of the device oi’ Fig. 6. - 2,411,041 3 . of the device of Fig. 6. 4 ing a thermojunction 2| formed by union-of two . Fig. 8 is a perspective view of the wet-air uni couple elements 22 and 23 secured with good _. thermal contact to a thermally conducting mem- l Fig. 9 is a fragmentary side elevation of a modification of the device of Fig. 6 in which the ber 24, the whole being -enclosed in an evapora tive envelope 25. The thermocouple electric cir cuit is completed through a sensitive d’Arsonval ltype ammeter 26, which forms the reference ' in section,.showing the movable element of myV junction. When the air to be measured is forced meter powered by opposed bimetal elements. past- .the wet-air unit 20 at the proper 'velocity Fig. 11 is 9, fragmentary perspective, partly in section, of a modification of my meter having 10 with the wick 25 suitably moistened and with the ammeter 26 at the dry-air temperature, the net dry-air temperature responsive deflection sensi M. F. of the circuit, and therefore the current tivity. _ in the circuit, is substantially proportional tothe Fig. 12 is a fragmentary sectional view on the depression of the wet-air temperature below the une' Iz-lz of Fig. 11. ` . Fig. 13 is a diagrammatic view of my meter 15 dry-air temperature. electric fan motor is replaced by a spring motor. Fig. 10 is a fragmentary perspective view, partly Greater currentfor the same wet-air depres with non-resilient proportionally variable scale. Referring now to the drawings, Fig. 3 is a graph sion may beobtained with the wet-air unit of Fig. 2, and thus make possible the use of a `much less in which _the unbroken lines are curves of con sensitive and therefore cheaper ammeter. In stant relative humidity, ordinates are dry-air or ambient temperatures, and abscissae are depres 20 place of the element 24 of Fig. 1, there are shown in Fig. 2, three elements 30, each the functional sions of wet-air temperature below dry-air tem equivalent of element 24. The thermocouples 3| perature. These data are plotted from the stand are arranged as a thermopile with intermediate ard ps_ychrometric tables in the United States junctions 32 at the same temperature as the ref Weather Bureau publication W. B. No. 235. l Corresponding respectively to the unbroken 25 erence junction at .the ammeter 33. The ele ments 30 and attached thermocouples are elec curves of constant relative humidity, I have trically insulated from each other by shellac or shown an equal plurality- of broken straight lines - other‘electric insulating coating which will not I‘8 diverging from a common point I9 on the axis of ordinates at about eight degrees Fahrenheit, one of said'straight lines being drawn to coin materially reduce the heat transfer from the ele 30 ments 30 to the evaporative envelope 34. cide generally with èach of the constant relative humidity curves between‘the ordinates 40 F. and 100 F. As is readily evident from Fig. 3, per One method of providing the desired variation of sensitivity of indication with change of am bient temperature is illustrated in Fig..4 in which a d’Arsonval ammeter 35 is shown comprised of a cent relative humidity can thus be represented with good fidelity by a series of straight lines on 35 permanent magnet 36, core 31, movable coil 38 and lpointer 39. In place of the'customary scale, a graph of dry-air temperature against depres however, there is provided a resilient scale 40 sion of wet-airtemperature. composed of a flat rubber band, or the like, 'An indicating instrument having a deflecting graduated in per cent relative humidity with the element responsive to difference between wet-air spacings of the graduations proportional to the and dry-air temperatures, `when calibrated at horizontal spacings of the broken relative hu constant dry-air temperature in accordance with the intersections o'f the relative _humidity lines I8 with the ordinate representing the dry-air . midity lines of Fig. 3. The left hand end 4I of differential be then made to vary inversely as the suitably connected to one of the wet-air units usual ambient temperatures as illustrated in Fig. f vthe abscissae of the graph of Fig. 3. The element 44 is constructed so that the movement of its end the scale 4I| is fixed relative to magnet 36, while ' the right hand end 42 is attached to the tempera temperature, will indicate with _good accuracy per cent relativehumidity at that temperature. 45 ture responsive or movable end 43 of a bimetal element 44 whose other end 45 is also fixed rela If the deflection of the element relative to the tive to magnet 36. When the ammeter 35 is scale for a given wet-air to dry-air temperature ypreviously described, the pointer 39 will deflect " dry-air temperature less approximately 8 F., the instrument will indicate percentage relative 60 proportionally to the depression of the wet-air temperature below ambient, or proportionally to humidity with good accuracyover'the range of '3. - Inother words, this effect is produced by mere ly making the sensitivity of indication of the meter inversely proportional to the dry-air tem perature above approximately 8 F. I use the term “sensitivity of indication” to express the 43, and therefore the stretchl of the resilient scale~ 40, in response to changes in ambient tempera ture, is such that for any ambient temperature between .40 F. and 100 F., the scale 40 will be stretched to correspond with thebroken lines of the graph of Fig. 3 at the particular ambient between `the lwet-air and dry-air temperatures. 