To build Schmermund's thermometer, contact a local glassblower to purchase a Pyrex tube 30 centimeters (12 inches) long and eight millimeters (0.3 inch) in diameter. At one end of the tube, have the glassblower form a receptacle that is five centimeters (two inches) long for the RTD sensor.
Next, attach lead wires to the sensor. If you solder the leads or use wires insulated with plastic, you'll be restricted to temperatures below the melting point of those substances. That's not a problem for many applications. To allow the maximum possible range of temperatures, however, Schmermund spot-welds the RTD to bare 10-mil nickel wires that he then insulates in thin Pyrex sleeves. He gets these sleeves in 46- centimeter lengths from a local glassblower, but capillary tubes, which are available from any scientific supply house, work equally well when strung on the wire like beads on a necklace.
For a thermometer that will be used with a four-wire ohm meter, Schmermund bundles four of his long tubes and delicately tapes them together at one end. He then bends two one-meter lengths of nickel wire in half and threads each half through a different tube from the untaped end. Finally, he spot-welds the RTD to the bends in the two wires. (Note: If you will not be making four-wire measurements, simply connect one wire to each of the RTD leads.)
To secure the insides of the device and to thwart convection currents from forming, Schmermund packs the instrument with tiny glass beads that are only about 25 microns in diameter. These are expensive and must be purchased from a scientific supply house. Fortunately, fine silica sand (grit 30 or greater) also does the job. You can purchase a 23-kilogram (50-pound) sack from a hardware store for just a few dollars.
Because any moisture that becomes trapped inside the thermometer will distort your readings, all water must be driven from both the filler and the glassware before assembly. Bake everything, including the entire sensor assembly, at 250 degrees F for approximately two hours.
Image: DANIELS & DANIELS
Hermetically seal the thermometer by topping off the sand with glue from a hot-glue gun. If you're using uninsulated wires, heat them with a hair dryer for a few seconds before the adhesive sets so that the wires will seat themselves into the glue.
To minimize signal interference, connect the probe to your ohm meter through a stereo microphone cable, which consists of two twisted pairs of wire shielded inside a metal sheath that you must ground. Use a four- wire terminal strip to connect each twisted pair across the device. Solder the wires and protect the strip inside a plastic canister from a roll of 35-millimeter film.
This homemade instrument, which is functional up to about 400 degrees C,
can open up fascinating avenues of research. Although the device is a
bit cumbersome for fieldwork, you can use it for accurately calibrating
other thermometers. In the laboratory, it will also help you probe the
nature of phase transitions and measure the strength of chemical bonds
(for ideas, see the March 1996 Amateur Scientist). With a little
imagination, this thermometer can become a powerful weapon in your
arsenal of research techniques.
As a service to the amateur community, the Society for Amateur Scientists is offering the Schmermund four-wire thermometer (not including an ohm meter) as a kit, complete with spot-welded nickel wire leads, prefabricated Pyrex tubes and insulating glass beads. The cost is $250. To order, send a check to SAS at 4735 Clairemont Square, Suite 179, San Diego, CA 92117, or call the society at 619-239-8807. For more information about this and other projects, check out the SAS Web page.
This article was originally published with the title A Homemade High-Precision Thermometer.