Faculty of Engineering

Mechanical Engineering Department

Triple Point Research

Prepared By

Project Team

Gönül Öykü Güngör 21001709

Eren Özgün 21000526

Onur Albayrak 21002125

Irmak Ece Ulusoy 21001540

İmge Karakoç 20901396

Şafakcan Uysal 21000680

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Instructor: Dr. Şakir Baytaroğlu

Due Date : 25.02.2014

1. Introduction                    

In nature, substances can exist in three different phases. There are some exact temperatures and pressures

which three phases of substance in a thermodynamic equilibrium. These conditions called as triple point

and many materials have their unique triple points. For example, the triple point of ethylene occurs at a

temperature of 38.8344 °C and a pressure of 0.2 mPa.

Additionally, the triple point between gas, liquid and solid phases, there may be triple points which

consists more than one solid phase for types of substances with multiple-polymorphs. Generally, a system

with p possible phases, there is a formula in the below which represents the triple points numbers.

In the terminology of measurement, zero celcius measured as 273.16 K and sea level defines as

611.73 pascal. At this condition, liquid water, water vapor and solid ice are occasionaly exist at the same

time because this is a triple point for the water. At this point, changing phases of the substance to a specific

phase is possible by changing the conditions slightly. However, if the total pressure is above the 611.73 Pa,

system can regenaretes itself to triple point for the water. In order to achieve that, the critic surfaces which

divides phases as stable, must be perfect and smooth that removes surface tensions.

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Even the water has a strange phase diagram, the definition of triple point is still be valid. At higher

pressure, water has various phases which consist different liquid and ice structures. At lower temperatures

and under compression, the liquid state still exist but water passes directly from gas to solid.

In triple point, boiling ice and soliditiation of water vapor can observable. Data and charts are

available in literature that is fixed for every possible substances.The phenomena of a substance occupying

three phases simultaneously (gas, liquid and solid) while in thermodynamic equilibrium is called the triple

point of that substance. Values of triple points are fixed temperature and pressure values. The triple point

of Carbon(graphite) can be seen at 4.765 K and 10.132 kPa while Oxygen’ s can be seen at 54.36 K and

0.152 kPa.

There can be triple points including more than one solid phases, when considered ice having 15

different phases, or in a sulfur system all triple points consists of two solid, one gas, one liquid system. To

define the number triple points in a system with possible n phases, the equation below is useful;

 

       

Water has a fairly complex phase diagram at high pressures, and more than one triple

points can be observed above certain pressures , the diagram and various triple points can be

seen below:

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                      Fig. 1: Phases of water at different temparatures and pressures.

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Fig. 2 : Temperature vs Pressure Fig

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(Water at its unique triple point condition when there is ice boiling)

2. Water Triple Point Cell

The triple point of water (TPW) is the unique physical state of water in which all three phases

(solid, liquid and vapour) coexist at thermodynamic equilibrium. The TPW is realized in practice

by using TPW cells in sealed borosilicate glass- or fused-silica- envelopes containing from 400

cm3 to 500 cm3 of high-purity water. A re-entrant well, located along the axis of the cell, allows

for insertion of the Standard Platinum Resistance Thermometer (SPRT) to be calibrated. An ice

mantle is produced around the thermometer well. After creating the ice mantle, the cell is

accommodated in a water maintenance bath controlled at a temperature close to 0.01 °C

(typically within few mK) or in a Dewar flask containing crushed ice, and the equilibrium between

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the three phases is automatically established within the cell and can be maintained for many

weeks. [1]

Figure 1 [2]

The Water Triple Point is the most important fixed point, the only point common to the ITS-90 and

the Thermodynamic Temperature Scale. It is an essential reference point for every temperature

laboratory. The Jarrett-Isotech cells are the best standard, all cells are not the same accept no

inferior device.

Triple Point of Water Cells are used in the calibration of thermometers. Although most commonly

used for the determination of the R of platinum resistance thermometers, they are also used for

calibration of quartz thermometers, thermistors, liquid in glass thermometers and as a reference

temperature for thermocouples. They are also used to establish the zero, and evaluate the

stability of other temperature sensing devices.