60 temperature ordinate. Because the broken linesl I8 are straight and _converge to a point this is a As an illustration, referring to Fig. 3, the reading simple matter. The departure in Fig. 3,V of, the or indication for a dry-air temperature of 80 broken lines vfrom the unbroken lines, between and a diiïerential wet-air to dry-air'temperature the ordinates 40 _F.- and~100 F., shows that the. of 20 F. would be the same as for a dry-air tem- Y degree to which’the'deflection of the element relative to the scale is responsive to the difference ' perature of 40 F. and a differential temperature 65 errors to be. expected in reading this meter di of 9 F., namely ‘30% relative humidity. The in strument would _thus have as great an indication _relative to the scale for a 9 F. differential at 40 F. dry-air temperature as would result from a 20 F. _differential temperature at 80 F. dry-air temperature. . rectly inrelative humidity are of a very low order. A pointer 46 is attached to the end 43 of birnetal _element 44 and is arranged to traverse a scale 41 fixed with respect to magnet 36 and suitably graduated in degrees of temperature so that the device also functions as a thermometer. _ In place of the ñat band 40, I have shown in the relative humidity meter 56 of Fig-5, an elas tic scale 5I composed of a ñattened helical spring -between wet-air and dry-air temperatures. Fig.> 1 shows a wet-air thermocouple unit 20 compris 75 52. The major or cardinal divisions are pro Figs. l and 2 illustrate thermoelectric circuits. having instruments responsive to the difference 2,411,041 V vided with index tabs -53 extending beyond the spring 52 and being marked with the appropriate value of per cent relative humidity. A stitl' internal core 55, suitably secured in fixed relationship to the frame 58 of the meter, sup ports the spring 52 in the desired scale shape, in this case illustrated as a circular arc. The left hand end 51 of the scale 5| is secured to ` ' 6 cent relative humidity in accordance with the broken lines of the graph of Fig. 3, as previously explained. ' ' ` ~ The illustrations of Figs. 6 and 'l depict .the container 12 as made of glass but it may be made of any other suitable material `such as metal or . a. resin. In place of the electric motor 81, a spring powered motor 90 with manual wind 9| , on a base core 55. IA bimetal element 44’ has one end 45' 65’ illustrated in Fig. 9, may be used to' drive the fixed to the frame 58, with the other end 43' 10 ian 56’ which is shown with a screen or filter 92 secured to the movable end 58 ofthe scale 5|. to minimize accumulation of dirt in the duct or 'I_‘he separations oi.' the individual turns of spring on the wet-air unit, although experiencevhas 52 are preformed to correspond to aliquot sub-> shown that the wet-air unit will operate with divisions of per cent relative humidity in accord good accuracy‘ when quite dirty. The meter is ance with the separations of the broken lines of 15 designed for both continuous operation and for the graph of Fig. 3, taking into account the shape spot readings. In the latter case the fan is of scale 5|. A pointer 59 attached to the end 58 started and the indicating pointer of the meter of scale 5| registers with an ambient tempera is observed until it reaches a minimum value of ture scale 60 fixed to the core 55V so that the relative humidity, the steady state condition be meter can also be used as a thermometer. 20 ing reached in a few seconds as a result of the My invention, as embodied in a. small self small size of the wet-air unit. Either of the wet air units illustrated in Figs. l or 2 may be used with this meter, a less sensitive ammeter being required if the wet-air thermopile of Fig. 2 is used. blower 68 driven by an electric motor 61 which is 25 In place of the thermocouple wet-air units of supplied with electricity via leads 68 and 69 from Figs. 1 and 2, the bimetal wet-air unit |00 of ' a suitable source of current supply (not illus the meter shown in Fig. 10 may be used. In this trated) which may be A.-C. or D.