NAT'L INST. OF STAND & TECH R.I.C. NIST

PUBLICATIONS

A111D5 Dfl7fi53

NIST SPECIAL PUBLICATION 260-127

U.S. DEPARTMENT OF COMMERCE/Technology Administration

National institute of Standards and Technology

Standard Reference Materials:

Standard Reference Material 1747:

Tin Freezing-Point Cell and

Standard Reference Material 1748:

Zinc Freezing-Point Cell.

Gregory F. Strouse and Ahmet T. Ince

QC

100

.U57

0.260-127

1997
rhe National Institute of Standards and Technology was established in 1988 by Congress to "assist industry in the development of technology . . . needed to improve product quality, to modernize manufacturing processes,

to ensure product reliability . . . and to facilitate rapid commercialization ... of products based on new scientific discoveries." NIST, originally founded as the National Bureau of Standards in 1901, works to strengthen U.S. industry's

competitiveness; advance science and engineering; and improve public health, safety, and the environment. One

of the agency's basic functions is to develop, maintain, and retain custody of the national standards of

measurement, and provide the means and methods for comparing standards used in science, engineering,

manufacturing, commerce, industry, and education with the standards adopted or recognized by the Federal Government.

As an agency of the U.S. Commerce Department's Technology Administration, NIST conducts basic and

applied research in the physical sciences and engineering, and develops measurement techniques, test methods, standards, and related services. The Institute does generic and precompetitive work on new and

advanced technologies. NIST's research facilities are located at Gaithersburg, MD 20899, and at Boulder, CO 80303.

Major technical operating units and their principal activities are listed below. For more information contact the Publications and Program Inquiries Desk, 301-975-3058.

Office of the Director

• National Quality Program

• International and Academic Affairs

Technology Services

• Standards Services

• Technology Partnerships

• Measurement Services

• Technology Innovation

• Information Services Advanced Technology Program

• Economic Assessment

• Information Technology and Applications

• Chemical and Biomedical Technology

• Materials and Manufacturing Technology

• Electronics and Photonics Technology

Manufacturing Extension Partnership

Program

• Regional Programs

• National Programs

• Program Development

Electronics and Electrical Engineering

Laboratory

• Microelectronics

• Law Enforcement Standards

• Electricity

• Semiconductor Electronics

• Electromagnetic Fields'

• Electromagnetic Technology 1

• Optoelectronics'

Chemical Science and Technology

Laboratory

• Biotechnology

• Physical and Chemical Properties 2

• Analytical Chemistry

• Process Measurements

• Surface and Microanalysis Science

Physics Laboratory

• Electron and Optical Physics

• Atomic Physics

• Optical Technology

• Ionizing Radiation

• Time and Frequency

1

• Quantum Physics'

Materials Science and Engineering

Laboratory

• Intelligent Processing of Materials

• Ceramics

• Materials Reliability

1

• Polymers

• Metallurgy

• NIST Center for Neutron Research

Manufacturing Engineering

Laboratory

• Precision Engineering

• Automated Production Technology

• Intelligent Systems

• Fabrication Technology

• Manufacturing Systems Integration

Building and Fire Research

Laboratory

• Structures

• Building Materials

• Building Environment

• Fire Safety Engineering

• Fire Science

Information Technology Laboratory

• Mathematical and Computational Sciences 2

• Advanced Network Technologies

• Computer Security

• Information Access and User Interfaces

• High Performance Systems and Services

• Distributed Computing and Information Services

• Software Diagnostics and Conformance Testing

' At Boulder. CO 80303.

2 Some elements at Boulder, CO.
NIST Special Publication 260-127

Standard Reference Materials:

Standard Reference Material 1747:

Tin Freezing-Point Cell and

Standard Reference Material 1748:

Zinc Freezing-Point Cell.

Gregory F. Strouse

Process Measurements Division

Chemical Science and Technology Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

and

Ahmet T. Ince

National Metrology Institute Gebze/Kocaeli

Turkey

U.S. DEPARTMENT OF COMMERCE, William M. Daley, Secretary

TECHNOLOGY ADMINISTRATION, Gary R. Bachula, Acting Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Robert E. Hebner, Acting Director

Issued August 1997
National Institute of Standards and Technology Special Publication 260-127

Natl. Inst. Stand. Technol. Spec. Publ. 260-127, 65 pages (Aug. 1997)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1997

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
Foreword

Standard Reference Materials (SRMs) as defined by the National Institute of Standards and

Technology (NIST) are well-characterized materials, produced in quantity and certified for one or more physical or chemical properties. They are used to assure the accuracy and compatibility of

measurements throughout the Nation. SRMs are widely used as primary standards in many

diverse fields in science, industry, and technology, both within the United States and throughout

the world. They are also used extensively in the fields of environmental and clinical analysis. In many applications, traceability of quality control and measurement processes to the national

measurement system is carried out through the mechanism and use of SRMs. For many of the

Nation's scientists and technologists, it is therefore of more than passing interest to know the

details of the measurements made at NIST in arriving at the certified values of the SRMs

produced. The NIST Special Publication 260 Series is a series of papers reserved for this

purpose.

The 260 Series is dedicated to the dissemination of information on different phases of the

preparation, measurement, certification, and use of NIST SRMs. In general, much more detail

will be found in these papers than is generally allowed, or desirable, in scientific journal articles. This enables the user to assess the validity and accuracy of the measurement processes

employed, to judge the statistical analysis, and to learn details of techniques and methods

utilized for work entailing greatest care and accuracy. These papers also should provide

sufficient additional information so SRMs can be utilized in new applications in diverse fields not foreseen at the time the SRM was originally issued.

Inquiries concerning the technical content of this paper should be directed to the author(s).

Other questions concerned with the availability, delivery, price, and so forth, will receive prompt

attention from:

Standard Reference Materials Program

Bldg. 202, Rm. 204

National Institute of Standards and Technology

Gaithersburg, MD 20899

Telephone: (301) 975-6776

FAX: (301) 948-3730

e-mail: srminfo@nist.gov, or www:http://ts. nist.gov/srm

Thomas E. Gills, Chief

Standard Reference Materials Program

iii
OTHER NIST PUBLICATIONS IN THIS SERIES

Trahey, N.M., ed., NIST Standard Reference

Materials Catalog 1995-96, NIST Spec. Publ.

260 (1995 Ed.). PB95-232518/AS

Michaelis, R.E., and Wyman, L.L., Standard

Reference Materials: Preparation of White Cast

Iron Spectrochemical Standards, NBS Misc.

Publ. 260-1 (June 1964). COM74-1 1061**

Michaelis, R.E., Wyman, L.L., and Flitsch, R.,

Standard Reference Materials: Preparation of

NBS Copper-Base Spectrochemical Standards,

NBS Misc. Publ. 260-2 (October 1964).

COM74-11063**

Michaelis, R.E., Yakowitz, H., and Moore, G.A.,

Standard Reference Materials: Metallographic

Characterization of an NBS Spectrometric

Low-Alloy Steel Standard, NBS Misc. Publ.

260-3 (October 1964). COM74-1 1060**

Hague, J.L., Mears, T.W., and Michaelis, R.E.,

Standard Reference Materials: Sources of

Information, Publ. 260-4 (February 1965).

COM74-11059**

Alvarez, R., and Flitsch, R., Standard Reference

Materials: Accuracy of Solution X-Ray

Spectrometric Analysis of Copper-Base Alloys,

NBS Misc. Publ. 260-5 (February 1965).

PB168068**

Shultz, J. I., Standard Reference Materials: Methods

for the Chemical Analysis of White Cast Iron Standards, NBS Misc. Publ. 260-6 (July 1965).

COM74-11068**

Bell, R.K., Standard Reference Materials: Methods

for the Chemical Analysis of NBS Copper-Base

Spectrochemical Standards, NBS Misc. Publ.

260-7 (October 1965). COM74-1 1067**

Richmond, M.S., Standard Reference Materials:

Analysis of Uranium Concentrates at the

National Bureau of Standards, NBS Misc. Publ.

260-8 (December 1965). COM74-1 1066**

Anspach, S C., Cavallo, L.M., Garfinkel, S.B., et

al., Standard Reference Materials: Half Lives of

Materials Used in the Preparation of Standard

Reference Materials of Nineteen Radioactive

Nuclides Issued by the National Bureau of

Standards, NBS Misc. Publ. 260-9 (November

1965). COM74-11065**

Yakowitz, H., Vieth, D.L., Heinrich, K.F.J., et al., Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric

Standards II: Cartridge Brass and Low-Alloy

Steel, NBS Misc. Publ. 260-10 (December

1965). COM74-11064**

Napolitano, A., and Hawkins, E.G., Standard

Reference Materials: Viscosity of Standard

Lead-Silica Glass, NBS Misc. Publ. 260-1 1**

(November 1966).

Yakowitz, H., Vieth, D.L., and Michaelis, R.E.,

Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric

Standards III: White Cast Iron and Stainless

Steel Powder Compact, NBS Misc. Publ. 260-12

(September 1966).

Spijkerman, J. J., Snediker, D.K., Ruegg, F.C., et al., Standard Reference Materials: Mossbauer

Spectroscopy Standard for the Chemical Shift of

Iron Compounds, NBS Misc. Publ. 260-13**

(July 1967).

Menis, O., and Sterling, J.T., Standard Reference

Materials: Determination of Oxygen in Ferrous

Materials (SRMs 1090, 1091, 1092), NBS Misc.

Publ. 260-14** (September 1966).

Passaglia, E. and Shouse, P. J., Standard Reference

Materials: Recommended Method of Use of

Standard Light-Sensitive Paper for Calibrating

Carbon Arcs Used in Testing Testiles for Colorfastness to Light, NBS Spec. Publ. 260-15

(July 1967). Superseded by SP 260-41.

Yakowitz, H., Michaelis, R.E., and Vieth, D.L.,

Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric

Standards IV: Preparation and Microprobe

Characterization of W-20% Mo Alloy Fabricated

by Powder Metallurgical Methods, NBS Spec.

Publ. 260-16 (January 1969). COM74-1 1062**

Catanzaro, E.J., Champion, C.E., Garner, E.L., et

al., Standard Reference Materials: Boric Acid;

Isotopic, and Assay Standard Reference

Materials, NBS Spec. Publ. 260-17 (February

1970). PB189457**

iv
Geller, S.B., Mantek, P.A., and Cleveland, N.G.,

Calibration of NBS Secondary Standards

Magnetic Tape Computer Amplitude Reference

Amplitude Measurement "Process A," NBS

Spec. Publ. 260-18 (November 1969).

Superseded by SP 260-29.

Paule, R.C., and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory

Measurements on the Vapor Pressure of Gold

(Certification of SRM 745). NBS Spec. Publ.

260-19 (January 1970). PB190071**

260-20: Unassigned

Paule, R.C., and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory

Measurements on the Vapor Pressures of

Cadmium and Silver, NBS Spec. Publ. 260-21

(January 1971). COM74-1 1359**

Yakowitz, H., Fiori, C.E., and Michaelis, R.E.,

Standard Reference Materials: Homogeneity

Characterization of Fe-3 Si Alloy, NBS Spec.

Publ. 260-22 (February 1971). COM74-1 1357**

Napolitano, A., and Hawkins, E.G., Standard

Reference Materials: Viscosity of a Standard

Borosilicate Glass, NBS Spec. Publ. 260-23

(December 1970). COM71-00157**

Sappenfield, K.M., Marinenko, G., and Hague, J.L., Standard Reference Materials: Comparison of

Redox Standards, NBS Spec. Publ. 260-24

(January 1972). COM72-50058**

Hicho, G.E., Yakowitz, H, Rasberry, S.D., et al., Standard Reference Materials: A Standard

Reference Material Containing Nominally Four

Percent Austenite, NBS Spec. Publ. 260-25

(February 1971). COM74-11356**

Martin, J.F., Standard Reference Materials:

NBS-U.S. Steel Corp. Joint Program for Determining Oxygen and Nitrogen in Steel, NBS

Spec. Publ. 260-26 (February 1971). PB

81176620**

Garner, E.L., Machlan, L.A., and Shields, W.R.,

Standard Reference Materials: Uranium Isotopic

Standard Reference Materials, NBS Spec. Publ.

260-27 (April 1971). COM74-11358**

Heinrich, K.F.J., Myklebust, R.L., Rasberry, S.D.,

et al., Standard Reference Materials: Preparation

and Evaluation of SRMs 481 and 482

Gold-Silver and Gold-Copper Alloys for Microanalysis, NBS Spec. Publ. 260-28 (August

1971) . COM71-50365**

Geller, S B., Standard Reference Materials:

Calibration of NBS Secondary Standard

Magnetic Tape (Computer Amplitude Reference)

Using the Reference Tape Amplitude

Measurement "Process A-Model 2," NBS Spec.

Publ. 260-29 (June 1971). COM71-50282**

Supersedes Measurement System in SP 260-18.

