A111Q5 0H7

NBS

OBLIGATIONS

^NATL If ST OF STAND & TECH A11107 E57717

NBS SPECIAL PUBLICATION 260-87

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards SRM 1969: Rubidium Triple - Point

Standard — A Temperature

Reference Standard

Near 39.30 °c

100

.U57

260-87 _

1933

- - 100 Boiling Point of H$0

58.08 SCN Triple Point

39.30 Rb Triple Point 37.0 Body Temperature

29.772 Ga Melting Point

- - 0 Ice Point
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology. THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry, and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers: Absolute Physical Quantities2 — Radiation Research — Chemical Physics —

Analytical Chemistry — Materials Science THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices; and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Manufacturing

Engineering — Building Technology — Fire Research — Chemical Engineering2 THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government. The Institute consists of the following centers: Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234.

J Some divisions within the center are located at Boulder, CO 80303.
Standard Reference Materials:

SRM 1969: Rubidium Triple-Point Standard —

A Temperature Reference Standard Near 39.30 °C

c I v

B. W. Mangum $ ^

V oo

Temperature and Pressure Division Center for Basic Standards '

. \

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director Issued December 1983
Library of Congress Catalog Card Number: 83-600621

National Bureau of Standards Special Publication 260-87

Natl. Bur. Stand. (U.S.), Spec. Publ. 260-87, 36 pages (Dec. 1983)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1983

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

Standard Reference Materials (SRM's) as defined by the National Bureau of Standards are wel 1 -characteri zed 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.

SRM's 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

are carried out through the mechanism and use of SRM's. 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 NBS in arriving at the certified

values of the SRM's produced. An NBS series of papers, of which this publication is a member, called the NBS Special Publication - 260 Series , is reserved for this purpose.

This 260 Series is dedicated to the dissemination of information on different phases of the preparation, measurement, certification and use of NBS-SRM's. 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 the greatest care and accuracy.

These papers also should provide sufficient additional infor- mation not found on the certificate so that new applications

in diverse fields not foreseen at the time the SRM was originally issued will be sought and found. Inquiries concerning the technical content of this paper

should be directed to the author. Other questions concerned

with the availability, delivery, price, and so forth will receive prompt attention from: Office of Standard Reference Materials

National Bureau of Standards Washington, D.C. 20234

Stanley D. Rasberry

Acting Chief Office of Standard Reference Materials
OTHER NBS PUBLICATIONS IN THIS SERIES

Catalog of NBS Standard Reference

(1981-83 edition).

R. W. Seward, ed., NBS Spec.

Publ .. 260 (November 1981). Michaelis, R. E. and Wyman, L. L., Standard Reference Materials: Preparation of White Cast Iron Spectrochemical Standards. NBS Misc. Publ . 260-1 (June 1964) C0M74-11061**

Michaelis, R. E., Wyman, L. L., and Flitsch, R., Standard Reference Materials: Prepararation of NBS Copper-Base

Spectrochemical Standards.

NBS Misc. Publ. 260-2 (October

1964) C0M74-11063**

Michaelis, R. E., Yakowitz, H., and Moore, G. A., Standard Reference Materials: Metal

lographic Characterization of Steel Standard. NBS Misc. Publ. 260-3 (October 1964) C0M74-11060**

Hague, J. L. Mears, T. W. , and Michaelis , R. E. , Standard Reference Materials: Sources

of Information. NBS Misc. Publ. 260-4 (February 1965) C0M74-11059

Alvarez, R. and Flitsch, R., Standard Reference Materials: Accuracy of Solution X-Ray Spectrometri c Analysis of Copper-Base Alloys. NBS Misc.

Publ. 260-5 (March 1965) PB168068**

Shultz, J. I., Standard Reference

Materials: Methods for the Chemical Analysis of White Cast

Iron Standards. NBS Misc. Publ. 260-6 (July 1975) C0M74-11068**

Bell, R. K., Standard Reference

Materials: Methods for the Chemical Analysis of NBS CopperBase Spectrochemical Standards.

NBS Misc. Publ . 260-7

(October 1965) C0M74-11067**

Richmond, M. S., Standard Reference

Materials: Analysis of Uranium

Concentrates at the National Bureau of Standards. NBS Misc.

Publ . 260-8 (December 1965) C0M74-11066**

Anspach, S. C, Cavallo, L. M., Garfinkel, S. 6., Hutchinson,

J. M. R. and Smith, C. N .

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) C0M74-11065**

Yakowitz, H., Vieth, D. L., Heinrich,

K. F. J. , and Michaelis, R. E.

Standard Reference Materials: Homogeneity Characterization on NBS Spectrometric Standards II: Cartridge Brass and Low-Alloy Steel,

NBS Misc. Publ. 260-10 (December

1965) C0M74-11064**

Napolitano, A. and Hawkins, E. G., Standard Reference Materials: Viscosity of Standard Lead-Silica

Glass. NBS Misc. Publ . 260-11 (November 1966) NBS Misc. Publ. 260-11**

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** Spijkerman, J. L., Snediker, D. K., Ruegg, F. C, and DeVoe, J. R., Standard Reference Materials: Mossbauer Spectroscopy Standard for

the Chemical Shift Compounds.

NBS Misc. Publ. 260-13** (July

1967) Menis, 0. and Sterling, J. T., Standard Reference Materials: Determination of Oxygen in Ferrous Materials - SRM 1090, 1091, and 1092. NBS Spec. 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 Textiles for Colorf astness

to Light. NBS Misc. Publ. 260-15

(June 1967) (Replaced by NBS Spec. 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) C0M74-11062**

Catanzaro, E. J., Champion, C. E., Garner, E. L., Marinenko, G., Sappenfield, K. M. and Shields,

W. R., Standard Reference Materials:

Boric Acid: Isotopic and Assay Standard Reference Materials. NBS Spec. Publ. 260-17 (February 1970)

Out of Print
Geller, S. B., Mantek, P. A., and Cleveland, N. G., Standard Reference

Materials: Calibration of NBS Secondary Standard Magnetic Tape (Computer Amplitude Reference)

Using the Reference Tape Amplitude Measurement "Process A." NBS Spec. Publ. 260-18

(November 1969) (See NBS Spec. Publ. 260-29.)

Paule, R. C. and Mandel, J., Standard Reference Materials: Analysis of Interlaboratory Measurements on the Vapor Pressure of Gold (Certification of Standard Reference Material

745). NBS Spec. Publ . 260-19 (January 1970) PB190071**

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) C0M74-11359**

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) C0M74-11357**

Napolitano, A. and Hawkins, E. G., Standard Reference Materials: Viscosity of a Standard Borosilicate Glass. NBS Spec. Publ. 260-23

(December 1970) C0M71-00157**

Sappenfield, K. M. , Marinenko, G. and Hague, J. L., Standard Reference

Materials: Comparison of Redox Standards, NBS Spec. Publ. 260-24 (January 1972) C0M72-50058**

Hicho, G. E., Yakowitz, H., Rasberry,

S. D., and Michaelis, R. E., Standard Reference Materials: A Standard Reference Material Containing Nominally Four Percent Austenite. NBS Spec. Publ. 260-25 (February 1971) C0M74-11356**

Martin, J. F., Standard Reference

Materials: National Bureau of Standards-US Steel Corporation

Joint Program for Determining

Oxygen and Nitrogen in Steel.