-C. A collar device a bimetal spiral |0| has its internal end bracket 10 is provided with an outward flange ñxed to a concentric shaft |02 rotatably mounted 1| which is attached by screws to the base 65. A 30 in a ball thrust bearing |03. The external end container 12 having a cylindrical lower reservoir of the spiral |0| is attached to a rigid arm |00 section 13, a duct section 14,l and a mouth section `fixed to a cylindrical sleeve |05 integral with or contained compact portable relative humidity in dlcator, is »illustrated in Figs. 6, 7 and 8. A base 85 has attached theretoa miniature centrifugal 15, is held in 'place by the bracket 10 which fits about .the reservoir 13; The duct 14 communi- ` cates with the reservoir 13 by way of an opening 18 rimmed by an annular shoulder 11. the mouth 15 being centered over the opening 18. A small wet-air thermocouple unit 18 which maybe of, secured to the stationary part of the bearing |03. The shaft |02 and the sleeve |05 are made of thermally non-conducting material such as Syn thetic resin or plastics. An evaporative envelope` |08 completely encloses the spiral |0| ard a part of lthe sleeve |05, in such manner that heat leak age into the interior of said envelope |00 via the thereof, but illustrated as of the thermopile type 40 Vsleeve A |05 is insignificant. 'I'he wet-air unit |00 of Fig. 2, has its conductor elements 19 extending comprises the envelope |08 and the mechanism through acork or rubber stopper 80 which iits enclosed therein. . snugly into the mouth 15 so that the body of The shaft |02 extends beyond the bearing |03 the wet-air unit 18 is suspended centrally within and carries at the outer end a bimetal spiral |01 the duct 14 and the dependent wick 8| of the unit 45 similar to the spiral |0| and similarlyl attached 18 passes through the opening 16 into the reser ' to the shaft |02. The external end of the spiral voir 13. A conical collar 82 of waxed >paper or |01 is rigidly linked to the lower end of a pointer other non-absorbent material, is fitted over the |08 by a connecting memberl |09 disposed parallel wick 8| so that said collar rests on the shoulder to shaft |02. The pointer |08 is carried on 'a 11, thereby sealing reservoir 13 and minimizing 50 shaft ||0 aligned with shaft |02 and journaled the rate at which water is evaporated therefrom. » in a bearing I I|. The entire indicating mech The exhaust vent 83 of the fan 66 abuts the inlet _ anism is mounted so that the upper or indicating end of the duct 1I and the joint is made airtight end I|2 of pointer |08 traverses the expansible with an elastic sleeve 84 which fits over both scale 40' which may be any of the scales shown parts. A d’Arsonval meter 85 constructed ac 55 in Figs. 4, 5, 13 or their equivalents. cording to the modifications of my invention When the wet-air unit |00 and the spiral |01 hereinbefore described or those disclosed in my are exposed to a current of the air to be meas aforesaid copending patent application Serial No. ured at the proper velocity, the spiral |0| will> 229,281 as being adapted to read directly in per ' soon reach the >wet-air temperature and the cent relative humidity when connected to a prop 60 spiral |01 the dry-air temperature. The pointer erly conditioned wet-air thermocouple unit, is |08 will be rotated by the member |09 to a. posi-I also attached to the base 55, the terminal leads tion corresponding to the differential rotation of 88 and81 oi' Ithe wet-air unit 18 being connected the spirals |0| and |01 which is proportional to thereto. ‘ ' the difference between the wet- and dry-air tem 'I‘he operation of this _meter -is as follows. The 65 peratures or the abscissae of the graph of Fig. 3. reservoir 13 is ñlled with water 88 through the In effect this device is a mechanical functional mouth 15 and opening 18. The assembly com equivalent of the thermocouple circuits of Figs. the type shown in Figs. l or 2 or the equivalent f. prising stopper 80, wet-air unit 18, wick 8|, and 1 and 2. ' ' collar 82 is slipped’into place through the mouth vIn Fig. 11 I have shown the wet-air unit |00'V 15. Electric current is then supplied to the motor 70 which is similar to the unit |00 except that the 51 causing the fan 58 .to'force a stream of air axis of rotation is vertical instead of horizontal . through the duct 14 over 'the unit 18. In a few seconds the temperature of the measuring junc and the spiral |0| ' is encased in a housing H5 of thinmetal of good thermal conductivity such as . tion of the unit 18 will be reduced to the' wet-air copper or silver. The housing ||5 prevents the temperature and the meter 85 will indicate per 75 envelope |00’ from interfering with the move aardbeiy 7 y . .. ment of the spiral I’0I’ and also protects the bi metal from corrosion by the moisture. The hous ing |I5 is preferably 'filled with alight oil to minimize any lag in reaching steady state tem - perature. A crank I I6 journaled in a fixed bear ing ||`| aligned with shaft |02’ has its oil'set end attached to the external end of the dry-air spiral. |01’. Another bearing I|8 is translatably mount ` ~ 8 - positioned in accordance with the lines I8 of Fig. 3 to represent the cardinal percentages of a rela _ tive humidity scale as indicated by the index tabs 53'. The positioning is accomplished by the bi metal element _ 44" which revolves a suitably guided circular-arc rack |32 about the point |9| as the ambient' temperature changes. Propor _ tional changes in spacing of the arms |90 are ob tained by means of pinions|93 rotatably