Gorozhanina, R.S., Freedman, AY., and Shaievitch,

A.B., (translated by M.C. Selby), Standard

Reference Materials: Standard Samples Issued in the USSR (A Translation from the Russian),

NBS Spec. Publ. 260-30 (June 1971).

COM71-50283**

Hust, J.G., and Sparks, L.L., Standard Reference

Materials: Thermal Conductivity of Electrolytic Iron SRM 734 from 4 to 300 K, NBS Spec.

Publ. 260-31 (November 1971).

COM71-50563**

Mavrodineanu, R., and Lazar, J.W., Standard

Reference Materials: Standard Quartz Cuvettes

for High Accuracy Spectrophotometry, NBS

Spec. Publ. 260-32 (December

1973).COM74-50018**

Wagner, H.L., Standard Reference Materials:

Comparison of Original and Supplemental SRM

705, Narrow Molecular Weight Distribution

Polystyrene, NBS Spec. Publ. 260-33 (May

1972) . COM72-50526**

Sparks, L.L., and Hust, J.G., Standard Reference

Material: Thermoelectric Voltage of Silver-28

Atomic Percent Gold Thermocouple Wire, SRM

733, Verses Common Thermocouple Materials

(Between Liquid Helium and Ice Fixed Points),

NBS Spec. Publ. 260-34 (April 1972). COM72-

50371**

Sparks, L.L., and Hust, J.G., Standard Reference

Materials: Thermal Conductivity of Austenitic

Stainless Steel, SRM 735 from 5 to 280 K, NBS

Spec. Publ. 260-35 (April 1972).

COM72-50368**

v
Cali, J. P., Mandel, J., Moore, L.J., et al., Standard

Reference Materials: A Reference Method for the

Determination of Calcium in Serum NBS SRM

915, NBS Spec. Publ. 260-36 (May 1972).

COM72-50527**

Shultz, J.I., Bell, R.K., Rains, T.C., et al., Standard

Reference Materials: Methods of Analysis of

NBS Clay Standards, NBS Spec. Publ. 260-37

(June 1972). COM72-50692**

Richard, J.C., and Hsia, J. J., Standard Reference

Materials: Preparation and Calibration of

Standards of Spectral Specular Reflectance, NBS

Spec. Publ. 260-38 (May 1972). COM72-

50528**

Clark, A.F., Denson, V.A., Hust, J.G., et al., Standard Reference Materials: The Eddy Current

Decay Method for Resistivity Characterization of

High-Purity Metals, NBS Spec. Publ. 260-39

(May 1972). COM72-50529**

McAdie, H.G., Garn, P.D., and Menis, O., Standard

Reference Materials: Selection of Differential

Thermal Analysis Temperature Standards

Through a Cooperative Study (SRMs 758, 759,

760), NBS Spec. Publ. 260-40 (August 1972)

COM72-50776**

Wood. L.A., and Shouse, P.J., Standard Reference

Materials: Use of Standard Light-Sensitive Paper

for Calibrating Carbon Arcs Used in Testing

Textiles for Colorfastness to Light, NBS Spec.

Publ. 260-41 (August 1972). COM72-50775**

Wagner, H.L., and Verdier, P.H., eds., Standard

Reference Materials: The Characterization of

Linear Polyethylene, SRM 1475, NBS Spec.

Publ. 260-42 (September 1972).

COM72-50944**

Yakowitz, H, Ruff, A.W., and Michaelis, R.E.,

Standard Reference Materials: Preparation and

Homogeneity Characterization of an Austenitic

Iron-Chromium-Nickel Alloy, NBS Spec. Publ.

260-43 (November 1972). COM73-50760**

Schooley, J.F., Soulen, R.J., Jr., and Evans, G.A.,

Jr., Standard Reference Materials: Preparation

and Use of Superconductive Fixed Point Devices,

SRM 767, NBS Spec. Publ. 260-44 (December

1972). COM73-50037**

Greifer, B., Maienthal, E.J., Rains, T.C., et al., Standard Reference Materials: Development of

NBS SRM 1579 Powdered Lead-Based Paint,

NBS Spec. Publ. 260-45 (March 1973).

COM73-50226**

Hust, J.G., and Giarratano, P. J., Standard Reference

Materials: Thermal Conductivity and Electrical

Resistivity Standard Reference Materials:

Austenitic Stainless Steel, SRMs 735 and 798,

from 4 to 1200 K, NBS Spec. Publ. 260-46

(March 1975). COM75-10339**

Hust, J.G., Standard Reference Materials: Electrical

Resistivity of Electrolytic Iron, SRM 797, and

Austenitic Stainless Steel, SRM 798, from 5 to 280 K, NBS Spec. Publ. 260-47 (February

1974) . COM74-50176**

Mangum, B.W., and Wise, J.A., Standard Reference

Materials: Description and Use of Precision

Thermometers for the Clinical Laboratory, SRM

933 and SRM 934, NBS Spec. Publ. 260-48

(May 1974). Superseded by NIST Spec. Publ.

260-113. COM74-50533**

Carpenter, B.S., and Reimer, G.M., Standard

Reference Materials: Calibrated Glass Standards

for Fission Track Use, NBS Spec. Publ. 260-49

(November 1974). COM74-51185**

Hust, J.G., and Giarratano, P. J., Standard Reference

Materials: Thermal Conductivity and Electrical

Resistivity Standard Reference Materials:

Electrolytic Iron, SRMs 734 and 797 from 4 to 1000 K, NBS Spec. Publ. 260-50 (June 1975).

COM75-10698**

Mavrodineanu, R., and Baldwin, JR., Standard

Reference Materials: Glass Filters As a SRM for Spectrophotometry-Selection, Preparation,

Certification, and Use-SRM 930 NBS Spec.

Publ. 260-51 (November 1975).

COM75-10339**

Hust, J.G., and Giarratano, P.J., Standard Reference

Materials: Thermal Conductivity and Electrical

Resistivity SRMs 730 and 799, from 4 to 3000 K, NBS Spec. Publ. 260-52 (September

1975) . COM75-11193**

Durst, R.A., Standard Reference Materials:

Standardization of pH Measurements, NBS

Spec. Publ. 260-53 (December 1978).

Superseded by SP 260-53 Rev. 1988 Edition.

PB88217427**

vi
Burke, R.W., and Mavrodineanu, R., Standard

Reference Materials: Certification and Use of

Acidic Potassium Dichromate Solutions as an

Ultraviolet Absorbance Standard, NBS Spec.

Publ. 260-54 (August 1977). PB272168**

Ditmars, D.A., Cezairliyan, A., Ishihara, S., et al., Standard Reference Materials: Enthalpy and

Heat Capacity; Molybdenum SRM 781, from

273 to 2800 K, NBS Spec. Publ. 260-55

(September 1977). PB272127**

Powell, R.L., Sparks, L.L., and Hust, J.G., Standard

Reference Materials: Standard Thermocouple

Material, Pt-67: SRM 1967, NBS Spec. Publ.

260-56 (February 1978). PB277172**

Cali, J. P., and Plebanski, T., Standard Reference

Materials: Guide to United States Reference

Materials, NBS Spec. Publ. 260-57 (February

1978). PB277173**

Barnes, J.D., and Martin, G.M., Standard Reference

Materials: Polyester Film for Oxygen Gas

Transmission Measurements SRM 1470, NBS

Spec. Publ. 260-58 (June 1979). PB297098**

Chang, T., and Kahn, A.H., Standard Reference

Materials: Electron Paramagnetic Resonance

Intensity Standard: SRM 2601; Description and

Use, NBS Spec. Publ. 260-59 (August 1978).

PB292097**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al., Standard Reference Materials: A Reference

Method for the Determination of Sodium in Serum, NBS Spec. Publ. 260-60 (August 1978).

PB286944**

Verdier, P.H., and Wagner, H.L., Standard

Reference Materials: The Characterization of

Linear Polyethylene (SRMs 1482, 1483, 1484),

NBS Spec. Publ. 260-61 (December 1978).

PB289899**

Soulen, R.J., and Dove, R.B., Standard Reference

Materials: Temperature Reference Standard for Use Below 0.5 K (SRM 768), NBS Spec. Publ.

260-62 (April 1979). PB294245**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al., Standard Reference Materials: A Reference

Method for the Determination of Potassium in Serum, NBS Spec. Publ. 260-63 (May 1979).

PB297207**

Velapoldi, R.A., and Mielenz, K.D., Standard

Reference Materials: A Fluorescence SRM

Quinine Sulfate Dihydrate (SRM 936), NBS

Spec. Publ. 260-64 (January 1980).

PB80132046**

Marinenko, R.B., Heinrich, K.F.J., and Ruegg, F.C.,

Standard Reference Materials:

Micro-Homogeneity Studies of NBS SRM, NBS

Research Materials, and Other Related Samples,

NBS Spec. Publ. 260-65 (September 1979).

PB300461**

Venable, W.H., Jr., and Eckerle, K.L., Standard

Reference Materials: Didymium Glass Filters for Calibrating the Wavelength Scale of

Spectrophotometers (SRMs 2009, 2010, 2013,

2014). NBS Spec. Publ. 260-66 (October 1979).

PB80104961**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al., Standard Reference Materials: A Reference

Method for the Determination of Chloride in Serum, NBS Spec. Publ. 260-67 (November

1979). PB80110117**

Mavrodineanu, R., and Baldwin, JR., Standard

Reference Materials: Metal-On-Quartz Filters as a SRM for Spectrophotometry SRM 2031, NBS

Spec. Publ. 260-68 (April 1980). PB80197486**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al., Standard Reference Materials: A Reference

Method for the Determination of Lithium in Serum, NBS Spec. Publ. 260-69 (July 1980).

PB80209117**

Marinenko, R.B., Biancaniello, F., Boyer, P. A., et

al., Standard Reference Materials: Preparation

and Characterization of an

Iron-Chromium-Nickel Alloy for Microanalysis:

SRM 479a, NBS Spec. Publ. 260-70 (May

1981). SN003-003-02328-1*

Seward, R.W., and Mavrodineanu, R., Standard

Reference Materials: Summary of the Clinical

Laboratory Standards Issued by the National

Bureau of Standards, NBS Spec. Publ. 260-71

(November 1981). PB82135161**

Reeder, D.J., Coxon, B., Enagonio, D., et al., Standard Reference Materials: SRM 900, Antiepilepsy Drug Level Assay Standard, NBS Spec.

Publ. 260-72 (June 1981). PB8 1220758

vii
Interrante, C.G., and Hicho, G.E., Standard

Reference Materials: A Standard Reference

Material Containing Nominally Fifteen Percent

Austenite (SRM 486), NBS Spec. Publ. 260-73

(January 1982). PB822 15559**

Marinenko, R.B., Standard Reference Materials:

Preparation and Characterization of K-4 1 1 and

K-412 Mineral Glasses for Microanalysis: SRM

470, NBS Spec. Publ. 260-74 (April 1982).

PB82221300**

Weidner, V.R., and Hsia, J. J., Standard Reference

Materials: Preparation and Calibration of First Surface Aluminum Mirror Specular Reflectance

Standards (SRM 2003a), NBS Spec. Publ.

260-75 (May 1982). PB82221367**

Hicho, G.E., and Eaton, E E., Standard Reference

Materials: A Standard Reference Material

Containing Nominally Five Percent Austenite

(SRM 485a), NBS Spec. Publ. 260-76 (August

1982). PB83115568**

Furukawa, G.T., Riddle, J.L., Bigge, W.G., et al., Standard Reference Materials: Application of

Some Metal SRMs as Thermometric Fixed

Points, NBS Spec. Publ. 260-77 (August 1982).

PB83 117325**

Hicho, G.E., and Eaton, E.E., Standard Reference

Materials: Standard Reference Material

Containing Nominally Thirty Percent Austenite

(SRM 487), NBS Spec. Publ. 260-78

(September 1982). PB83 115576**

Richmond, J.C., Hsia, J.J., Weidner, V.R., et al., Standard Reference Materials: Second Surface

Mirror Standards of Specular Spectral

Reflectance (SRMs 2023, 2024, 2025), NBS

Spec. Publ. 260-79 (October 1982).

PB84203447**

Schaffer, R., Mandel, J., Sun, T., et al., Standard

Reference Materials: Evaluation by an ID/MS

Method of the AACC Reference Method for Serum Glucose, NBS Spec. Publ. 260-80

(October 1982). PB842 16894**

Burke, R.W., and Mavrodineanu, R., Standard

Reference Materials: Accuracy in Analytical

Spectrophotometry, NBS Spec. Publ. 260-81

(April 1983). PB832 14536**

Weidner, V.R., Standard Reference Materials: White

Opal Glass Diffuse Spectral Reflectance

Standards for the Visible Spectrum (SRMs 2015

and 2016), NBS Spec. Publ. 260-82 (April

1983). PB83220723**

Bowers, G.N., Jr., Alvarez, R., Cali, J. P., et al., Standard Reference Materials: The Measurement

of the Catalytic (Activity) Concentration of

Seven Enzymes in NBS Human Serum (SRM

909), NBS Spec. Publ. 260-83 (June 1983).

