A SELF-REGULATING ELECTROLYTIC IMMERSION HEATER FREDERIC E. HOLMES Cincinnati, Ohio
IN
THE heater herein described, heat is produced by the well-known action of an electric current passing through on aqueous solution of u sult. Thr unique featurn is the incorporntion i u the heater itsrlf of the means for making and breaking the circuit in response to changes in the temperature of the surrounding fluid, thus eliminating the need for a separate controlling device. This is accomplished very simply by enclosing one or both of the electrodes in a gas-tight chamber in which expansion of the enclosed air pushes the electrolyte away from the electrode. The application of these principles is almost self-evident in Figure 1 and in Figure 2, A. Heaters similar to that shown in Figure 1have proved quite satisfactory for control of temperature of aquaria within ranges suitable for tropical fish. Other uses for which it may be found to. be suitable are suggested: for the control of temperature of aquaria for experimental aquatic animals in biological, physiological, and biochemical laboratories; for water baths for the promotion or inhibition of enzyme action or bacterial
growth; for promotion of chemical reactions a t moderately elevated temperatureefor example, for acceleration of biological or spontaneous chemical oxidations which do not involve precise gas measurements in the bath; for volatilization of low-boiling liquids in solvent extractions; and for other purposes for which a variation of plus or minus 1to 2%. may be tolerated. Although i t is not designed to facilitate measurement and study of its operating characteristics, the apparatus itself may be of interest to students of elementary physics and physical chemistry. Some of the advantages of the apparatus may be enumerated as follows: ease of construction, from readily available material and a t low cost, making i t especially adaptable for use in school laboratories; operation a t low temperature, which is important where living specimens may come in contact with the heater; absence of radio "interference" (clicks and crackling produced in receivers by sparking of bimetallic regulators); and possibly some minor advantages in efficiency, characteristic of electrolytic immersion heaters operating a t low temperatures. CONSTRUCTION
Glass is the obvious material for the c o n t a i ~ n vessel g because of its electrical insulating property and chemical resistance. The simplest satisfactory form, shown in Figure 2, C, consists of an 18-mm. tube with plain bends. Figure 2, B, shows in perspective a form which fits conveniently into the corner of a rectangular tank and provides an extensive horizontal heating surface. In common with other forms having a large closed arm (25 to 28 mm.) attached to a salt bridge of smaller tubing (15 to 18 rnrn.), it gives slightly closer temperature control. A vessel has also been used (not shown here) in which the parts have a triangular arrangement similar to C, but tubing larger than that of the salt bridge is used for the open and closed arms. Plain U-tubes were tested, but were found to lack sufficient heating capacity. In all models made, the tops of the two arms have been brought into close proximity, as in Figure 2, B and C, for convenience in wiring. Heaters of greater capacity have been made, having longer and larger tubing (28 mm.) in the salt bridge. Most amateur glass blowers will find difficulty in making smooth, uniform bends in large tubing. Figure 2, D, represents an attainable compromise, not entirely satisfactory because of the constriction, but mechanically strong, having an outer wall that is not too thin and a smooth profile on the inside of the bend, free of i6
APRIL, 1947
lumps and ridges which often introduce dangerous stresses. The walls have been thickened by heating and gently pushing the tubing toward the middle of the heated portion until the walls are about five times their original thickness and the lumen has shrunk to about one-third. This heated portion has then been drawn out slightly, bent to the desired angle, and blown out again to a little more than half its original diameter. Vessels have been tested which have closed arms of diameter larger than that of the salt bridge and open arms of the same tubing as the bridge. They function well in the narrow range of temperature encountered during operation, hut may draw in air on cooling when not in use. Electrodes have been made of carbon rods (arc or welding carbons) or of carbon rods attached to brass rods, because of the resistance of carbon to corrosion in salt solution. Rods of 3/16-, and S/16-in~h diameter (4.7-, 6.3-, and 7.8-mm.) have been used, depending on the diameter of tubing of the glass parts. The lead-in wires may be attached to all-carbon electrodes by passing them through a tiny hole drilled transversely through the carbon and then covering the joint tightly with tape or a short piece of snug rubber tubing to hold them in place. A '/*-inch (3-mm.) brass rod may be attached to the VI6-inch carbon by drilling a slightly larger hole in the end of the carbon rod and puddling solder into the space between brass and carbon. The brass rod is easily thrust through a small tight hole in the supporting rubber stopper and furnishes a suitable terminal for soldering to the lead-in wires. During insertion of the stopper carrying the electrode in the closed arm, it may be manipulated to bring the end of the electrode into the center of the tube. This is important, to promote a clean break and prevent deposition of salt crystals a t a point where they may form a bridge between electrode and electrolyte. For best performance in vessels having two sizes of tubing, the lower end of the electrode should be in the funnel-shaped portion joining the two tubes and about 3 mm. above the end of the smaller tube. The carbon portion of the other electrode, 8, should extend nearly the entire length of the open arm. It is not important whether it is centered. Figure 1 shows these details. The conducting path between electrodes in the salt solution may be of any length between 50 mm. and 300 mm. for heaters having a salt bridge of 15- to l&mm. tuhing. Greater length in proportion to the square of the diameter is suitable in larger tuhing. SALT SOLUTION
For 110-volt (alternating) current in a salt bridge of 15- to 18-mm. tubing, the electrolyte consists of a solution of sodium chloride in water containing between 1.5 and 3 per cent of salt for each one inch (25 mm.) of length of path between electrodes. Convenient concentrations may be obtained by diluting a saturated solution of sodium chloride: one-third saturated (approximately 12 per cent) for a path of 4 to 6 inches (100 to 150 mm.); and one-half saturated (approxi-
Fiwm 2.