NBS Spec. Publ. 260-26 (February

1971) 85 cents* PB81176620

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) C0M74-11358**

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

R. E., Standard Reference Materials: Preparation and Evaluation of SRM's

481 and 482 Gold-Silver and Gold- Copper Alloys for Microanalysis.

NBS Spec. Publ. 260-28 (August 1971) C0M71-50365**

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) C0M71-50282

Gorozhanina, R. S., Freedman, A. Y., 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) C0M71 -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) C0M71 -50563** Mavrodineanu, R. and Lazar, J. W., Standard Reference Materials: Standard Quartz Cuvettes for High Accuracy Spectrophotometry. NBS Spec. Publ. 260-32 (December 1973)

55 cents* SN003-003-01213-1

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) C0M72-50526**

Sparks, L. L. and Hust, J. G., Standard Reference Materials: Thermoelectric Voltage. NBS Spec. Publ. 260-34 (April 1972) C0M72-50371**

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) 35 cents* C0M72-50368**

Cali, J. P., Mandel, J., Moore, L. J., and Young, D. S., Standard Reference

Materials: A Referee Method for the Determination of Calcium in Serum,

NBS SRM 915. NBS Spec. Publ. 260-36

(May 1972) C0M72-50527**

Shultz, J. I., Bell, R. K. , Rains,

T. C, and Menis, 0., Standard Reference Materials: Methods of Analysis of NBS Clay Standards.

NBS Spec. Publ. 260-37 (June 1972) C0M72-50692**

v
Richmond, 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) C0M72-50528**

Clark, A. F., Denson, V. A., Hust J. G. , and Powell, R. L., Standard Reference Materials: The Eddy Current Decay Method for Resistivity Characterization of High-Purity

Metals. NBS Spec. Publ . 260-39

(May 1972 C0M72-50529**

McAdie, H. G . , Garn, P. D. , and Menis, 0., Standard Reference

Materials: Selection of Thermal Analysis Temperature Standards

Through a Cooperative Study (SRM

758, 749, 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 Colorf astness

to Light. NBS Spec. Publ. 260-41

(August 1972) C0M72-50775**

Wagner, H. L. and Verdier, P. H., eds., Standard Reference Materials: The Characterization of Linear Poly- ethylene, SRM 1475. NBS Spec. Publ. 260-42 (September 1972) C0M72-50944**

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

R. E., Standard Reference Materials: Preparation and Homogeneity Characterization of an Austenitic Iron-ChromiumNickel Alloy. NBS Spec. Publ. 260-43 (November 1972) C0M73-50760**

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

Materials: Preparation and Use of Superconduti ve Fixed Point Devices,

SRM 767. NBS Spec. Pub. 260-44 (December 1972) C0M73-50037**

Greifer, B., Maienthal, E. J., Rains,

T. C, and Rasberry, S. D., Standard Reference Materials: Powdered Lead- Based Paint, SRM 1579. NBS Spec. Publ. 260-45 (March 1973) C0M73-50226**

Hust, J. G. and Giarratano, P. J., Standard Reference Materials: Thermal Conductivity and Electrical Resistivity

Standard Reference Materials: Austenitic Stainless Steel SRM's 735

and 798, from 4 to 1200 K. NBS Spec. Publ. 260-46 (March 1975) SN-003-003-

01278-5

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) C0M74-50176**

Mangum, B. W. and Wise, J. A., Standard Reference Materials: Description and Use of Precision Thermometers for the for the Clinical Laboratory, SRM 933

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

(May 1974) 60 cents* SN003-003-01278-5

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

Standard Reference Materials Calibrated

Glass Standards for Fission Track Use. NBS Spec. Publ. 260-49 (November 1974) C0M74-51185

Hust, J. G. and Giarratano, P. J., Standard Reference Materials: Thermal Conductivity

and Electrical Resistivity Standard Reference Materials: Electrolytic Iron, SRM's 734 and 797 from 4 to 1000 K. NBS

Spec. Publ. 260-50 (June 1975) $1.00* SN003-003-01425-7

Mavrodi neanu , R. and Baldwin, J. R., Standard Reference Materials: Glass Filters As a Standard for Spectro- photometry; Selection; Preparation; Certification; Use-SRM 930. NBS Spec. Publ. 260-51 (November 1975) $1.90* SN003-003-01481-8

Hust, J. G. and Giarratano, P. J., Standard Reference Materials: Thermal Conductivity and Electrical Resistivity

Standard Reference Materials 730 and 799, from 4 to 3000 K. NBS Spec. Publ. 260-52 (September 1975) $1.05* SN003-003-01464-8

Durst, R. A., Standard Reference Materials: Standardization of pH Measurements. NBS Spec. Publ. 260-53 (December 1975, Revised)

$1.05 SN003-003-01551-2

Burke, R. W. and Mavrodi neanu , R., Standard Reference Materials: Certification and Use of Acidic Potassium Dichromate Solutions as an Ultraviolet Absorbance

Standard. NBS Spec. Publ. 260-54

(August 1977) $3.00* SN003-003-01828-7

Ditmars, D. A., Cezairliyan, A., Ishihara,

S., and Douglas, T. B., Standard Reference

Materials: Enthalpy and Heat Capacity: Molybdenum SRM 781, from 273 to 2800 K. NBS Spec. Publ. 260-55 (September 1977) $2.20* SN003-003-01836-8

Powell, R. L., Sparks, L. L., and Hust,

J. G. , Standard Reference Materials: Standard Thermocouple Materials, Pt.67: SRM 1967. NBS Spec. Publ. 260-56 (February 1978) $2.20* SN003-003-018864

Cali, J. P. and Plebanski , T., Guide to United States Reference Materials. NBS

Spec. Publ. 260-57 (February 1978) $2.20*

PB 277173 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) $2.00* SN003-003-02077
Chang, T. and Kahn, A. H., Standard Reference Materials: Electron Paramagnetic Resonance Intensity

Standard; SRM 2601. NBS Spec. Pub 1

260-59 (August 1978) $2.30* SN003- 003-01975-5

Velapoldi, R. A., Paule, R. C, Schaffer,

R . , Mandel, J., and Moody, J. R., Standard Reference Materials: A Reference Method for the Determination

of Sodium in Serum. NBS Spec. Pub 1

260-60 (August 1978) $3.00 SN003-003- 01978-0

Verdier, P. H. and Wagner, H. L., Standard Reference Materials: The Characterization of Linear Polyethylene

(SRM 1482, 1483, 1484). NBS Spec.

Publ. 260-61 (December 1978) $1.70* SN003-003-02047-8

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) $2.30* SN003-003-02047-8

Velapoldi, R. A., Paule, R. C, Schaffer,

R., Mandel, J., Machlan, L. A., and Gramlich, J. W., Standard Reference

Materials: A Reference Method for the Determination of Potassium in Serum.

