This copied from
Scientific American. This is a great magazine, guys, well worth reading and supporting with real money.
Quote:
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Originally Posted by Scientific American
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
PLASTIC CANISTER protects a terminal strip that connects the thermometer to a cable leading to a digital multimeter.
You must complete the next steps while everything is hot, so be sure to exercise the proper care by wearing gloves, an eye shield and protective clothing. Secure the large tube in a vise. A clean rag wrapped around each jaw will allow you to hold the glass tube firmly without breaking it. Insert the RTD assembly into the tube and use a small glass funnel to pour in enough of the desiccated sand to cover the sensor completely. Lift the assembly just a bit to make sure the RTD is suspended about two millimeters above the bottom of the well, without it touching the glass wall. Remove the tape and slowly fill the tube with hot sand to within about half a centimeter from the top, stopping frequently to tap the glass with a pencil to consolidate the material
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.
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There's more there, so please follow the link. I'm unable to find out if you can get all of Scientific American's Amateur Scientist column on CD-ROM though. The canonical collection is sold out, but a $29.95 version for "all" the projects might be available here at
Amazon. Search for science fair and tinkers guild (no apostrophe in tinkers).
Now that solves an issue of a single point, if that is accurate enough. For multiple point calibration, you need to either use something that is known to be linear temperature through the interesting range (is a mercury thermometer linear enough?), or use other temperature set points like the melting point of mercury and watch the calibrated probe as the mercury does the phase change. I think that mercury is less sensitive to atmospheric pressure than water, although, you could put it into a sealed cell and try to measure the temp through the glass walls.
Anyways, hope this is interesting.