A CONVENIENT SYSTEM OF WATER THERMOSTATS FOR LABORATORY INSTRUCTION* ROBERT T m ,UNIVERSITY OF KANSAS, LAWRENCE, KANSAS
There have been described in the existing literature a number of automatically controlled constant-temperature baths. In many cases these have been designed for high precision work where variations of not more than one- or two-thousandths of a degree were required. Such instruments can also be had commercially but like all precision instruments are exceedingly expensive. On the other hand, many devices for controlling bath temperatures have been proposed which do not work satisfactorily for any great length of time or require so much care and attention that their operation practically amounts to hand regulation of the bath. For use in a laboratory devoted to teaching general physical chemistry the following requirements of an automatically controlled temperature bath may be enumerated: 1. Constancy of temperature to within a few hundredths of a degree. Extreme precision of control is not necessary but since many of the properties to be measured change appreciably with temperature, this must be kept sufficiently constant so that errors introduced by temperature fluctuation are reduced a t least equal to, or below, the precision of the least precise measurements. For example, quite a number of properties vary as much as 2% per degree. Fluctuation of *0.05°C. might produce uncertainties in the measured value of *O.lyo. While it is generally true that a precision of 0.1% is difficultly attainable for the ordinary student, nevertheless, since it is easily preventable, there is no need of introducing the additional error which may result from lack of temperature control. 2. A range of temperatures is desirable. In order to determine temperature coefficients or to show the dependence of a given property upon temperature, the bath should be easily adjusted to any desired temperature or several baths operating at different temperatures should be provided. In the system described below three baths are used, one operating at 15'C., a second at 30°C., and a third a t 45'C. 3. The bath should. have transparent sides. Since many of the measurements to be made depend upon seeing the system kept at constant temperature (such as viscosity, drop number, moving boundary measurements, etc.), it is essential that the bath be made of glass. For the best vision, flat plate glass is the most satisfactory. 4 . Operation of the bath should require the minimum attention of the instructor. A busy instructor has sufficient demands upon his time without requiring that every day or so he take time to overhaul and readjust the regulator or other parts of the constant temperature device. 5. Lastly the system should be as inexpensive as possible. As remarked * Presented before the Kansas Academy of Science, Hays, Kansas, April 18,1930. 2953
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before, precision instruments are expensive, but by assembling individual stock pieces of equipment and employing local mechanical help the cost of such systems as those herein described can be considerably reduced. The cost of the entire system described below will he in the neighborhood of two hundred dollars for the best quality pieces. Very satisfactory equipment could be obtained a t ouehalf to three-fourths of this estimate. Complete commercial assemblies for equipment of the capacity described below would cost between four and six hundred dollars, and would fall considerably short of some of the desirable features enumerated above. As a result of some years' experience in the l a b o r a t o r y w i t h such equipment the author has assembled the equipmeut described below but before FIGURE1.-DIAGRAMMATIC ARRANGEMENT 01 APPARATUS* starting on this description A Metastatic thermoregulator a brief outline of the B Circuit breaker method of automatically C Heating bulb D Circuit breaker key controlled temperature L Lamp (50watt) baths might be helpful for M Resistance (4 ohm) RI Variable resistance (200 ohm) the less experienced. R2 Variable resistance (40 ohm) The regulation in such On heating, the Hg expands until i t completes systems is obtained by the circuit a t A. The current magnetizes B which in~ -the attracts D and throws. in more resistance supplying heat to the heating circuit, thereby decreasing the current in C. bath (such as a glowing On cooling, the circuit A is broken. This demagnetizes B, and D falls to its original place where electric lamp) controlled i t short circuits the current around R, and inby a device called the creases the current in C again. regulator. The regulator is usually a bulb filled with mercury, in the neck of which are sealed two contacts. (See Figure 1.) As the temperature of the bath rises ~
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* Thanks are due to my colleague, Mr. J e w Stareck, for the preparation of this drawing.