NBS Spec. Publ. 260-63 (May 1979) $3.75* SN003-003-02068

Velapoldi, R. A. and Mielenz, K . 0., Standard Reference Materials: A Fluorescence Standard Reference

Material Quinine Sulfate Dihydrate

(SRM 936). NBS Spec. Publ. 260-64

(January 1980) $4.25* SN003-003-02148-2

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

Materials: Micro-Homogeneity Studies

of NBS Standard Reference Materials,

NBS Research Materials, and Other Related Samples. NBS Spec. Publ. 260-65 (September 1979) $3.50* SN003- 003-02114-1

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

,

Standard Reference Materials: Didymium Glass Filters for Calibrating

the Wavelength Scale of Spectrophotometers (SRM 2009, 2010, 2013). NBS Spec. Publ. 260-66 (October 1979) $3.50* SN003-003-02127-0

Velapoldi, R. A., Paule, R. C, Schaffer,

R., Mandel, J., Murphy, T. J., and Gramlich, J. W., Standard Reference

Materials: A Reference Method for the Determination of Chloride in Serum.

NBS Spec. Publ. 260-67 (November 1979) $3.75* SN003-003-02136-9

Mavrodi neanu , R. and Baldwin, J. R., Standard Reference Materials: Metal

On-Quartz Filters as a Standard Reference Material for Spectrophoto- metry-SRM 2031. NBS Spec. Publ. 260-68 (April 1980) $4.25* SN003-003- 02167-9

Velapoldi, R. A., Paule, R. C, Schaffer,

R., Mandel, J., Machlan, L. A., Garner,

E. L., and Rains, T. C, Standard Reference Materials: A Reference Method

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

1980) $4.25* SN003-003-02214-4

Marinenko, R. B. Biancaniello, F., Boyer,

P. A., Ruff, A. W., DeRobertis, L.

,

Standard Reference Materials: Preparation

and Characterization of an Irop-Chromi urn- Nickel Alloy for Microanalysis. NBS Spec. Publ. 260-70 (May 1981) $2.50* SN003-003-02328-1

Seward, R. W. and Mavrodi neanu , R., Standard Reference Materials: Summary of the Clinical Laboratory

Standards Issued by the National Bureau of Standards. NBS Spec. Publ. 260-71 (November 1981) $6.50* SN003- 003-02381-7

Reeder, D. J., Coxon, B., Enagonio, D., Christensen, R. G., Schaffer, R .

,

Howell , B. F., Paule, R. C, and Mandel, J., Standard Reference Materials:

SRM 900. Antiepilepsy Drug Level Assay

Standard. NBS Spec. Publ. 260-72

(June 1981) $4.25* SN003-003-02329-9

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) $2.75* SN003-003-02386-8

Marinenko, R. B., Standard Reference

Materials: Preparation and Characterization of K-411 and K-414

Mineral Glasses for Microanalysis:

SRM 470. NBS Spec. Publ. 260-74

(April 1982) $3.50 SN003-003-023-95-7

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) $3.75 SN003-003-023-99-0
Hicho, G. E. and Eaton, E. E., Standard Reference Materials: A Standard Reference Material Containing Nominally Five Percent Austenite

(SRM 485a). NBS Spec. Pub 1 . 260-76

(August 1982) $3.50 SN003-003-024-33-3

Furukawa, G. T., Riddle, J. L., Bigge,

W. G., and Pfieffer, E. R . , Standard Reference Materials: Application of Some Metal SRM's as Thermometric Fixed Points. NBS Spec. Publ. 260-77 (August

1982) $6.00 SN003-003-024-34-1

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) $3.75 SN003-003-024-35-0

Richmond, J. C, Hsia, J. J., Weidner,

V. R., and Wilmering, D. B, Standard Reference Materials: Second Surface

Mirror Standards of Specular Spectral Reflectance (SRM's 2023, 2024, 2025).

NBS Spec. Publ. 260-79 (October 1982) $4.50 SN003-003-024-47-3

Schaffer, R., Mandel, J., Sun, T., Cohen,

A., and Hertz, H. S., Standard Reference

Materials: Evaluation by an ID/MS Method of the AACC Reference Method for Serum Glucose. NBS Spec. Publ. 260-80

(October 1982) $4.75 SN003-003-024-43-1

Burke, R. W. and Mavrodi neanu , R. (NBS retired), Standard Reference Materials: Accuracy in Analytical Spectrophotometry.

NBS Spec. Publ. 260-81 (April 1983) $6.00 SN003-003-024-84-8

Weidner, V. R., Standard Reference

Materials: White Opal Glass Diffuse

Spectral Reflectance Standards for the Visible Spectrum (SRM's 2015 and 2016).

NBS Spec. Publ. 260-82 (April 1983). $3.75 SN003-003-024-89-9

Bowers, G. N . , Jr., Alvarez, R., Cali, J.

P. (NBS retired), Eberhardt, K. R., Reeder, D. J., Schaffer, R., Uriano, G. A., Elser, R., Ewen, L. M. , McComb, R. B., Rej, R., Shaw, L. M., Standard Reference

Materials: The Measurement of the Catalytic

(Activity) Concentration of Seven Enzymes

in NBS Human Serum SRM 909. NBS Spec. 260-83 (June 1983) $4.50 SN003-003-024-99-6

Gills, T. E., Seward, R. W. , Collins,

R. J., and Webster, W. C, Standard Reference Materials: Sampling, Materials Handling, Processing, and Packaging of NBS Sulfur in Coal Standard Reference Materials, 2682, 2683, 2684, and 2685. NBS Spec. Publ. 260-84 (August

1983) $4.50 SN003-003-025-20-8

Swyt, D. A., Standard Reference Materials:

A Look at Techniques for the Dimensional Calibration of Standard Microscopic

Particles. NBS Spec. Publ. 260-85 (September 1983) $5.50 SN003-003-025-

21-6

Hicho, G. E. and Eaton, E. E., Standard Reference Materials: A Standard Reference Material Containing Two

and One-Half Percent Austenite,

SRM 488. NBS Spec. Publ. 260-86

(In Press). Mangum, B. W., SRM 1969: Rubidium Triple-Point Standard - A Temperature Reference Standard Near 39.30 °C. NBS Spec. Publ. 260-87 (In Press).

* Send order with remittance to Superintendent of Documents, US Government Printing Office, Washington, DC 20402. 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 22151.

v i i i
TABLE OF CONTENTS

PAGE

I. INTRODUCTION 1 II. EXPERIMENTAL DETAILS 5 II. 1. Samples 5 11. 2. Sample Holders

.