attached ed in a guide ||9 for constrained movement in to the outer ends of the arms |90 so that they` alignment with shaft |02’ as imposed by a engage both the rack |92 and individual sectors U-shaped bimetal element |22 having its-ends of a fixed annular gear led. The individual pin,attached respectively to the bearings I I‘l and | I3. ions |93 and sectors of gear |94 are severally pro A shaft |2| is journaled in bearing lI I8 which is portioned to move the arms |39 to the desired of the thrust type to prevent axial movement of the shaft. The shaft |2| is provided with a slot 15 positions for any particular ambient temperature. For intermediate scale divisions, elements of ted sleeve |22 at its lower end to receive the upper wire or other suitable material may be attached end of the crank shaft H5 which carries a trans verse pin |23 engageable inthe slot toY prevent relative rotation of the two shafts while permit to'or be guided by the median intersections of lazy tongs connected between pairs of the arms tingv relative axial movement. The upper end of - 20 |9ß. This construction is shown in Fig. 13 for scale subdivisions between 10% and 20% relative shaft |2I is bent at right angles to form a lever humidity, the elements |95 representing scale |24 whose end engages a radial slot |25 in the subdivisions being positioned by the two sets of upper end of a gear sector |25. ~ lazy tongs |96 and |91. The space enclosed in The sector |26 is fixed to a shaft |27 Journaled in a fixed bearing §28, theishaft |21 being at 25 the dashed lines is the portion of the scale in tended to be exposed to view in the assembled tached to the sector at a point between the slot meter. The indicating element for this device |25 and the lower or gear end I 29 so that the axis may be that of Figs. l, 2, 10 or their equivalents. of rotation of the sector is relatively very close It should be realized that the foregoing ther to the slot |25'as compared with the end |29. The gear |29 engages a pinion |30 fixed to a. shaft |3| 30 moelectric powered meters are suitable for re which extends through and is journaled in a , mote indication of relative humidity. It is well known that in place of the dry-air responsive bearing |32 and carries a pointer |33 operating ' thermosensitive elements, generally shown herein over a ñxed scale |3d. A ñne spiral spring |35 is as bimetal devices, remotely actuated thermosen attached to shaft |3| and bearing |32 to elimi 35 sitive devices, such as that illustrated in Fig. 6 of nate backlash. . the aforesaid Patent No. 2,293,064 or its func This device functions as follows: The shaft IIB rotates proportionally to the difference between tional equivalent may be used. Suchvconstruc Vtion permits the meter in its various forms to be used for air in ducts ’or other inaccessible places plained for the device of Fig.- 10. This rotation isy communicated through shaft |2| to the sector 40 and the location of the meter itself to suit the convenience of the user. |26, but the point of engagement of the lever |24 Although the compact self-contained device of in thevslot |25 is determined by the bimetal ele Figs. 6 and 7 is shown with a thermoelectric pow _ment |20, That is, the higher the ambient teni' ered differential unit, it is obvious that the bi perature, the higher the bearing ||8 will ascend metal powered diüerential units of Figs. l0 and in its guide I I9 and the less will be the rotation 11 maybe used instead with either the expansible of the sector |26 and hence of the pointer |33. scale or the variable deflection sensitivity con The object of the proportioning of the sector |26 to give a high mechanical advantage of the order Although the invention has been described in `of V10 to 1 or more is to provide a relatively great considerable detail, such description is intended movement of the pointer |33 for avery small as illustrative rather than limiting, as numerous4 deflection of the lever |24 in the slot' |25. Not embodiments will be apparent to those skilled in only does this construction eliminate the need for - the art. My invention, therefore, is not to belim excess clearances in the slot and lever connection ited except insofar as is necessitated by the prior as shown in Fig. 12, but the proportionality of angular movement is maintained at that connec 55 art or by the spirit of the appended claims. tion because only very small angles are involved. I claim: » i 1. In a direct reading relative humidity meter, In this device it is- obvious -that the shaft IIE a deñecting element responsive to the difference could be operated from the rotating elements of between the wet-air and dry-air temperatures of the ammeters of the therm'ocouple circuits of ' Figs. 1 and 2 as well as from the mechanical dif 60 the air to be measured, ‘a scale adapted to be varied in length, a pointer adjacent to said scale ‘ vferential unit illustrated. and constructed and arranged to be deilected over 'I'he principle 9! the- elastic scale in connection wet- and dry-air temperatures