PB83239509**

Gills, T.E., Seward, R.W., Collins, R.J., et al., Standard Reference Materials: Sampling,

Materials Handling, Processing, and Packaging

of NBS Sulfur in Coal SRMs 2682, 2683, 2684,

and 2685, NBS Spec. Publ. 260-84 (August

1983). PB84109552**

Swyt, DA., Standard Reference Materials: A Look

at Techniques for the Dimensional Calibration of

Standard Microscopic Particles, NBS Spec.

Publ. 260-85 (September 1983). PB841 12648**

Hicho, G.E., and Eaton, E.E., Standard Reference

Materials: A SRM Containing Two and

One-Half Percent Austenite, SRM 488, NBS

Spec. Publ. 260-86 (December 1983).

PB84 143296**

Mangum, B.W., Standard Reference Materials:

SRM 1969: Rubidium Triple-Point - A

Temperature Reference Standard Near 39.30° C,

NBS Spec. Publ. 260-87 (December 1983).

PB84149996**

Gladney, E.S., Burns, C.E., Perrin, D R., et al., Standard Reference Materials: 1982 Compilation

of Elemental Concentration Data for NBS

Biological, Geological, and Environmental

Standard Reference Materials, NBS Spec. Publ.

260-88 (March 1984). PB842 18338**

Hust, J.G., Standard Reference Materials: A FineGrained, Isotropic Graphite for Use as NBS

Thermophysical Property RMs from 5 to 2500

K, NBS Spec. Publ. 260-89 (September 1984).

PB85 112886**

Hust, J.G., and Lankford, A.B., Standard Reference

Materials: Update of Thermal Conductivity and

Electrical Resistivity of Electrolytic Iron, Tungsten, and Stainless Steel, NBS Spec. Publ.

260-90 (September 1984). PB851 15814**

viii
Goodrich, L.F., Vecchia, D.F., Pittman, E.S., et al., Standard Reference Materials: Critical Current

Measurements on an NbTi Superconducting

Wire SRM, NBS Spec. Publ. 260-91 (September

1984). PB85 118594**

Carpenter, B.S., Standard Reference Materials:

Calibrated Glass Standards for Fission Track

Use (Supplement to NBS Spec. Publ. 260-49),

NBS Spec. Publ. 260-92 (September 1984).

PB85 113025**

Ehrstein, J.R., Standard Reference Materials:

Preparation and Certification of SRM for Calibration of Spreading Resistance Probes,

NBS Spec. Publ. 260-93 (January 1985).

PB85 177921**

Gills, T.E., Koch, W.F., Stolz, J.W., et al., Standard

Reference Materials: Methods and Procedures

Used at the National Bureau of Standards to Certify Sulfur in Coal SRMs for Sulfur Content,

Calorific Value, Ash Content, NBS Spec. Publ.

260-94 (December 1984). PB85 165900**

Mulholland, G.W., Hartman, A.W., Hembree, G.G.,

et al., Standard Reference Materials:

Development of a 1mm Diameter Particle Size

Standard, SRM 1690, NBS Spec. Publ. 260-95

(May 1985). PB95-232518/AS**

Carpenter, B.S., Gramlich, J.W., Greenberg, R.R., et

al., Standard Reference Materials: Uranium-235

Isotopic Abundance Standard Reference

Materials for Gamma Spectrometry

Measurements, NBS Spec. Publ. 260-96

(September 1986). PB87108544**

Mavrodineanu, R., and Gills, T.E., Standard

Reference Materials: Summary of the Coal, Ore,

Mineral, Rock, and Refactory Standards Issued

by the National Bureau of Standards, NBS Spec.

Publ. 260-97 (September 1985). PB861 10830**

Hust, J.G., Standard Reference Materials: Glass

Fiberboard SRM for Thermal Resistance, NBS

Spec. Publ. 260-98 (August 1985).

SN003-003-02674-3*

Callanan, J.E., Sullivan, S.A., and Vecchia, D.F.,

Standard Reference Materials: Feasibility Study

for the Development of Standards Using

Differential Scanning Calorimetry, NBS Spec.

Publ. 260-99 (August 1985). PB86 106747**

Taylor, J.K., Trahey, N.M., ed., Standard Reference

Materials: Handbook for SRM Users, NBS Spec

Publ. 260-100 (February 1993). PB93 183796**

Mangum, B.W., Standard Reference Materials:

SRM 1970, Succinonitrile Triple-Point Standard

A Temperature Reference Standard Near 58.08°

C, NBS Spec. Publ. 260-101 (March 1986).

PB86197100**

Weidner, V.R., Mavrodineanu, R., Mielenz, K.D., et

al., Standard Reference Materials: Holmium

Oxide Solution Wavelength Standard from 240

to 640 nm - SRM 2034, NBS Spec. Publ.

260-102 (July 1986). PB86245727**

Hust, J.G., Standard Reference Materials: Glass

Fiberblanket SRM for Thermal Resistance, NBS

Spec. Publ. 260-103 (September 1985).

PB86109949**

Mavrodineanu, R., and Alvarez, R., Standard

Reference Materials: Summary of the Biological

and Botanical Standards Issued by the National

Bureau of Standards, NBS Spec. Publ. 260-104

(November 1985). PB86 155561**

Mavrodineanu, R., and Rasberry, S.D., Standard

Reference Materials: Summary of the

Environmental Research, Analysis, and Control

Standards Issued by the National Bureau of

Standards, NBS Spec. Publ. 260-105 (March

1986) . PB86204005**

Koch, W.F., ed., Standard Reference Materials:

Methods and Procedures Used at the National

Bureau of Standards to Prepare, Analyze, and

Certify SRM 2694, Simulated Rainwater, and

Recommendations for Use, NBS Spec. Publ.

260-106 (July 1986). PB86247483**

Hartman, A.W., and McKenzie, R.L., Standard

Reference Materials: SRM 1965, Microsphere

Slide (10 urn Polystyrene Spheres), NIST Spec.

Publ. 260-107 (November 1988).

PB89153704**

Mavrodineanu, R., and Gills, T.E., Standard

Reference Materials: Summary of Gas Cylinder

and Permeation Tube Standard Reference

Materials Issued by the National Bureau of

Standards, NBS Spec. Publ. 260-108 (May

1987) . PB87209953**

ix
Candela, G.A., Chandler-Horowitz, D., Novotny,

D.B., et al., Standard Reference Materials:

Preparation and Certification of an

Ellipsometrically Derived Thickness and

Refractive Index Standard of a Silicon Dioxide

Film (SRM 2530), NIST Spec. Publ. 260-109

(October 1988). PB89133573**

Kirby, R.K., and Kanare, H.M., Standard Reference

Materials: Portland Cement Chemical

Composition Standards (Blending, Packaging,

and Testing), NBS Spec. Publ. 260-1 10

(February 1988). PB88193347**

Gladney, E.S., O'Malley, B.T., Roelandts, I., et al., Standard Reference Materials: Compilation of

Elemental Concentration Data for NBS Clinical,

Biological, Geological, and Environmental

Standard Reference Materials, NBS Spec. Publ.

260-111 (November 1987). PB88 156708**

Marinenko, R.B., Blackburn, D.H., and Bodkin,

J.B., Standard Reference Materials: Glasses for Microanalysis: SRMs 1871-1875, NIST Spec.

Publ. 260-112 (February 1990). PB90215807**

Mangum, B.W., and Wise, J. A., Standard Reference

Materials: Description and Use of a Precision

Thermometer for the Clinical Laboratory, SRM

934, NIST Spec. Publ. 260-1 13 (June 1990).

PB90257643**

Vezzetti, C.F., Varner, R.N., and Potzick, J.E., Standard Reference Materials: Bright-Chromium

Linewidth Standard, SRM 476, for Calibration

of Optical Microscope Linewidth Measuring

Systems, NIST Spec. Publ. 260-1 14 (January

1991). PB91167163**

Williamson, MP., Willman, N.E., and Grubb, D.S.,

Standard Reference Materials: Calibration of

NIST SRM 3201 for 0.5 in. (12.65 mm) Serial

Serpentine Magnetic Tape Cartridge, NIST Spec.

Publ. 260-115 (February 1991). PB91 187542**

Mavrodineanu, R., Burke, R.W., Baldwin, J.R., et

al., Standard Reference Materials: Glass Filters as a Standard Reference Material for Spectrophotometry-Selection, Preparation,

Certification and Use of SRM 930 and SRM

1930, NIST Spec. Publ. 260-1 16 (March 1994).

PB94-188844/AS**

Vezzetti, C.F., Varner, R.N., and Potzick, J.E., Standard Reference Materials: Antireflecting-Chromium Linewidth Standard, SRM

475, for Calibration of Optical Microscope

Linewidth Measuring Systems, NIST Spec. Publ.

260-1 17 (January 1992). PB92-149798**

Williamson, M.P., Standard Reference Materials:

Calibration of NIST Standard Reference

Material 3202 for 18-Track, Parallel, and

36-Track, Parallel Serpentine, 12.65 mm (0.5

in), 1491 cpmm (37871 cpi), Magnetic Tape

Cartridge, NIST Spec. Publ. 260-1 18 (July

1992). PB92-226281**

Vezzetti, C.F., Varner, R.N., and Potzick, Standard

Reference Materials: Antireflecting-Chromium

Linewidth Standard, SRM 473, for Calibration

of Optical Microscope Linewidth Measuring

System, NIST Spec. Publ. 260-1 19 (September

1992)

Caskey, G.W., Philips, S.D., Borchardt., et al., Standard Reference Materials: A Users' Guide to NIST SRM 2084: CMM Probe Performance

Standard, NIST Spec. Publ. 260-120 (1994)

Rennex, B.G., Standard Reference Materials:

Certification of a Standard Reference Material

for the Determination of Interstitial Oxygen

Concentration in Semiconductor Silicon by

Infrared Spectrophotometry, NIST Spec. Publ.

260-121 (1994) PB95-125076/AS

Gupta, D., Wang, L., Hanssen, L.M., Hsai, J.J., and

Datla, R.U., Polystyrene Films for Calibrating

the Wavelength Scale of Infrared

Spectrophotometer (SRM 1921). NIST Spec.

Publ. 260-122 (1995) PB95-226866/AS

Development of Technology and the Manufacture of

Spectrometric SRMs for Naval Brasses (MC62

M63). NIST Spec. Publ. 260-123 (IN PREP).

Strouse, G.F., SRM 1744: Aluminum Freezing Point

Standard. NIST Spec. Publ. 260-124 (1995)

SN003-003-03342-1

Schiller, S.B, Standard Reference Materials:

Statistical Aspects of the Certification of

Chemical Batch SRMs. NIST Spec. Publ. 260-

125 (1996) PB96-210877/AS

x
Guenther, F.R., Dorko, W.D., Miller, W.R.,

et al., Standard Reference Materials: The

NIST Traceable Reference Material

Program for Gas Standards, NIST Spec.

Publ. 260-126 (1996) PB96-2 10786/ AS

Zhang, Z.M., Gentile, T.R., Migdall, A.L.,

and Datla, R.U., Transmission Filters with

Measured Optical Density at 1064 nm

Wavelength-SRM

2036. SRM Spec. Publ. 260-128 (IN

PREP).

Potzick, J.E., Antireflecting-Chromium

Linewidth Standard, Standard Reference

Material 473 , for Calibration of Optical

Microscope Linewidth Measuring Systems,

NIST Spec. Publ. 260-129 (1997) PB97-

151922/AS.

Zarr, R.R., Standard Reference Materials:

Glass Fiberboard, Standard Reference

Material 1450c, for Thermal Resistance

from 280K to 340K, NIST Spec. Publ.

260-130 (IN PREP).

Ehrstein, J.R., and Croarkin, M.C., The

Certification of 100 mm Diameter Silicon

Resistivity SRMs 2541 through 2547 Using

Dual-Configuration Four-Point Probe

Measurements. NIST Spec. Publ. 260-131

(IN PREP).

Send order with remittance to: Superintendent

of Documents, U.S. Government Printing

Office, Washington, DC 20402-9325.

Remittance from foreign countries should

include an additional one fourth of the

purchase price for postage.

*May be ordered from: National Technical

Information Services (NTIS), Springfield,

VA 22161.