5 11. 3. Tests of Samples 7 11. 4. Apparatus Used in Testing 7 a. Constant-Temperature Bath 7 b. SRM 1969 Cell Holder 9 c. Thermometers 9 d. Temperature Measurement System 11 III. RESULTS AND DISCUSSION 11 IV. RECOMMENDED PROCEDURE FOR USE OF SRM 1969 IN CALIBRATION

OF THERMOMETERS 20

V. SUMMARY AND CONCLUSIONS 21 VI. ACKNOWLEDGMENTS 22 VII. REFERENCES 23

i x
LIST OF TABLES

TABLE NO. PAGE

I. Defining Fixed Points of the IPTS-68 2

II. Some Secondary Reference Points 3

III. Analytical Report of Rubidium Used to Fill SRM 1969 Cells. . . 6

IV. Ancillary Equipment Needed When Using SRM 1969 to Calibrate Thermometers 20

x
LIST OF FIGURES

FIGURE NO. PAGE

1. Cross-sectional drawing of SRM 1969, Rubidium

Triple-Point Standard 8

2. Drawing of Plexiglas holder assembly for SRM 1969 cells .... 10

3. Typical melting curves for two SRM 19 69 cells in an oil bath at 39.450 °C 12 4. Typical melting curves for two SRM 1969 cells in an oil bath at 39.450 °C 13

5. Melting curves for three SRM 1969 cells in an oil bath

at 39.400 °C 14

6. Histogram of the observed midpoint temperatures of

100 SRM 1969 cells 15 7. Typical melting curves of rubidium obtained with an SRM 1969 cell in baths at three different temperatures 17 8. Typical melting curves of rubidium obtained with an SRM 1969 cell in baths at three different temperatures 18 9. Typical melting curves of rubidium obtained with an SRM 1969 cell in baths at three different temperatures 19

x i

SRM 1969: Rubidium Triple-Point Standard - A Temperature Reference

Standard Near 39.30 °C

B. W. Mangum

Center for Basic Standards National Bureau of Standards

Washington, DC 20234

ABSTRACT Previous work has demonstrated the practicability of using the triple

point of rubidium as a temperature reference point. As a result of that work,

a Standard Reference Material (SRM) has been developed. It is designated SRM

1969 - the Rubidium Triple-Point Standard. This publication reports results of an investigation of 100 SRM 1969 cells; it describes SRM 1 9 6 9- , the tests

which were performed on the cells, the conditions under which the cells were

tested, the results of the tests, and the recommended procedure for the use of SRM 1969 in calibrating thermometers. For the 100 cells, the temperatures of

the midpoints of the plateaus of the melting curves ranged from 39.280 °C to 39.313 °C, with a mean value of 39.303 °C. The melting ranges of the samples

varied from approximately 8 mK to approximately 24 mK, with a mean value of

14 mK. Only two cells had melting ranges greater than 20 mK, one being 21 mK

and the other 24 mK. An estimated uncertainty of t0.010 °C is assigned to the midpoint temperatures of the plateaus of all cells except for those two cells.

Thus, in using SRM 1 969 cells for the calibration of thermometers, a calibration at any point on the plateau of the melting curve should provide a temperature (the value specified on the Certificate accompanying each cell)

with an uncertainty no greater than ^0.0 10 °C. With special care, a thermometer can be calibrated against an SRM 1969 cell to an uncertainty as small as 10.005 °C in the National Bureau of Standards (NBS) assigned value.

The use of these SRM 1969 rubidium triple-point temperature standards in clinical and biomedical laboratory applications should significantly enhance

the accuracy of temperature measurements in the region of body temperature.

I. INTRODUCTION Melting-point, freezing-point and triple-point temperatures of highpurity metals are widely used as temperature fixed points in high-precision

thermometry. 1 -4 Some fixed points serve as defining fixed points for the

International Practical Temperature Scale of 1968 (IPTS-68)5 (see Table I) and

a large number of others serve as secondary fixed points5>6 ( see Table II for

a list of some of them). Within the past few years, the melting point of gallium has been developed?"^ as an easy-to-use temperature fixed point for use in the clinical, biomedical and chemical laboratories, and to provide a point needed for the calibration of or for checking the calibration of

thermometers. 1 » TO-12 Clinical and biomedical laboratories need temperature fixed points as references for specific tests, for calibration of thermometers, and for checking thermometers for calibration drift. The feasibility of the use of

1
Table I. Defining Fixed Points of the IPTS-68a

Assigned value of International

Practical Temperature

Fixed points T68 (K) t68 ( °C)

Triple point of equilibrium hydrogen 0 Boiling point of equilibrium hydrogen at a pressure of 33330.6 Pa (25/76 standard

atmosphere) D > c Boiling point of equilibrium hydrogen 0 ' c Boiling point of neon0 Triple point of oxygen

Triple point of argond

c d Condensation point of oxygen ' Triple point of water

Boiling point of watere Freezing point of Tine Freezing point of zinc Freezing point of silver

Freezing point of gold

13.81

17.042 20.28

27. 102 54.361 83.798

90. 188 273.16

373.15

505.1181 692.73

1235.08 1337.58

-259.34

-256. 108 -252.87 -246.048

-218.789 -189.352

-182.962

0.01

100 231.9681 419.58

961.93

1064.43

aExcept for the triple points and the equilibrium hydrogen point at 17.042 K, the assigned values of temperature are for equilbrium states at a pressure of

101325 Pa (1 standard atmosphere). If differing isotopic abundances could significantly affect the fixed point temperatures, the abundances are specified.

°Equilibrium hydrogen means that the hydrogen has its equilibrium orth-para

composition at the relevant temperature. "Ortho" and "para" are the designations

for the molecular configurations (nuclear spin arrangements) of hydrogen.

fractionation of isotopes or impurities dictate the use of boiling points

(vanishingly small vapor fractions) for hydrogen and neon and condensation point

(vanishingly small liquid fraction) for oxygen. ^The triple point of argon may be used as an alternative to the condensation

point of oxygen.

e The freezing point of tin may be used as an alternative to the boiling point

of water.

2
Table II. Some Secondary Reference Points

International Practical Temperature

Equilibrium state T 68^ K ^ t6s(°C)

Triple point of normal hydrogena 13.956 -259. 194 Boiling point of normal hydrogen3 20.397 -252.753

Triple point of neon 24 .56

1

-248.589

Triple point of nitrogen 63.146 -210.004 Boiling point of nitrogen 77.344 -195.806

Boiling point of argon 87.294 -185.856 Sublimation point of carbon dioxide 194.674 . - 78.476 Freezing point of mercury 234.314 - 38.836

Ice point^ 273.15 0 Triple point of phenoxybenzene

(diphenyl ether) 300.02 26.87 Melting point of gallium 302.922 29.772 Triple point of gallium 302.924 29.774

Triple point of benzoic acid 395.52 122.37 Freezing point of indium 429.784 156.634 Freezing point of bismuth 544.592 271.442 Freezing point of cadmium 594.258 321.108 Freezing point of lead 600.652 327.502 Boiling point of mercury 629.81 356.66 Boiling point of sulfur 717.824 444.674 Melting point of the copper-aluminum eutectic 821.41 548.26 Freezing point of antimony 903 .905 630.755 Freezing point of aluminum 933.61 660.46 Freezing point of copper 1358.03 1084.88 Freezing point of nickel 1728 1455 Freezing point of cobalt 1768 1495 Freezing point of palladium 1827 1554 Freezing point of platinum 2042 1769 Freezing point of rhodium 2236 1963 (continued)

3
Table II. Some Secondary Reference Points (cont'd)

International Practical Temperature

Equilibrium state T 68<K) t 6 8(° c >

Melting point of aluminum oxide 2327 2054 Freezing point of iridium 2720 2447 Melting point of niobium 2750 2477 Melting point of molybdenum 2896 2623 Melting point of tungsten 3695 3422

a Normal hydrogen is a mixture of 15% orthohydrogen and 25% parahydrogen.

