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Патент USA US2411041

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Nov. 12, -1946'.
Filed‘Aug. 7, 1942
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Filed Aug. '7. 1942
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I» Patented Nov. -l2, 1946l Y
Frank Kahn, Philadelphia, Pa. -
Application August 7, 1942, serial N6. 454,009
12 claims. (ci '1s-33s)
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
The invention relates to hygròmetry, and, more
particularly, relates to a direct reading relative>
humidity meter.
Still another object of my invention :is to pro
vide a bimetal wet-air temperature responsive
' 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,
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
- 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,
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.
The primary object of my invention is to pro
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
Fig. 7 isa fragmentary front elevation of the
device oi’ Fig. 6.
of the device of Fig. 6.
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
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.
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
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.
left hand end 51 of the scale 5| is secured to
cent relative humidity in accordance with the
broken lines of the graph of Fig. 3, as previously
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
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
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
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:
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
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
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
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.
wet- and dry-air unit comprising an evaporative 40
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