Calibrating with Cold
Shawn Carlson shows how to fine-tune a laboratory thermometer
Image Credit--DANIELS & DANIELS
WIDE-MOUTH THERMOS containing a solution of alcohol and frozen
carbon dioxide chills a test tube filled with mercury to its freezing point, ¿34.8 degrees Celsius. This simple apparatus thus serves as a low-temperature calibration standard for
a precision thermometer.
After having lived in balmy California all my life, I just moved to Rhode Island. I love it, but the frigid weather
here requires some getting used to. Wintering in the
Northeast has also forced me to check the operation of some of my laboratory instruments, especially my outdoor electronic thermometers. In general, one calibrates a
thermometer by finding out what it reads at two known temperatures and interpolating between them. Until now, I've always calibrated my thermometers down to 0 degrees
Celsius. That strategy worked well because the weather around my former home in San Diego rarely dipped below freezing. The same can't be said for New England. So when
accuracy counts, amateur meteorologists living in colder climates must be able to perform a truly chilly temperature calibration. My own temperature station relies on a J-type
thermocouple. The thermocouple wire costs $10 for a spool (Omega; see www.omega. com or call 800-872-9436). Just strip the
insulation off and twist the ends together to form the sensor. The integrated circuit to which it attaches runs about $24 (Analog Devices AD594CQ, available from
Pioneer-Standard Electronics; check www. ied.pios.com/onestop/ or call 440-720-8500), which may seem a bit pricey, but it is
worth every penny, because the device automatically compensates for several subtle effects that otherwise complicate thermocouple measurements. This simple setup is
accurate to about one degree if you interpolate temperature values from the table [below].
Image Credit--DANIELS & DANIELS
INTEGRATED CIRCUIT, the Analog Devices AD594CQ, makes a J-type
thermocouple into a highly accurate thermometer for a home weather station.
But you can do about 10 times better by calibrating your thermometer.
My usual procedure was to record the output voltage of the circuit when the thermocouple was immersed in a slurry of distilled water
and ice chips, which gave me 0 degrees
Celsius, and also in distilled boiling water, which gave me 100 degrees Celsius. The latter remains a useful high-temperature end point for the calibration. Be aware, however,
that the boiling temperature of water depends on atmospheric pressure (which changes with altitude and with the weather). So you will need to know the barometric
pressure to determine the exact boiling temperature of water; for this task you could use one of the handy calculators available on the Internet
(for example, www.biggreenegg.com/ boilingPoint.htm).
To learn how to make a truly low-temperature standard, I
turned to my good friend George Schmermund. After perfecting his triple-point-of-water cell (described in The Amateur Scientist of February 1999), George began developing
other temperature standards, including one based on the freezing point of mercury, which falls at ¿34.8 degrees C. Mercury is somewhat expensive (the 500 or so grams you
will need costs more than $100), but it is widely available (try Thomas Scientific; www.thomassci.com or 800-345-2100). And
although it is a potent poison, it can be handled with complete safety by strictly following a few commonsense precautions.