• A WCCD-EIEMENT HyGRCSTAT January 1946 Nc. PI €02 U.S. DEPOSITORY UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison 5, Wisconsin In Cooperation with the University of Wisconsin A VOOD-ZLEMENT KYJRCSTAT By SEYMOUR J. J0&N80N, Laboratory Aid and E. F. RASM'SSEII, Engineer Introduction Several types of wood-element hygrostats-- — have beer, .^ade and used by the Forest Products Laboratory. All have demonstrated their usefulness in maintaining accurate control of relative humidity in dry kilns and con- ditioning rooms, particularly when conditions of constant equilibrium mois- ture content are desired. All operate on the same basic principle, the inherent tendency of wood to absorb or lose moisture with changes in rela- tive humidity, which causes the controlling wood element to shrink or swell, In this article is presented information on the functioning of these hygrostats; their operational faults, and corrective measures used in over- coming them; and details of their construction and u-ethods of calibrating. Early Type of Hygrostat Operation An early type of Forest Products Laboratory hygrostat controls hu- midity conditions by means of a wood element installed with a slight curva- ture (fig. l). One end of the wood element is fastened to a fixed block "i," the other end to an adjustable block "j." As the humidity increases, the wood element absorbs moisture and begins to swell. 3ecause it is securely fastened at both ends, its degree of curvature increases as it swells. This causes the lever arm "a," pivoted at "b" and connected to the wood element by linkage "c," to ;..ove away from the air nozzle "d, ! ' h is connected to a bellows "e" by means of a s.:.all copper tube :, f." Zhe apt ing of the air nozzle "d" releases the air pressure in the bellows "e," 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 Element Hygroetat," by W, Earl Loughborough and R. ), Rietz. IHSTHDHBJTS, June 1932. p -"Moisture Fluctuations in Lumber Within Closed Storage Sheds Controlled with Electrical Equipment," Forest Products Laboratory Report -Co. R1140. Report Ho. R1602 -1- air mechanism "h," shutting off tho air supply to the air-operated diaphragm valve "n" which controls the flow of steam 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 "d" by the lever arm "a." This builds up pressure in bellows "g," expanding it and pushing pin "g" with its attached flanges clear of the air port in the auxiliary air mechanism "h. " Air pressure on the diaphragm of the air-operated valve overcomes the resistance offered by the compres- sion snring 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 tho sliding block n .; it j • 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 oausing 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 v/ill 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 humidity conditions can be rapidly attained. 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 No. R1602 -?,- Improved hygrostats Design 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 snows the design with electrical contact points while figure 3 shov/s an arrangement suitable for air control. Although quite suitable for interior drying compartments, this hygrostat 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 Liade by 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 (No, 2 In fig. 4) upon which the su£)port rod rests. By turning the control rod, which is attached to the cam snaft and extends outside the drying or conditioning room, the wood element can be raised or lowered to any desired setting on the indicator dial (Jo. 19 in fig. 4). The dial is numbered from 1 to IOC so that 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. By corresponding numbers, each part is described in the parts list given in the appendix. All dimensions necessary for assembly are shown in figure 4. Location and Assembly The hygrostat is mounted in a vertical position (fig. 4) to .:. i..side 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 hygrostat. 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 marking on the dial should be vertically above Report No. R1602 -3- the adjusting rod, which runs through the holes provided and is attached to the hygrostat cam shaft (Mo. 