A wood-element hygrostat


Material Information

A wood-element hygrostat
Physical Description:
Johnson, Seymour J
Rasmussen, Edmund Frederick, 1903-
Forest Products Laboratory (U.S.)
United States -- Forest Service
University of Wisconsin
Dept. of Agriculture, Forest Service, Forest Products Laboratory ( Madison, Wis )
Publication Date:

Record Information

Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 29227624
oclc - 698204865
System ID:

Full Text


January 194(C

No. I1(62

U . *' ... ] -:'Y

Madison 5, Wisconsin
In Cooperation with the University of Wisconsin


SEYM14OUR J. JOHNSON, Laboratory Aid
E. F. RASIOSSSE., Engineer


Several t;.pes of wood-element hy.rostats- 2 have beer. ziade ar.d used
by the Forest Products Laboratory. All have demonstrated their usefulness
in maintaining accurate control of relative humidity in dry kilns arnd cor.-
ditior.irLg rooms, particularly when conditions of constant equilibrium mois-
ture content are desired. All operate on the same bIrasic principle, the
inherent tendency of '.ood to absorb or lose moisture with changes in rela-
tive humidity, which causes the controlling- o-.cod element to shrink or swell.

In this article is presented information on the functioning; of these
hygrostats: their operational faults, and corrective measures used ii. over-
coming them; and details of their construction and wethodo of cclilratrlng.

Early Type of Hyfrostat


An early type of Forest Products Laboratory vhygrostat controls h'-
midity conditions by means of a wood element installed with a slight curva-
ture (fig. 1). One end of the wood element is fastened to a fixed block
"i," the other eAnd to an adjustable block "j." As the humidity increases,
the wood element absorbs moisture and begins to swell. Because it is
securely fastened at both ends, its degree of curvature increases as it
swells. This causes the lever arm "ta,' pivoted at "b" and connected to the
wood element by linkage "c," to :..ove away from the air nozzle "d." which is
-onnected to a bellows "e" by mLeans of a s..all copper tube ,f." -he open-
ir.g of the air nozzle "d" releases the air pressure in the bellows "'e,"1
causing it to contract and pull on pin "g," to which it is attached.
Flanges attached to pin "g" are pulled against an air port in the auxiliary

-"A Sensitive Wood 2lementt Hy,-rostat," by W. Karl Loughborough and R. C.
Rietz. I:STRU.T:TS, June 1932.
2"Moistlre Fluctuations in Lumber Within Closed Storage Sheds Controlled
with -lectrical Equipment," Forest Products Laboratory Report l'o. Rll40.

Report No. R1602


air mechanism "h," shutting off the air supply to the air-operated diaphragm
valve "n"' which contrqls the flow of stearm into the steam spray lines. The
release of air pressure against the diaphragm of valve "n" allows the com-
pression spring attached to the valve stem to close the valve, thereby
shutting off the steam spray. As the humidity within the dry kiln or con-
ditioning room decreases, the wood element loses moisture and begins to
shrink, decreasing the amount of curvature which results in the closing of
air nozzle "dt" by the lever arm "a."'t This builds up pressure in bellows
"a," expanding it and pushing pin "g" with its attached flanges clear of the
air port in the auxiliary air mechanism "th." Air pressure on the diaphragm
of the air-operated valve overcomes the resistance offered by the compres-
sion spring attached to the valve stem, thus opening the valve and allowing
steam to flow again into the spray pipe within the dry kiln or conditioning
room. The relative humidity can be increased or decreased by controlling
the curvature of the wood element by means of the adjusting knob "k" at-
tached to the adjusting rod "m," which is threaded into the sliding block
IJ. It

The auxiliary air mechanism "h" is similar to that used in some com-
mercial temperature recorder-controllers. It may also be used to control
a diaphragm motor that operates the ventilating dampers of a kiln or condi-
tioning room.

This principle of humidity control can also be used in conjunction
with electrically operated equipment by installing contact points connected
by wires to a motor-operated valve on the steam spray line. One of these
points is in a fixed position, the other attached to the wood element. As
the curvature of the wood element increases or decreases in response to
changes in moisture content, the movable contact point makes or breaks
contact with the fixed point, thereby causing the motor to open or close
the steam spray valve. A single (2-wire) or double (3-wire) contact system
may be required, depending on the type of control valve being used.