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the mercury expands and eventually closes the space between the two contacts with mercury. These contacts are in the circuit of a relay and when the gap between them is filled with mercury the circuit is closed, thus bringing the relay into action. This is so arranged that it then shuts off or reduces the current in the heating circuit. The bath begins to cool, the mercury contracts, the regulator circuit is broken and the heating circuit is again brought into play. The essential features of such a bath are shown in Figure 1. A description of the component parts of the system as we have assembled them follows. The Baths These are fish aquaria having dimensions 30" X 12" X 12".* They have plate glass sides mounted in steel-enameled frame with a suitable cement. This cement begins to soften a t temperatures in the neighborhood of 45% so that it is not advisable to operate them above this temperature. As we employ them there are three baths placed end to end, thus occupying a table space approximately 8 feet long. Mounted on the wall behind these baths is a shaft turned by a motor. The stirrers for each bath are operated by belts and pulleys from this common shaft. The relays, resistances, etc., are all placed in a cupboard beneath the table and locked to prevent students tinkering with them. In order that an understanding of the operation of the baths may he conveniently obtained by the interested students, an enlargement of Figure 1 has been framed and placed on the wall behind the baths. The baths are placed on benches 6" high which serve the purpose of elevating the baths so that observations are more easily made and also because the clear space beneath the benches is required to place the bases of stands used to support equipment and apparatus placed within the baths. The Bath Liquid Distilled water must, of course, be used for the bath liquid, as the continued evaporation would leave considerable residue if tap water were used. We have not attempted to fit the baths with constant-level devices but have formed the habit of filling them each morning as the loss, even in the one operating at 45'C., is not more than several liters. Another constant source of annoyance in connection with the bath water is the growth of mold, soon producing a cloudy bath, although the 45" bath is free from this growth. We have added a number of substances to prevent this growth, e. g., chloroform, phenol, mercuric chloride, and copper sulfate. Of these copper sulfate appears to be the most effective. If copper sulfate is used care must be taken that all iron surfaces are thoroughly protected with *Obtainablefrom the Central Scientific Co.,Chicago.
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enamel or lacquer. One of the defects of using copper sulfate is that it makes the bath an electrical conductor and therefore a possible source of leakage if electrical measurements are made. It will be recalled that in precise electrical measurements oils are almost invariably used as bath liquids.
Heaters As heaters we have employed (a) ordinary electric lamps, (6) electric lamps known as radiant heaters,* (c) a metallic knife type immersion heater,** and (d) a flexible metal-cased heating coil.? Of these we prefer types (a) and (b) as their heat capacities are apparently less than the other types of heaters and therefore they do not continue to add heat to the bath after the current is shut off. Types ( c ) and particularly (d) are useful as the space taken up in the bath is considerablyless than that required for the lamps. Type (d) may he bent in any shape desired and can therefore be placed along the side and bottom of the bath, thus taking the minimum of space. These types of heaters are also useful where the emission of light would prove harmful to the system under observation. We have found it advisable to choose a heater of somewhat greater capacity than demanded by the size of the bath in order to make them usable over a considerable range of temperature. For the baths described here a 250-watt radiant type heater is employed. This is not used at its full rating, as the current flowing through it is regulated by an external resistance placed in series with it. (Cf. Figure 1.) I n order to secure a greater uniformity of temperature throughout the bath, two lamps in parallel$ are generally used at opposite ends of the bath. The bath operating at 15'C. is always below room temperature and therefore removal of heat is required. As the automatic control of heat removal is more difficult than that of heat addition, it is customary to cool the bath somewhat lower than required and obtain the constancy of temperature by the control of a heating circuit as above described. We obtain the cooling by passing tap water through three coils of 3-8" thin copper tubing placed at the top and inside of the bath, i. e., in the bath liquid itself. We have not had occasion to use this bath when the tap water rises above 15"C., but are planning on precooling the water before it passes through the bath by immersing a portion of the copper tubing in a large porcelain jar filled with ice placed between the tap and the bath.