For information phone (703-487-4650)

To Place an Order with PB# phone (800-

553-6847)

TABLE OF CONTENTS

PAGE

1. Introduction 2

2. Samples and Filling Procedure of the SRM Freezing-Point Cells 2

2.1 SRM samples 2

2.2 SRM freezing-point cell specifications and design 2

2.3 Filling of the SRM freezing-point cells 3

3. Certification Procedure for SRM 1747 and SRM 1748 6

3.1 Freezing-point realization 6

3.2 Melting-point realization

3.3 Direct comparison measurements

4. Analysis of Results 8

4.1 Analysis of SRM freezing-point curves 8

4.2 Analysis of SRM melting-point curves 19

4.3 Analysis of SRM direct comparisons 19

5. Uncertainties 32

5.1 Uncertainty of direct comparison measurements 32

5 .2 Uncertainty assigned to SRM fixed-point cells 33

6. Application 34

7. Conclusions 34

8. References 36

9. Appendix A 37

10. Appendix B 45

xiii
LIST OF FIGURES

FIGURE NO. PAGE

1 . Schematic of an SRM fixed-point cell assembly 4

2. Freezing curves for SRM 1747 fixed-point cell Sn 95-1 9

3. Freezing curves for SRM 1747 fixed-point cell Sn 95-2 10

4. Freezing curves for SRM 1747 fixed-point cell Sn 95-3 11

5. Freezing curves for SRM 1747 fixed-point cell Sn 95-4 12

6. Freezing curves for SRM 1747 fixed-point cell Sn 95-5 13

7. Freezing curves for SRM 1748 fixed-point cell Zn 95-1 14

8. Freezing curves for SRM 1748 fixed-point cell Zn 95-2 15

9. Freezing curves for SRM 1748 fixed-point cell Zn 95-3 16

1 0. Freezing curves for SRM 1 748 fixed-point cell Zn 95-4 17

1 1 . Freezing curves for SRM 1 748 fixed-point cell Zn 95-5 18

12. Melting curves for SRM 1747 fixed-point cell Sn 95-1 20

13. Melting curves for SRM 1747 fixed-point cell Sn 95-2 21

14. Melting curves for SRM 1747 fixed-point cell Sn 95-3 22

15. Melting curves for SRM 1747 fixed-point cell Sn 95-4 23

16. Melting curves for SRM 1747 fixed-point cell Sn 95-5 24

17. Melting curves for SRM 1748 fixed-point cell Zn 95-1 25

18. Melting curves for SRM 1 748 fixed-point cell Zn 95-2 26

1 9. Melting curves for SRM 1 748 fixed-point cell Zn 95-3 27

20. Melting curves for SRM 1 748 fixed-point cell Zn 95-4 28

2 1 . Melting curves for SRM 1 748 fixed-point cell Zn 95-5 29

22. Direct freezing plateau comparison results of Sn 95-1, Sn 95-2, Sn 95-3,

Sn 95-4 and Sn 95-5 with Sn 88A (laboratory standard) 30

23. Direct freezing plateau comparison results of Zn 95-1, Zn 95-2, Zn 95-3,

Zn 95-4 and Zn 95-5 with Zn 93-4 (laboratory standard) 31

xiv
APPENDIX

PAGE

A. Certificate for SRM 1747: Tin Freezing-Point Cell 37

B. Certificate for SRM 1748: Zinc Freezing-Point Cell 45

xv

Standard Reference Material 1747: Tin Freezing-Point Cell and

Standard Reference Material 1748: Zinc Freezing-Point Cell

Gregory F. Strouse

National Institute of Standards and Technology

Process Measurements Division

Gaithersburg, MD 20899

Ahmet T. Ince

National Metrology Institute

Gebze/Kocaeli, Turkey

Abstract

The freezing points of Sn (231.928 °C) and Zn (419.527 °C) are defining fixed points of the

International Temperature Scale of 1990 (ITS-90). Realization of these freezing points is performed

using fixed-point cells containing high-purity (> 99. 9999% pure) metal. The five Sn and five Zn

freezing-point cells constituting Standard Reference Material (SRM) 1747 and SRM 1748,

respectively, have been constructed and certified as suitable for use in the realization of the freezingpoint temperatures of Sn and Zn for the ITS-90. The expanded uncertainties (k=2) assigned to the

freezing-point temperatures of the fixed-point cells SRM 1747 and SRM 1748 do not exceed

0.40 m°C and 1.1 4 m°C, respectively. In this document, the methods used for the construction of the

SRM Sn and SRM Zn freezing-point cells and their certification are described.

Disclaimer

Certain commercial equipment, instruments or materials are identified in this paper in order to

adequately specify the experimental procedure. Such identification does not imply recommendation

or endorsement by NIST, nor does it imply that the materials or equipment identified are necessarily

the best available for the purpose.

Acknowledgment

The authors wish to thank the Standard Reference Materials Program for their support in the

development ofthese new SRMs and William Guthrie of the Statistical Engineering Division for his

helpful discussions on uncertainties.

1
1. Introduction

The two freezing points required to realize the International Temperature Scale of 1990 (ITS-90)

from 0 °C to 420 °C are that of Sn (23 1 .928 °C) and Zn (419.527 °C)[1]. These freezing points are

realized by using thermometric fixed-point cells containing high-purity (> 99.9999% pure) metals.

Sn and Zn freezing-point cells, with the triple point of water (TPW) cell (0.01 °C), are used for the

ITS-90 calibration of standard platinum resistance thermometers (SPRTs) from 0 °C to 419.527 °C,

for part ofthe calibration of SPRTs from 0 °C to 660.323 °C, and for part of the calibration of hightemperature SPRTs (HTSPRTS) from 0 °C to 961.78 °C. To provide certified fixed-point cells for

this purpose, we have developed two Standard Reference Materials (SRMs), the Sn Freezing-Point

Cell (SRM 1747) and the Zn Freezing-Point Cell (SRM 1748).

Five fixed-point cells of each type (designated Sn 95-1, Sn 95-2, Sn 95-3, Sn 95-4 and Sn 95-5;

Zn 95-1, Zn 95-2, Zn 95-3, Zn 95-4 and Zn 95-5) were certified as SRMs. The certification

procedure of each cell included the evaluation of freezing and melting curves and the direct

comparison with the appropriate laboratory freezing-point standard (Sn 88A or Zn 93-4) in the NIST

Platinum Resistance Thermometry (PRT) Laboratory. Four of each of the five cells were selected

as SRMs available through the Standard Reference Materials Program with the fifth cell (Sn 95-1 and

Zn 95-4) kept in the NIST Platinum Resistance Thermometer Laboratory as an SRM reference cell. The certification procedure and results of the two SRMs are given below.

2. Samples and Filling Procedure of the SRM Freezing-Point Cells

2.1 SRM samples

A 6 kg lot (M3821) of high-purity (>99.9999% pure) Sn metal to be used for SRM 1747 and a 6 kg

lot (M3371) of high-purity (>99.9999% pure) Zn metal to be used for SRM 1748 were purchased

from Johnson Matthey Company of Spokane, Washington. The metals were in teardrop form

(nominally 3 mm o.d.) placed in plastic bottles, each containing 1 kg of Sn or Zn sealed in an Ar

atmosphere. The Johnson Matthey Company emission spectrographic assay of the Sn (Appendix A)

shows the total impurity level to be 0.6 ug/g (0.6 parts per million), resulting from 0.3 ug/g of Sb,

0.2 ug/g of Bi and 0. 1 ug/g of Ag; and the emission spectrographic assay of the Zn (Appendix B)

shows the total impurity level to be 0.3 ug/g (0.3 parts per million), resulting from 0.2 ug/g of Sn and

0.1 ug/g of Cu.

2.2 SRM freezing-point cell specifications and design

Five thermometric fixed-point cells of each metal were constructed for certification as ITS-90

freezing-point standards. Each SRM 1747 cell contains 1071 g of the high-purity Sn metal from

randomly selected bottles of lot M3821 and each SRM 1748 cell contains 1031 g of the high-purity

Zn metal from randomly selected bottles of lot M3371.

2
As shown in figure 1, the metal (K) was contained within a high-purity graphite crucible (L), with

a high-purity graphite cap (I) and a high-purity graphite re-entrant well (J). The graphite assembly

was placed inside a precision-bore borosilicate-glass envelope (H) (46.2 cm long, ground to 5 cm

o.d. with a wall thickness of 0.4 cm). Axially located in the annular space between the graphite

crucible and the borosilicate-glass envelope was a piece of ceramic fiber blanket (M) (24.3 cm long,

14.1 cm wide and 0.15 cm thick) for thermal insulation between the borosilicate-glass envelope and

the graphite crucible and to provide cushioning for the graphite assembly. Above the graphite cap,

there was a matte-finished borosilicate-glass guide tube (F) (1.0 cm o.d. with a wall thickness of

0.1 cm), 1.2 cm thick washed-ceramic fiber disks (E) and two graphite heat shunts (G). The first heat shunt was placed approximately 3.2 cm and the second heat shunt was placed approximately

10.8 cm above the top of the graphite cap and are snug fitting in the borosilicate-glass envelope. A

space of about 1.8 cm between the top of the borosilicate-glass envelope and the ceramic fiber

insulation allows for the silicone rubber stopper to be glued into place using silicone-rubber sealant

in the top of the borosilicate-glass envelope. This silicone rubber stopper (D) has (1) a modified

compression fitting with a silicone rubber O-ring (C) for inserting and sealing the SPRT into the

fixed-point cell and (2) a stainless steel gas filling tube (B) (4.3 cm long and 0.3 cm o.d.) for

evacuating and backfilling the cell with an inert gas. Additionally, the gas filling tube allows for a

slight overpressure (0.25 kPa above atmospheric pressure) of an inert gas (Ar or He) in the cell to prevent contamination of the metal.

The immersion depth for the SRM cells was 1 8 cm from the sensor mid-point of the SPRT to the

top of the liquid metal surface (distance from the bottom of the graphite re-entrant well to the top

of the liquid level is 20.5 cm). The pressure in the cells during use was 101 325 Pa ±27 Pa.

The high-purity (> 99.9999% pure) graphite pieces (crucible, well, cap and heat shunts) used for

SRMs 1747 and 1748 were purchased from Carbone of America, Ultra Carbon Division, Bay City,

Michigan. The usable volume space, allowing for a 1 cm head space between the liquid metal and

the underside of the graphite top, is 149 cm3

.

The fixed-point cell assembly of the Sn and Zn freezing-point cells was placed inside a three-zone

furnace for evaluation. A description of the furnace may be found in Ref. [2].

2.3 Filling of the SRM freezing-point cells

Any handling procedure ofhigh-purity material is apt to introduce contamination. The teardrop form

is convenient for handling and filling during freezing-point cell construction, while a solid

cylindrical sample may require cutting and cleaning. By using one-time use polyethylene gloves,

efforts were made to maintain the purity of the metal and the fixed-point cell components that come

in contact with the metal.

Prior to filling the graphite crucible with the metal sample, the graphite crucible assembly (crucible,

cap, re-entrant well) and graphite heat shunts were placed in a silica-glass furnace tube and "bakedout" at 650 °C under vacuum for 4 hours. This "bake-out" of the graphite was a final purification

3
Figure 1. A schematic of an SRM freezing-point cell showing: (A) a 25 .5 Q SPRT; (B) fill tube to

inert gas (Ar or He) supply and pressure gauge; (C) thermometer gas seal (a modified Swagelok

fitting with a silicone rubber O-ring); (D) silicone rubber stopper; (E) thermal insulation (1 .2 cm thick

washed Fiberfrax disks); (F) matte-finished borosilicate-glass guide tube; (G) two graphite heat

shunts; (FT) precision-bore borosilicate-glass envelope; (I) graphite cap; (J) graphite re-entrant well;

(K) metal sample; (L) graphite crucible; (M) thermal insulation between the borosilicate-glass

envelope and the graphite crucible.

4
to remove hydrocarbons and other contaminants that might have been present from the fabrication

process. The vacuum system used in the fabrication of the freezing-point cells is described in

Ref. [2].

After the graphite assembly was heated and then cooled to ambient temperature under vacuum, the

furnace tube was purged with purified Ar gas and the graphite pieces removed and placed inside a clean polyethylene bag for storage. To remove any contaminants from the "bake-out," the silicaglass furnace tube was (1) cleaned in hot, soapy water; (2) rinsed with copious amounts of water;

(3) the inside soaked in 20% nitric acid-80% distilled water (volume) for 2 hours; (4) rinsed with

copious amounts of distilled water and then (5) dried. The silica-glass furnace tube was cleaned after each use ("bake-out" or "fill").

In order to introduce 1071 g of Sn or 1031 g of Zn into the crucible and insert the graphite re-entrant

well, two fillings were required. Approximately, 760 g of Sn or 800 g of Zn could be poured directly

into the graphite crucible for the first "fill." The graphite crucible with the graphite cap containing

the appropriate metal sample was placed into the cleaned furnace tube and the furnace tube was

placed in the furnace. The system was evacuated for 1 hour and then back-filled with purified Ar

to a pressure of about 34 kPa. This process of pumping and flushing the system was carried out

three times, with the system finally under vacuum. For Sn, the furnace was turned on and the

temperature was brought to 235 °C to melt the sample. For Zn, the furnace was turned on and the

temperature was brought to 424 °C to melt the sample. When the Zn metal reached a temperature

of about 345 °C, the vapor pressure of the Zn was high enough to cause vapor deposition of the Zn

on the inner wall of the furnace tube. To prevent that vapor deposition, the furnace tube was

backfilled to a partial pressure of about 34 kPa of purified Ar. After about 3 hours, the sample was

completely melted and the furnace was then allowed to cool to ambient temperature before removal

of the graphite crucible from the furnace tube.