Operational Precautions

Extreme changes in relative humidity conditions will occasionally
cause severe distortion, reversal of curvature, or breakage of the wood
element in this instrument, When the instrument is thus thrown out of
adjustment, kiln conditions will be upset, Because of the danger that the
instrument will be so affected, it is impossible to calibrate the instru-
ment so that the desired relative humdity conditions can be rapidly

These faults are difficult to overcome but can be minimized by gradu-
ally increasing or decreasing relative humidity conditions. Furthermore,
if a drying chamber that is operated at a high relative humidity is to be
entered for extended periods of time, with the door left open, the wood
element should be covered with a damp cloth so that it will not break as
a result of rapid drying and shrinking.

Report NTo, 1602'


Improved Hygrostats


To overcome the difficulty caused by the breakage of the curved wood
element, another instrument was designed with a straight, free-floating wood
element (figs. 2 and 3). This hygrostat was designed to be mounted through
the roof of a drying compartment, with the wood element inside and the
control arms and contact points outside the kiln. Figure 2 shows the design
with electrical contact points while figure 3 shows an arrangement suitable
for air control. Although quite suitable for interior drying compartments,
this hyf-rostat cannot be calibrated for rapid adjustments of relative hu-
midity conditions because the control points are located above the roof.

These disadvantages are largely overcome in the design shown in
figures 4 and 5. A patent application on this design has been Lade ty, the
Forest Products Laboratory for free use by the public.

The straight wood element in this hygrostat is installed in a verti-
cal position, with its support rod resting upon a cam and the upper con-
necting rod free to move up or down as the wood element swells or shrinks
with changes in relative humidity.

A feature of this hygrostat is the cam (io. 2 in fig. 4) upon which
the support rod rests. By turning the control rod, which is attached to
the cam shaft and extends outside the dr-inj or conditioning; room, tL.e wood
element can be raised or lowered to any desired setting on the indicator
dial (No. 19 in fig. 4). The dial is numbered from 1 to 10O so tnat the
instrument can be calibrated in terms of equilibrium moisture content. The
calibration procedure is described later in this report.

All parts are numbered on figure 4. 2y correspondi.-g numbers, each
part is described in the ..arts list given in the appendix. All diension.s
necessary for assemrtly are shown in figure 4.

Location and Asseb!y

The hygrostat is mounted in a vertical position (fig. 4) to an inside
wall of the drying or conditioning room in such a manner that the air can
circulate freely around the wood element at all times, A hole is then
drilled through the kiln wall for the adjusting rod. This hole must be in
line with the cam shaft of the h1-yrostat.

A panel board containing a hole slightly larger than the adjusting
rod is fastened on the outside of the kiln with the hole in line with the
hygrostat cam shaft and the hole drilled through the kiln wall,

The dial is fastened to this panel board by: means of small screws,
so that the hole in the center of the dial is directly over the hole drilled
in the panel board. The zero markinr.g on the dial should be vertically above

Report 1:o. R1602


the adjusting rod, which runs through the holes provided and is attached to
the hygrostat cam shaft (No. 23 in fig. 4) by means of the adjusting rod
coupling (No. 24 in fig. 4).

In order to comply with the calibration procedure given in this
report, the cam should be placed on the cam shaft so that by turning the
adjusting knob to the left (counterclockwise) the wood element is railed.
The tip of the air nozzle should be level with the top of the pivot .nou.t-
ing (1To. 14 in fig. 4) so that the lever arm will be horizontal when in
contact with the tip of the air nozzle, thereby reducing the possibility of
air leakage. The pivot mounting should be securely fastened by a lock nut,
so that the lever arm is fixed directly over the air nozzle.


For convenience in calibrating this hygrostat, it is suggested that
the moisture content of the wood element first be brought to approxim-ttely
7 to 8 percent.

