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LIBRARY
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UNITED STATES DEPARTMENT OF AGRICULTURE
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Contribution from the Bureau of Animal Industry JOHN R. MOHLER, Chief
Washington, D. C. PROFESSIONAL: PAPER ~ © June 4, 1921
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METHODS FOR CLOSE AUTOMATIC CONTROL OF INCUBATING TEMPERATURES IN LABORATORIES.
By JoHN T. Bowen, Technologist, Dairy Division.
CONTENTS. Page. Page. Construction of temperature-control Description of temperature-control COMpAbMent, | ee es a eS 2 Sy Stenis 225 tes chick CEs eed VERA eo 7 Construction of TFOoOmM sso ss = 2 Temperature recorders______ 171 Ekeatinewequipment, === == = aes 5 Costof operaition= eae eae x 16
The automatic control of temperature within close limits is very important in many lines of commercial work. It is of special im- portance, in fact, is a necessity, in laboratories where chemical, bac- teriological, and physiological investigations are conducted. In many instances success in the maintenance of cultures depends upon the ability continuously to maintain practically constant temperature. Pathogenic microorganisms, as a.rule, do not grow except within very narrow temperature limits, the optimum temperature being about wipes (99.5- FE.)
The Bureau of Animal Industry, United States Department of Agriculture, produces tuberculin on a large scale; about 16,000 culture flasks of 300 c. c. capacity are maintained to produce the required quantity. The tubercle bacillus is very sensitive te temperature changes. In order to obtain the best growths, therefore, the tempera- ture during the period of incubation should be maintained practically constant at the optimum of about 37.5° C. To hold the temperature in all parts of the chamber approximately constant requires con- siderable care, especially if the chamber is large.
Other work carried on by the bureau includes the study, by bacteriological means, of other diseases of domestic animals. The
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2 BULLETIN 951, U. S. DEPARTMENT OF AGRICULTURE.
cultures made from the diseased tissues are usually incubated at about the same temperature as that mentioned for the tubercle bacillus. There are, however, certain thermophilic organisms that may be en- countered in bacteriological investigations of diseased tissues, and in order to be able to investigate these organisms smaller compartments are provided. These are located inside the larger incubating com- partment and are furnished with separate heating devices. The sys- tem used for controlling the temperature is identical with that of the larger room, the controlling device being adjusted to maintain the temperature at a higher point.
CONSTRUCTION OF TEMPERATURE-CONTROL COMPARTMENT.
In the control of temperatures the proper construction of the com- partment is of equal importance with that of the controlling ap- paratus. The walls, floors, and ceiling should be so constructed as to minimize the transfer of heat from the inside to the outside of the room, thus economizing on the quantity of heat required to maintain the desired room temperature. Not only will the proper construction of the room reduce the quantity of heat necessary to maintain the desired temperature, but should the supply of heat be discontinued the insulated walls of the room will assist in holding the temperature near the desired point for a considerable time. Figure 1 shows the rate at which the temperature falls when the source of heat is entirely cut off. The curve shows that for a period of approximately one week the drop in temperature was only about 6° C. The temperature sur- rounding the incubator room—that is, the temperature in the labora- tory during this time—averaged about 21.1° C. It is obvious, there- fore, that in a well-installed compartment cultures may continue to grow several days after the supply of heat has been discontinued.
The dimensions and construction of incubating compartments used im laboratories of the bureau at Washington are illustrated in figure 2. The finished inside dimensions of the rooms are 7 feet wide by 7 feet 64 inches deep by 8 feet 5 inches high. The capacity of each room, in terms of standard 300 ¢. c. flasks, is about 2,600. In all there are 7 rooms of these dimensions.
CONSTRUCTION OF ROOM.
Floors.—On the laboratory floor one course of 1-inch cork board was laid down in hot asphalt. All transverse joints were broken and all joints were slushed with hot asphalt and made tight by butting the sheets of cork board closely together. The top surface of the cork board was flooded with hot asphalt about one-eighth inch thick, and directly on top of this a wearing concrete floor was laid, the thick- ness of the floor being 2 inches in the center of the room and 3 inches
CONTROL OF INCUBATING TEMPERATURES IN LABORATORIES. — 3
at the walls. The floor was sloped to the center to facilitate cleaning and disinfecting.
