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A Furnace in a Thermos
by Shawn Carlson
Because the same process that cooks organic material out of soil will also remove it from the surface of glass, a furnace that approaches 500 degrees C can be used to clean the most intricate laboratory glassware. Likewise, baking sorbents at this temperature drives away chemical contaminants and recharges them for reuse in, say, pumps for producing ultraclean vacuums (the topic of the October 1996 column). Such a
furnace would have other uses as well, including melting enamels, activating glass beads for use in chemical separators, annealing glass and metals, and making electrical feed-throughs for laboratory glassware.
So you can see why I was thrilled to learn that Roger Daschle, a talented musician and hiking buddy of mine, had developed a small furnace that is safe to operate at these temperatures. It consumes a scant 80 watts, heats up in less than an hour and can be built for as little as $60.
Roger and I are part of an informal ensemble of self-absorbed iconoclasts who hike every Friday in the San Diego County foothills to get away from our offices and talk tech. His ingenious innovation came to him while he was pouring a cup of hot chocolate during a lull in our discussions of chaos theory and homemade infrared detectors. Roger wanted to build a stout furnace to service a small chemical separator he was developing. When he poured a cup of cocoa from his thermos and saw the rising steam in the cold afternoon air, he realized that he had found the perfect container. A thermos is inexpensive and has negligible thermal mass. He knew that if he could secure a high-temperature electric heater inside a suitable thermos and plug the top with an insulator, he would have a fully functional and highly efficient desktop furnace.
Roger showed off his invention at our next hike. He had purchased a Stanley-brand wide-mouth thermos (the kind typically used to hold soup) for $25 from a local discount store. But the brand doesnŐt matter. Just make sure the vacuum bottle is made of steel and not glass, which might break, or aluminum, which might soften and implode. Roger got things cooking with a rope heater: a prefabricated bundle of Nichrome wire wrapped around an insulating core and covered with an insulating sheath. These cords run on wall current and are much safer than bare wire. Omega Engineering sells them in three-foot lengths for $22 (www.omega.com, part no. FGR-030). The rope is rated for operation at 480 degrees C (900 degrees F). This sets the safe operating temperature of the furnace. The device will get much hotter if you run too much current through it. You can keep the current at a safe level by wiring in a household dimmer switch and monitoring the temperature. As a precaution, Roger wisely wired in a one-hour mechanical timer to make sure that his unit could not be accidentally left on.
To install the heater in the thermos, Roger fashioned a cylinder out of a wide-mesh steel screen, available at a well-stocked hardware store. He loosely coiled the heating rope around the cylinder and covered the entire assembly with a centimeter-thick blanket of Fiberfrax, a clothlike material made of spun alumina fibers. (Because you canŐt purchase Fiberfrax in small quantities, the Society for Amateur Scientists will provide it for $5.) Muffler packing, available at a motorcycle parts store, would also do. The whole thing snugs into the thermos through its wide mouth. Finally, Roger tightly rolled a strip of Fiberfrax into a plug that just fit into the thermos mouth. A single twist of steel wire wrapped around the plug prevents it from unraveling.
The most economical way to measure the temperature is a K-type thermocouple, which produces a voltage in proportion to the temperature. The voltage can be read with a high-end digital voltmeter that has internal circuitry to interpret this sensor. Otherwise, you can estimate the temperature by measuring the voltage developed between the leads using a digital voltmeter. The temperature in Celsius is given approximately by multiplying the voltage in millivolts by 27.7; for Fahrenheit, multiply by 50. I tested RogerŐs furnace using a bare-wire thermocouple from Omega Engineering (part no. CHAL-015). I insulated it using a short length of Nextel sleeving (Omega part no. XC4-116) and installed the sensor near the top of the furnace. At 480 degrees C inside, the exterior was uncomfortably warm but not too hot to touch. When, as a safety test, I pushed the device to 600 degrees C using an ultrahigh-temperature heating tape, the outer casing got far too hot to handle.
So make sure to monitor the temperature at all times and keep it at or below 480 degrees C. Keep it well away from curious children and pets. Wire in a timer switch. And connect the heating unit through a ground-fault switch, such as those often seen in bathroom wall outlets these days. These switches contain an internal circuit breaker that blows when a short circuit occurs. That way, if the furnace should overheat and short out, the power will be cut off.
Using the furnace, you can easily measure the organic content of soil. First, carefully weigh about 100 grams of dirt from your garden and dry it in your kitchen oven for one hour at 120 degrees C (about 250 degrees F). Then weigh it again. The soil in my garden turned out to contain 33.2 percent water by weight. Tightly wrap the dry soil in aluminum foil and bake it in your thermos furnace for two hours at 480 degrees C. The charring organics liberate a ghastly waft of smelly smoke, so use a fume hood or keep the device outdoors. A final weigh-in revealed that my garden dirt is 8.6 percent (dry weight) organic material. Sand from a nearby playground weighed in at just 3.2 percent water and contained a scant 0.7 percent organics (dry weight).
It would also be interesting to monitor the weight continuously, in order to look for physical processes that occur at different temperatures.
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