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 to use.

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

Third, for users who characterizeprobes using ratios (that is, they use the

ratios of the resistances at various ITS-90fixed points to the resistance of the ther-mometer at the triple point of water, indi-cated by “W”), interim checks at the triplepoint of water allow for quick and easyupdates to the characterizations of criticalthermometer standards, which can beused to extend calibration intervals.

And lastly, the triple point of water iswhere the practical temperature scale(ITS-90) and the thermodynamic tempera-ture 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 cellsachieve this temperature with expandeduncertainties of less than 0.0001 °C andreproducibilities within 0.00002 °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 repetitivelydistilled ocean water and are meticulouslyevacuated and sealed to maintain an iso-topic composition nearly identical to theinternational standard, “Vienna StandardMean Ocean Water,” or “VSMOW.”

The oxygen atoms found in most waterare predominantly comprised of eight pro-tons and eight neutrons (16O). Some oxy-gen atoms, however, have an extraneutron (17O) or two (18O). Similarly, thehydrogen atoms in water normally containonly a single proton (1H), but sometimescontain a neutron also (2H), resulting in“heavy” water. These isotopes coexist invarying proportions in ocean water, polarwater, and continental water, with oceanwater 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 uncer-tainty contribution due to the effect of de-viation from VSMOW in Hart cells is lessthan ± 0.000007 °C. That’s sevenmicrokelvin!

Hart offers two options for verifying theisotopic 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 sampleto you in a sealed ampoule for you to con-duct your own tests. We can even providemultiple samples from the same cell (virtu-ally as many as you’d like) so you cancheck for changes over time.

ImpuritiesFurther, the potential for errors due to wa-ter impurity is even greater than the errorsfrom isotopic composition. Hart cells un-dergo multiple distillation processes andutilize special techniques to retain waterpurity. Among other things, our primary

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

Triple point of water cells

standards scientists are able to connectquartz cells directly to the glass distillationsystem without using coupling hardwarethat may invite contamination.

Glass vs. quartzMost Hart water cells may either be pur-chased with borosilicate glass or withfused silica (“quartz”) housings. What’s thedifference? Glass is less expensive thanquartz, but it’s also more porous, allowingimpurities to pass through it over time. Re-search indicates that glass cells generallydrift about 0.000006 °C per year whilequartz cells drift less.

Many sizesHart cells come in four general sizes. Mod-els 5901A, 5901C, and 5901D each comein either quartz or glass shells and include265 mm of thermometer immersion depth.The primary difference between thesemodels (other than the arm on the 5901A)is the inside diameter of the probe well.(See chart on page 16 and note that theinside diameter of the 5901C cells varieswith the shell material). A variety of bathsis available, which can maintain the triplepoint within these cells for many weeks.Accredited (NVLAP) test certificates areavailable with any cell under our Model1904-TPW.

5901A cells include an arm that can beused 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 significantly smallerthan the other cells. It is designed for usein our Model 9210 Maintenance Appara-tus, which automates the realization andmaintenance of the TPW. The 9210-5901B combination is perfect for both cal-ibrating thermometers and providing peri-odic checks of sensor drift.

AccessoriesFor simplest realization of the TPW in ourlarger cells, the Model 2031A “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 matches theinner diameter of the reentrant well of thecell and the outer diameter of the SPRT.Additionally, a small piece of foam (<0.5cm) may be placed beneath the bushing toisolate it from the bottom of the cell whichresearch has shown is slightly colder thanthe 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 a nomi-nal fee, we’ll insure your cell in one-yearincrements. If something goes wrong, justlet us know and we’ll replace your cell. Noquestions asked.

Temperature Calibration Equipment 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.

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 (main-

tains four cells)7312 TPW Maintenance Bath (main-

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

Apparatus2028 Dewar (for TPW ice bath)2031A “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)

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