Walls and ceiling—Expanded-metal walls and ceiling were con- structed, as shown in figure 2. These consisted of 1-inch channel studs, tied at the floor and ceiling and thoroughly crosstied. The studs were covered with No. 24 gauge high-rib expanded-metal laths which were used instead of channels in order to give greater strength.
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Fie. 1.—Chart showing drop in room temperature for one week, with heat supply entirely shut off.
Directly on the expanded metal a 4-inch course of cement mortar was appled, mixed in the proportion of 1 part of Portland cement to 2 parts clean, sharp sand.
While the mortar was green a course of 2-inch cork board was erected on it and the joints were slushed with hot asphalt. Against the exposed surface of the cork board a Portland-cement plaster finish about one-half inch in thickness was applied in two coats.
The first coat, about one-fourth inch in thickness, was mixed in the
4 BULLETIN 951, U. S. DEPARTMENT OF AGRICULTURE.
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proportion of one part Portland cement to two parts of clean, sharp sand, and it was rough scratched. After this coat had thoroughly dried, the second MXssS QYEN AYA YVAN AIO Ws Cat, ap plex —+oh mately one-fourth inch in thickness and mixed in the proportion of one part of Portland cement to one and one-half parts of clean, sharp sand, was applied and brought to a float finish. Shelves.—Shelves were originally con- structed (fig. 2) of l-inch_ galvanized- metal pipe, covered with galvanized- wire mesh made of No. 16 wire with four meshes to the inch. The wire mesh was securely soldered to the pipe frame, care being taken to leave no projections that might cut or scratch those working in the compartment. It was found, how- ever, that the wire- mesh covering of- fered more or less resistance to plac- ing the trays on the shelves, and it was subsequently removed and the trays containing the flasks are now placed directly on the pipes. AII the trays containing the
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CONTROL OF INCUBATING TEMPERATURES IN LABORATORIES 5
flasks are of the same size and fit on the pipe frame. Where different- sized crates are used, however, the wire covering is advisable.
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HEATING EQUIPMENT.
Electricity is used in all cases as the heating medium. Two systems of tenrperature control, however, are used. The temperature-control
6 BULLETIN 901, U. S. DEPARTMENT OF AGRICULTURE.
apparatus for the first incubating compartment was designed and for the most part constructed in the bureau. The temperature-con- trol system for the other compartment consists largely of assembled com- mercial apparatus. The heating units consist of resistance tubes or coils fed from a 220-volt, direct-current line. The electric current is turned on and off the resistance units by means of a ther- mostat in the incu- bator compartment. The thermostat is adjusted to turn the electric current on and off with a variation of room temperature of 1° Cs That is) 0 “above and 0.5° below the optimum. Tt is possible to ad- just the thermostat to turn the electric | current on and off “a between much closer limits, but the hmits of 0.5° above and 0.5° below the optimum tempera- ture give satisfac- tory results, and the frequency of operation and hence the wear on the controlling appara- tus are decreased.
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CONTROL OF INCUBATING TEMPERATURES IN LABORATORIES. a
designed by the bureau demonstrated on tests that the temperature inside the room could be maintained between limits of +0.1° C.
In order that all parts of the room may be kept at approximately the same temperature it is necessary to provide means for thoroughly and continuously stirring the air in the room. This is accomplished by an electrically driven fan, as shown in figure 4, with the piping so arranged that the air is blown down over the heating coils in one corner of the room and drawn from near the floor at the opposite corner and from the top of the room at the other two corners. With this arrangement the air is stirred thoroughly and the temperature is maintained approximately the same in all parts of the room. Since it is necessary to operate the fan continuously it is belt-driven at about half-normal speed, or at a speed that will insure a thorough mixing of the air and hence a uniform temperature. By using an oversize fan and operating it at reduced speed the chances of a break- down of this important part of the equipment are greatly lessened.