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 raised. The tip of the air nozzle should be level with the top of the pivot mount- ing (No, 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. Calibration For convenience in calibrating this hygrostat, it is suggested that the moisture content of the wood element first be brought to approximately 7 to 8 percent. Turn the adjusting knob to the left (counterclockwise) until the small orojection 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 arm 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 adjusting knob, checking the dial readings against hygrometer readings. When the desired condition is obtained however, the hygrostat will accurately main- tain this condition until another change is necessary. Report No, R1602 -4- By keeping an accurate record of the dial settings and resultant equilibrium moisture content conditions, the operator will soon have suffi- cient data v/hich 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. Operation The 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 nave been obtained. As soon as the relative humidity falls below that desired, the wood element will begin to shrink and the projecting nut will move downward against the lever arm, v/hich is pivoted at the pivot mounting. This move- ment will raise the lever arm clear of the air nozzle, t'r.erebv opening the steam-spray control valve. As the relative humidity within the dry kiln or conditioning room increases, the v: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 hygrostat 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 damper 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 attachments are not heavy enough to operate the switch. Operational Precautions The wood element will not become distorted under normal drying con- ditions. It should not, however, be subjected to extended periods of Report No. 10.602 -5- 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 occur 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, sandpaper, or emery cloth. Lubricating oils should never be used, as they v/ill 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 >e obtained. The instrument will accurately control drying conditions within a 1° ?. 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 Kygrostat The hygrostat controlling air-operated valves can be mounted in a horizontal position and operated from the roof of the kiln by adding 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- pression spring should be placed over the support rod between the bottom guide plate and a pin inserted through a l/ 16-inch hole drilled in the rod just above the frame. A l/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 Ho. B1602 -6- APPENDIX Parts List for Forest Products Laboratory Improved Hygroatat Part No. rminoer required Part name Size and description- 8 9 10 11 12 Wood element Can Coupling Adjusting rod Air tube support Support rod Lever arm (air) Air nozzle Projecting nut (pro- vided with lock nut) Switch support Lever arm (electric) Air line 5/32" x 1" x 10".- l/lc" thick. Minimum radius 1". Maximum radius 1-3/4". 5/16" by 5/16" by 1". Slotted 5/ 3 ' d- for wood element and drilled for con- necting bolts on one end; tapped for attaching the support and connecting rods on the other end. l/'r" diameter. Any length desired. Threaded on one end for adj knob and for lock nuts on each side of indicator. 3/8" x 3/8" x 1-5/8" slotted on c. end for attachment to top plate and drilled at other end for air line. rl/4" x 3-1/4" threaded one end for coupling and slotted l/4" deep and l/8" wide on other end. 1/32" x 1/4" >: 4" fitted with pivot pin attachment ll/l6" from one end. Any type with small air opening. 1/8" thick by 3/4" vide threaded for connecting rod. Size to be determined by size of switch. 1/4" x 1/4" x 3-3/6" channel with pivot pin attachment or. or.e end, :!/=" copper tubing cut to required length. Report ho. R1602 -7- Parts List for Forest Products Laboratory Improved Hygrosta t (continued) Part No. Number required Part name Size and description - 13 1 [Connecting rod 1/4" x 4-1/4" threaded both ends. 14 1 Pivot mounting l/2" x 5/8" tapped for frame rod and ! slotted on one end for lever arm and . pivot pin. Pivot pin l/l6" diameter. 