* Obtainahle from Westinghouse Electric Co., East Pittsburgh, Pa. Style No. 102,747. ** Obtainahle from Central Scientific Co., Chicago. t Obtainable from General Electric Co., Schenectady, N. Y. Trade name "Helicoil" units. $ Putting them in series with each other reduces the current to such an extent that the IZRvalue is insufficient to keep the bath at the higher temperatures.
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Regulators As regulators we have employed a commercial form known as the Metastatic Thermoregulator.* This is illustrated diagrammatically at A in Figure 1. The chief advantages of this type of regulator over the more familiar forms of mercury or mercury-toluene regulator are (a) adjustability over a wide range of temperatures and (b) absence of oxygen (air) within the system. As shown in the diagram, there is a reservoir for excess mercury in the upper bulb, i. e., the amount of mercury in the regulator proper can be varied and hence be used over a range of temperatures. For use at a higher temperature than that for which it is originally set it is necessary to warm it slightly above the temperature desired, thus expelling mercury from the regulator into the upper reservoir. For use at a lower temperature, the regulator is inclined until the capillary in the upper reservoir is beneath the mercury and the bulb of the regulator is then warmed until the mercury in the capillary makes contact with that in the reservoir, the bulb then being slowly cooled to slightly above the desired temperature. The regulator is then returned to its vertical position. The slight elevation of temperature above that desired for each setting may be determined approximately as follows: assume that the regulator has been set for 45'C. and it is desired to set it at 30°C. Mercury is drawn from the reservoir as described above by placing the regulator in water a t 30°C. When the regulator has reached this temperature it is turned into a vertical position and a small drop of mercury clmging to the end of the reservoir is shaken off into the reservoir. Coolmg is then continued further until contact is broken between the upper platinum contact and the mercury meniscus; breaking being detected by the relay and not visually. The temperature at the break is then noted; let us say in this case it was 29.10°C. The ditference in temperature, i. e., 30-29.1' or 0.Q0,is the constant of the instrument. That is the temperature a t which the drop was shaken off should have been 30.9' in order to obtain regulation at 30'. This constant can be used for any subsequent setting. Setting to an exact temperature requires a number of trials. These regulators can be obtained in several sizes-we employ for these baths an 8" size. The sensitiveness, i. e., the length of the temperature interval between make and break, will depend upon the total volume of the mercury and the diameter of the capillary portion of the regulator, greater sensitivity being secured by a large volume and area of mercury and a small capillary. We have noticed considerable difference in the diameter of the capillary for the same sized bulb. For work where the greatest precision obtainable is desired the smallest size of capillary should_be chosen. * Obtainable from American Instrument Co., 774 Guard St., N. W., Washington, D. C., or Hiergesell Brothers,2518 N. Broad St., Philadelphia,
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The second advantage mentioned for these regulators was the absence of air. The greatest defect of the open-to-air type of mercury regulator lay in the fact that at the break of the regulator circuit the current, due to its inertia, continued on across the small air gap, producing a spark. This continued sparking produced mercury oxide which contaminated the surface of the mercury and interfered with good electrical contact as the mercury rose in the capillary. Consequently to obtain any pretense of precise temperature control required frequent cleaning and resetting of the regulator. In the metastatic type of regulator air is removed and hydrogen substituted. The oxide formation is thus prevented and further hydrogen has the effect of damping the electric spark. After continued use for some nine months several of our regulators have had to be replaced, due to the fact that the mercury in the capillary would separate during the cooling period. We asaibed this to slight oxide formation (due to incomplete removal of air) on the walls of the glass in the capillary which caused the sticking of a small fragment of mercury as the mercury descended the capillary, thus producing an uncertainty in the setting. Sparking can