For the second "fill," the remaining metal was added to metal in the graphite crucible from the first

"fill." The graphite re-entrant well was inserted in the graphite crucible as far as possible through

the hole in the graphite cap and the assembly was placed in the newly cleaned furnace tube. A silica- glass push rod extended from the bottom of the graphite re-entrant well through a vacuum seal at the

top of the furnace tube and for a sufficient distance above the seal at the top of the furnace tube to allow the graphite well to be pushed into place when the metal was molten. Finally, using the

method described above, the furnace tube was placed in the furnace and pumped and flushed with

purified Ar and the metal sample was melted. When the sample had melted, the graphite re-entrant well was slowly inserted into the molten metal by pushing down the silica-glass push rod. When

the graphite well was fully inserted, the furnace was turned off, the system allowed to cool to ambient temperature, and the filled graphite crucible assembly removed and placed inside a clean

polyethylene bag for storage. The filled graphite crucible assembly was then placed into its borosilicate-glass envelope and fixed-point cell assembly that was described earlier.

5
3. Certification Procedure for SRM 1747 and SRM 1748

The first step in certifying the fixed-point cells was to obtain three freezing and three melting curves

for each of the specimens in the cells, using a 25.5 Q standard platinum resistance thermometer

(SPRT). The second step in certifying the fixed point cells was to obtain three direct comparisons

with the appropriate laboratory standard by simultaneously freezing both the SRM cell and

laboratory standard. An Automatic System Laboratories (ASL) F18 30 Hz ac resistance ratio bridge,

with a thermostatically controlled (25 °C ±0.01 °C) Tinsley 5685A ac/dc 100 Q reference resistor,

was used to measure the SPRT. A description of the measurement system used in the PRT

Laboratory may be found in Ref 3

.

3.1 Freezing-point realization

In the realization of the freezing point, the recommended "induced inner freeze" method [4,5] was

used. Due to the deep supercool of approximately 25 K below the freezing-point temperature of

high-purity Sn, the technique for the realization of the Sn freezing point is different from that of the

other ITS-90 freezing-point metals. The freezing point was achieved by heating the cell overnight

to approximately 5 K above the freezing-point temperature and then setting the furnace temperature

about 0.5 K below the freezing-point temperature of the metal and monitoring the temperature of the

metal with the check thermometer. When the Sn had cooled to its freezing-point temperature, the

freezing-point cell with the check thermometer was removed from the furnace until the start of

recalescence. At the beginning of that recalescence, the fixed-point cell was placed back into the

furnace, the check thermometer removed and two fused-silica glass rods were inserted three minutes

each into the reentrant well of the cell to induce an inner solid-liquid interface.

The freezing point of Zn was achieved by heating the cell overnight to approximately 5 K above the

freezing point temperature and then setting the furnace temperature about 1 K below the freezingpoint temperature of the metal and monitoring the fixed point with the check thermometer during

the supercool of the metal and the subsequent recalescence. At the beginning of the recalescence,

the furnace control temperature was set to approximately 0.5 K below the fixed-point temperature

of the metal, the check thermometer removed and two fused-silica glass rods were inserted five

minutes each into the reentrant well of the cell to induce an inner solid-liquid interface.

Finally, for both the Sn and Zn SRM cells, the "cold" thermometer was reinserted into the cell and,

after equilibrium was obtained, measurements begun. Using an excitation current of 1 mA,

thermometer readings were recorded continuously until the freezing was complete. After each

freezing-point realization the SPRT was measured at the TPW to ensure that the thermometer had

not changed.

From the analysis of the freezing curve plateaus, an estimation of the total impurity level in the metal

sample may be made using Raoult's Law of dilute solutions [6]. The total impurity level obtained

from the assay of the metal sample and Raoult's Law of dilute solutions gives a calculated estimation

of the depression in temperature over the first 50% of a freezing curve. The calculated and the

6
experimentally determined temperature depressions may be compared to confirm the overall purity

of the metal sample in the fixed-point cell. Differences between the calculated and the

experimentally determined temperature depressions may indicate either that Raoult's Law of dilute

solutions is not exact, that the sample contained additional impurities not accounted for in the

emission spectrographic assay or perhaps that additional impurities were inadvertently added to the

metal during the construction of the fixed-point cells. In estimating the expected temperature

depression, Raoult's Law of dilute solutions is intended to provide a guideline and does not strictly

apply. Additionally, there is an uncertainty in the extrapolation method chosen to derive the

temperature depression.

3.2 Melting-point realization

After the metal sample was slowly and completely frozen in the above manner, the furnace

temperature was set at about 1 °C above the freezing-point temperature to slowly melt the metal over

a time of approximately 10 hours. Using an excitation current of 1 mA, thermometer readings were

recorded continuously until the melting was complete. After each melting-point realization, the

SPRT was measured at the TPW to ensure that the thermometer had not changed.

The temperature range of a melting curve is not indicative of the purity of the metal. Using the

melting curve to indicate purity is complicated by the fact that the shape and range of a melting

curve will depend upon the history of the previous freezing of the metal in the fixed-point cell [7].

A slow freeze (> 10 h) causes the impurities to be segregated, which in turn causes a large melting

range. A fast freeze (<30 min) causes a homogenous mixture of the impurities within the metal

sample, which in turn causes a small melting range. A fast freeze is realized by removing the fixedpoint cell to ambient and allowing the molten metal to quench freeze.

During the melting of the sample, two liquid-solid interfaces are formed. One liquid-solid interface

is next to the inner wall of the graphite crucible and the second liquid-solid interface is near the

graphite re-entrant well. This second liquid-solid interface is formed where the lower-purity metal

solidified at the end of the previous slow freeze. The lower-purity metal has a slightly lower

freezing and melting temperature causing this second liquid-solid interface to form during the melt.

Analysis of the freezing and melting curves, shows that their temperature ranges are about the same,

which is expected if an inner melt is formed during the melt [8].

3.3 Direct comparison measurements

The second part of the certification was a direct comparison of the fixed-point cells under test with

the laboratory standard fixed-point cell to determine their freezing-point temperatures relative to that

of the reference cell. The five SRM Sn cells and five SRM Zn cells were directly intercompared

with the NIST laboratory reference cells Sn 88A and Zn 93-4, respectively. This was obtained by

realizing simultaneous freezes for the two cells in two separate but identical furnaces and making

three sets of alternate measurements, at equal time intervals, on their freezing-curve plateaus, using an SPRT. This ensures that the comparison measurements on the two cells were made at

7
approximately the same liquid-solid ratio of the metal samples. Ideally, the equivalent temperature

difference between measurements of each of the pairs would be identical. However, due to small

differences in sample purity, only the first of the three pairs of measurements on the cells was used

for the comparison. The other two pairs of measurements on the cells provided information on the

progress of the freezes. The fraction of metal frozen during a set of three measurements of the

freezing-point temperature of each cell did not exceed 20%. The SPRT was measured with

excitation currents of 1 mA and 1 .414 mA to permit extrapolation to zero-power dissipation (0 mA).

Corrections were made for any differences in pressure and hydrostatic head effects in each cell. Each cell was measured using an SPRT three times during the direct comparison and this procedure

was repeated two times. Following each set of direct comparison measurements, the SPRT was

measured at the TPW to ensure that the thermometer had not changed.

For the metal to be certified as a freezing-point standard, the fixed-point cells containing samples

of the metal should have a freezing-point temperature that is in agreement with the laboratory

reference cell containing high-purity metal to within the uncertainties of the measurements. If the

purity of the metal in the new fixed-point cell is greater than that of the reference cell, then it may

be of even higher quality, as indicated by being "hotter" than the laboratory standard. A cell that is "hotter" usually has fewer impurities, since impurities in these samples will usually decrease the

freezing-point temperature.

4. Analysis of Results

4.1 Analysis of SRM freezing-point curves

Figures 2 to 6 show the freezing curves for each of the five Sn fixed-point cells and figures 7 to 1

1

show the freezing curves for each of the five Zn fixed-point cells (the region of supercooling and

recalescence are not shown, as the curves begin after the reinsertion of the thermometer). Using an

excitation current of 1 mA, thermometer readings were recorded continuously until the freezing was

complete. The average length of a freeze for the five Sn cells was 1 0 hours and it was 1 2 hours for

the five Zn cells. The time-temperature relationship shown in the figures are calculated from the

change in resistance of the SPRT during a freezing-point realization. For comparison purposes, the

three freezing curves for each cell shown the figures were normalized so that the maximum SPRT

resistance obtained during the freezing-point realization is equivalent to the 0 m°C point on the

graph. The calculated temperature depression when 50% of the metal was frozen and the shapes

from the freezing-curve plateaus were used to estimate and confirm the overall purity of the sample

in the cell.

In most cases, impurities present in the high-purity samples will cause a depression in the

temperature of a freezing point. The total mole fraction impurity level of the metal sample

(7.5 x 10 7 mol for Sn and 4.6 x 10 7 mol for Zn) and Raoult's Law of dilute solutions give a

calculated estimation of the depression in temperature of 0.2 m°C for Sn and 0.3 m°C for Zn. As

experimentally determined from the freezing curve plateaus in figures 2 to 6, the average estimated

temperature depression from the extrapolated 0% frozen metal (time of recalescence) to the 50%

8
M

f a

jiIf

II

I

1

•

•

•

I

L

©

00©

©

i m

Onc

CO

©' c

e

N

eg

no

</> O

fio

©

©

o

o

o

© (N r<->

i i i i 30

ui 'ajnjBjaduisj juiod-psxij ui aSueip

in

3

'2

~CO

<uc

'»-> u

N

ac

-oo

3

c

a) J3

<—

>

SOc

'So3

Onc

o

-GU

X

r00

.2

V

I

3O

60c

"n

c>

CD

l— H

u

3

00









frozen metal of the flat section of the plateaus was 0.0g m°C, 0.09 m°C, 0.1 3 m°C, 0.1, m°C and

0.1, m°C for Sn 95-1, Sn 95-2, Sn 95-3, Sn 95-4 and Sn 95-5, respectively. For the Zn freezing curve plateaus in figures 7 to 1 1, the average estimated temperature depression from the extrapolated

0% frozen metal (time of recalescence) to the 50% frozen metal of the flat section of the plateaus

was 0.1 0 m°C, 0.09 m°C, 0.09 m°C, 0.12 m°C and 0.12 m°C for Zn 95-1, Zn 95-2, Zn 95-3, Zn 95-4

and Zn 95-5, respectively.

For both the Sn and Zn freezing-point cells, the experimentally determined temperature depressions

are smaller than the calculated value. Differences between the calculated and the experimentally

derived temperature depression may indicate either an uncertainty in using Raoult's Law of dilute

solutions, an uncertainty in the extrapolation method chosen to derive the temperature depression

or an uncertainty in the impurities specified in the emission spectrographic assay.

4.2 Analysis of SRM melting-point curves

Figures 12 to 16 show the melting curves for each of the five Sn fixed-point cells. Using an

excitation current of 1 mA, thermometer readings were recorded continuously until the melting was

complete. The average length of a melt for both the five Sn cells and five Zn cells was 8 hours. The

time-temperature relationship shown in the figures are calculated from the change in resistance of

the SPRT during a melting-point realization. For comparison purposes, the three melting curves for

each cell shown the figures were normalized so that SPRT resistance equivalent to 50% melted

sample passes through the 0 m°C point on the graph. From the graphs, the average temperature range

of the melting curves were 0.4 m°C, 0.5 m°C, 0.5 m°C, 0.4 m°C and 0.4 m°C for Sn 95-1, Sn 95-2,

Sn 95-3, Sn 95-4 and Sn 95-5, respectively. Figures 17 to 21 show the melting curves for each of

the five Zn fixed-point cells. From the graphs, the average temperature range of the melting curves were 0.4 m°C, 0.4 m°C, 0.6 m°C, 0.5 m°C and 0.6 m°C for Zn 95-1, Zn 95-2, Zn 95-3, Zn 95-4 and

Zn 95-5, respectively.

While the temperature range of a melting curve is not indicative of the purity of the metal, an

analysis of the difference in the liquidus-point temperatures obtained from a slow freeze (> 10 hours)

and from a melt after a fast freeze may be performed. A good cell will have a difference that is less

than 0.2 m°C [9]. The differences in the liquidus-point temperatures for the Sn cells were about

0.1 m°C and about 0.2 m°C for the Zn cells.

