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JOE RICH Blackhawk Christian School 7400 East State Boulevard Fort Wayne. IN 46815
Low-Cost Water-Bath Temperature CI Gary D. Mercer Boise State University Boise, ID 83725
This article describes the construction of a temperature controller for the student laboratory that is low-cost, totally solid-state, and reasonably precise. When combined with a magnetic stirrer or small circulating pump, the unit will control a 1- to 4-Lwater bath with a precision of a t least M.2 T over an ambient temperature range of 20-80 -C. The cost of construction, including the heater and enclosure, is less than $30. I have also found satisfactory results using the wntroller for keeping a constant air temperature (M.2 ~ C in ) a closed chamber, although the controller was not specifically designed for this. Construction The controller is constructed around the popular LM741 operational amplifier acting as a comparator to control the heater through an optocoupler with a triac. The heater is an inexpensive 200-W "coffeecup" heater generally available a t discount department stores. All electronic components are readily available from most electronics parts outlets.' Figure 1shows the schematic diagram. The heating coil and thermistor should be mounted together to form a single unit. The leads of the thermistor should be soldered to the ends of a two-conductor cable. (Twenty-twogauge stranded speaker wire works well.)
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Figure 2. Photograph showing the reiationship between the heater and the thermistor. The connection is then insulated using 118-in. heatshrink tubing so that one end covers the thermistor up to the tip and the other end is above the water line. The thermistor should he 0.5 cm from the heater coil and directly above it for the most stable operation. These two should be taped or banded together at the handle of the heating coil so that they will not move relative to one another during use. Figure 2 shows the heater-thehnistor assembly that I constructed. The figure also shows that the plug was removed from the end of the heater in my version. Then both its leads and the thermistor leads were wired to a 5-pin DIN connector. The panel temperature control (5-kQ potentiometer) should be wired so that it will be a t its lowest temperature setting when the control is in the extreme counter-clockwise position. This can be done by connecting the thermistor lead to the right-hand terminal and by connecting the 4.7442 resistor to the left-hand terminal, as viewed from the shaft end. Operation The controller is used with a magnetic stirrer or small circulating pump, for example, a fish tank pump or an evaporative air-cooler pump. The bath vessel can be a large evaporating dish, a beaker, or a flat-bottom plastic bowl. Before the controller can he used it must be calibrated. Immerse the heater and sensor in the water bath. Then turn the unit on, and set the panel temperature control to 50% of its maximum position. Then adjust the calibration trimmer (50-kR potentiometer) so that it regulates at 35 T. The controller will now show a usable temperature range of about 20-80 ' C . The temperature range can be shifted by simply readjusting the calibration trimmer. The controller is now ready for use.
Figure 1. Schematic diagram of the temperature controller
568
Journal of Chemical Education
Modification for Air-Temperature Control Modification of the controller for controlling air temperature is done by simply substituting a light bulb for the immersion heater and a fan for the stirrer. Again, the sys-
tern is set up so that the thermistor is reasonably close to the light bulb (2-4 cm). For a cardboard-box enclosure (about 0.1 m3) that is kept at about 10 O C above ambient temperature, I found a 50-w bulb and a 4-in. "muffm" fan to be quite adequate.
Acknowledgment
I would like to thank the College of Arts and Sciences at Boise State University for funding to complete this project through a minidevelopment grant program.
Volume 69 Number 7 July 1992
569