be considerably reduced by making the current in the regulator circuit as small as possible and by placing a condenser in parallel with the two contacts of the regulator as shown in Figure 1. As a matter of theory it should be possible by the choice of a condenser of suitable capacity to completely eliminate sparking; actually it is rather difficult to do so. If the capacity of the condenser is too small it will not "absorb" all of the electricity which would pass in the spark; if it is too large sparking occurs on "make," producing still more serious troubles. We have explained this last effect as due to the discharge of the condenser just before "make" across a very small air gap; the current from the condenser is added to that coming from the applied potential, thus inaeasing the current momentarily with the result that the mercury surface is actually moved. Making and breaking may occur several times before the current steadies down. There is thus an optimum value for the capacity of the condenser and this can only be found by trial and error. Those we have employed are telephone condensers having a capacity of 2 microfarads. Another remedy suggests itself. Most of the relays we have employed function only on direct current. A specially wound relay to operate on alternating current would diminish the sparking very materially. Relays or Circuit Breakers In the older types of automatic temperature regulation a telegraph relay was used to make and break the heating circuit as we have shown in Figure 1. The contact points at which the heating circuit was made and broken, as in the regulating circuit, were always a source of grief. The current being larger in this circuit caused continual corrosion or sticking of the
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points of contact which were made of metal or of carbon. The recent introduction of commercial mercury circuit breakers has solved very largely this problem. We have tried two types of these circuit breakers. In one type* a small cylinder of soft iron rises and falls by magnetic attraction in a glass tube filled with mercury. This in turn causes the mercury to fall and rise in a U-shaped limb. Two tungsten contacts are sealed through at the two ends of the U and these are placed in series with the heating circuit. As the mercury falls in the U the circuit is broken and when it again rises the circuit is closed, making and breaking of the circuit taking place in an air-free atmosphere. The iron cylinder is actuated by a solenoid placed externally about the glass tube containing it. This solenoid is in series with the regulating circuits; when the regulating circuit is closed the solenoid produces a magnetic field sufficient to cause the iron cylinder to rise, which shuts off the heating circuit as above described. When the regulating circuit is open the iron core drops down. The second form** of circuit breaker employed resembles the familiar telegaph relay more closely. It consistsof "a cylindrical glass tube partly filled with mercury and having' two non-deteriorating electrical contacts sealed into one end. The tube is exhausted and filled with an inert gas which serves to prevent oxidation of the contacts and mercury and tends to prevent any tendency toward arcing. The tube is mounted in a saddle a t a slight angle to the horizontal so that the mercury runs to the end of the tube, which carries the contacts. Upon energizing the relay the tube is tilted so that the mercury flows to the other end of the tube, thus disconnecting the contacts and cutting off the current." Both types of circuit breakers have their advantages. The first type operates on a current of 70 milliamperes whereas the second requires about a third of an ampere, the last type thereby considerably increasing the sparking in the regulator described above. On the other hand, the second type operates more rapidly than does the first, producing no appreciable time lag in the control of temperature, and for that reason we have more generally employed this type. The first type may be operated directly on 110 a. c. or 110 d. c. but we have added snfficient resistance in series with it to reduce the current to its lowest operating value. The second type requires about 2 volts d. c. We have obtained this by the potential divider arrangement shown at L and M in Figure 1. Of course dry cells could be used for its operation.
Stiig The constancy of temperature throughout the bath depends largely upon the rate of stirring; the greater the rate of stirring the more nearly will
* Obtainable from Westinghouse Elec. Co., Style No. 352,397.
*'
Obtainable from the American Ins. Co.