15 1 : Frame : 1/8" x 2-3/8" x 11-3/4" bent as shown and drilled for base plate, cam shaft, frame rods, and supijort rod. 16 1 Top plate •1/8" x 2-3/8" x 6" drilled for frame ' rods, connecting rod, air tube support, and wall bracket. 17 2 : Frame rods ! 1/4" x 15-1/2" threaded approximately 2-1/2 inches on each end. 19 2 Connecting bolts 1/8" x 1/2", screw head. 19 : 1 ! Indicator dial 1/16" x 3" numbered from 1 to 100. 20 1 Indicator Any size or shape desired. 21 2 Top and bottom guide i 1/8" x 1-1/8" x 2-3/8" drilled for plates frame rods and connecting and support rods. 22 1 : Base plate ;l/8" x 2-3/3" x 6" drilled for frame ' and wall bracket. 23 1 : Cam shaft •5/16" x 2-1/4". 24 1 .Adjusting rod 1/2" x 3/<±". Threaded on cam shaft coupling and fastened to adjusting rod with a lock screw. 25 1 .Cam collar .Any size allowing free movement of : shaft; fastened to cam shaft with • lock screw and riveted to cam. 26 ! 1 :Cam shaft collar :5/l6" x 1-1/2" drilled to fit cam : shaft and attached to it by a lock : screw. Report No. R1602 -8- Fart6 List for F o rest Products Laboratory Improved Hygrostat (continued) Fart V.o. l.'uinber required Fart name | Size and description- 27 28 : 1 1 > Adjusting knob Counterweight for electric hook-up l/<±" x 1"; threaded on adjusting rod. Of a size to insure free movement between frame rods and heavy enough to trip the control switch. "All parts are of brass unless otherwise specified. 2 ^an be of spruce, birch, basswood, or maple. 1-iaple is preferred. Consists of a transverse section cut from a flat-sawn 1-inch board, 10 inches in width. Thickness of 5/32 inch is recommended. Reuort Ho. R1602 -9- g 1 1 a m £ a : d *A~* c CO 3 ■ ; i ft S, ! tn i 8i R3 « - - C II d ■ ■ nci i". : en - o , tn in \$i on si!* n p i r tn 9 f r - d i oo - ci *1rn f an en V- I r-; ". i ?T> !TE Tg" - 0] ffl (So o r> ki ft 3 tn |fn T *r ■*> ■a o 1 n- : r ■'»' 8 d C ip c ri 5 en 6 V cn i? 1 1 "S ! «! i ?"? 1 : 1 j O ( i»S » 8 r . jO S hT P Of ■ a on ^ -■ i : fl «i 1 /' 1 i ^ ! S . ; 3 r ' ; n " *•■ 1 ' : » t- >5 89 - d a ri 4 t- S 3 ?! n- !S '■ ■ in in igl *«; •S ! a;, t IJ! 3: J 1 5" 8 c- Q - r- -j CO n h on ~ * s s s in - 1 ! n ^ . i S* c in up- . a 1 ; i J ! 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S 'r -" B s s s ; s- 1 ! * S S ■j HttlYJKHII O C3 99 c -r c io £ £ g g g s £ s « g Lfi O O \£ o CM fi D So o 2 8 g Q a: o CO = I > I. 0) w 3 t- o c~ a> -H ^3 ct) ■P Oil *J c CD ■»-> C o o 0) (- ■p (0 W •H c o c R T-l (fl c CO = Hi ■H u t_ p. £i 0) •H •c -H •H A 3 H cr 3 a> A ■d 4J CJ - C -P «J ■H •o Ifl •H 0) F h 3 £ P aj 0) h > CD ■H a •P E etj V H ■P 4! cc b. 1 O CD ■J Table 2. — Dial settings with corresponding eauilibriuc .joisture content value s for improved Torest Products Laboratory hygrostat— ; Equilibrium moisture Dial setting content of wood- : Percent 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. —Average values within *1.5 percent, Report To. R1602 ^ k m 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; i, fired block; J_, adjustable block; k, adjusting knob; m, adjusting rod; n, control valve. 2 M 66676 F CONTACT POIM *M ( WOUl TO* ADMS1IHC HUTS L LCVt* A*** 1/ WOOD CLCUCHT » 1 * irijir .^* pivot noum >nq I - AOJUSTIMC HUT AlA Tuat St»WT •ooo clcixnt (ft ft ■..,)' II ... " I lH rHi ELECTRIC CONTROL M 1 1 ;< n ¥ U V /J/P CONTROL Figure 2. — Improved F.P.L. hygrostat to be used with electrically oper- ated control valves and free- floating wood element. Figure 3. --Improved F.P.L. hygrostat with free-floating wood element, to be used with air-operated control valves . z y ^a?? f !fc . c T?Ft- ^\\\\\\\\\\\SX ^W^\\\\|,\y\\\\\\\\\\\\\\'7W^^A\\\\\\\V\\\\\\\\U^\_\_\U|\ j \\^ V ^S£*^ A T i 0> h u •P u c 0) (0 -I «3 OJ ft. I -o o o * X n w o c p. u a pi e o •H u — h •♦J to B c fc. u o 4) > C c 0) p. & CO a3 t- 0) C J3 iH •P | P C (0 C *J — I •H t. «j d ill C C- -P C 72 E C O o — +J 0) E ■d c «j [0 -H c O T3 •H 0) 10 .O C "H 01 B o Ifl 0) *o 0J 10 tO O 10 £ V -p o 0) C C -P -i x: t- H 4) 4 aj -rt 43 - * = a3 S* ■P O to o -c t. c &c o fc. & ■C to Figure 5.--A, Free-floating wood element hygrostat to be used with elec- trically operated control valves. B, similar hvgrostat used with air-operated control valves. Z H 65679 F UNIVERSITY OF FLORIDA 3 1262 08924 2985