One of the horrible truths ofscientific research is thatsimple and inexpensive tech-

niques will get you just so far. Beyondsome point, increasing accuracy can beobtained only with a disproportionalrise in expense, sweat and frustration.That’s partly because accurate measure-ments require an extremely well cali-brated instrument, and providing suchan exact scale can be a vexing challenge.

Consider thermometers. You mightthink they would be easy to calibrate:just determine what they read at twoknown temperatures, like the boiling

and freezing points of water. But it’s notso simple. These temperatures cannotbe reproduced accurately, because theydepend on factors that are difficult tocontrol, like atmospheric pressure. Forprecise work, researchers must resort tomore sophisticated techniques.

One method is based on awonderfully repeatable proper-ty of water: the unique temper-ature, called the triple point, atwhich water can exist with itssolid, liquid and gas phases allin equilibrium. To reproducethis temperature, defined to

be exactly 0.01 degree Celsius, re-searchers rely on a special Pyrex flaskfilled with ultrapure water, evacuatedwith a vacuum pump and then hermeti-cally sealed with a blowtorch. At$1,000 apiece, such “triple-point cells”are beyond the budgets of most homelaboratories.

But that is about to change, thanks toGeorge Schmermund, a gifted amateurscientist in Vista, Calif. His device re-mains within about 0.0001 degree C ofthe triple point for days and costs lessthan $50 to build.

The cell is simple to construct. Startwith a Pyrex straight-walled flask aboutfive centimeters (two inches) in diame-ter and at least 17 centimeters (seveninches) long. Schmermund hires a glass-blower to thicken and angle the open-ing slightly for a snug fit between theflask and a large rubber stopper. With-out these modifications the lip can shat-ter explosively. As a precaution, wrapthe top two centimeters of the flaskwith electrical tape.

Drill a hole in the stopper and insert along Pyrex test tube so that it reaches towithin two centimeters of the bottom of

The Amateur Scientist98 Scientific American February 1999

Tackling the Triple Point

by Shawn Carlson

T H E A M A T E U R S C I E N T I S T

WATERVAPOR

TRIPLE POINT

LIQUIDWATER

ICE

PRES

SUR

E

0.01

INSULATEDCONTAINER WITH CRUSHEDICE AND WATER

TRIPLE-POINTCELL

THERMOMETER TO BE CALIBRATED

THERMOMETER CALIBRATIONcan be performed with a triple-point cell(left), which settles at 0.01 degree Cel-sius: the unique temperature at whichwater can exist in its solid, liquid andgas phases all in equilibrium (bottom).Note that a portion of the crushed ice,which should cover the cell, has been

removed to provide a better view of the apparatus.

ILLU

STR

ATIO

NS

BY

DA

NIE

LS &

DA

NIE

LS

Copyright 1999 Scientific American, Inc.

the flask. Then hermetically seal thejoint with silicone cement. To ensure atight fit between the stopper and theflask, spread a thin film of silicone vac-uum grease uniformly around the bot-tom two thirds of the stopper.

Although professional units containultrapure, triple-distilled water, Schmer-mund has discovered that ordi-nary distilled water from a grocerystore works just fine. Fill the flaskuntil the water comes to aboutfive centimeters below the stopperwhen assembled.

Next, you must remove airfrom the chamber atmosphere aswell as any gases dissolved in thewater. Schmermund eliminates theneed for a vacuum pump by sim-ply boiling the water—the ex-panding steam will force out theair molecules. First, though, toprevent the water from boiling tooviolently, shatter a clean test tubeinside a towel and drop a fewshards into the flask to act as nu-cleation sites for the forming bub-bles. Then, secure the cell in a ringstand and gently rest the stopperon top of the flask to allow thesteam to escape.

Heat the flask’s bottom with apropane torch until the waterboils gently. Dissolved gases in theflask will form visible bubbles onthe inner test tube. Keep the waterboiling until the convection cur-rents have swept them away anduntil you no longer see any con-densation inside at the top. Thecondensation will disappear whenthe internal atmosphere has beencompletely replaced by hot steam.