The ice point is a very close approximation to the temperature defined as being 0.01 K below the triple point of water.

4
the triple-point temperature of rubidium as a temperature reference point near

body temperature has been investigated previously by us^3 and others^. We

found"1 3 that it would meet the need for such a reference point, although the uncertainty in the definition of its triple point is considerably greater than that of some other metals, e.g., gallium"!. The uncertainty is, nevertheless,

not so great as to make the triple-point temperature of rubidium unsuitable as

a reference point, particularly if each cell is measured and the reference

temperature is certified, as is the case for SRM 1 969- Based on our previous work 13, we have now developed a rubidium triplepoint Standard Reference Material (SRM). It is designated SRM 1969 - the Rubidium Triple-Point Standard - and it is available from the Office of

Standard Reference Materials of the National Bureau of Standards (NBS). Pro- vided that care is exercised in using these SRM devices, the user can confidently expect to achieve a calibration point near 39.30 °C for which the

uncertainty is no greater than +0.010 °C. The most reproducible point of the melting curve for calibration work is the midpoint temperature of the plateau.

Since the variation of this temperature among the SRM 1969 cells is larger

than either the irreproducibili ty of the midpoint temperature of any given

cell or the melting ranges of the samples in the cells, each SRM 1969 is accompanied by a calibration certificate, which includes the value of the midpoint temperature and a copy of the melting curve of the sample, as obtained at NBS. The remainder of this publication describes SRM 1969, the tests performed

on the SRM 1969 cells, the conditions under which the cells were tested, the results obtained from testing 100 SRM 1969 cells, and the recommended procedure for using the cells in calibration work.

II. EXPERIMENTAL DETAILS

11. 1. Samples

The rubidium used in preparing SRM 1969 was supplied by MSA Research

Corporation. All cells were prepared from the same lot of material, which was

stated by the supplier to be 99-9+% pure rubidium; the emission spectroscopic

analysis (provided by MSA Research Corporation) of that material is given in Table III. Although the rubidium used in preparing the SRM 1969 was taken

from the same lot of material and presumably had the same chemical composition, the SRM 1969 stainless-steel containers themselves may have contaminated the rubidium put into them and by different amounts. This would be reflected in the triple-point temperature and also in the melting range of

each cell. Preliminary measurements on the SRM 1969 cells indicated that the plateaus of their melting curves occurred over a range of temperatures which was considerably greater than the irreproducibility of any given cell, and

this made it necessary to test the melting behavior of each cell so that a certificate could be provided for it. 11. 2. Sample Holders

The Type 304 stainless-steel containers were fabricated and filled with

the 99.9+% pure rubidium by MSA Research Corporation. The containers are of

5
Table III. Analytical Report of Rubidium Used to Fill SRM 1969 Cells

Tvno nf M:a t" or» i'al I Id L/Ci idl nuuiui mil T Wn n_UU7_Q8n_

1

\jl due KJL Ma t" t** "i a 1 L let L-CI id -L

PPm PI pmpn t* PPm PPm

Fe _5 Cr _5 Sr _1 B _5 Si _5 Ba -? Co -5 Ti -5 Ca 1 Mn -1 Ni -5

Al -5 Mo -5 Na -5 Mg -2 V -5 K -5

Sn -5 Be -1 Rb Balance

Cu -2 Ag -1 Cs 700

Pb -5 Zr -10 °2

C

Remarks

:

The prefix indicates less than.

Metallic impurity levels reflect analysis of the chloride form of the metal.

Reference: Emission Spec Analysis - NUMEC Plate No. 12850-51-63

6
an all-welded construction and their dimensions are as indicated in Figure 1. They were thoroughly cleaned, evacuated and then baked to remove adsorbed

oxygen and other volatile materials. While under vacuum and at a temperature

above the melting point of rubidium, the cells were completely filled with

liquid rubidium and the stainless-steel bellows valve of each cell was then tightly closed. The rubidium in the tube above the valve was removed and the tube sealed with a stainless-steel plug in an argon atmosphere. Each cell

contains approximately 154 g of rubidium. The plug should not be removed nor should the valve be opened. Extreme caution is essential in handling

rubidium . Rubidium ignites on contact w ith air and reacts violently w ith

water, the liberated hydrogen being burned simultaneously. Consequently, the valve must be kept closed and the plug in place.

11. 3. Tests of Samples The melting behavior of each SRM 1969 cell was determined in a wellstirred constant-temperature oil bath at a temperature of 39.450 °C. This

temperature was selected for testing after obtaining melting curves of several samples at 39.400 °C, 39-450 °C, 39.500 °C and 39-700 °C. Prior to the beginning of the melting of the rubidium in each of the SRM

1969 cells, calibrated thermistor thermometers were inserted into the ther- mometer wells of the cells while the cells were in air at about 22 °C. The

thermometers were positioned such that they rested on the bottom of the well. The cells containing the thermometers were than placed in plastic cell holders

located in the oil bath. Measurements of the temperatures with the thermistor

thermometers were then begun, and measurements continued to be made every four minutes until most, or all, of the rubidium had melted. This process was repeated at least once for each SRM 1969 cell. The temperature of the midpoint of the plateau of the melting curves, which is specified on the SRM Certificate as the temperature calibration point, was obtained from these

melting experiments. Reproductions of the melting curves are provided as part

of the SRM certificates. The freezing behavior of several cells was obtained by placing them,

containing the thermistor thermometers, in an oil bath maintained at

39.100 °C. The cells were held in the bath by plastic cell holders. Prior to putting the cells in the 39.100 °C bath, the rubidium samples had been totally

melted by placing the cells in an oil bath at about 70 °C, and leaving them

there until they reached the bath temperature, as indicated by a small plati- num resistance thermometer.

11. 4. Apparatus Used in Testing

a. Constant-Temperature Bath

The constant-temperature bath used for all of the melting and freezing

experiments on the SRM 1969 cells was a commercially available bath having a volume of about 14 liters. A low-viscosity oil (a dimethylpolysiloxane) was

used as the bath fluid. The temperature of the oil bath was maintained at the selected values to within +0.002 °C by means of a commercially available

proportional controller, using a thermistor as the sensor. Compressed air,

which passed through a heat exchanger (located in an ice bath) before entering

7
65 mm

21 mm

53 mm

158 mm

30 mm

27 mm

1/4" Plug, SS

1/4" SS Tubing

(0.035" wall)

Nupro Valve

(SS 4HTSW)

1/4" SS Tubing

(0.035" wall)

Thermometer

Weil

1/4" SS Tubing

(0.010" wall)

/1 1/4" O.D. SS / Tubing (0.035" wall)

3/4" O.D. Plate,

0.010" Thick

(welded at 3 places

to tube)

Figure 1. Cross-sectional drawing of SRM 1969, Rubidium Triple-Point Standard.

