Tackling the Triple Point

 THERMOMETER CALIBRATION can be performed with a triple-point cell (above), which settles at 0.01 degree Celsius: the unique temperature at which water can exist in its solid, liquid and gas phases all in equilibrium (below). Note that a portion of the crushed ice, which should cover the cell, has been removed to provide a better view of the apparatus.

One of the horrible truths of scientific research is that simple and inexpensive techniques will get you just so far. Beyond some point, increasing accuracy can be obtained only with a disproportional rise in expense, sweat and frustration. That's partly because accurate measurements require an extremely well calibrated instrument, and providing such an exact scale can be a vexing challenge.

Consider thermometers. You might think they would be easy to calibrate: just determine what they read at two known temperatures, like the boiling and freezing points of water. But it's not so simple. These temperatures cannot be reproduced accurately, because they depend on factors that are difficult to control, like atmospheric pressure. For precise work, researchers must resort to more sophisticated techniques.

One method is based on a wonderfully repeatable property of water: the unique temperature, called the triple point, at which water can exist with its solid, liquid and gas phases all in equilibrium. To reproduce this temperature, defined to be exactly 0.01 degree Celsius, researchers rely on a special Pyrex flask filled with ultrapure water, evacuated with a vacuum pump and then hermetically sealed with a blowtorch. At \$1,000 apiece, such ¿triple-point cells¿ are beyond the budgets of most home laboratories.

But that is about to change, thanks to George Schmermund, a gifted amateur scientist in Vista, Calif. His device remains within about 0.0001 degree C of the triple point for days and costs less than \$50 to build.

The cell is simple to construct. Start with a Pyrex straight-walled flask about five centimeters (two inches) in diameter and at least 17 centimeters (seven inches) long. Schmermund hires a glassblower to thicken and angle the opening slightly for a snug fit between the flask and a large rubber stopper. Without these modifications the lip can shatter explosively. As a precaution, wrap the top two centimeters of the flask with electrical tape.

Drill a hole in the stopper and insert a long Pyrex test tube so that it reaches to within two centimeters of the bottom of the flask. Then hermetically seal the joint with silicone cement. To ensure a tight fit between the stopper and the flask, spread a thin film of silicone vacuum grease uniformly around the bottom two thirds of the stopper.

Although professional units contain ultrapure, triple-distilled water, Schmermund has discovered that ordinary distilled water from a grocery store works just fine. Fill the flask until the water comes to about five centimeters below the stopper when assembled.

Next, you must remove air from the chamber atmosphere as well as any gases dissolved in the water. Schmermund eliminates the need for a vacuum pump by simply boiling the water--the expanding steam will force out the air molecules. First, though, to prevent the water from boiling too violently, shatter a clean test tube inside a towel and drop a few shards into the flask to act as nucleation sites for the forming bubbles. Then, secure the cell in a ring stand and gently rest the stopper on top of the flask to allow the steam to escape.

Heat the flask's bottom with a propane torch until the water boils gently. Dissolved gases in the flask will form visible bubbles on the inner test tube. Keep the water boiling until the convection currents have swept them away and until you no longer see any condensation inside at the top. The condensation will disappear when the internal atmosphere has been completely replaced by hot steam.

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