The fans used in these installations operate at a speed of about 1,400 R. P. M. and deliver about 190 cubic feet of air a minute, or about 26 complete changes of air in the room every hour. ‘The motors are shunt-wound, 230-volt, direct current. Two sizes of motors are used. Where a single motor drives two circulating fans a one-half horsepower motor is used, and where a motor drives only one fan a
-one-fourth-horsepower motor is used. The motor power is much in
excess of that actually required to drive the fans; but since they oper- ate continually and receive little attention, it is advisable to have considerably more power than is actually required.
DESCRIPTION OF TEMPERATURE-CONTROL SYSTEMS.
A general view of the temperature-control board. which was de- signed and constructed in the bureau, is shown in figure 5, and a diagram of the system is shown in figure 6. The electrical connec- tions covering the motor for driving the fan are simple and require no explanation.
In the temperature-control system proper there are four coils of 110 ohms each. ‘Two of the coils are connected in series and directly across the main line, with a switch for cutting the coils out if desired. In cold weather the coils are brought into service. They are so pro- portioned that when continuously in service they will maintain the temperature in the room slightly below the point desired. The other two coils, which are connected in series, furnish the additional heat required to bring the temperature of the room up to the desired point. The latter two coils are connected to and operated by the controlling system. These coils are connected through the relay to the other side of the 220-volt line through a variable resistance in series with the instrument. This resistance is sufficient to reduce the voltage in
8 BULLETIN 951, U. S. DEPARTMENT OF AGRICULTURE.
the relay circuit to four volts, and thus prevent arcking and burning
the thermostat and relay contacts. Condensers are placed in the circuits where shown (fig. 6) for the purpose of absorbing the induc- tive discharge from the coils when the circuits are broken.
Fic. 5.—Temperature-control board of incubating room in Pathological Division.
The relay is connected for reverse action; that is, when the electric circuit is completed through the thermostat, owing to rising tempera- ture in the room, the relay operates to break the main heating-coil circuit and thus cut off the supply of heat. On the other hand, when the relay circuit is broken in the thermostat, owing to a lowering
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CONTROL OF INCUBATING TEMPERATURES IN LABORATORIES. 9
temperature in the room, the relay closes the heating-coil circuit. The overload device, shown in the diagram, has for its object the cut- ting out of the whole system in case the temperature-control appa- ratus fails to operate. In principle it is identical with the tempera-
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ture-control equipment. ‘The overload thermostat is adjusted to close the electric circuit through the overload relay at a room temperature a few degrees above the normal operating temperature, the relay in turn breaking the circuit through the overload coil in the main line, thus cutting off the electric current in the entire system.
10 BULLETIN 951. U. 8. DEPARTMENT OF AGRICULTURE.
The heat-control system installed in the small high-temperature boxes, which are used for the investigation of thermophilic organisms, is identical with that used in the large room, except that on account aR the small size of these boxes no means are provided for circulating the
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inclosed air. These small boxes, the construction of which is shown in figure 7, are placed inside the large room; consequently the tem- perature surrounding them is that of the large room, 37.5° C. (99.5° F.). Owing to their small size and to the fact that their temperature
CONTROL OF INCUBATING TEMPERATURES IN LABORATORIES. 11
is maintained only a few degrees above that of the surrounding air, only small-capacity resistance units are required.
The special thermostat used in the incubator room is shown in figure 8. It is capable of making and breaking the electric circuit on a variation in temperature of about 1/100 of a degree F. Tem- perature adjustments are made by transferring mercury from the small reservoir to the capillary tube, and vice versa.
A general view of the temperature- control boards made up of commercial apparatus is shown in figure 9, and a diagram of the elec- trie connections in figure 10. Six rooms pole Eee y are equipped with (QQgssssttiiassinlsancsnes ie Seerie eine this apparatus. Re- ys a) Gis ferring to the dlia- gram (figure 10), one side of the line is connected directly to the heating coil and from the heating coil to the line-magnet coil. The current in this coil is controlled by the pilot coil, which in turn is con- trolled by the ther- ‘ie mostat. When the i :- pilot circuit is closed * through the thermo- stat the pilot coil is energized and closes the circuit through the line coil, which acts to close the line circuit through the heating cous. When the temperature in the incubation room rises to the upper limit the thermostat makes contact with the stop point, thus breaking the pilot circuit and opening the line coil, and hence the current in the heating coils.