8
the bath cooling coil, was used to provide the requisite cooling. Although

the temperature of the oil bath was maintained within 10.002 °C of the temperature selected for a melting or freezing experiment for the duration of the experiment, such precise control is not necessary in the normal use of SRM

1969. This will become evident in Section III and is indicated graphically by the figures of that section.

b. SRM 1969 Cell Holder Holders for the rubidium SRM 1969 cells, shown in Figure 2, were con- structed of Plexiglas tubes attached to a Plexiglas base plate (attached by

using a glue made by dissolving Plexiglas in dichlorome thane). This assembly

was suspended in the constant-temperature oil bath from a Plexiglas top plate

by means of three Plexiglas rods. Holes were drilled in the base plate, as shown in Figure 2, to permit oil flow around the cells. The cell holders were immersed sufficiently deep in the bath oil that the tops of the stainlesssteel plugs on the tops of the SRM 1969 cells were about 2 cm below the top

surface of the oil. All immersion problems then were negligible and temperature gradients along the cells were minimized (except at the boundary where

the cells extended from the cell holders). The cell holders (i.e., the

Plexiglas tubes) provided some slight insulation around the bottom 5 cm of the cells and this caused the duration of the melts to be somewhat greater than it would have been if the oil were constantly flowing over the entire cell. Plexiglas tubes which covered more of the cells could have been used to

further extend the duration of the melt, but this would have caused an undue amount of time to be spent in testing the cells.

c. Thermometers

The thermometers used in this investigation were bead-in-glass probe-type

thermistor thermometers^, 16 Which had been calibrated over the temperature

range from 0 °C to 70 °C against a standard platinum resistance thermometer^

(SPRT) in a temperature-regulated oil bath. The thermistor thermometers and the SPRT were located in adjacent wells of a copper block in the oil bath

during the calibration. This insured temperature equilibrium among the thermometers and also damped any sudden temperature fluctuations. Measurements of the SPRT resistances were made with an ac resistance bridge.^ The SPRT had been calibrated previously by the NBS Platinum Resistance Thermometer

Calibration Laboratory using the same ac bridge. Fluctuations in temperature

during calibrations at a given calibration temperature, as measured by the

SPRT, were about 10.1 mK. The uncertainty in the resistance measurements of the thermistor thermometers corresponded to about +0.25 mK. By fitting the equation

1/T = A + B log R + C(log R) 2 + D(log R) 3 where T is the temperature in kelvins and R is the thermistor resistance in Ohms, to the calibration data, the constants A, B, C and D were determined.

The temperature value then derived from this equation for a measured

thermistor-thermometer resistance agreed with that measured with the SPRT to within 11 mK.

9

d. Temperature Measurement System

The temperature measurement system used in testing the SRM 1969 cells was

an automated system consisting of a microcomputer, a 6-1/2-digit digital

voltmeter, a constant-current source, a standard resistor, and the calibrated

thermistor thermometers.

IIll RESULTS AND DISCUSSION Some typical melting curves are shown in Figures 3> 4, and 5. Included

in these figures are melting curves of some of the samples having the lowest

and the highest midpoint melting temperatures. The two curves of Figure 3(a) represent the melting behavior and the reproducibility of that behavior for a single sample. Similar data for three other samples are presented in Figures 3(b), 4(a) and 4(b). Three melting curves of one sample are shown in Figure 4(b). The melting curves of most of the samples (about 70 of them)

exhibit an initial behavior such as that shown in Figure 3(a), i.e., the

initial rise in temperature to a fairly broad peak. This peak is attributed

to the hot bath oil which flowed into the thermometer well of the cell when

the cell was placed in the oil bath for melting. After some time, temperature

equilibrium was reached as the rubidium melted. The temperature in the well decreased slightly as equilibrium was approached. Following the peak, the temperature was rather constant for a time before increasing relatively

rapidly. The latter temperature rise is typical behavior of samples melting

when perhaps 75% of the sample has melted and the remaining solid is at or near the bottom part of the cell. The top part of the sample, which was

liquid, was warming from effects of the bath and the sample's finite thermal

conductivity, and the thermometer was beginning to sense the bath. When all

of the sample had melted, the sample temperature rose exponentially to the

bath temperature. All of these features of melting are depicted in Figure 5, which has curves which represent the melting behavior of three samples of

rubidium. Note the rapid temperature rise of the samples to the bath temperature after all of the solid had melted. Also note the good agreement of the melting curves of these three samples.

Although the melting-point temperature of a sample is the temperature at

the liquidus point, the position along the melting curve that is most reproducible and, consequently, most useful for calibration purposes is not at the liquidus point but at or in the region of a point approximately midway along the plateau of the melting curve (i.e., in the vicinity of a point approximately midway between the liquidus and the solidus points). In the region of that point, which we will call the midpoint temperature, the curve is most

flat and any small changes in the fraction of material melted has only a small

effect on the liquid-solid equilibrium temperature. The Certificate gives the value of the midpoint temperature. The reproducibility of the melting curves for each of the 100 SRM 1969 cells is rather good. For the worst case, the spread in the values of the temperature at the midpoints of the plateaus of

the melting curves of that sample was less than 10 mK. The spread was 1 mK to

3 mK for most of the cells. The distribution of midpoint temperatures for the 100 cells is shown in Figure 6. The mean value of the midpoint temperatures is 39-303 °C an <* tne

1 1
39.350

39.325

39.300

9 39.275

- (a)

i—i—

r

i—i—

r

J I L J I L

39.250 I L 50 100

Time (min.)

Figure 3. Typical melting curves for two SRM 1969 cells in an oil bath at 39.450 °C. The two curves in (a) are for one cell; the two curves in (b) are for another cell.

12
oo0

<Da

E

0

39.350

39.325

39.300

39.275

39.350

39.325

39.300

39.275

39.250

1 1 1

1 1

i

1 1

1

1 T^l J (a)

1 I I I 1 I 1 i I i—

r

(b)

J I L I 1 I

1 I I

1 I I I 50 100

Time (min.)

150

Figure 4. Typical melting curves for two SRM 1969 cells in an oil bath at 39.450 °C. The two curves in (a) are for one cell and the three curves in (b) are for another cell.

13

l i

i r i i

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^ _ ^^^^^^^s _ E -

I I I I I I 1 I LO O LO o

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15
median value is 39.305 °C. The highest midpoint temperature was 39.313 °C and

the lowest was 39-280 °C, there being a total spread of 0.033 °C in the midpoint temperatures for the 100 cells. The melting range of a sample, as well as its melting point, is an indication of the purity of the sample, the smaller the range, the higher the purity. For the 100 SRM 1969 cells tested, the melting ranges varied from

8 mK to 24 mK, with a mean value of 14 mK. This is an indication that the rubidium samples comprising the 100 cells of SRM 1969 have a higher purity

than the samples tested previously. 1 3 Higher melting-point temperatures for the SRM 1969 cells indicate higher purity also. The duration of a melt for any given sample (which requires a fixed

quantity of heat for fusion) depends on the difference between the temperature

of the environment of the cell and the melting-point temperature. Results

obtained during melting of three samples in oil baths at three different

temperatures are shown in Figures 7, 8 and 9. The bath temperatures for these experiments were 39.400 o c> 39.500 °C and 39-700 °C, giving temperature

differences of approximately 0.11 °C, 0.21 °C, and 0.41 °C, respectively. Al- though there is some scatter in the midpoint temperatures of the plateaus, it

is not large, the spread being approximately 3 mK for the sample of Figure 7, approximately 2 mK for the sample of Figure 8, and approximately 6 mK for the sample of Figure 9. One can see from Figures 7, 8 and 9 that, when a cell was placed in a bath maintained constant at 39-400 °C, the temperature in the well of the cell was at the plateau of the melting curve for about 2 hours.