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The Jarrett-Isotech cells have been in production since  1958. A independent comparison* in

1981 showed the  first cell to be within 0.000006°C of the reference cell. The  most recent

international study organized by BIPM consistently shows labs using both recent and older

Jarrett-Isotech cells tightly grouped, tens of µK around  the BIPM reference value. [3]

The capability of a triple point of water cell to provide an accurate, stable and reproducible

temperature depends upon the purity of the water in the cell. Jarrett-Isotech cells are carefully

cleaned and aged by a special procedure. They are then filled with water that has been purified

by an elaborate 12 step process designed to eliminate the possibility of contamination while

avoiding change in isotope proportions.

Jarrett-Isotech Cells use water with an isotopic content essentially similar to Standard Mean

Ocean Water. Following research by the international science community into the make up of

Standard Mean Ocean Water, and V-SMOW the BIPM recommended in 2005* that “The triple

point of water is now defined as the equilibrium temperature of vapour, liquid and solid water,

with the liquid water having the isotopic composition defined by the following amount of

substance ratios:

0.00015576 mol 2H per mol 1H

0.000379 mol 17O per mol 16O

0.002005 mol 18O per mol 16O”

Since early in 2000 Jarrett-Isotech Cells have been made to this definition and our cells have

been within +10 and -40μK of it. During 2005, subsequent to the CCT definition, the waterin our

triple point cells has been further enriched and our latest cells meet ±20μK of the above

definition. Further we can provide Isotopic Analysis giving the exact composition with an

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uncertainty of ±3μK. Samples of the actual water used in a particular cell can be supplied for

purity analysis.

Cells made from Borosilicate Glass may drift lower in temperature by up to 0.1mK after 10 to 20

years. Cells made from Quartz are expected to last 10 times longer. [3]

Manufacture of a triple point of water cell

The main steps in the manufacture of a TPW cell cover:

- The cell envelope (borosilicate or fused-silica) which is annealed to release strains

and leak-tested to ensure that it is vacuum tight.

- A thoroughly cleaned and pre-conditioned cell-envelope to prevent its inner

surface from dissolution into the water.

- A high-purity source water which is distilled, degassed and transferred into the cell

envelope by distillation or by gravity.

- Flame-sealing of the cell at a constricted section of the filling tube.

- An isotopic and chemical analysis.of a sample of the same water sealed in the cell . [1]

3. Experimental Setup To Obtain Water Triple Point Cell

- A water triple point cell

- A container of crushed ice

- A cooling material

- 8 mm diameter stainless steel rod

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- High purity alcohol or acetone

- A syringe

- A transparent container of water [5]

4. How The Experiment is Done

Firstly, cell should be placed into the crushed ice container. Then, reentrant tube should be half

filled with alcohol or acetone through using syringe. Nextly, cell should be inverted to displace

any water particle that may be present in the reentrant tube. Now disgard this liquid and refill the

tube with fresh alcohol. Next, steel rod will be placed in the cooling material to chill. Then, steel

rod should be placed into reentrant tube. This cooling will cause small shim of ice to form in

reentrant tube. Now, the mantle can be built but first using a syringe to adjust the level of alcohol

in the reentrant tube to just above the meniscus of the water. Then, same procedure will be done

again. Steel should be cooled and placed in the reentrant tube. Appearing ice bridged can be

removed by warm of the hands. Mantle will be progressing by placing it into the water container.

This cancels out the refraction of light caused by the water within the cell and allows us to see the

true size  of the mantle. [5]

There are three reasons for uncertainity in a water triple point cell

A) Entrapped Air: A water triple point cell should be at the vapour pressure of the water it

contains.

B) Impurities in Water : All impurities depress the triple point of water. Fortunately when water

is solidified in a triple point cell the ice rejects the impurities, concentrating them in the

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remaining water. By turning 50% of the water into ice around the re-entrant tube of the cell,

the impurities within the water are approximately doubled. To measure the temperature within

the made up water triple point cell, a rod of copper at ambient temperature is placed in the

water filled re-entrant tube and as it cools a thin film of ice is melted around the tube. This thin

film is very pure, being water melted from the ice. After measuring the cell’s temperature the

cell is inverted three times, allowing the ice mantle to float and suck in impure water from the

cell to replace the clean water from the melted ice.