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Fic. 8—Mercury thermostat used in temperature-control system of Pathological Division.
TEMPERATURE RECORDERS.
Each incubation room is fitted with a continuous 7-day recording thermometer. The recording elements are mounted on the wall out- side the room and the capillary passes through the wall, the bulb
42 BULLETIN 951, U. S. DEPARTMENT OF AGRICULTURE.
being located near the top and in the center of the room. The charts taken from the recorders are examined and any variations in tem- perature beyond the prescribed limits are noted with the time at which the variations occurred. With these data at hand it is nearly always possible to find the cause of an excessive variation in tem-
Fic. 9.—The temperature-control boards used by the Biochemic Division.
perature and also to fix the responsibility, if any is due. The mere fact that a permanent, continuous, and exact record is kept of the temperature conditions inside the incubator room is a stimulus to those in charge to do their best to maintain the conditions desired.
CONTROL OF INCUBATING TEMPERATURES IN LABORATORIES.
HEATING COVE
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Fic. 10.—Wiring diagram of temperature-control system in Biochemic Division.
13
14 BULLETIN 951, U. S. DEPARTMENT OF AGRICULTURE.
The vaiue of a continuous-temperature record in the process of incubating cultures is great. Such records are not only of value in showing the temperature at all times throughout the period of incu- bation, but they may be dated and filed for reference, thus making it possible to determine with accuracy the temperature maintained in
the incubating process on any previous date.
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Fie. 11.—Chart showing the temperature maintained in the incubating room of the Pathological Division, January 22 to 29, 1918. Note that night temperatures especially are practically without fluctuation.
The chart in figure 11, taken from the incubator room equipped with the temperature-control system designed in the bureau, shows the temperature maintained for the week January 22 to 29, 1918— typical of the winter season. Attention is invited to the fact that during the night periods, when the room is closed, there is scarcely any variation in the temperature. During the day periods, however, when the door is opened for storing or taking out specimens, tem- perature variations occur. But im no instance do the variations exceed 1° C., and they exist for only a few minutes at a time. Even
CONTROL OF INCUBATING TEMPERATURES IN LABORATORIES. 15
these temperature variations may be materially lessened if the door is quickly opened and closed and kept closed while storing or remov- ing specimens. A similar chart, figure 12, covering the week August 6 to 13, 1918, shows the results obtained during a typical summer | period. The incubator room was entered only about twice during this week; consequently the temperature was maintained practically - constant throughout the period.
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Fie. 12.—Chart showing temperatures maintained in incubating room of Pathological Division, August 6 to 13, 1918. Note that variations are exceedingly small.
The chart in figure 13 was, taken in one of the incubator rooms equipped with a control system made up largely of assembled com- mercial apparatus. The curve shows that while the average tem- perature was about that for which the control was adjusted it was not so uniform as the temperatures shown in the previous charts. While this latter temperature-control system is not so sensitive or reliable as the former, and hence requires closer attention and more frequent adjustments, it has the advantage of being assembled from commercial apparatus.
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16 BULLETIN 951, U. S. DEPARTMENT OF AGRICULTURE.
COST OF OPERATION.
The cost of electric energy for operating the heating equipment and the motor-driven circulation fan at 6 cents a kilowatt hour averages about 50 cents a day of 24 hours for each room.
Prior to the installation of the present incubating rooms a large number of gas-heated incubators were used, the temperature being controlled by means of a bimetallic thermostat which regulated the
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Fic. 13.—Chart of temperature maintained in incubating room, Biochemic Division, April 7 to 14, 1919. The system of control used is not so sensitive as that used in the Pathological Division.
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flow of gas. These incubators occupied a large floor space, the con- trol of temperatures was poor, and the fire hazard was great. With the present installation of incubating rooms the capacity has been about. doubled and the space occupied has been reduced about one- half. Furthermore, the cultures have grown better than ever before, a result mainly of the very uniform temperature which has been maintained in the incubating rooms.
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