That was the amount of time available for calibrating a thermometer with those cells. Over a span of 2 hours from the time the cells were placed in the

39-400 °C bath, the maximum change in the observed temperature of either of

these three cells was about 9 mK (Figure 7). The changes were about 5 mK for the cells of Figures 8 and 9. Consequently, if one had calibrated a thermometer or thermometers in either of these three cells and assumed the cell temperature to be the midpoint temperature, the maximum error incurred would

have been no greater than 6 mK. When the three samples of Figures 7, 8 and 9, as well as the other 97 SRM

1969 cells, were placed in a bath at 39-450 °C and held in holders as shown in Figure 2, the amount of time that the temperature in the well of any given

cell was at the plateau of the melting curve was reduced to about 100 minutes.

This, then, is the amount of time available for calibrating thermometers with

the SRM 1969 cells when the cells are used in the manner just indicated. If the SRM 1969 cells are placed in baths at temperatures other than the four

reported here, the length of time that the temperatures of the samples are at the plateaus of the melting curves, and, thus, the cells are at the tempera- tures for calibrating thermometers, can be estimated from the curves of Figure 7, 8 or 9- The uncertainties in the temperatures measured in our experiments were estimated as follows. The resolution of the thermistor thermometer system was approximately +0.0001 °C, and the overall statistical uncertainty in the temperatures measured with that system was estimated to be approximately

+0.00025 °C. There is a systematic uncertainty of approximately 10.0006 °C relative to the IPTS-68, due to the calibration 17 of SPRTs and the fact that the platinum elements are "real" (as opposed to theoretically performing)

materials. 1 The errors due to differences in self-heating in the calibrations and in the experiments, and those due to immersion of the thermistors are

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negligible. Thus, the total uncertainty in our determination of temperatures

on the IPTS-68 with the thermistor thermometers was about 10.0015 °C.

IV. RECOMMENDED PROCEDURE FOR USE OF SRM 1969 IN CALIBRATION OF THERMOMETERS

To obtain the best results in the calibration of thermometers using SRM

1969, the rubidium cells must be totally immersed in a well-stirred fluid bath (preferably a light purified mineral oil) maintained at 39.^5 ± 0.05 °C. Since the SRM 1969 cells are approximately 30 cm long, a fluid bath somewhat

deeper than 30 cm should be used for the calibration. The cell or cells

should be mounted in a holder such as that shown in Figure 2 (which holds 3 cells). If only one cell is being used, then a holder similar to that shown

in Figure 2, but modified to hold only one cell, could be used (such a holder

is provided with each SRM 1969 cell). As described in Section II, the cell

holders are easy to construct. A tabulation of the equipment which is needed

to obtain the best results when using SRM 1969 in the calibration of thermometers is given in Table IV.

Table IV. Ancillary equipment needed when using SRM 1969 to calibrate

thermometers.

Well-stirred fluid (oil) bath, approximately 32 cm in depth from top of fluid surface.

Light purified mineral oil for the bath.

Temperature controller (±0.02 °C perferably, but could use +0.05 °C).

SRM 1969 holder (e.g., Plexiglas), such as provided with the cell.

Means of suspending SRM 1969 holder in the bath (e.g., Plexiglas rods attached

to a bath cover plate, which could be Plexiglas also).

Thermometer(s) to be calibrated (diameter less than 5 mm).

20
The preferred procedure for calibrating a thermometer through the use of an SRM 1969 rubidium triple-point cell is as follows. First, get the wellstirred oil bath to a temperature of 39.45 ± 0.05 °C and maintain it at that

point. Put the thermometer to be calibrated in the thermometer well of the SRM 1969 cell (which has previously been placed in the Plexiglas holder).

Next, place (suspend) the cell holder, with the SRM cell and thermometer, in the temperature-controlled oil bath. Wait 10 to 15 minutes, then read the

indication of the thermometer being calibrated. The correct temperature of the thermometer is the value indicated on the certificate for the particular SRM 1969 cell being used for the calibration. If other thermometers are to be calibrated, they may then be placed successively in the thermometer well of the SRM cell. There should be a minimum time of about 70 minutes available

for calibration from the time the cell was placed in the oil bath. If more

than one thermometer is to be calibrated, the second and succeeding thermometers should be allowed 5 to 10 minutes time to reach temperature equilibrium

with the melting rubidium in the cell before performing the calibration. The smaller the mass of a thermometer, the shorter the length of time required for equilibration.

After the calibrations have been completed, or if the length of time that the SRM cell has been in the bath is approaching that value at which the temperature is near the end of the plateau region, remove the cell from the

oil bath and permit it to cool. The rubidium will freeze fairly rapidly and the cell will be ready within 10 to 15 minutes to put back into the oil for

further calibration work. This melting and freezing process may be repeated

as often as required.

V. SUMMARY AND CONCLUSIONS

In testing 100 SRM 1969 rubidium triple-point cells, we have found that the samples in the different cells were not of uniform purity, although the rubidium

metal used to fill the cells was taken from only one lot of material. Either

the rubidium became slightly contaminated during filling or by the presence of different amounts of contaminants in the cells themselves.

The total range of the temperatures of the midpoints of the plateaus of the melting curves was from 39.280 °C to 39-313 °C, or a spread of 0.033 °C, with a mean value of 39.303 °C. The melting ranges of the samples varied from approximately 8 mK to approximately 24 mK among the cells, with a mean value of 14 mK. Only 2 cells had a melting range greater than 20 mK, one being 21 mK and the

other 24 mK. Thus, with the exception of those two cells, an estimated uncertainty of +0.010 °C in the midpoint temperature of the plateau of the melting curve of each SRM 1969 cell will encompass the entire melting range of each

cell. Since the value of the midpoint temperature and a copy of the melting

curve will be provided with each of the SRM 1969 cells, the user will be able to calibrate thermometers easily at the rubidium point to an uncertainty no greater

than 10. 010 °C. If the entire melting curve of an SRM 1969 cell is obtained with a thermometer to be calibrated and if the midpoint of the plateau of that curve

is used as the reference, it is possible for a user to calibrate that

thermometer to an uncertainty that is no greater than ±0.005 °C. For a pure material with infinite thermal conductivity and no supercooling, the plateau

2 1
is perfectly flat throughout the liquid -solid transition. For a slightly

impure material that has a finite thermal conductivity, however, the plateau

will not be flat and the material will have a finite melting range. 8 The

midpoint of that range is the point of reference here and the thermometer

indication at that point of the plateau, rather than that at some arbitrarily

selected point anywhere along the plateau, is the value to be used in the calibration. The temperature value in the vicinity of that midpoint is the

temperature specified on the calibration certificate for that particular SRM. Thus, by ensuring that a thermometer is calibrated at the same, or very nearly

the same, position on the melting curve of an SRM 1969 cell as that determined

by NBS, rather than at just any position within the melting range, the uncertainty of calibration can be reduced to ±0.005 °G. In conclusion, by following the procedure outlined in Section IV, the user of SRM 1969 will be able to quickly and easily realize the rubidium

triple-point temperature and use that to calibrate his thermometers. The

calibration procedure is straightforward and does not require the use of complicated and expensive apparatus or the expertise of trained workers. The calibration point provided by any one of the SRM 1969 cells is at an important

value for clinical laboratory applications, for use in hyperthermia applications, for use with clinical (fever) thermometers, and for general laboratory

calibrations. Through the use of these devices, the accuracy of temperature measurements in this region of temperature should be significantly enhanced.