4.3 Analysis of SRM direct comparisons

The second part of the certification was a direct comparison of the fixed-point cells under test with

the laboratory standard fixed-point cell (Sn 88A or Zn 93-4) to determine their freezing-point

temperatures relative to that of the reference cell. Figure 22 shows the results of the direct

comparison of the five SRM Sn cells with the laboratory standard. The set of matching symbols

(open and closed) are for the direct comparison measurements of Sn 95-X (where X is 1, 2, 3, 4 or 5)

compared with Sn 88A. The average temperature difference of the first readings of each direct

comparison showed that Sn 95-1, Sn 95-2, Sn 95-3, Sn 95-4 and Sn 95-5 were 0.05 m°C, 0.09 m°C.

19
GO

ONo

00o

o

noo

©

o

COo

o

ONG

1/3

T3

s

"E.s

c/s O

eo

'*-> o

O

©

<L> .>

'*-> o

4>Oco

<L> Ui

0) -> -oo

NO 00

30ui 'ajnjBJsduiaj juiod-paxij ui 9§iraip

20

u,3

00
00

O

CO©

o

©

o

o

o

CN©

i o

e

a.) £

"a,1

00

C

Oo

TO

5

a.

22

-a

0)

eg

toB

u

TO

N

<D

TO

.„

_oI

I o>c

CO

o

o

I oo

30iu '9jn^j9diuaj juiod-paxij ui sSireip

21

i-



1 '

1 1 I L_ L—l L

o

oo©

o

O

©

©

o

O

inOscN

.5T3

:—I 13

B

JH

a.£

o

c

.2

"i—

o

ed

f

1-

O

O

i i i 30ui '9jnjBJ9dui9i juiod-poxg ui sSueqo

2u&

-o

eg

E

w

N

o

0360C

'Io

I

I IT, o

cN

"ou

o

a.

i X

COr00

XI

.2 »

to s

& 3> ° £ 00 o

.£ <u

.±2 n

H 1/5

25

. —
: 5 on S oo £ os

111 ill iii , , I i _| L_i

ONo

00o

©

NO©

©

o

©

o

I OncN

<u 13

cmo

c

.2**-> o

<a

M-H

o

o OOOvO'3-<NO<N'3- _i i i

lOOO i i 30ui c 9jnjBJ9diu9j juiod-paxg ui agueip

>

t3Qg

J3

•*-> T3

.2

<uS

O

«J W

<uN

<U

0)

eta

"c/3 edc

*£

I

<N

i ir> ONcN

&>o

-»-> a

'o

I T3

<UX

tB

00

00

c2 00

on 93

f S

N

S £

.2 o

26

00

J I I L_J I I I L_ I I I I I I L_J L

ONo

OO©

o

O

©

©

©

©

OncN

.s

d>

+-» *q3S

ju

"S, £

(/> O

C

O

4—

O

«J

<4-l

o

o

,—I 111

30ui '9jnjBJodui9; juiod-paxij in 9§ueip

I o

>

<ue

c2

1

o

W

<uN

<U

<L>

o

atc

'I

ONGN

13

o

3

ni T3

<DX

r4) JC•»->

° £ op o

.S o

,±5 N

<L> <L) E £

.3 O

28

o

U

V o o a b

w> On o o U

UU o o o £ £ £

NO

OO

O »-H 1-H o o o o o < ^ <N m

oo

m >/"> oo On On c es c

GO GO GO

»r> 10 »n

On On On c c c

GO GO GO / //< J

r°

y^ i i f i 0s 4 ISJ

.. ._j i i j ooo©

Oo

in

CNO

co ooo

lO

CNON

OO

IT") ON

COin

CN

On

CO

O

<^

COm

<N

On

CO

CO

CO

IT) CNOn

00

g6u3CO£co

"C

ex£ooou

<L>

0)oc

§-

c/i t-< c

CD£

t-H 3

in 61) 4) ON ctf

-4—t

C/5

[(Do 10'0)W(

US)>J]A\

30

CM

CN

CD

u.3

00
rON

VO

oo

NO

in o o £ £ Oi ©i u

uu o o o £ £ £

in cn no m co -^r © © ©

—I co in

m On On On C C e N N NM

m in

1/1 in 1/1 ON ON On C C C N N N

HH

H • : • < sj < :/ c

a" 1 0=

"«

— <^

_ 1 1 1 °1 1 1 oIT) oOo C£

43

U,

=3Ul«3

43Ecoc

a*£oa

*-> ou

43OU

43 T3

03Oa

033

CP

03 <—

0)s

oo

cd

13

NOOn

VO

0O

NOm

m

ON

VO

00

NOm

OnNO

00

NOin

mON

NO00

NO

CN

[(Do io"oya/(«z)ulAV

in

g

in

O

ON ifa C 43 N o

"0 43 C bi i -a S £ N 3

in c

On

g

C ~ N

<N 2 . ' 43 in > ON £ N J= c

03E

03

i—3

00au5co

'Ca

Q.5ouo

03

'3

03

00

03= T c m 4> ON $ e <- O o C 43 "5 "

T3

43O

03

43 c .£

43 § 43 £

00

oo

03u

m e

,43

43 ISCO

13

—: O

0300 J=Owc

00

CO «J

00 -rt '5 c o0 bo

q.

N

0) £ 03 0 tfc: -> % O 3

43

O

bi 43 ^ C

J5 5

00 43

43 3 £

43 J5 « -O

.b .ts 43 ks Q £ •£ £3

31

CO

<N

43

u,3

DO • —
0.06 m°C, 0.1 0 m°C and 0.1 0 m°C colder than the laboratory standard. Figure 23 shows the results

of the direct comparison of the five SRM Zn cells with the laboratory standard. The set of matching

symbols (open and closed) are for the direct comparison measurements of Zn 95-X (where X is 1, 2, 3, 4 or 5) compared with Zn 93-4. The average temperature difference of the first readings of each

direct comparison showed that Zn 95-1, Zn 95-2, Zn 95-3, Zn 95-4 and Zn 95-5 were 0.3 7 m°C,

0.44 m°C, 0.3 5 m°C, 0.32 m°C and 0.46 m°C colder than the laboratory standard. Each set of symbols

(open or closed) connected by lines are for the direct comparison measurements made during the

simultaneous freezes.

5. Uncertainties

The expanded uncertainty U assigned to the measurements was calculated from the equation:

where k is the coverage factor, s is the Type A standard uncertainty calculated from the standard

deviation of the mean of the W [W=R(t90)/R(0.01 °C)] values obtained from the direct comparison

measurements and u(i) is the estimated Type B standard uncertainty for each known component in

the measurement process that cannot be directly measured [10,1 1].

5.1 Uncertainty of direct comparison measurements

Many of the systematic effects in measurement process cancel because the measurements being

analyzed are from the direct comparisons of fixed-point cells. There were two known contributions

to the Type A standard uncertainty; one from the instrumental measurements themselves and the

second from the handling of the SPRT during transfer from cell to cell.

The value of the Type A standard uncertainty for the measurements was at most 0.0

1

0 m°C for

SRM 1747 and at most 0.01 8 m°C for SRM 1748. The two contributions attributed to the Type A

standard uncertainty were at most 0.008 m°C and 0.006 m°C from the instrumentation and at most

0.008 m°C and 0.01 7 m°C from handling of the SPRT for SRMs 1747 and 1748, respectively.

There were three known contributions to the Type B standard uncertainty in the direct comparison

measurements. These were the uncertainty in the exact immersion depth of the SPRT due to the

uncertainty in the position of the thermometer sensor during measurements, the uncertainty in the

immersion depth of the thermometer due to the uncertainty in the exact fraction of the metal sample

frozen, and the uncertainty in the adequacy of immersion of the thermometer to eliminate the

thermometer stem conduction during the intercomparisons.

The Type B standard uncertainty from the three known contributions in the direct comparison

measurements for both cells (SRM cell and the reference cell) was 0.009 m°C for SRM 1747 and

0.01 3 m°C for SRM 1748. The uncertainty in the exact immersion depth of the SPRT due to the

(1)

32
uncertainty in the position of the thermometer sensor during measurements gives an uncertainty of

1.3 u°C for SRM 1747 and 1.6 u°C for SRM 1748. The uncertainty in the immersion depth of the

thermometer due to the uncertainty in the exact fraction of the metal sample frozen gives an

uncertainty of 0.2 u°C for SRM 1747 and 0.6 u°C for SRM 1748. The uncertainty in the adequacy

of immersion of the thermometer to eliminate the thermometer stem conduction during the

intercomparisons gives an uncertainty of 6.5 u°C for SRM 1747 and 9.2 u°C for SRM 1748.

The expanded uncertainty (k=2) in the intercomparison measurements of SRM 1747 is 0.027 m°C

and 0.044 m°C for SRM 1748.

5.2 Uncertainty assigned to SRM fixed-point cells

The expanded uncertainty (k=2) [10,1 1] assigned to each of the SRM 1747 Sn freezing-point cells

is given in Table 1. The Type A standard uncertainty of 0.15 m°C is the standard deviation of W^)

values of repeated measurements of the laboratory standard cell with a check SPRT [12]. The

Type B standard uncertainty is obtained from the estimated uncertainty of 0.09 m°C in the freezingpoint temperature of the laboratory standard calculated from the impurities listed in the metal assay

[12], the temperature difference between the SRM cell and the laboratory standard cell as determined

from the intercomparison measurements (see Section 4.3), and the uncertainty in those

intercomparison measurements (see Section 5.1).

The expanded uncertainty (k=2) [10,1 1] assigned to each of the SRM 1748 Zn freezing-point cells

is given in Table 2. The Type A standard uncertainty of 0.3 m°C is the standard deviation of W(t90) values of repeated measurements of the laboratory standard cell with a check SPRT [12]. The

Type B standard uncertainty is obtained from the estimated uncertainty of 0.17 m°C in the freezingpoint temperature of the laboratory standard calculated from the impurities listed in the metal assay

[12], the temperature difference between the SRM cell and the laboratory standard cell as determined

from the intercomparison measurements (see Section 4.3), and the uncertainty in those

intercomparison measurements (see Section 5.1).

Table 1. Estimate of uncertainties assigned to SRM 1747 Sn freezing-point cells. SRM 1747 Type A Type B Expanded

Freezing-Point Standard Uncertainty Standard Uncertainty Uncertainty, k=2

Cell m°C m°C m°C

Sn 95-1 0.15 0.10 0.36

Sn 95-2 0.15 0.13 0.39

Sn 95-3 0.15 0.11 0.37

Sn 95-4 0.15 0.14 0.40

Sn 95-5 0.15 0.14 0.40

33
I

Table 2. Estimate of uncertainties assigned to SRM 1748 Zn freezing -point cells. SRM 1748 Type A Type B Expanded

Freezing-Point Standard Uncertainty Standard Uncertainty Uncertainty, k=2

Cell m°C m°C m°C

Zn 95-1 0.3 0.41 1.0,

Zn 95-2 0.3 0.47 l.l 2 Zn 95-3 0.3 0.39 0-98 Zn 95-4 0.3 0.36 0.94 Zn 95-5 0.3 0.49 l.l 4

6. Application

Prior to use of one of these SRM cells, the cell should be evacuated and backfilled with an inert gas

(He or Ar) several times to remove any air present in the cell. During use, the cell should have a

slight overpressure of an inert gas (He or Ar) of about 250 Pa above atmospheric to prevent air from

entering the cell. To prevent possible breakage of the fixed-point cell and to ensure correct formation and advancement of the solid/liquid interfaces, the temperature gradient in the furnace

used for realizing the fixed point should be small (< 10 m°C) over the crucible length (see Section

3.1).

In using one of these SRM cells, a long-stem SPRT should be associated with the fixed-point cell to act as a check thermometer for use during calibration. The check thermometer should be used to measure the beginning of each freeze prior to measurement of test thermometers and then at the end

of the freeze to ensure that the freeze plateau has progressed as expected. As a continuing check on

the overall purity of the metal sample contained in the fixed-point cell, melting and freezing curves should be obtained every 6 months and compared with those obtained previously.

In assigning a temperature value to a realization of a freezing-point cell, corrections must be applied

for the depth of immersion (0) of the thermometer sensing element below the surface of the metal

(dt/dfi = 2.2 x 10' 3 °C/m for Sn and 2.7 x 10 3 °C/m for Zn) [l]. Also, if the pressure (p) over the

cell during the measurements is not controlled at 101 325 Pa (l standard atmosphere), a correction

(dt/dp = 3.3 x 10~ 8 °C/Pa for Sn and 4.3 x 10" 8 °C/Pa for Zn) must be made for the difference in

pressure [l]. The immersion depth for the SRM cells was 18 cm from the sensor mid-point of the

SPRT to the top of the liquid metal surface (distance from the bottom of the graphite re-entrant well

to the top of the liquid level is 20.5 cm). The pressure in the cells during use was 101 325 Pa

±27 Pa.