Turn the adjusting knob to the left (counterclockwise) until the
small projection on the high point of the cam strikes against the support
rod. The indicator can then be moved until it points to zero on the dial
and be securely fastened in this position by means of lock nuts. Set the
indicator at 30 on the dial. Enter the drying or conditioning chamber and
turn the projecting nut down or up so that the lever arz just clears the
air nozzle. Place a good hygrometer adjacent to the hygrostat and leave
the kiln.

The steam can now be turned on and the hygrostat will be calling for
steam spray. Maintain this setting until the steam-spray control valve has
opened and closed several times. Again enter the chamber and read the
temperatures on the hygrometer. If these readings result in an equilibrium
moisture content of 7.5 percent, as found in table 1, no further adjustment
is necessary. If the readings show an equilibrium moisture content lower
than 7.5 percent, the projecting nut should be turned down until the proper
condition is reached. If the readings show an equilibrium moisture content
greater than 7.5 percent, the projecting nut should be turned up, Give the
wood element plenty of time to reach equilibrium. As soon as the drying
condition is definitely at 7.5 percent equilibrium moisture content, the
projecting nut is securely locked in place with the lock nut. 3e careful
that it is not moved, as this initial setting, if properly made, will elimi-
nate later adjustments of the wood element.

From this point on, the dial settings given in table 2 can be em-
ployed. The data given in this table, however, are based on average values,
and consequently the dial settings shown are only reliable to within 1.5
percent equilibrium moisture content. To obtain the desired conditions,
the operator will have to make minor adjustments by means of the a&justing
knob, checking the dial readings against hygrometer readings. When the
desired condition is obtained however, the hy6rostat will accurately main-
tain this condition until another change is necessary,

Report iNo, R1602


By keeping an accurate record of the dial settings and resultant
equilibrium moisture content conditions, the operator will soon have suffi-
cient cdalta which will reduce, if not entirely eliminate, minor adjustments
in the dial settings of his particular instrument in order to obtain a
given equilibrium moisture content condition.

If the wood element must be replaced, the entire calibration pro-
cedure must be repeated and new dial settings obtained.


Tne indicator is set at a point on the indicator dial that will give
the desired equilibrium moisture content as determined from table 1. As
already pointed out, it may be necessary to adjust the indicator until the
wet- and dry-bulb temperatures show that the desired conditions have been
obtained. As soon as the relative huLility falls below that desired, the
wood element will begin to shrink and the projecting nut will move downward
against the lever arm, which is pivoted at the pivot mounting. This move-
ment will raise the lever arm clear of the air nozzle, thereby opening the
steam-spray control valve. As the relative humidity within the dry kiln or
conditioning room increases, the w..ood element begins to swell, raising the
projecting nut and allowing the lever arm to contact and close the air
nozzle and thereby the steam-spray control valve.

The relative humidity can be controlled by turning the adjusting
knob. The dial settings, with their approximate equilibrium moisture con-
tent values as determined by means of table 1, will assist in making changes
during drying operations.

This hy1rostat operates the same air mechanism described for the
original hy.,rostat. It can also be connected to a diaphragm motor that
operates ventilating dampers.

It has been found that the wood element will reach practical equi-
librium with the drying atmosphere in approximately 30 minutes for a 2
percent, and in approximately 1 hour for a 4 percent equilibrium moisture
content change.

This instrument can also be used to control the opening or closing
of electrically operated spray valves or vent daoper motors by employing a
sensitive electric switch (figs. 4 and 5,A). The switch is wired. to a motor
operating the control mechanism. The principle of operation is identical
to that of air operation. The counterweight (figs. 4 and 5,A) is necessary,
as the wood element and its attachKments are not Leavy enough to operate the

Operational Precautions

The wood element will not become distorted under normal dryin.1- con-
ditions. It should not, however, be subjected to extended periods of

Report No, R1602


uncontrolled steaming and should never be wrapped or covered with wet
cloths, as these treatments will cause distortion of the wood element and
subsequent binding of the support and connecting rods at the holes in the
top and bottom guide plates.

