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Calibrating with Cold |
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by Shawn Carlson |
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 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].
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 Ð34.8 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. Begin by filling a wide-mouth stainless-steel thermos (the kind used to keep soup warm) about halfway with a solution of 91 percent isopropyl alcohol. (This is the least dilute variety George can buy at his drugstore.) Next obtain a small block of frozen carbon dioxide from a nearby liquor store. (Check the Yellow Pages under "dry ice" to find a local supplier.) Wrap the frosty mass inside a towel and hammer it into small fragments. Transfer some of the chips to the thermos using tongs and stir the concoction with a wooden spoon until it stops bubbling.
The effervescence gives off a highly flammable cloud of alcohol vapor, so work only where a fan blows plenty of fresh air over your work area. And, obviously, keep cigarettes and all open flames well away. Position the thermos inside a plastic food-storage container to catch any liquid that might escape. Note that you should never pour room-temperature alcohol into the cold solution. The chilled liquid contains a lot of carbon dioxide, and if suddenly warmed by the addition of more alcohol, CO2 vapor will burst out of solution, causing a frothy foam to erupt from the thermos. Before you go to the trouble of making up the cold liquid, though, you'll need to fashion a "thermometer well"--something to protect your thermocouple when it is immersed in the mercury. You can buy just the right piece of glassware as part of a kit from the Society for Amateur Scientists, or you can make your own from a slender Pyrex tube by following the procedure for fabricating test tubes that was given in the May 1964 installment of this department. With the appropriate thermometer well and cold bath, a temperature calibration at the freezing point of mercury is easy to perform. Secure a large test tube and a matching rubber stopper from a purveyor of scientific supplies (one is Fisher Scientific; www.fisher scientific.com or 800-766-7000). Make sure the stopper comes with a hole that is the right size to accept the thermometer well, stuck just far enough in that it hangs about two centimeters from the bottom of the test tube. Fill the test tube two thirds full with mercury. Pour some alcohol in the well and insert your thermocouple so that it is about one centimeter from the bottom and, if possible, not touching the glass. Then immerse the well in the mercury and push the stopper snug. Finally, place the entire assembly gently into the thermos containing the chilled alcohol solution. Be careful! The vapor pressure of mercury at room temperature is sufficiently high that prolonged exposure can cause brain damage. Don't allow pregnant women or children anywhere near, work only in a well ventilated area, and wear protective clothing and safety glasses. Also, keep your containers of mercury tightly sealed. And because spills are notoriously difficult to clean up, think through all the ways that an accident might happen before you begin. Make sure, for example, that you can fully contain any spills if one does take place by keeping the mercury-filled test tube low to the table and over a large food-storage container. Once contaminated, plastic can never be completely cleaned, so clearly and permanently label it "DANGER! MERCURY CONTAMINATED--DO NOT USE FOR FOOD." I should emphasize that mercury is rightly classified as hazardous waste, and by law it must be disposed of safely. Because regulations vary regionally, you'll have to contact the hazardous materials office of your local fire department for guidelines. To carry out the calibration, carefully monitor the output of your thermometer as the temperature plummets. When the mercury begins to freeze, the voltage will remain nearly constant and won't drop again until all the mercury has solidified. After you see the voltage fall for a second time, remove the test tube and place it in a sturdy stand. Then record the output of the circuit as the mercury melts. Again, it should linger at one voltage for a while. If the voltage stabilized at the same value as before, you can be confident that the temperature plateau occurred at exactly Ð34.8 degrees C. If the two voltages (at freezing and at melting) were different, your mercury is contaminated. To purify it, submerge the test tube in the chilled alcohol and wait until about half the metal has solidified. Pour the remaining liquid into a separate container for waste mercury. Repeating these steps a second time should yield mercury that is at least 99.99 percent pure. Produce enough of it for your needs and then perform the calibration as described. Once you have determined the output voltages for the freezing point of mercury and for the freezing and boiling points of water, check these numbers against the table below. This comparison will immediately reveal the corrections you should apply to the tabulated values at Ð34.8, 0 and 100 degrees. You can then interpolate the appropriate changes to all temperatures within this range, which brackets even the coldest winter nights that Rhode Island is ever going to see. Now my home weather station is operational again--just as long as I don't ever decide to move to the South Pole.
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