Laboratory Apparatus for Producing a Controlled Temperature Program WM. B. WARREN,Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa.
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IC'CONNECTION with studies of the influence of the rate
of heating in the carbonization of coal, a control system has been developed to procure definite, predetermined, and reproducible rates of temperature rise in an electric furnace, and to provide a record of the actual temperature a t all times. The design of the apparatus is relatively simple, permitting it to be constructed from readily available parts, and the control mechanism has great flexibility so that it may be used for many purposes. I n the particular application of the control apparatus to be described, its performance has been satisfactory during 2 years of operation. The temperature of a furnace, drawing as high as 2250 watts a t 110 volts, has been raised from room temperature to 1000" C. in 3,6,12, and 24 hours, respectively, at constant rate throughout, and the temperature subsequently maintained constant a t 1000" C. for 1 hour. The recorded temperature never departed from the predetermined temperature a t any instant by more than 10" C. Since the retort in the furnace has a fairly high heat capacity, the variation of the retort temperature itself was somewhat less. A diagrammatic sketch of the apparatus used is shown in Figure 1. The furnace temperature is measured by a thermocouple and recorded on the chart of a standard recording potentiometer. The controller is built onto the recorder as an integral part and consists essentially of a mandril, a control chart, two brushes, and a stylus. The mandril is identical with that which carries the chart under the recording pen and is supported so that both mandrils are rotated by the recorder-driving mechanism. The control chart is carried by this mandril and is constructed from a portion of the recorder chart by first drawing upon it the desired time-temperature curve and then cementing onto the paper strips of aluminum foil in such manner that the time-temperature curve lies in a slit 0.06 inch (0.16 em.) wide between strips of foil. (Artists' rubber cement has proved a very convenient adhesive, as it does not wrinkle the paper. Good electrical contact was obtained between two foil strips cemented together with it, in spite of the rubber.) T o each of these strips is connected a second strip which is 1 inch (2.5 cm.) wide, laid along the adjacent edge of the paper parallel to the time axis. The brushes are supported above and make contact with the control chart through the marginal strips. The stylus is also supported above the control chart and is moved parallel to the temperature axis by the same mechanism which moves the recording pen, so that it will lie in the slit or on one or the other of the foil strips, depending upon the temperature and the time. Two relays, one connected to each brush and to a source of potential through the stylus, complete the set-up. I n use the control chart is placed upon its mandril a t zero time reading, the brushes and stylus are lowered upon it, power is turned on to the furnace, and the recorder motor is started. The recorder pen and the stylus moving with it then take up a position which indicates the temperature. If this is the proper temperature for this instant of, the program, the stylus will lie on the slit between the foil strips and will not make contact with either; thus, neither relay is actuated and a medium value of current is provided by the shunt around resibtance Ra and part of R2. If the tem-
perature continues to rise according to the desired program, the stylus will move so as always to lie on the slit as the chart is moved forward. However, if a t any given time the temperature is too low, the stylus will make contact with the left-hand or low-temperature foil, completing the circuit through the battery to relay 1, and thereby causing an increase in current, since the shunt around Rs, Rz,and part of R1 is completed through closing CI. Likewise, if the temperature is too high, contact will be established between the stylus and the right-hand or high-temperature foil, completing the circuit through the battery to relay 2, opening Cz while C1 remains open and thus providing a decrease in current, since both shunts are open and the-only path for current is through Ra, R2, and part of R1. Therefore, the system operates so as automatically and continuously to choose one of three graded values of current according to the instantaneous requirements of the program. 11%
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FIGURE1. DIAGRAM OF APPARATUS I n practice the low value of current is chosen, through adjustment of Rs, such that the required rate cannot be maintained even at lowest temperature, while the high value is chosen, through adjustment of R1, so that the rate of rise will be too rapid even a t highest temperature. The medium value is then set half way between by adjustment of R2. I n the case of the author's furnace of 6-ohm resistance, 5, 10, and 15 amperes were the values used for a program of 6 hours to 1000" C. This use of three different controlling conditions is particularly valuable where the range of temperature over which the controller operates is great, since thereby "hunting" is considerably reduced. In case of a narrower range of temperature it is possible that the edge of a single foil strip and one relay, hence two controlling conditions, would suffice. As a matter of fact, it is observed that during the early part of a program, the system alternates between the low and medium currents and toward the end between the medium and high currents, while during an intermediate range of temperature three values of current are used. I n the above paragraphs an apparatus has been described
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ANALYTICAL EDITION
for controlling the rate of heating of an electric furnace over the temperature interval from room temperature to 1000" C. However, the control system is not limited to this use but may be adapted to procure any desired temperature program or to control the rate of heating or cooling in metallurgical and chemical problems, wherever the method of adding or removing energy can be controlled by operation of electrical
Vol. 5, No. 4
relays. While the description of the apparatus and its use has been limited to programs involving temperature as a function of time, it appears evident that only slight modifications would be necessary to use the same system for controlling other variables as a function of time. RECEIVED March 29, 1933.