C) Isotopic Composition : We think of water as H2O, more precisely we should write 1H2

16O. This has been termed ‘light’ water and is unobtainable. If it were obtainable it’s triple

point would be approximately +0.008°C. In practice what we drink from the tap, and what is in

the ocean is a soup of 1H, 2H with 16O, 17O and 18O. This means real water does not have

a single triple point temperature. Its ‘combination triple point’ will vary as the isotopic

composition varies. [4]

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Figure 2 [4]

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5. ITS-90

Measurements on the Kelvin and Celsius temperature scales are made with an equipment, ITS-

90 which is The Inernational Temperature Scale of 1990. ITS–90 is an approximation of the

thermodynamic temperature scale that enables the similarities and relevance of temperature

measurements internationally. There are predefined calibration points in ITS-90 which has a

range of 0.65 K to 1358 K. The scale is also other points of calibration which other multiple

temperature ranges overlap.ITS-90 is references the absolute zero and it represents the

thermodynamic (absolute) temperature scale as possible inside its range. There are different

thermometer designs which covers the entire range. For instance, helium gas thermometers,

helium vapor pressure thermometers , standard platinum resistance thermometers and

monochromatic radiation thermometers.The triple point of water, temperature scales Celcius and

Kelvin are defined by absolute zero(0 K), unfortunately it is almost illogical to use this use this at

temperatures when the temperature is very different from the triple point of water. ITS–90 has

many defined points, these are all based different thermodynamic equilibrium states of fourteen

pure chemical elements and one compound , namely water. Defined points are decided by phase

transitions; such as the melting and freezing points of pure chemical elements. However, the

lowest temperature(cryogenic) points are based exclusively on the vapor pressure and

temperature relationship of helium and its isotopes, the rest of the points that are less than room

temperature are based on triple points. Fixed points on ItS-91 can be seen below;V:vapour

pressure point T:triple point G:Gas thermometer point. M,F: Melting point, freezing

pointThermometers are calibrated by ITS–90, one should use formulas for interpolation between

these defined points. ITS–90 indicates control over variables to make sure that from lab to lab

there is reproducibility. For example, even a small change in athmospheric pressure has drastic

effects on melting poings, ITS-90 compensates for this. ITS–90 also defines “freezing” and

“melting” points clearly. This distinction depends on the heat, during measurement it is checked

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whether the heat is going into or out of the sample. There are thirteen elements with triple points

and freezing melting points on the ITS-90 standard and these values are precisely fixed, so for

different temperature measurements these data are known. There maybe small differences

between measurements calibrated by ITS–90 and thermodynamic temperature. But in general by

ITS-90 in a lab across the world for example in New Zealand a scientists can simply measure the

very same temperature by the help of international calibration standard, ITS–90. Temperatures

measured may be expressed using any temperature scale, Celsius, Kelvin, Fahrenheit, or

Rankine. For example, a temperature can be measured in kelvin-based ITS–90 standard, and

then this value can be converted to Fahrenheit scale.

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5. REFERENCES

[1] http://www.bipm.org/utils/common/pdf/its-90/SInf_Section_2_2_2013.pdf

[2] http://nettuno.ogs.trieste.it/jungo/cto/imagini/PTH2O_eng.png

[3] http://www.isotech.co.uk/files/product_file1-10.pdf

[4] http://www.isotech.co.uk/files/document_library_file-50.pdf

[5] https://www.youtube.com/watch?v=RyjOCqSSpuc video recorded by Isote

[6] http://nvlpubs.nist.gov/nistpubs/jres/80A/jresv80An3p505_A1b.pdf

[7] http://www.iapws.org/faq1/temper.htm

[8] http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/examples/triplpt.html

[9] http://goldbook.iupac.org/T06502.html

[10] http://www1.lsbu.ac.uk/water/phase.html

[11] https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Triple_point.html

[12] http://en.wikipedia.org/wiki/Triple_point

[13] http://en.wikipedia.org/wiki/Water_(molecule)#Triple_point

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