Triple Point of Water Cells

The triple point of water (TPW) is not onlythe most accurate and fundamental tem-perature standard available, it’s also oneof the least expensive and simplest touse.

Water cells are essential!Triple point of water cells fill four criticalpurposes. First, they provide the most re-liable way to identify unacceptable ther-mometer drift between calibrations—including immediately after a calibrationif the thermometer has been shipped. In-terim checks are critical for maintainingconfidence in thermometer readings be-tween calibrations. Second, they providea critical calibration point with unequaleduncertainties.

Third, for users who characterizeprobes using ratios (that is, they use theratios of the resistances at various ITS-90fixed points to the resistance of the

thermometer at the triple point of water,indicated by “W”), interim checks at thetriple point of water allow for quick andeasy updates to the characterizations ofcritical thermometer standards, which canbe used to extend calibration intervals.

And lastly, the triple point of water iswhere the practical temperature scale(ITS-90) and the thermodynamic temper-ature scale meet, since the triple point ofwater is assigned the value 273.16 K(0.01 °C) by the ITS-90 and the Kelvin isdefined as 1/273.16 of the thermody-namic temperature of the triple point ofwater.

Good triple point of water cells containonly pure water and pure water vapor.(There is almost no residual air left inthem.) When a portion of the water isfrozen correctly and water coexists withinthe cell in its three phases, the “triplepoint of water” is realized. Hart water

cells achieve this temperature with ex-panded uncertainties of less than0.0001 °C and reproducibilities within0.00002 °C.

In simple terms, water cells are madefrom just glass and water, but there’smuch more to it than that! For starters,that’s not just any water in there.

Heavy waterHart cells contain carefully and repeti-tively distilled ocean water and are me-ticulously evacuated and sealed tomaintain an isotopic composition nearlyidentical to the international standard,“Vienna Standard Mean Ocean Water,” or“VSMOW.”

The oxygen atoms found in most wa-ter are predominantly comprised of eightprotons and eight neutrons (16O). Someoxygen atoms, however, have an extraneutron (17O) or two (18O). Similarly, thehydrogen atoms in water normally con-tain only a single proton (1H), but some-times contain a neutron also (2H),resulting in “heavy” water. These isotopescoexist in varying proportions in oceanwater, polar water, and continental wa-ter, with ocean water being the heaviest.

The ITS-90 recommends that watercells be made from water with “substan-tially the isotopic composition of oceanwater.” Research has shown that TPW er-rors associated with isotopic compositioncan be as large as 0.00025 °C. The un-certainty contribution due to the effect ofdeviation from VSMOW in Hart cells isless than ± 0.000007 °C. That’s sevenmicrokelvin!

Hart offers two options for verifyingthe isotopic composition of any purchasedwater cell, both at nominal costs. We cansubmit a sample of water taken from yourown cell to a testing laboratory (after itwas completely manufactured, so you geta valid comparison) and give you the testreport. Or, we can send that water sam-ple to you in a sealed ampoule for you toconduct your own tests. We can evenprovide multiple samples from the samecell (virtually as many as you’d like) soyou can check for changes over time.

ImpuritiesFurther, the potential for errors due towater impurity is even greater than theerrors from isotopic composition. Hartcells undergo multiple distillation pro-cesses and utilize special techniques toretain water purity. Among other things,our primary standards scientists are ableto connect quartz cells directly to theglass distillation system without using

14 Primary Standards

● Easy-to-use, inexpensive standard with uncertainty better than±0.0001 °C

● Four sizes and two shells (glass and quartz) to choose from● Isotopic composition of Vienna Standard Mean Ocean Water

Test Equipment Depot - 800.517.8431 - 99 Washington Street Melrose, MA 02176

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coupling hardware that may invitecontamination.

Glass vs. quartzMost Hart water cells may either be pur-chased with borosilicate glass or withfused silica (“quartz”) housings. What’sthe difference? Glass is less expensivethan quartz, but it’s also more porous, al-lowing impurities to pass through it overtime. Research indicates that glass cellsgenerally drift about 0.000006 °C peryear while quartz cells drift less.

Many sizesHart cells come in four general sizes.Models 5901A, 5901C, and 5901D eachcome in either quartz or glass shells andinclude 265 mm of thermometer immer-sion depth. The primary difference be-tween these models (other than the armon the 5901A) is the inside diameter ofthe probe well. (See chart on page 16and note that the inside diameter of the5901C cells varies with the shell mate-rial). A variety of baths is available, whichcan maintain the triple point within thesecells for many weeks. Accredited (NVLAP)test certificates are available with any cellunder our Model 1904-TPW.

5901A cells include an arm that canbe used as a handle, a hook, or a McLeodgauge to demonstrate how much residualair is trapped in the cell. Carefully devel-oped manufacturing processes at Hartkeep the air bubble in a quartz cell assmall as the air bubble in glass cells.

