How to build homemade high-precision thermometer

by Shawn Carlson

Last month I described how to build a triple-point cell, a device that reproduces the unique temperature, defined to be exactly 0.01 degree Celsius, at which water can exist with its solid, liquid and vapor phases all in equilibrium. The cell can be used for calibrating state-of-the-art thermometers, but few amateur scientists can afford such expensive instruments, which cost thousands of dollars.

Fortunately, George Schmermund, the creative genius from Vista, Calif., who developed our triple-point cell, has also designed a thermometer capable of measuring temperature to within a few thousandths of a degree C. What is more, you can build this remarkable instrument for less than $100.

Schmermund's thermometer uses something called a resistance temperature detector (RTD), which relies on the fact that the resistance of platinum changes with temperature in a precisely known way. For each degree C of temperature change, these sensors typically change their resistance by 0.00385 ohm per ohm of resistance. For example, if your RTD has a resistance of 100 ohms, each degree C change in temperature will alter the resistance by 0.385 ohm. So if you know the probe's resistance at a particular temperature, such as the triple point of water, you can then convert any measured resistance into a corresponding temperature.

EXACT TEMPERATURE MEASUREMENTS to within millidegrees can be made with a thermometer that relies on a resistance temperature detector (RTD)--a sensor that exploits how the electrical resistance of platinum changes as the material becomes hotter or colder. The relation is linear and is given by the equation shown (above), where a (Alpha) is typically 0.00385 and RTP is the resistance of the sensor at 0.01 degree Celsius: the triple-point temperature of water. A digital multimeter measures the RTD's resistance.

In the past, RTDs were always made of wire. Because the wire had to be thick enough to withstand the manufacturing processes and because a larger-diameter wire has less resistance than a smaller one made of the same material, the operating resistance was limited to about 100 ohms. Recently, though, a new breed of RTDs has been constructed by laying an ultrathin platinum coating on a ceramic substrate. The resistance of some of these devices tops 2,000 ohms. You'll find a smorgasbord of these marvels in the catalogue of Omega Engineering in Stamford, Conn. (; 800-826-6342). For this project you'll need a model like the F3141, a small, unencapsulated 1,000-ohm unit that sells for $19.

These new RTDs can bring exquisite sensitivity into the home-based laboratory. Using a high-quality handheld digital multimeter that can measure 1,000 ohms of resistance to within 0.02 ohm, amateurs can now resolve temperatures to within 0.005 degree C, or 0.009 degree Fahrenheit. That performance bests liquid-filled thermometers by 20 times and trumps any thermocouple by a factor of 10.

And you can do much better. In practice, the sensitivity of an RTD-based thermometer is limited by how accurately you can determine its resistance, which is measured by observing the voltage drop associated with a known current. With a typical digital multimeter, the lead wires are part of the circuit, and so their resistance affects the results. This error can be eliminated by measuring the voltage drop directly across the resistor with an independent set of wires. Such instruments, called four-wire ohm meters, have separate inputs for a current source and a volt meter.

Hewlett-Packard's snazzy HP 34401A multimeter (priced at $995) uses the four-wire technique to measure 1,000 ohms to within 0.001 ohm. And top- of-the-line instruments costing $30,000 apiece enable professionals to resolve temperature differences as slight as 10 millionths of a degree C. In such a four-wire configuration, an RTD-based thermometer is called a standard platinum resistance thermometer (SPRT).

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.

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.

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.