llarEh 19/ J ET-235 ue:?ary STATE PLANT BOAJ^^®^ states Lepartment of Agrioulture Agricultural Research Administration Bureau of Entomology and Plant Quarantine INEXPENSIVE REPLACEMENT ELEMENTS FOR LABORATORY riOT PLATES By Harold W. Rusk Division of Insecticide Investigations The hot plates generally available from supply houses do not fill the unusual needs of some laboratories, being limited either by physical sire or temperature range. Inexpensive replacement elements, which are available in 600-and lOOO-watt sizes, may be used as versatile heating units for custom-designed hot plates. These elements consist ol' two equal arms wound with ribbon re- sistance wire over a mica core, enclosed in sheets of mica and connected to form a waferlike unit with leads at the end of each arm coil and at the common center. At low heat both arms are connected in series, at medium heat only one arm is across the line, the other arm being dead (^this causes uneven surface temper- atures), and at high heat both arms are across the line in paral- lel. A hot plate that would accommodate six 500-ml. Erlenmeyer flasks, and produce a temperature sufficient to reflux 50- to 100- ml. samples of benzene solutions was needed. Since no hot plate of proper size and heat range could be found in the catalogs, it was decided to construct a unit for this purpose. The framework of the hot plate consisted of 20-gage sheet metal for the front, back, and sides, 1-by 1-inch angle iron for corner posts, and 3/4- by 2/4-inr.h angle iron for the heater sup- port rails. The support rail5^ were riveted on the inside of the front and back pieces about l/2 inch below the top. The heating unit was constructed from a piece of cold-rolled steel 3/16 by 5 by 35 inches for the top, three 5-inch, 5-heat replacement ele- ments for the heat source, and a base or backing plate of Transite (an asbestos-cement insulating board) of the same dimensions as the top. Each element was separated oy cutting the mica separator strip at the end havin,? the two leads. The mica at the center- tapped end was tnen cut and the lead cut off, with care not to cut zhe wire connecting the two arms. Each elemert was spread out and the distance between the two arms adjusted to about 1 inch by pull.n , the connectinf; wire slightly. After the three elements had been treated as abov'e, they were then placed in position on the Transite base, the leaos were puiltd through their holes as shown in figures 1 and 2, ard all j-paces not occupiec by the ele- irients were filled with asbestos paper thick enough to keep the elemtnts in place and prevent buckling. The resistance wire con- necting the orms of each element was insulated vvith asbestos paper and tne entire heating urit placed in position on the support -2- rails and drawn up tightly with the stove bolts, as shov/n in figure 2, after i^ich the sides and corner posts were bolted on, switches and terminal block secured in place, ana the wiring done according to the diagram in figure 3. r.s wired the hot plate has an output of 450 watts with a current consumption of 3.y amperes. The design of this hot plate permits using each element individually through its respective switch, however, many vari- ations are possible. If a higher plate temperature is desired, a few turns may be removed from each arm of each element, or, conversely, if a lower temperature is desirable, fixed resistors may be placed in series with each element. A suitable variaole resistor coalc also be placed in the main lead to provide a wide range of temperatures . By using 4-position rotary switches and utilizing the center-tapped leads, the hot plate can be changed to a 3-heat unit similar to the usual laboratory hot plate, however, consideration must be given to the current consumption of such an arrangement, since the three elements on high heat ^800 watts) would draw a current of about 16 amperes. Variation of the described design proved practical in the construction of a 2-heat unit, 10 \fz by 13 inches, made of materials similar to those in the 4£0-watt unit. Two heating elements, spread apart and placed one behind the other to give uniform distribution of heat over the v/orking surface, were employed in this hot plate and controlled by a double-pole, double-throw icnife switch to obtain two operating heats (300 and 1200 watts;. In the first, or low, heat the two arms of each element are in series and the two elements in parallel, while at high heat each arm of each element is in parallel. This hot plate has proved satisfactory for general laboratory use such as reducing the volume of organic solvents or boiling down aqueous solutions. Its total cost is about $10. -3- MATEfilALS NEEDED I-OR A 450-WAn' PLATE Heating Unit: Top— 1 piece z/l6 x 5 x 36 inches, steel plate, drilled and OQuntereunk to take 3/l6-inch stove bolts Elements— 3 replacement units for 6- inch, S^heat stove, 600<-watt Base — 1 piece Transite, 3/I6 x 5 x 36 inches* drilled as in figure 1 P'rama: Sheet-metal, 20-gage — 2 pieces 6 x 36 inches (front and back) 2 pieces 5x6 inches (sides) Angle iron, 1x1 inch— 4 pieces 1x6 inches (comer posts) 3/4 X 3/4 inch — 2 pieces 3/4 x 35 1/2 inches (heater unit supports) Switches~3 recessed snap switches Appliance cord-— 6-ft, asbestos-covered cord with receptacle plug Terminal block — 1 piece Transite 2x3 inches 2 small metal angles Bolts, nuts, washers, and rivets as needed ^s^N^V^ ^p^ ? l^ V9 o ••*n T- o 1 O o ) Q 1 G 1 O ( O v^ o It ) ^^^ o • n •i o 1 o mm^ . ,' > • -t> ^ UNIVERSITY OF FLORIDA Il'"!»f''l1f'''f1l''lllllillllll 3 1262 09240 3822 ^ (0 ® H -P Q •H Si (a ^ i CO o o 0) §« « o • o ^ o C 4J O -H 5,°