V ~ i 0 " . other form. of the appar.tu.
mately 18 per cent) for 6 to 12 inches (150 to 300 mm.). Approximately five per cent of glycerin is added to prevent deposition of salt on electrode El. In sizes of tubing larger than 18 mm. the salt bridge may be made longer in direct proportion to the square of the diameter, or the solution may be diluted proportionally. The stopper in the open arm is lifted and an amount of electrolyte estimated to be sufficient is poured into the vessel. With the stopper again in place and with a finger over the vent, the heater is tipped until the solution just touches the end of electrode 2%. More electrolyte may be added to bring the level in the open arm to a little above the middle, or about 20 mm. below the top of the carhon of carbon-brass electrodes. ADJUSTMENT OF TEMPERATURE CONTROL
The operating temperature is raised by transferring a small bubble from the closed to the open arm, or lowered by transferring one in the opposite direction. The heater will approach stability in a few hours, but may require a few days to become completely adjusted. Immediate adjustment to the maximum temperature is di5cult. It is easier to make small adjustments when the bath goes above the desired maximum or below the minimum temperature. Error in measurement of bath temperature due to position of the thermometer should be considered before attributing deviation from the desired range to the adjustment of the heater, especially
JOURNAL OF CHEMICAL EDUCATION
in the case of thermometers having their bulbs on or near the bottom in aquaria in which artificial circulation is not maintained. Once reached, the adjustment is remarkably stable except for minor variation due to barometric change. UNSATISFACTORY HEATERS
Consideration of the principles of operation of the foregoing forms of heaters may lead to attempts to use forms which have been found to be unsatisfactory. They are presented here to enable the reader to avoid modifications which have been found to be inferior in operation. The extremely simple heater shown in Figure 2, A, may be convenient for laboratory demonstration but is not practical for its intended purpose. It utilizes the water of the aquarium, or tap water, or other dilute electrolyte solutions in a bath, as the conducting and heating medium between the closely placed electrodes. It is dangerous to fish in aquaria, probably because of continuous liberation of gas from solution which eventually exhausts the dissolved oxygen to a degree insufficient to support life. It is unsatisfactory for other heated baths because of a tendency to drift to lower temperatures due to accumulation of gas and to accumulation of distilled water around the electrodes. To attain various anticipated minor advantages, a vessel was built in which the circuit was opened by breaking the conducting path in the electrolyte a t some point in the salt hridge. The temperature control was found to fluctuate widely because of deposition of wet salt crystals a t the point of separation. Efforts to utilize the water vapor generated in the closed arm as a medium of heat transfer by increasing the surface area of the closed arm did not seem to offer any advantage. Diffusion is slow and localized. The temperature gradient appears to be too low. DISCUSSION OF OPERATION
The major mechanism of operation was mentioned in the introduction. Heaters will not continue to operate on direct current because of electrolysis. On 60-cycle alternating current, production of gas by electrolysis cannot be detected. Formation of bubbles a t the electrodes during the initial period of operation is due to liberation of dissolved gases and does not persist. Rise in temperature of the electrolyte produces an increase of both air pressure and vapor pressure in the closed arm in advance of that due to rise in temperature of the aquarium or bath. Consequently, the heater itself does not reach excessively high temperature, and the maximum temperature of the bath is approached bv a series of on-and-off oscillations of thc hearer which prevent overheating of the hath. Local overheating produces both beneficial and deleterious effects. At the end of electrode El a t the time of break, the effectis to speed the break, preventing or diminishing arcing and possibly decreasing the interval
between periods of closedcircuit by producing less vapor (at higher pressure) with a higher temperature and less latent heat. On the other hand, an electrode of too small cross-section area probably decreases heat production by shortening the period of closed circuit. Local overheating in the salt hridge below the closed arm is probably necessary for the production of convection currents in the salt solution which prevent accumulation of refluxed distilled water around the electrode El. For this purpose the effect of lower resistance in the shorter path on the inside of the bend of the salt bridge is sufficient. Constrictions in the glass tubing which narrow the conducting path often produce excessive local heating, result in breaking the circuit before sufficient heat can be produced in the rest of the salt solution, and are no longer used to promote convection currents. Overheating in parts of the salt bridge remote from the closed arm has little or no effect on the temperature control. Since the amount of heat produced and transferred to the bath is dependent upon the ratio between duration of closed circuit and the intervening intervals of open circuit, which is in turn dependent upon the difference between the temperature of the air and vapor in the closed arm and that of the bath, some drop in temper* ture of the hath must occur to bring the production of heat into equilibrium with loss from the bath when the temperature of the environment becomes lower. Although these conditions have not been carefully measured, aquaria have been held to a drop of less than 3'C. (5.4"F.) when the room temperature went down to 5 to 10°C. (40 to 50°F.) overnight. The foregoing tendency toward lowering of temperature may be compensated for, or even overbalanced, by the effect of cold air on the portion of the closed arm which extends above the water of the bath. The temperatures of aquaria on a winter morning have occasionally been found to be above those of the previous evening. Ordinary changes in harometric pressure produce variations in temperature through a range of about 2.5'C. (4 to 5°F.). Further incrcasc in the size of the closed arm and the rolumr of air enclosed in it would be rxnccted to make the control more accurate and increase the load which the heater can carry. It would not eliminate the variations due to harometric changes. Convection currents from the salt hridge carry warm salt solution throughout the open arm, thereby including this portion of the vessel in the total area through which heat is delivered to the bath. Practicable devices have been considered for compensating for harometric change, improvement of accuracy, and extension of heating surface. Their incorporation would sacrifice the chief advantage over more elaborate apparatus now available, simplicity and low cost, without gaining any other advantages. Patents are pending covering the structures disclosed in this paper. Permission is granted to teachers to build the apparatus for classroom instruction.