Under certain conditions, dirt may accumulate or corrosion may o"cur
on the support and connecting rods and in the holes through which they move.
In this event, they should be cleaned by fine steel wool, sandpaj er, or
emery cloth. Lubricating oils should never be used, as they will collect
dust which will eventually prevent free movement of the rods.

If an electrical hook-up is used and the counterweight (fig. 4) is
not heavy enough to trip the switch control, improper control will result.
Furthermore, this counterweight should not at any time be allowed to rest
upon the nuts on the frame rods located just above the base plate. If this
occurs, the support rod will not engage the cam and proper conditions cannot
1e obtained.

The instrument will accurately control drying conditions within a
1 F. wet-bulb depression once they have been established, between 3 and 19
percent equilibrium moisture content. Good control cannot be assured,
however, above 19 percent equilibrium moisture content.

Horizontal Placement of Hygrostat

The hygrostat controlling air-operated valves can be mounted in a
horizontal position and operated from the roof of the kiln by add in,, a
compression spring to hold the support rod firmly against the cam and by
changing the location of the pivot mounting and air-tube support. The com-
pressioa spring should be placed over the support rod between the bottom
guide plgte and a pin inserted through a 1/16-inch hole drilled in the rod
Just above the frame. A 1/4-inch hole should be drilled in the top plate
for the air tube support and pivot mounting, so that the lever arm when in
position will extend downward., The lever arm, however, should be slightly
less than vertical when in contact with the air nozzle. A small counter-
weight, such as a drop of solder, added to the lever arm near the air
nozzle will reduce the possibility of air leakage.

All that is required for horizontal placement and operation of the
hygrostat equipped to control electrically operated valves, is a compression
spring just sufficiently strong to hold the support rod firmly against the
cam while the shrinking wood element trips the control switch. This spring
is mounted in the same manner as described in the preceding paragraph.

Beport No. B1602


Parts List for Forest Products Laboratory Izproved HL;rostat

Part : Uuuiber : Part name Size and description1
17o. :required:
- - - --.: - - - - - - - - - - - - - - - - - - - - - - - - --- - - - -

1 :'Wood element

1 Cam

2 Coupling













1 :Air tube sup .ort

1 :Support rod

1 :Lever arm (air)

1 :Air nozzle

:5/32" x 1" x 10".

:1/16" thick. Minimum radius 1*1.
Maximum radius 1-3/4".

:5/16" br- 5/16" by 1", Slotted 5/'def-p
: for wood element and drilled for con-
: necting bolts on one end; tipped for
: attachin.F-" the suprPort and cirinn.cting
: rods orn the other end.

:1/4" diaxieter. Ary lengCth desired.
T:.rc';ded on one c-nd for adjrti.
: knob and for lock nuts on each side
: of ir.dicaitor.

:3/3" x 3/8" x 1-5/E: slotted on one
: end for attachment to to? late and
: drilled at other end for air line.

.1/4" x 3-1/4" threaded one end for
: coupling and slotted 1/4"'1 deep and
: 1/9" wide on other end.

:1/32" x 1/4" x 4" fitted with pivot
: pin attachment 11/16" from one end.

:Any t;,rpe with small air opening.

1 :Projecting nut (pro- :1/8" thick by 3/4" wide threaded for
: vided with lock nut): conr.ectinf- rod.

1 : Switch support

:Size to re deter.-ined by size of
: switch.

1 :Lever arm (electric) :1/4" x 1/4" x 3-3/8" channel with
: : pivot pin attach--ent on one end.

1 :Air line

:1/8" copper ti'in cr-:t to required
: length.

Report io. S1602

1 :Adjusting rod


Parts List for Forest Products Laboratory Improved Hygrostat

Payt : Number : 1:
Ne'0 requirede: Part name : Size and description-
--.- ,_ : -- - - - - .... -- -- -- --------

1 :Connecting rod

1 :Pivot mounting

1 : Frame

1 :Top plate















1 :Cam shaft collar

:1/4" x 4-1/4" threaded both ends.

:1/2"11 x 5/8" tapped for frame rod and
: slotted on one end for lever arm and
pivot pin. Pivot pin 1/16" diameter.