7. Conclusions

The certification process of the freezing-point cells of SRM 1 747 and SRM 1 748 has shown that the

cells contain high-purity (> 99.9999%) metal samples and are acceptable for use as a defining fixed

points of the ITS-90. Direct comparisons of the SRM cells with the appropriate laboratory standard

34
have shown that the SRM cells contain metal samples of slightly lower purity than the laboratory

standard. A copy of a certificate for SRM 1747 and SRM 1748 is given in appendix A and

appendix B, respectively.

35
8. References

[ 1 ] H. Preston-Thomas, "The International Temperature Scale of 1 990 (ITS-90)," Metrologia,

Vol. 27, pp. 3-10, (1990); ibid. p. 107.

[2] G.T. Furukawa, J.L Riddle, W.R. Bigge and E.R. Pfeiffer, "Application of Some Metal

SRM's as Thermometric Fixed Points," NBS SP260-77, 140 pp. (1982).

[3] G.F. Strouse, "NIST Implementation and Realization of the ITS-90 Over the Range 83 K

to 1235 K. Reproducibility, Stability, and Uncertainties," Temperature. Its Measurement

and Control in Science and Industry, Edited by J.F. Schooley, Vol. 6, pp. 169-174,

(American Institute of Physics, New York, 1992).

[4] E.H. McLaren, "The Freezing Points of High Purity Metals as Precision Temperature

Standards," Canadian J. of Phys., 35, pp. 1086-1 106 (1957).

[5] B.W. Mangum and G.T. Furukawa, "Guidelines for Realizing the International

Temperature Scale of 1990 (ITS-90)," NIST Technical Note 1265, 190 pp. (1990).

[6] S. Glasstone, Thermodynamics for Chemists, p. 322, (D. Van Nostrand Co., Inc., New

York, 1947).

[7] J.V. McAllan and M.M. Ammar, "Comparison of the Freezing Points of Aluminum and

Antimony," Temperature. Its Measurement and Control in Science and Industry, Edited

by H.H. Plumb, Vol. 4, pp. 275-285, (Instrument Society of America, Pittsburgh, 1972).

[8] Private communication with G.T. Furukawa.

[9] B. W. Mangum, P. Bloembergen, M. V. Chattle, P. Marcarino and A. P. Pokhodun,

"Recommended Techniques for Improved Realization and Intercomparisons of Defining

Fixed Points", Report to the CCT by WG1, Comite Consultatif de Thermometrie, 19e

Session (1996).

[10] ISO, Guide to the Expression of Uncertainty in Measurement, International Organization

for Standardization, Geneva, Switzerland (1993).

[11] B.N. Taylor and C.E. Kuyatt, "Guidelines for Evaluating and Expressing the Uncertainty

of NIST Measurement Results," NIST Technical Note 1297, 17 pp. (1993).

[12] G.F. Strouse and W.L. Tew, "Assessment of Uncertainties of Calibration of Resistance

Thermometers at the National Institute of Standards and Technology," NISTIR 5319,

16 pp. (1994).

36
9. Appendix A

Certificate for SRM 1747: Tin Freezing-Point Cell

37

(Kertt fixate

Standard Reference Material

®

1747

Tin Freezing-Point Cell

Serial No. Sn 95-2

Certified Freezing Point (231.928 ± 0.000 39) °C

International Temperature Scale of 1990 (ITS-90)

This Standard Reference Material (SRM) is intended primarily for use as one of the defining fixed points of the International Temperature Scale of 1990 (ITS-90) [1]. The value of 231.928 °C is the temperature assigned to the freezing-point of pure tin [1]. The fixed-point temperature is realized as the plateau temperature of the freezing curve

of the high purity tin when it is frozen slowly. SRM 1747 consists of approximately 1071 g of tin with impurities of

0.4 mg/kg contained in a high purity graphite crucible containing a high-purity graphite re-entrant well. The design

of the metal fixed-point cells and their assemblies have been described previously [2-5]

.

An expanded uncertainty (k = 2) [6,7] of 0.39 m°C is assigned to Sn 95-2. The Type A standard uncertainty

component of 0. 15 m °C is the standard deviation of W(ft)o) values of repeated measurements of the laboratory standard

cell with a check Standard Platinum Resistance Thermometer (SPRT) [8]. The Type B standard uncertainty components

are obtained from the estimated uncertainty of 0.09 m °C in the freezing-point temperature of the laboratory standard

calculated from the impurities listed in the metal assay [8], the temperature difference of 0.09 m °C between the SRM

cell and the laboratory standard cell as determined from the intercomparison measurements and the uncertainty of

0.014 m°C in those intercomparison measurements [9].

Source of Material: The tin metal (Lot M3821) for this SRM was obtained from Johnson Matthey Co., Spokane,

WA 99216.

Notice and Warnings to Users: This fixed-point cell is not of the sealed-cell type. Prior to use, the fixed-point cell should be evacuated and back-filled with an inert gas (Helium or Argon) at least three times. The gas should be

controlled to a pressure slightly larger than atmospheric pressure to prevent contaminants from entering the cell.

Temperature studies on the fixed-point cell were performed by G.F. Strouse of the NIST Process Measurements

Division.

Statistical consultation was provided by W.F. Guthrie of the NIST Statistical Engineering Division.

The technical and support aspects involved in the preparation, certification, and issuance of this SRM were

coordinated through the NIST Standard Reference Materials Program by J.C. Colbert.

Gaithersburg, MD 20899 Thomas E. Gills, Chief

Certificate Issue Date: March 31, 1997 Standard Reference Materials Program

Page 1 of 6
Instructions for Use: In assigning a temperature value to fixed-point cells for calibration purposes, corrections must

be applied for the average depth of immersion (?) of the thermometer sensing element below the surface of the metal

(dt/de = (2.2 x 10"3) °C/m). The immersion depth of this cell is 18 cm for SPRTs. Using a NIST-manufactured

three-zone furnace, the cell has sufficient immersion to track the hydrostatic-head effect over the lowest 10 cm of

immersion. If the pressure (p) over the cell during the measurements is not controlled at 101 325 Pa (1 standard

atmosphere) [10], a correction (dt/dp = (3.3 x 10~8) °C/Pa) must be made for the difference in pressure.

Certification Testing: The thermal tests for the certification of this SRM were performed in a manner similar to that described in reference [3]. The cell contains approximately 1071 g of tin obtained from randomly-selected bottles of lot M3821.

The freezing points were prepared using the recommended "induced inner freeze" method [2]. With the metal

completely melted, the furnace was set to control at 0.5 °C to 0.75 °C below the freezing-point temperature. The

sample cooled and when the SPRT indicated that the cell temperature was within about 10 m °C of the freezing-point

value, the cell and the SPRT were withdrawn from the furnace. The cell cooled rapidly and when the SPRT detected

recalescence, the cell was replaced in the furnace. In order to freeze a thin mantle of solid around the thermometer

well, the SPRT was withdrawn from the cell well, and two fused-silica glass rods, each initially at room temperature, were inserted successively in the well for about 3 min each. Then, the cool SPRT was re-inserted into the cell. Three

freezing curves are shown in Figure 1 of this certificate (the region of supercooling and recalescence is not shown,

as the curve begins after the reinsertion of the thermometer).

After the metal was slowly and completely frozen, the furnace was set at about 2 °C above the freezing-point

temperature to slowly melt the metal over a time of approximately 8 h. Thermometer readings were recorded

continuously until the melting was complete. Three melting curves obtained under such conditions are shown in Figure 2

of this certificate.

Following the freezing and melting curve measurements, the plateau temperature of a freezing curve of the test cell was compared directly with that of the standard tin freezing-point cell of the Platinum Resistance Thermometer

Calibration Laboratory, using a 25.5 Q SPRT. Three pairs of measurements, with the SPRT being transferred directly

from one cell to the other, were made on each of three separate freezes of the two fixed-point cells being

intercompared. Although only the first of the three pairs of measurements on a given freeze of a sample was used

to determine the temperature difference between the two cells, the other two measurements on the cells provided

information on the progress of the freezes (see Figure 3). (Ideally, the difference between the measurements of each

of the pairs would be identical.) Only the first 25 % of the freezing curves were used for the intercomparisons. To

remove any bias in the measurements, the cell measured first in the sequence was changed from comparison to comparison. The method of direct comparison is described in detail in reference [9]

.

The electronic measurement equipment for the direct intercomparisons included an Automatic System Laboratories

(ASL) F181 resistance-ratio bridge, operating at a frequency of 30 Hz, and a temperature-controlled Tinsley 5685A

100 Q reference resistor. This reference resistor was maintained at a temperature of (25.00 ± 0.010) °C. Freezing curve and melting curve measurements were made with an excitation current of 1 mA. Direct comparison

measurements of the thermometer resistance were conducted at two excitation currents, 1 mA and \I2 mA with a 25.5 Q

SPRT, to allow analysis of the results at zero-power dissipation. A computer-controlled data acquisition system was used

to acquire the ASL F18 bridge readings through the use of an IEEE-488 bus.

Certain commercial materials and equipment are identified in order adequately specify the experimental

procedure. Such identification does not imply a recommendation or endorsement by the National Institute of Standards

and Technology, nor does it imply that the materials or equipment are necessarily the best available for this purpose.

Page 2 of 6
REFERENCES

[1] Preston-Thomas, H., "The International Temperature Scale of 1990 (ITS-90)," Metrologia 27, pp. 3-10 and

p. 107 (1990).

[2] Mangum B.W. and Furukawa, G.T., "Guidelines for Realizing the International Temperature of 1990 (ITS90)," NIST Tech. Note 1265, 190 pages, (1990).

[3] Furukawa, G.T., Riddle, J.L., Bigge, W.R., and Pfeiffer, E.R., "Standard Reference Materials: Application

of Some Metal SRMs as Thermometric Fixed Points," Natl. Bur. Stand. (U.S.), Special Publication 260-77,

140 pages, (1982).

[4] Riddle, J.L., Furukawa, G.T., and Plumb, H.H., "Platinum Resistance Thermometry," NBS Monograph 126,

U.S. Gov't. Printing Office, Washington, D.C. 20402, (1973).

[5] Mangum, B.W., "Platinum Resistance Thermometer Calibrations," NBS Special Publication 250-22, U.S.

Gov't. Printing Office, Washington, D.C. 20402, 364 pages, (1987).

[6] Guide to the Expression of Uncertainty in Measurement, ISBN 92-67-10188-9, 1st Ed. ISO, Geneva,

Switzerland, (1993).

[7] Taylor, B.N. and Kuyatt, C.E., "Guidelines for Evaluating and Expressing the Uncertainty of NIST

Measurement Results," NIST Tech. Note 1297, U.S. Government Printing Office, Washington, D.C,

(1994).

[8] Strouse, G.F. and Tew, W.L., "Assessment of Uncertainties of Calibration of Resistance Thermometers at the National Institute of Standards and Technology," NISTIR 5319, 16 pages, (1994).

[9] Mangum, B.W., Pfeiffer, E.R., and Strouse, G.F. (NIST); Valencia-Rodriguez, J. (CENAM); Lin, J.H. and

Yeh, T.I. (CMS/ITRI); Marcarino, P. and Dematteis, R. (IMGC); Liu, Y. and Zhao, Q. (NIM); Ince, A.T.

and Cakiroglu, F. (UME); Nubbemeyer, H.G. and Jung, H.J. (PTB), "Intercomparisons of Some NIST FixedPoint Cells with Similar Cells of Some Other Standards Laboratories," Metrologia 33, (1996).

[10] Taylor, B.N., "Guide for the Use of the International System of Units (SI)," NIST Special Publication 811,

1995 Ed., (April 1995).

Page 3 of 6


< i 00 IT) 00 On c C

X!O

w

h

i 1

?

< »

-

-A

0£

1 1 1 1 1 -J 1 1 J

4

ON

00

o 2G

£

3

V5

C«

0>B

co1/1 •c

o,S

o

o

+-> o

<L>

NO

CO

<u£

OOn

c)On

CO

On

oo

I/") CN

ON

00

o

on

oo

in

<NOn

OO

inoo

oom

ON) ON

OO

9- 6

[(3o I00)W(uS)ttlA\

I—

NO

M-lO

NO

0160

CO

P-.
UNITED STATES DEPARTMENT OF COMMERCE

National Institute of Standards and Technology

Gaithersburg. Maryland 20899O001

6 November 1996

NIST

SRMP

Gaithersburg, MD 20899

Subject: SRM 1747 (Tin freezing-point cell): Sn95-2

Dear Sir:

A direct comparison of your tin freezing-point cell (Sn 95-2) was made against our laboratory

standard tin freezing-point cell (Sn 88A). The measurement system included an ASL Model F18

operating at a frequency of 30 Hz with a 100 Q Tinsley Model 5685 reference resistor and a 25.5 Q

SPRT. The immersion depth ofyour fixed-point cell is 18 cm for a 25.5 Q SPRT (distance from top

of liquid level to bottom of cell well is 20.5 cm). An expanded uncertainty (k=2) of 0.39 m°C is estimated in the realized value (231.928 °C) of Sn 95-2 to account for the uncertainty in the

laboratory standard, the temperature difference between Sn 95-2 and Sn 88A as determined from the

direct comparison measurements, and the uncertainty in those intercomparison measurements.