A fourth size, the 5901B cell, comes ina glass version and is significantlysmaller than the other cells. It is designedfor use in our Model 9210 Maintenance

Apparatus, which automates the realiza-tion and maintenance of the TPW. The9210-5901B combination is perfect forboth calibrating thermometers and pro-viding periodic checks of sensor drift.

AccessoriesFor simplest realization of the TPW in ourlarger cells, the Model 2031 “Quick Stick”Immersion Freezer uses dry ice and alco-hol to facilitate rapid formation of an icemantle within the cell without requiringconstant intervention while the mantleforms.

For best results, use a 3901 bushingwith your triple point of water cell. Abushing is used to improve the thermalcontact between your SPRT and the icemantle of your water triple point cell. Besure to choose a bushing that matchesthe inner diameter of the reentrant wellof the cell and the outer diameter of theSPRT. Additionally, a small piece of foam(<0.5 cm) may be placed beneath thebushing to isolate it from the bottom ofthe cell which research has shown isslightly colder than the rest of the cell.

Insurance is also available for eachwater cell purchased from Hart. Watercells are not difficult to handle nor is theTPW difficult to realize, but they are deli-cate and accidents do happen. For anominal fee, we’ll insure your cell in one-year increments. If something goeswrong, just let us know and we’ll replaceyour cell. No questions asked.

There is no tool available to tempera-ture metrologists more valuable than a re-liable triple point of water cell. Hart cellsuse the right water, right enclosures, andright manufacturing methods to ensureyou get the best cells available in theworld.

Ordering Information

5901A-G TPW Cell, 12 mm ID withhandle, glass shell

5901A-Q TPW Cell, 12 mm ID withhandle, quartz shell

5901C-G TPW Cell, 13.6 mm ID, glassshell

5901C-Q TPW Cell, 14.4 mm ID, quartzshell

5901D-G TPW Cell, 12 mm ID, glassshell

5901D-Q TPW Cell, 12 mm ID, quartzshell

5901B-G TPW Cell, mini, glass shell7012 TPW Maintenance Bath

(maintains four cells)7312 TPW Maintenance Bath

(maintains two cells)9210 TPW (5901B-G) Maintenance

Apparatus2028 Dewar (for TPW ice bath)2031 “Quick Stick” Immersion

Freezer1904-TPW Accredited Cell

IntercomparisonINSU-5901 TPW Cell Insurance, one-year5901-ITST Isotopic Composition Analysis,

TPW Cell5901-SMPL Water Sample, TPW Cell

(comes in a sealedglass ampoule)

3901-11 TPW Bushing, 5901/5901A to7.5 mm

3901-12 TPW Bushing, 5901/5901A to5.56 mm (7/32 in)

3901-13 TPW Bushing, 5901/5901A to6.35 mm (1/4 in)

3901-21 TPW Bushing, 5901C to7.5 mm

3901-22 TPW Bushing, 5901C to5.56 mm (7/32 in)

3901-23 TPW Bushing, 5901C to6.35 mm (1/4 in)

www.hartscientific.com 15

Triple Point of Water Cells

The 2028 Dewar has inside dimensions of 20 cm by50 cm (7.75 in x 19.5 in), and outside dimensions of25 cm by 61 cm ( 9.75 in x 24 in).

Accessories like the “Quick Stick” Immersion Freezerand 3901 bushings add simpler realization and im-proved thermal contact.

16 Primary Standards

Triple Point of Water Cells

5901B-G5901A-G/Q

450

mm

50 mm

12 mm

265

mm

5901C-G

420

mm

265

mm

60 mm

13.6 mm

5901D-G/Q

420

mm

265

mm

60 mm

5901C-Q

420

mm

265

mm

60 mm

12 mm14.4 mm

8 mm

30 mm18

0 m

m

118

mm

Specifications

5901A-G 5901A-Q 5901C-G 5901C-Q 5901D-G 5901D-Q 5901B-G

Expanded Uncertainty (k=2) < 0.0001 °C < 0.0002 °C

Reproducibility 0.00002 °C 0.00005 °C

Dimensions 50 mm OD12 mm ID

450 mm long

60 mm OD13.6 mm ID

420 mm long

60 mm OD14.4 mm ID

420 mm long

60 mm OD12 mm ID

420 mm long

30 mm OD8 mm ID

180 mm long

Immersion Depth (watersurface to well bottom)

265 mm 118 mm

Material BorosilicateGlass

Fused Silica(Quartz)

BorosilicateGlass

Fused Silica(Quartz)

BorosilicateGlass

Fused Silica(Quartz)

BorosilicateGlass

Water Source Ocean

DVSMOW ±10 ‰ (±1 %) ±20 ‰18OVSMOW ±1.5 ‰ (±0.15 %) ±3 ‰

Effect of Deviation fromVSMOW

±7 µK ±14 µK

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