:1/8" x 2-3/8" x 11-3/4" bent as shown
: and drilled for base plate, cam
: shaft, frame rods, and support rod.

:1/8" x 2-3/8" x 6" drilled for frame
: rods, connecting rod, air tube
: support, and wall bracket.

:1/4" x 15-1/2" threaded approximately
: 2-1/2 inches on each end.

:1/8" x 1/2", screw head.

:1/16" x 3" numbered from 1 to 100.

:Any size or shape desired.

:1/8" x 1-1/8" x 2-3/8" drilled for
: frame rods and connecting and support
: rods.

;1/8" x 2-3/8" x 6" drilled for frame
and wall bracket.

:5/16" x 2-1/4".

:1/2" x 3/4". Threaded on cam shaft
: and fastened to adjusting rod with
: a lock screw.

:Any size allowing free movement of
: shaft; fastened to cam shaft with
: lock screw and riveted to cam.

:5/16" x 1-1/2" drilled to fit cam
shaft and attached to it by a lock

Report No. R1602

2 :Frame rods

2 :Connecting bolts

1 :Indicator dial

1 :Indicator

2 :Top and bottom guide

1 :Base plate

1 :Cam shaft

1 :Adjusting rod

1 :Cam collar
1 : Cam collar

Parts List for Forest Products Laboratory Improved Hgrostat

Part : "umber : 1
rt :reubed: Part name Size and description-
lno. :required:
: :

27 1 :Adjusting knob :1/4" x i"; threaded on adjusting rod.

28 1 :Counterweight for :Of a size to insure free movement
electric hook-up : between frame rods and heavy enough
: to trip the control switch.

-All parts are of brass unless otherwise specified.
-Can be of spruce, birch, basswood, or maple, l-aple is preferred. Consists
of a transverse section cut fro,;. a flat-sawn 1-inch board, 10 inches in
width. Thickness of 5/32 inch is recont.ended.

ReLUort -Io. P1602



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llativei huIidity valuee in rtmin type.

%qailibriau moisture content values in italic type.

Table 1.--Relative humidity1 and equilibrium moisture content2 table for use with dry-bulb
temperatures and wet-bulb depressions
Z M 66e8o p


Table 2.-Dial settins with corresponding
esquiibrium noisture content
values for proved Forest
Products Laboratory hygrostat-

: Equilibrium moisture
Dial setting : content of wood2


20 5
24 6
28 7
32 8
36 9
40 10
44 11
47 12
51 13
54 14
58 15
61 16
64 17
67 18
70 19

-A maple wood element was used.
2Average values within 5 percent.
"Average values within A1.5 percent.

Report oc. R1602

- .

4 J /--...:..



~, !~


Figure 1.--Early type of F.P.L. hygrostat showing curved wood element and
method of connecting hygrostat to auxiliary air mechanism and air-
operated diaphragm valve. Parts as labeled are: a, lever arm;
b, pivot; c, linkage; d, air nozzle; e, bellows; f, copper tube; g, pin;
h, auxiliary air mechanism; 1, fired block; 1, adjustable block;
k, adjusting knob; m, adjusting rod; n, control valve.
Z M 66676 r

(Am:- &I

SN 4U .- J "

vo*- CQOOC7O0

11 1



"l I1 w
I 11


""'"n ?(/ T "-<,' K

f f -. z C c c 7%-?

Figure 2.--lmpronved F.P.L. hv rostat
to be used with electrically oper-
ated control valves and free-
floating wood element.

AIR j:'.L2

Figure 3.--Improved F.P.L. hvgrostat
with free-float ing wood element, to
be used with air-operated control

Z X' w.Bf77 F

F ~






'-I I

L .....J

Figure 4.--Detail drawinig and method of mounting improved F.P.L. free-floating wood-element
hygrostat, with necessary dimensions and part numbers for construction purposes. Parts corre-
spond by number to those described in parts list in appendix.

Z M 66678 F




4* ft


Figure 5.--A, Free-floatinrig wood element ,\I-gr,.stat to be used with elec-
trically operated control valves. B, similar h' :rostat ,used with
air-operated control valves.
Z H 565e9 F



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