A Microextractor LESLIETITUSAND V. W. MELOCHE,Chemical Laboratory, University of Wisconsin, Madison, Wis.
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N CONJUNCTION with the biological and chemical study of the inland lakes of Wisconsin conducted by the Geological and Natural History Survey under the direction of E. A. Birge and C. Juday, it was found desirable to determine the ether extract of the lake water residues prepared by the evaporation of known volumes of water. Since the water of a large number of the lakes was soft, it was not convenient t o prepare large residues, for this in turn would require the evaporation of large quantities of water. It was therefore necessary t o select an extraction method which was applicable to small samples. The macro-Soxhlet could not be used and early study was directed toward the use of a small-size Soxhlet extractor, as shown in Figure 1. The siphon C measures 4 cm. from the seal to its top and functions when it contains 8 cc. The extraction thimble was made by sealing a porous alundum disk into a section of glass tubing, X. A 15-mg. dried sample of lake residue was transferred to the thimble, 15 cc. of dry redistilled ether were placed in the flask, and the ether was refluxed over the sample for 24 hours. The flask Y was then placed over fused calcium chloride in a vacuum desiccator and the ether removed by evaporation. After the flask and extract had dried in this system for 12 hours, the flask was allowed to stand in the balance for 15 minutes and weighed. The increase in weight of the flask represented the weight of the extract. Although this system proved satisfactory, particularly for samples containing a reasonable amount of extractive material, certain difficulties appeared which made an extension of the study imperative. The purpose of the present paper is to describe a new microapparatus and procedure which developed from a study of these difficulties. NEW MICROAPPARATUS The condenser and lower part of the extractor shown in Figure 2 are of Pyrex glass. The lower part, AS,was blown from a standard Eck and Krebs 40-mm. joint, the outer member of the joint, R, sealed to the condenser, and the tube of the inner member A sealed to form a cup. The extraction thimble T is made from a 16-mm. Pyrex tube. The inner part of the joint shown in the diagram is made by grinding the end of a short section of 15-mm. tube to fit inside the end of the longer 16-mm. tube. Hooks are sealed on the top of the thimble to support it on the aluminum ring E. An 18-mm. disk of filter paper is cut from larger sections of Whatman Yo. 50 or S.and S. No. 575. The disk is wet with water, laid over the ground end of the outer tube of the thimble, and forced into place by firmly pressing the inner ring into the ground end of the larger tube, thereby closing the bottom of the larger tube with paper. I t is important that the surfaces of the joint be very carefully
ground so that when the paper is in place finely divided particles of sample cannot leak through the joint. The weighing bottle D was blown from soft glass and has a total weight of 4 grams. When the bottle is in position in the extractor, the clearance between the bottle and the thimble is at least 1 mm. The outer joint of the extractor is dry and must be ethertight in order that extraction may proceed a t reduced pressure without the loss of ether. In earlier models, a mercury seal was provided to prevent loss of ether a t the joint, but this has since been eliminated. In order to prevent condensation of ether a t the joint, it is necessary to provide a moderate amount of heat. This is accomplished by passing one strand of No. 26 nichrome wire around the top of the joint a t P , fastening it by means of adhesive tape, and maintaining a temperature of about 50" C. by making an alternating current connection and adjusting the resistance in the line (contribution by Walter Militzer, limnology assistant). The adhesive tape must cover the wire in order to keep the joint warm. SUCTION. An ordinary water pump is used to reduce the pressure in the apparatus. The connection is made from the pump to an expansion flask, to the manometer, and finally to the extractor. A plug of cotton is placed in the outlet of the extractor so that dirt cannot be drawn into the extractor when the stopcock is opened a t the end of the extraction procedure. Connection may be so arranged that the one pump and manometer will serve several extractors. (The authors use a battery of six extractors.) HEATING. The extractor is heated by mean8 of a 40-watt lamp placed in a small box which is covered with an asbestos board. The top of the box has a 3-inch (8-cm.) opening which makes it possible to place the extractor close enough to the lamp to provide the proper rate of distillation. ETHER. Ethyl ether was allowed to stand over fused calcium chloride for 2 weeks and finally distilled. Metallic sodium was added to the distillate and the dry ether was then distilled as needed. The fraction distilling between 34" and 36" C. was used in the extractor. Thirty cubic centimeters of the freshly distilled ether gave no detectable residue upon evaporation.
PROCEDURE Before beginning a determination of ether extract, the entire apparatus is cleaned thoroughly with "cleaning solution" and finally rinsed with water, alcohol, and ether in the order named. The apparatus, with the exception of the thimble T and weighing dish D, then needs no further cleaning between determinations, since nothing but ether vapor comes in contact with it. The weighing dish D is cleaned in a similar manner, and dried in a vacuum' desiccator over fused