Sincerely,

Dr. B. W. Mangum

Leader, Thermometry Group

Process Measurements Division

Nisr
10. Appendix B

Certificate for SRM 1748: Zinc Freezing-Point Cell

45
; ^Naitnnal ^Institute of J^tanimrfrs 8c tEecfyntflogg

Certificate

Standard Reference Material

®

1748

Zinc Freezing-Point Cell

Serial No. Zn 95-5

Certified Freezing Point (419.527 ± 0.001

4

) °C

International Temperature Scale of 1990 (ITS-90)

This Standard Reference Material (SEM) is intended primarily for use as one of the defining fixed points of the International Temperature Scale of 1990 (ITS-90) [1]. The value of 419.527 °C is the temperature assigned to the freezing-point of pure zinc [1]. The fixed-point temperature is realized as the plateau temperature of the freezing curve of the high purity zinc when it is frozen slowly. SRM 1748 consists of approximately 1031 g of zinc with

impurities of 0.3 mg/kg contained in a high purity graphite crucible containing a high purity graphite re-entrant well. The design of the metal fixed-point cells and their assemblies have been described previously [2-5]

An expanded uncertainty (k = 2) [6,7] of 1.1 4 m °C is assigned to Zn 95-5. The Type A standard uncertainty

component of 0.3 m °C is the standard deviation of W(/90) values of repeated measurements of the laboratory standard

cell with a check Standard Plantinum Resistance (SPRT) [8]. The Type B standard uncertainty components are obtained from the estimated uncertainty of 0.17 m °C in the freezing-point temperature of the laboratory standard

calculated from the impurities listed in the metal assay [8], the temperature difference of 0.46 m °C between the SRM

cell and the laboratory standard cell as determined from the intercomparison measurements and the uncertainty of

0.022 m °C in those intercomparison measurements [9].

Source of Material: The zinc metal (Lot M3371) for this SRM was obtained from Johnson Matthey Co., Spokane,

WA 99216.

Notice and Warnings to Users: This fixed-point cell is not of the sealed-cell type. Prior to use, the fixed-point cell should be evacuated and back-filled with an inert gas (Helium or Argon) at least three times. The gas should be

controlled to a pressure slightly larger than atmospheric pressure to prevent contaminants from entering the cell.

Temperature studies on the fixed-point cell were performed by G.F. Strouse of the NIST Process Measurements

Division.

Statistical consultation was provided by W.F. Guthrie of the NIST Statistical Engineering Division.

The technical and support aspects involved in the preparation, certification, and issuance of this SRM were

coordinated through the Standard Reference Materials Program by J.C. Colbert.

Gaithersburg, MD 20899 Thomas E. Gills, Chief

Certificate Issue Date: March 31, 1997 Standard Reference Materials Program

Page 1 of 6
Instructions for Use: In assigning a temperature value to fixed -point cells for calibration purposes, corrections must

be applied for the average depth of immersion ({) of the thermometer sensing element below the surface of the metal

(dt/d{ = (2.7 x 10"3 ) °C/m). The immersion depth of this cell is 18 cm for SPRTs. Using a NIST-manufactured

three-zone furnace, the cell has sufficient immersion to track the hydrostatic-head effect over the lowest 8 cm of

immersion. If the pressure (p) over the cell during the measurements is not controlled at 101 325 Pa (1 standard

atmosphere) [10], a correction (dt/dp = 4.3 x 10"8 ) °C/Pa must be made for the difference in pressure.

Certification Testing: The thermal tests for the certification of this SRM were performed in a manner similar to that described in reference [3]. The cell contains approximately 1031 g of zinc obtained from randomly-selected bottles of lot M3371.

The freezing points were prepared using the recommended "induced inner freeze" method [2]. With the metal

completely melted, the furnace was set to control at about 5 °C below the freezing-point temperature permitting the sample to supercool until recalescence was observed with the SPRT. Then, the furnace temperature was set to 0.5 °C

to 0.75 °C below the freezing-point temperature and the SPRT was removed from the cell. In order to freeze a thin mantle of solid around the thermometer well, two fused-silica glass rods, each initially at room temperature, were

inserted successively in the well for about 5 min each. Then, the cool SPRT was re-inserted into the cell. Three

freezing curves are shown in Figure 1 of this certificate (the region of supercooling and recalescence is not shown,

as the curve begins after the reinsertion of the thermometer).

After the metal was slowly and completely frozen, the furnace was set at about 2 °C above the freezing-point

temperature to slowly melt the metal over a time of approximately 9 h. Thermometer readings were recorded

continuously until the melting was complete. Three melting curves obtained under such conditions are shown in Figure 2

of this certificate.

Following the freezing and melting curve measurements, the plateau temperature of a freezing curve of the test cell was compared directly with that of the standard zinc freezing-point cell of the Platinum Resistance Thermometer

Calibration Laboratory, using a 25.5 Q SPRT. Three pairs of measurements, with the SPRT being transferred directly

from one cell to the other, were made on each of three separate freezes of the two fixed-point cells being

intercompared. Although only the first of the three pairs of measurements on a given freeze of a sample was used

to determine the temperature difference between the two cells, the other two measurements on the cells provided

information on the progress of the freezes (see Figure 3). (Ideally, the difference between the measurements of each

of the pairs would be identical.) Only the first 25 % of the freezing curves were used for the intercomparisons. To

remove any bias in the measurements, the cell measured first in the sequence was changed from comparison to comparison. The method of direct comparison is described in detail in reference [9].

The electronic measurement equipment for the direct intercomparisons included an Automatic System Laboratories

(ASL) F181 resistance-ratio bridge, operating at a frequency of 30 Hz, and a temperature-controlled Tinsley 5685A

100 Q reference resistor. This reference resistor was maintained at a temperature of (25.00 + 0.010) °C. Freezing curve and melting curve measurements were made with an excitation current of 1 mA. Direct comparison

measurements of the thermometer resistance were conducted at two excitation currents, 1 mA and v/2 mA with a 25.5 Q

SPRT, to allow analysis of the results at zero-power dissipation. A computer-controlled data acquisition system was used

to acquire the ASL F18 bridge readings through the use of an IEEE-488 bus.

Certain commercial materials and equipment are identified in order to adequately specify the experimental

procedure. Such identification does not imply a recommendation or endorsement by the National Institute of Standards

and Technology, nor does it imply that the materials or equipment are necessarily the best available for this purpose.

Page 2 of 6
REFERENCES

[1] Preston-Thomas, H., "The International Temperature Scale of 1990 (ITS-90)," Metrologia 27, pp. 3-10 and

p. 107 (1990).

[2] Mangum B.W. and Furukawa, G.T., "Guidelines for Realizing the International Temperature of 1990 (ITS90)," NIST Tech. Note 1265, 190 pages, (1990).

[3] Furukawa, G.T., Riddle, J.L., Bigge, W.R., and Pfeiffer, E.R., "Standard Reference Materials: Application

of Some Metal SRMs as Thermometric Fixed Points," Natl. Bur. Stand. (U.S.), Special Publication 260-77,

140 pages, (1982).

[4] Riddle, J.L., Furukawa, G.T., and Plumb, H.H., "Platinum Resistance Thermometry," NBS Monograph 126,

U.S. Gov't. Printing Office, Washington, D.C. 20402, (1973).

[5] Mangum, B.W., "Platinum Resistance Thermometer Calibrations," NBS Special Publication 250-22, U.S.

Gov't. Printing Office, Washington, D.C. 20402, 364 pages, (1987).

[6] Guide to the Expression of Uncertainty in Measurement, ISBN 92-67-10188-9, 1st Ed. ISO, Geneva,

Switzerland, (1993).

[7] Taylor, B.N. and Kuyatt, C.E., "Guidelines for Evaluating and Expressing the Uncertainty of NIST

Measurement Results," NIST Tech. Note 1297, U.S. Government Printing Office, Washington, D.C,

(1994).

[8] Strouse, G.F. and Tew, W.L., "Assessment of Uncertainties of Calibration of Resistance Thermometers at the National Institute of Standards and Technology," NISTIR 5319, 16 pages, (1994).

[9] Mangum, B.W., Pfeiffer, E.R., and Strouse, G.F. (NIST); Valencia-Rodriguez, J. (CENAM); Lin, J.H. and

Yeh, T.I. (CMS/ITRI); Marcarino, P. and Dematteis, R. (IMGC); Liu, Y. and Zhao, Q. (NIM); Ince, A.T.

and Cakiroglu, p. (UME); Nubbemeyer, H.G. and Jung, H.J. (PTB), "Intercomparisons of Some NIST FixedPoint Cells with Similar Cells of Some Other Standards Laboratories," Metrologia 33, (1996).

[10] Taylor, B.N., "Guide for the Use of the International System of Units (SI)," NIST Special Publication 811,

1995 Ed., (April 1995).

Page 3 of 6


i m

i On ON c c N N

1

o

W

og

1

On

00

NO

C

eu

3c/i as

<DE

c

o

•c

a,£

o

o

-4-> o

<L> *"5o

S

CO c

ON

oo^O

ON

NO

OONO

IT)

ONNO

OO

NO

v>

<N

ON

NO

OONOm

csj

On

NO

OONOm

04

[(Do lO O)Wte)^]M

4)U

vO

o

<u60
UNITED STATES DEPARTMENT OF COMMERCE

National Institute of Standards and Technology

Gaithersburg, Maryland 20899-O001

6 November 1996

N1ST

SRMP

Gaithersburg, MD 20899

Subject: SRM 1748 (Zinc freezing-point cell): Zn 95-5

Dear Sir: A direct comparison of your zinc freezing-point cell (Zn 95-5) was made against our laboratory

standard zinc freezing-point cell (Zn 93-4). The measurement system included an ASL Model F18

operating at a frequency of 30 Hz with a 100 Q Tinsley Model 5685 reference resistor and a 25.5 Q

SPRT. The immersion depth ofyour fixed-point cell is 18 cm for a 25.5 Q SPRT (distance from top

of liquid level to bottom of cell well is 20.5 cm). An expanded uncertainty (k=2) of 1.1 4 m°C is estimated in the realized value (419.527 °C) of Zn 95-5 to account for the uncertainty in the

laboratory standard, the temperature difference between Zn 95-5 and Zn 93-4 as determined from the

direct comparison measurements, and the uncertainty in those intercomparison measurements.

Sincerely,

Dr. B. W. Mangum

Leader, Thermometry Group

Process Measurements Division

Nisr

I

I

Technical Publications

Periodical

Journal of Research of the National Institute of Standards and Technology—Reports NIST research and development in those disciplines of the physical and engineering sciences in which the Institute is active. These include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement methodology and the basic technology

underlying standardization. Also included from time to time are survey articles on topics closely related to the Institute's technical and scientific programs. Issued six times a year.

Nonperiodicals

Monographs—Major contributions to the technical literature on various subjects related to the

Institute's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) devel- oped in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications—Include proceedings of conferences sponsored by NIST, NIST annual reports, and

other special publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical

properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NIST under the authority of the National Standard Data Act (Public

Law 90-396). NOTE: The Journal of Physical and Chemical Reference Data (JPCRD) is published

bimonthly for NIST by the American Chemical Society (ACS) and the American Institute of Physics (AIP).

Subscriptions, reprints, and supplements are available from ACS, 1155 Sixteenth St., NW, Washington, DC

20056.

Building Science Series—Disseminates technical information developed at the Institute on building

materials, components, systems, and whole structures. The series presents research results, test methods, and

performance criteria related to the structural and environmental functions and the durability and safety

characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of

a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NIST under the sponsorship of

other government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce

in Part 10, Title 15, of the Code of Federal Regulations. The standards establish nationally recognized

requirements for products, and provide all concerned interests with a basis for common understanding of

the characteristics of the products. NIST administers this program in support of the efforts of private-sector

standardizing organizations.

Order the following NIST publications—FIPS and NISTIRs—-from the National Technical Information

Service, Springfield, VA 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series

collectively constitute the Federal Information Processing Standards Register. The Register serves as the

official source of information in the Federal Government regarding standards issued by NIST pursuant to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of

Title 15 CFR (Code of Federal Regulations).

NIST Interagency Reports (NISTIR)—A special series of interim or final reports on work performed by

NIST for outside sponsors (both government and nongovernment). In general, initial distribution is handled

by the sponsor; public distribution is by the National Technical Information Service, Springfield, VA 22161,

in paper copy or microfiche form.
uu

E

©

O C/5

oo

o

I OS

Os

00o

E 3

I'l

Q g

. o

CZ5 -

>,5

_2 04

32 SO

3

03