Remove the flame and quicklypress the stopper down to form a vacu-um-tight seal. Before doing so, be sure toprotect your hands and arms by wearinglong sleeves and a pair of hot-watergloves used by professional dishwashers.Also, hold a towel against the flask.

If you immerse the hot, tightly sealedcell in a cool bath, the water inside theflask will boil again. This delightful ef-fect occurs as the water vapor withinthe cell condenses, lowering the internalpressure, which then decreases the boil-ing temperature. When the cell coolscompletely, you should test the qualityof the vacuum by giving the cell a gentlevertical shake. (Be careful, because a

vigorous jolt could shatter the glass.)You should hear a sharp “snap” causedby the so-called water-hammer effect:the water, uncushioned by air, will slamfull-force into the glass. If you don’thear the sound, regenerate the vacuum.

To reach the triple point, first chill thecell overnight in a refrigerator. Next,

you’ll need to form a thick ice mantlearound the inner test tube. Professionalsusually pour a frigid mixture of dry iceand alcohol into the inner well, butSchmermund gets fantastic results withliquid nitrogen, which is much colder.You’ll find both refrigerants at your localwelder’s supply store. Before you add thecoolant, dry the inner surface of the testtube thoroughly because the glass couldcrack if ice forms inside the well. Keep inmind that refraction will make the icemantle appear to grow faster than it ac-tually does. When the mantle looks likeit is nearly touching the flask, dump outthe remaining refrigerant.

Using liquid nitrogen entails a compli-cation. The ice mantle will form fastest atthe bottom where it is in contact with thenitrogen for the longest time. To make themantle more even, Schmermund periodi-cally lets all the nitrogen boil away andthen drops in progressively longer wood-en dowels. Additional nitrogen boils ener-

getically around the dowel, and theexpanding gases tend to keep thecoolant above the dowel’s top.

Separate the mantle from the testtube by filling the well with distilledwater and 10 percent isopropyl al-cohol to melt the mantle’s inner sur-face. Don’t be alarmed, though,when the ice cracks violently. Ifwhen you rotate the flask the man-tle stays put, the ice is no longerstuck to the glass. When that hap-pens pour off enough of the water-alcohol mixture so that its level istwo centimeters below the top ofthe ice.

Last, place the cell inside an insu-lated drink container filled withcrushed ice and water. Because theice mantle is buoyant, it will press upagainst the bottom of the inner testtube, making this spot slightly colderthan the triple point. A cutting froma pencil eraser makes an ideal spacer.Rest the thermometer on top of theeraser cutting inside the well. Inabout an hour, the thermometer willsettle on the triple point.

To determine temperature, thebest thermometers work by measur-ing the resistance across a thin plat-inum wire. Because the change in re-sistance caused by a given tempera-ture difference is well known forplatinum, the triple point is all you’llneed to calibrate the instrument.

Sadly, such thermometers are very expen-sive. But Schmermund has an answer forthat too, as you’ll see in my next column.

For more information about this andother amateur projects from “The Ama-teur Scientist,” check the on-line discus-sion at web2.thesphere.com/SAS/WebX.cgion the World Wide Web. As a service tothe amateur community, the Society forAmateur Scientists is making the Schmer-mund triple-point cell available for $75.Send a check to the society at 4735Clairemont Square, Suite 179, SanDiego, CA 92117, or call the society at619-239-8807.

The Amateur Scientist Scientific American February 1999 99

TRIPLE-POINT CELLis a vacuum-tight flask containing water vapor, liquid water and an ice mantle that has formed

around an inner test tube.

WATER VAPOR

WATER-ALCOHOLMIXTURE

WATER

FLASK

ELECTRICALTAPE

RUBBERSTOPPER

TEST TUBE

ICE MANTLE

Copyright 1999 Scientific American, Inc.