as previously ex trol. - ` said scale proportionally to the deñection of said element, and means to vary the length of the In place of the elastic or resilientv scales of Figs. 5 scale such that the sensitivity of indication of said pointer relative to said scale is in inverse propor 4 and 5, the scale may be constructed in the man- . tion to the dry-air temperature in degrees F. less ner of a folding fan or it may be constructed sim.. approximately 8 F. ilarly to the iris of a camera; with the compo 2. In a direct reading relative humidity meter. nent elements representing major divisions of . with my relative humidity meter is not intended to be limited to thev use of an elastic medium. per cent relative humidity and arranged to be 70 a differential unit including a pointer responsive to the difference between the wet-air and the dry for the various ambient temperatures by a suit- ` >air temperatures of the air to be measured, a scale adjacent to the path of said pointer, said scale able bimetal element or the equivalent, An ex being adapted to be varied in length and the di ample of such construction is illustrated in Fig. 13 in which arms |90 pivoted at a point |9| are 75 visions of said scale to beproportionally clon- >positioned in accordance with the graph of Fig. 3 gated vor contracted as the scale is lengthened orl shortened. and means to vary the length of said scale as a function of temperature. 3. In an indicating instrument, a frame, a scale, a deilection element mounted Aon said frame and CI ` 10 . envelope adapted to be subjected to standard evaporating conditions, a ñrst bimetal spiral en closed within said envelope, a ñxed cylindrical ' sleeve member piercing said envelope, a. shaft dis posed coaxially within said member, a second bi having a pointer adjacent said scale and movable metal spiral disposed externally to said envelope, with respect thereto, said scale having one end , the inner end of each of said spirals being respec ñxed to said frame and being adapted to be varied tively fixed to each end of said shaft and coaxial in length, and means for varying the length of therewith and the outer end of said ñrst spiral said scale, said means being mounted on said 10 being ilxed relative to said member, and means frame independently of said element. to support said shaft rotatably within said mem 4. In an indicating instrument, >a frame, a scale, '_ ber whereby the outer end of said second Spiral iS a 'deflection element mounted on said frame and having a pointer adjacent saidscale and movable - deflected in response to the diii’erence between the wet- and dry-air temperatures of the air being with respect4 thereto, said scalehaving one end 15 fixed to said frame and being .adapted -to be elas 9. In a direct reading relative humidity meter, tically stretched, and means for stretching said the combination of a thermocouple circuit com scale. said means being mounted on said frame prising (1) a wet-»air thermocouple unit adapted independently of said element. 5. In a hygrometer, a wet-air unit comprising 20 to be exposed under standard evaporating condi tions to the air to be measured, (2) a reference an evaporative envelope adapted tobe subjected thermojunction in said air. and (3) an electric in to standard evaporating conditions, a bimetal strument having an indicating element responsive , spiral enclosed within said envelope, a sleeve to the current in said circuit. said instrument in member piercing said envelope and having a cy cluding a scale adapted to be varied in length. lindrical opening, a shaft rotatablyA disposed in 25 and means for varying the length of said scale as~ the opening in said member. said shaft being fixed a function of temperature. to the inner end of said spiral and said member 10. The invention set forth in claim 9` charac being iixed relative to the outer end of said spiral. terized in that the divisions of said scale are 6, The invention set forth in claim 5 character 'adapted to beproportionally elongated and con ized in that said member and said shaft are com 30 tracted as the scale is lengthened and shortened. posed of thermally non-conducting material, and 11. 'I'he invention set forth in claim 9 charac said spiral being proportioned to the evaporative termed in that said scale is elastic and that said' properties of the envelope such that said spiral means is adapted to stretch said scale. is cooled substantially to the temperature of the 12. The invention set forth in claim 2 charac surface of the envelope. ' » . 35 terized- in that said meterincludes means for di- . 7. The invention set forth in claim 5 character recting a current of the air to be measured ized in that said spiral is enclosed withina hous against the wet-air component of said diiïerential ing of thin metalof good thermal conductivity. unit, and reservoir means for supplying moisture 8. In a relative humidity meter, a diiïerential to said wet-air component. measured. wet- and dry-air unit comprising an evaporative 40 ’ _ ' ` ’ FRANK KAHN. .