VI . ACKNOWLEDGMENTS The partial financial support by the Office of Standard Reference

Materials for some of the work leading to the development of SRM 1969 is gratefully acknowledged.

DISCLAIMER

In order to describe materials and experimental procedures adequately, it was

occasionally necessary to identify commercial products by the manufacturer's

name or label. In no instances does such identification imply endorsement by the National Bureau of Standards nor does it imply that the particular

products or equipment are necessarily the best available for that purpose.

22
VII. REFERENCES

1. B. W. Mangum and D. D. Thornton, Determination of the Triple-Point

Temperature of Gallium. Metrologia _15_, 201-215 ( 1979).

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 Special

Publication 260-77, August 1982, Superintendent of Documents, U. S. Government Printing Office, Washington, DC 20402.

3. E. H. McLaren, The Freezing Points of High Purity Metals as Precision

Temperature Standards, in American Institute of Physics, Temperature , Its Measurement and Control in Science and Industry , (Reinhold Publishing

Corp., New York, NY, 1962), Vol. 3, Part 1, pp. 185-198.

4. E. H. McLaren and E. G. Murdock, The Freezing Points of High Purity

Metals as Precision Temperature Standards. V., Thermal analysis on 10 Samples of Tin with Purities Greater than 99-99+ Percent. Can. J. Phys.

38, 100-118 (1960).

5. The International Practical Temperature Scale of 1968, Amended Edition of

1975. Metrologia _12, 7-17 (1976).

6. Proces Verbaux du CCT, Document No. CCT/82-31 , 1982; also, Proces Verbaux

du CCT, Document No. CCT/80-12, 1980.

7. D. D. Thornton and B. W. Mangum, Proces Verbaux du CCT, Document No. CCT/76-13, 1976.

8. B. W. Mangum and D. D. Thornton, Editors, The Gallium Melting Point

Standard. NBS Special Publication 481, June 1977, Superintendent of Documents, U. S. Government Printing Office, Washington, DC 20402.

9. H. E. Sostman, Melting Point of Gallium as a Temperature Calibration

Standard, Rev. Sci. Instrum. _48, 127-130 (1977).

10. B. W. Mangum, Triple Point of Gallium as a Temperature Fixed Point, in Temperature , Its Measurement and Control in Science and Industry , (American Institute of Physics, New York, NY, 1982), Vol. 5, pp. 299-309.

11. M. V. Chattle, R. L. Rusby, G. Bonnier, A. Moser, E. Renaot, P. Marcarino, G. Bongiovanni and G. Frassineti, An Intercomparison of Gallium Fixed Point Cells, in Temperature , Its Measurement and Control in Science and Industry , (American Institute of Physics, New York, NY, 1982), Vol. 5, pp. 311-316.

12. J. Bonhoure et R. Pello, Temperature du Point Triple du Gallium. Metrologia J_9 , 15-20 (1983).

23
13. J. M. Figueroa and B. W. Mangum, The Triple Point of Rubidium: A Temperature Fixed Point for Biomedical Applications, in Temperature , Its Measurement and Control in Science and Industry (American Institute of

Physics, New York, NY, 1982), Vol. 5, pp. 327-337.

14. D. L. Martin, Specific Heat Below 3 K, Melting Point, and Melting Heat of Rubidium and Cesium. Can. J. Phys. 48, 1327-1339 (1970).

15. M. Sapoff, Thermistors for Biomedical Use, in Temperature , Its Measurement and Control in Science and Industry , (Instrument Society of

America, Pittsburgh, PA, 1972), Vol. 4, pp. 2109-2121.

16. H. B. Sachse, Semiconducting Temperature Sensors and Their Applications

(Wiley-Interscience, New York, NY, 1975).

17. J. L. Riddle, G. T. Furukawa and H. H. Plumb, Platinum Resistance

Thermometry, NBS Monograph 126, April 1 973 > Superintendent of Documents,

U. S. Government Printing Office, Washington, DC 20402.

18. R. D. Cutkosky, An ac Resistance Thermometer Bridge. J. Res. Nat'l. Bur. Stand. (U.S.) 74C, 15-18 (1970).

24
NBS-H4A (REV. 2-BC) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR REPORT NO. NBS SP 260-87

2. Performing Organ. Report No. 3. Publication Date December 1983

4. title and subtitle standard Reference Materials:

SRM 1969: Rubidium Triple-Point Standard - A Temperature Reference Standard

Near 39.30 °C

5. AUTHOR(S)

B. W. Mangum

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered WASHINGTON, D.C. 20234 Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Same as item 6,

10. SUPPLEMENTARY NOTES Library of Congress Catalog Card Number: 83-600621

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a significant bibliography or literature survey, mention it here) Previous work has demonstrated the practicability of using the triple point of

rubidium as a temperature reference point. As a result of that work, Standard Refer- ence Material (SRM) 1969 - the Rubidium Triple-Point Standard, has been developed. This publication reports results of an investigation of 100 SRM 1 969 cells; it describes SRM 1969, the tests which were performed on the cells, the conditions under which the cells were tested, the results of the tests, and the recommended procedure

for the use of SRM 1969 in calibrating thermometers. For the 100 cells, the temperatures of the midpoints of the plateaus of the melting curves ranged from 39.280 °C to 39.313 °C, with a mean value of 39-303 °C. The melting ranges of the samples varied

from approximately 8 mK to approximately 24 mK, with a mean value of 14 mK. Only two

cells had melting ranges greater than 20 mK, one being 21 mK and the other 24 mK. An estimated uncertainty of +.0.010 °C is assigned to the midpoint temperatures of the plateaus of all cells except for those two cells. Thus, in using SRM 1 9 69 cells for

the calibration of thermometers, a calibration at any point on the plateau of the melting curve should provide a temperature (the value specified on the Certificate

accompanying each cell) with an uncertainty no greater than +0.010 °C. With special

care, a thermometer can be calibrated against an SRM 1969 cell to an uncertainty as

small as +0.005 °C in the National Bureau of Standards (NBS) assigned value. The use of SRM 1 969 in clinical and biomedical laboratory applications should significantly

enhance the accuracy of temperature measurements in the region of body temperature.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Melting-point temperature; rubidium; SRM 1969; temperature fixed point;

temperature reference point; triple-point temperature.

13. AVAILABILITY

r_^] Unlimited For Official Distribution. Do Not Release to NTIS

[X] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402. Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF PRINTED PAGES

36

15. Pr

USCOMM-DC 6043-P80



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