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the temperature be constant throughout the bath. The upper limit of speed must be such that water is not thrown out of the batb or that the water moves so rapidly as to disturb apparatus placed withm the batb. Our stirrers consist of a brass shaft 3-8" indiameter and 14"long rotating in a hollow brass sleeve 5-8' in diameter and some nine inches in length. Such a sleeve can be beld in a clamp and is practically free from vibration. The two bearings a t each end of the sleeve are of hard wood rather than metal as we have found that metal bearings soon cut the shaft at the lower end. Wooden bearings will last for years. The lower one should be drilled somewhat large to take care of the expansion of wood in water. A wooden pulley is glued to the upper end of the shaft. Several grooves of different diameter are cut in the pulley to allow some variation of the speed of rotation. A four bladed propeller cut from sheet copper of approximately 3'/2" diameter is mounted onthe lower end by threading the end to take a small bolt. By the use of two washers the blades may be firmly beld. Such a stirrer is centrally placed in the bath, the blades being near the bottom. The blades should be bent and the direction of rotation should be such as to produce a downward current of bath liquid a t its center, but a spreading and rising current a t the sides. An upward current a t the center would tend to produce greater differencesof temperature between the upper center of the bath and the bottom or sides due to the natural tendency of the water in contact with the heater to rise. The rate of stirring is about 300 revolutions per minute, although this is increased in the bath operating at 45°C. To secure this rate we have used a slow-speed d. c. motor operating at 400 r. p. m. Such motors are not stock items and ours have been made to our specifications.* Stock motors usually operate on a. c., at 1700 to 1800 r. p. m. As this rate is entirely unnecessary and as our motors are employed continuously for long periods, some running without stop for eleven months out of the year, it was thought best to employ the slower speed d. c. motors described above. With frequent oiling and occasional cleaning of contacts such motors have lasted for years. As stated before, but one motor is used to operate a counter shaft from which all three baths are stirred. Electrical Circuits Some of the essential features of the electrical circuits have already been discussed and are fully diagrammed in Figure 1. I t will be noted that a condenser is also placed in parallel across the heating circuit for the same reason that it was employed in the regulating circuit. It might be worthwhile to call attention to the fact that the two circuits are entirely iudependent of each other. The reader has doubtless noted this already as we
* These are 1-20 h. P. built by the Robbins and Meyers Co.. Springfield, Ohio.
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have employed the terms "regulating circuit" and "heating circuit" considerably. As will be seen, the heating circuit operates on 110 a. c., the regulating circuit on 110 d. c. An examination of the circuit including RI and Rt and a consideration of the legend included in Figure 1 shows that the heating current is not completely shut off when the regulating circuit is complete, but that the introduction of more resistance simply reduces the current flowing through the heater. Such an arrangement produces smaller fluctuations of temperatures than when the current is completely shut off. The connections shown in the heating circuit have been somewhat modified from that usually recommended, by altering the position of the sliding contact on Rz. With the arrangement as shown it is possible to alter the position of the contact on R1without causing a compensating change in the contact on Rz,i. e., the current during the cooling period may be adjusted to any desired value while the current during the heating period remains constant. This arrangement is especially helpful where room temperatures fluctuate and readjustment of current thus becomes necessary. The resistances R1and Rz are the familiar variable rheostats of nichrome wire mounted on a porcelain-covered steel tube. These can be obtained from a number of scientific supply houses. Performance These baths will rnn for weeks at a time with very little attention. If the room temperatures fluctuate considerably adjustment of the beating current is necessary by altering the values of R,,and when the temperature falls considerably, of Ra. When the heating current is once adjusted to a suitable value, the temperature of the bath remains quite constant. A Beckmann thermometer placed in the 30' bath and observed shortly after the end of each heating and cooling period for over a period of several hours showed a maximum variation of 0.02OC., that is, the temperature of the bath was 30.00° * O.OlO. For the baths operating considerably above and below room temperature the fluctuations were +0.03O from the set value. I t would be possible by taking greater care in the adjustment of the heating current, by reducing the cooling rate (i.e., surrounding the bath, save for an observation opening, with a heat insulator) to increase considerably the precision of control, but we have felt that the values obtained were sufficient for our purpose. Cost
An estimate of the cost of the equipment is given below. This does not include cost of heaters, or of employment of help in mounting. The heaters cost from 45 cents for a 150-watt Mazda lamp to somewhat over five dollars for the knife type heaters.
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Total for three baths
DECEMBER, 1930
at $24.00
$ 72.00
at 10.00 at 12.00 at 6.00 at 1.120
35.00 30.00 36.00 36.00 3.00 $212.00
As stated previously, this cost could be considerably reduced. Cheaper types of baths are easily obtainable; an a. c. motor could be used and geared down, homemade resistances could be employed, but when the instructor's time and the results are considered the above combinations are, in our experience, the most satisfactory yet found.
