RARY state; irLAt^^T BOA^O ET-162 .:.::.; September 1940 United States Department of Agriculture Bureau of Entomology and Plant Quarantine CONSTRUCTION OF A HAIR-HYGROSTAT WITH MAGNETIC SWITCH FOR HUMIDITY CONTROL IN AN INCUBATOR ROOM By K. D. Arbuthnot, Division of Cereal and Forage Insect Investigations Introduction A hair-hygrostat capable of operating for long periods of time without attention either for adjustment or maintenance is described herein. The construction is simple and inexpensive and can be done with a few coir.mon tools usually found in a laboratory or work shop. Detailed descriptions and instructions for fabrica- tion of the instrument are presented in the following pages, Hair-hygrostats which employ exposed electrical contact points are uncertain in operation, owing to oxidation and sticking of the contact points. In addition to this uncertainty of opera- tion there is a hazard to humidifying equipment, such as bearings, valves, and motors, due to excessive operation when the hygrostat fails to operate properly. This hair-hygrostat has a hermetically sealed mercury contact switch which is not affected by dust, dirt, or corrosion. The switch is operated by a magnet mounted outside the glass tube con- taining the switch. t This hair-hygrostat has been in constant operation in an incubator room at the New Haven, Conn., field station for a period of 5 months, during which time its reliability has been demonstrated. Humidity has been maintained within a range of 2 percent, from 92 to 94 percent relative humidity (hygrograph record), in a room 13 feet long, 8 feet wide, and 7 feet high, utilized in conducting genetic studies of the European corn borer. Drops in humidity of greater magnitude (below 92 percent relative humidity) sometimes occur when the incubator-room entrance door is opened at too fre- quent intervals. However, the humidity conditions are again regu- lated without attention to the instrument when the room is closed. - 2 - Material Required for Construction The following materials were used in constructing the hygrostat; Item Amount Use Specific essential parts : Hermetically sealed mercury contact switch with supporting clamp, having electrode and armature actuated by a per- manent magnet A permanent magnet and mount- ing arm designed to operate the mercury switch Stabilizer and guide with stop head for magnet mounting Nonspecific parts : Presswood board Enamel paint Small brass cabinet hinge Brass post with threaded hole in basal end and slotted horizontal hole near top Music-wire spiral spring Round-head machine screws 1/16" x 3/4" 1/16" X li" Wire nails i" Strand of human hair 1 unit Hygrostat switch 1 unit 1 unit Switch operator Switch operator guide •g-" X 11" X 18" Nonconducting base i pint 1 2 1" long and back Finish for base and back Hinge for magnet arm Stationary mounting for hair and spring adjusters Differential adjuster Fastening parts to 6 base and back 2 Hair and spring adjuster 2 dozen Fastening presswood together to form base and back 50 hairs Moisture-sensitive 9" long unit to operate switch - 3 - Description of Specific Essential Parts The hermetically sealed mercury contact switch is one which reirains in a fixed position and operates by magnetic attraction through the glass wall of the switch tube. Specifications are: Electrical capacity 9/10 amperes, 24 volts; maximuffi capacity 1/60 horsepower. The perR;anent icagnet should be one designed to operate the sv/itch which is used. The iragnet is mounted on an arm to be at- tached to the actuating mechanism of a thermostat and can be easily attached to a hinge for use in this hygrostat. The stabilizer and guide with stop head for iiagnet are made to fit on the upper end of the magnet arm and maintain the magnet in its proper position in relation to the switch. Method of Construction The construction will be clarified by reference to figures 1 and 2, v/herein the various parts discussed are labeled. The base of the hygrostat, which is 2f inches by 13-| inches, and the back, 3-^ inches by of inches, are constructed by nailing together 3 layers of presswood board and nailing the back to the base as illustrated. After the necessary holes for machine bolts are made, as indicated below, the base and back are painted with several coats of black enamel. This provides an inexpensive, rigid, insulated frame for supporting the operating parts. The mercury switch is a type comm.orily used with a bimetallic coil in commercial heating plants for dv/ellings. The essential features for the present purpose are the hermetically sealed mercury contact points and the magnetic actuating device. A clamp for sup- porting the switch (glass tube) is attached by machine screws to the back so that the switch is supported at the proper angle for operation. A wire, 1/32 inch by f inch, with ends curved to serve as hooks for attaching the hair and spring, is soldered into a hole f inch from the lower end of the magnet arm. The tagnet arm is then soldered to one side of a hinge. The proper position of the magnet and its supporting arm for operating the switch is detei^mined by holding the hinge on the base with the hand. When the proper position has been determined, the hinge is fastened to the base with a machine screw. The stabilizer, guide, and stop head consist of a single unit as used in a therm^ostat employing this type of switch. This unit is placed in the proper position in relation to the magnet and attached to the back by a machine screw. The con- necting wires from the switch are fastened with machine screws to the face of the back. These screws project through the back and serve as binding posts for wires to the relay transformer which actuates the humidifying unit. - 4 - The two brass posts are 1-g- inches long and f inch in diameter, with a hole in the basal end threaded to receive a 1/16 inch by f inch machine bolt and with a slotted hole, i.e., straight on one side, to receive a 1/16 inch bolt. They are attached with machine screv/s near the ends of the base and in line with the hinge attach- ment. The two long machine screws are flattened so that they can slide through but not rotate in the slots at the tops of these posts. They are provided with hooked ends for attachment of the hair and spring. Tv/o nuts on each of these screws hold the screws at desired distances through the slots and serve as spring and hair adjusters. The spring and hair are attached to their respective adjusters and to the hooks near the base of the magnet arm. The strand of hair consists of 50 human hairs, treated vdth ether to remove all fats and dirt, looped at the ends, and fastened v/ith household cement to form eyes for attachment. Operation and Adjustment As humidity in the room decreases, the strand of hair dries. This causes the hair to contract, pulling the arm and magnet toward the moveable electrode (armature) of the switch. The magnetic field of the magnet pulls the moveable electrode toward the magnet, and the mercury on this electrode contacts with the stationary electrode. thus completing the circuit to the relay. The relay then closes the circuit which operates the huff.idifier. As the humidity in the room increases, the strand of hair becomes moist. This causes the hair to expand, and the spring palis the arm and magnet away from the moveable electrode. When the magnet has moved away from this electrode until its magnetic field is weaker than the pull of gravity on the electrode, the electrode drops back to its original position and breaks the circuit to the relay, thus stopping the humidifier. The hygrostat is set by simultaneously adjusting the spring tension, which controls the differential within which the instru- ment operates, and the hair adjustment, which controls the set of the instrument until the desired percentage of humidity and dif- ferential are obtained. Utility of the Instrument The hermetically sealed switch has a long life under the most strenuous usage. The instrument has a wide range of possible adjustments, since the spring tension and hair balance for various ranges and settings are practically unlimited. By reversing the relay switch and connecting a drier (any dehumidifier in which air is circulated through an apparatus designed to remove moisture from the air, with the air-circulating device actuated by the hygrostat, can be adapted for this purpose), the same hygrostat can be used for de- humidification where this type of control is desired. Digitized by the Internet Archive in 2013 http://archive.org/details/constructionofhaOOunit uy» o a. CL D CO H LJ Z Q Z < I (O o I- u z o < o (O J g uu u cr u _IO QC < Q (flop ? O o2 t <-> O U QC < U 2 iiiS fe -1 u ' ' "T~" >• D O 2 u < CD C\J O UNIVERSITY OF FLORIDA 3 1262 09239 5218