NATL INST. OF STAND & TECH

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NBS SPECIAL PUBLICATION 481

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Gallium

Melting-point

Standard
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards^ was established by an act of Congress 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

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- JUN 0 1977

The Gallium Melting-Point Standard <k>

B. W. Mangum and D. D. Thornton, Editors

Ml

1911

Heat Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

^^^NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued June 1977
Library of Congress Catalog Card Number: 77-600017

National Bureau of Standards Special Publication 481

Nat. Bur. Stand. (U.S.), Spec. Publ. 481, 35 pages (June 1977) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

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

(Order by SD Catalog No. CI 3. 10:481). Stock No. 003-003- Price$i.40 (Add 25 percent additional for other than U.S. mailing).
Foreword

The investigation of the melting point of galHum as a thermometric reference ("fixed point") and the preparation of a supply of well-characterized, very pure gallium as a NBS Standard Reference material

(SRM 1968) have been carried out as part of the Temperature program in the Heat Division. This

particular project is designed to address specifically the temperature measurement problems en- countered in the medical, chnical, and biomedical areas of U.S. technology. The four articles re- produced from the journal Clinical Chemistry and collected here* are being made available to interested parties as a further service to the measurement community within this specialized area, with

the hope that they will contribute to improved metrological practice in the identified fields and hence to the quality of the nation's health care. We are grateful to Dr. J. Stanton King, Executive Editor of Clinical Chemistry, for kindly making

available negatives of the four articles and thus greatly facilitating the preparation of this Special

Publication.

R. P. Hudson

Chief, Heat Division

•References within the aiticles to specific commercial products do not in any way imply endorsement thereof by the National Bureau of Standards.

iii
Contents

page*

1. Introduction—The Gallium Melting-Point Standard:

A New Fixed Point to Assure the Accuracy of Temperature

Measurements in the Clinical Laboratory

George N. Bowers 709

2. The Gallium Melting-Point Standard:

Its Role in Our Temperature Measurement System

B. W. Mangum 711

3. The Gallium Melting-Point Standard:

A Determination of the Liquid-Solid Equilibrium Temperature

of Pure Gallium on the International Practical Temperature

Sacle of 1968

Donald D. Thornton 719

4. The Gallium Melting-Point Standard:

Its Role in Manufacture and Quality Control of Electronic

Thermometers for the Clinical Laboratory

Henry E. Sostman 725

5. The Gallium Melting-Point Standard:

Its Application and Evaluation for Temperature Measurements

in the Clinical Laboratory

George N. Bowers, Jr. and Stanford R. Inman 733

*Page numbers reflect original pagination in Clinical Chemistry, Vol 23. No. 4, 1977.

iv
Abstract

This Special Publication contains a series of papers published in Clinical Chemistry

concerning our temperature measurement system, the gallium melting-point as a thermometric fixed-point, how the gallium melting-point fits into the measurement system, and

applications of the gallium melting-point standard.

Keywords: Clinical laboratory; fixed-point; gallium; melting-point standard; temperature; thermometric fixed-point.

V

THE GALLIUM

MELTING-POINT STANDARD

The Gallium Melting-Point Standard: A New Fixed Point to Assure

the Accuracy of Temperature Measurements in the Clinical

Laboratory

Introduction by George N. Bowers^

This series of papers is written to introduce the

readers of Clinical Chemistry to the use of the melting

point of galHum as a means of improving the accuracy

of temperature measurements in our nation's cHnical

laboratories. Gallium, the 32nd most abundant element

in the earth's crust, is a silver-grey metal, widely dis- tributed in trace amounts in many rocks and ores. Its name, gallium (Lat., f;allia, France), honors the dis- covery of this element by a French chemist in 1875, just four years after Mendeleev predicted its probable ex- istence from a blank space in his newly described peri- odic table. M. Paul Emile Lecoq de Boisbaudran, al- though by vocation the owner-operator of a distillery business in Cognac, privately maintained his own

spectroscopy laboratory, and in addition to discovering

gallium he also discovered scandium in 1879 and ger- manium in 1886. Today, ultra-high-purity gallium metal

is manufactured primarily for the electronics industry

to make the semiconductor materials, gallium arsenide and gallium phosphide, used in rectifiers, photocells,

transistors, lasers, etc. Interestingly, this metallic ele- ment is unique in that it has the widest liquid range of any metal, melting at 29.77 °C and boiling at slightly above 2400 °C. The melting point of highly pure gallium

is very sharply defined and is easily maintained constant

for many hours. Fortunately, this melting point lies

' This serie.s of papers has been made possible by cooperative efforts and discussions between many individuals in addition to the authors.

I wish to acknowledge the valued contributions and help of Paul Cali, Robert Rej, Carl Burtis, Robert McComb, and Donald Young.

Hartford Hospital, Hartford, Conn. 06115. Received and accepted Jan. 8, 1977.

close to the center of the ranges of usual interest in the

clinical laboratory.- No costly or complex equipment

is required to realize 29.77 °C to ±0.01 °C in any clinical laboratory. Thus a small and relatively inexpensive

gallium melting-point cell provides an ideal means of

calibrating the thermistor probes of electronic ther- mometers. These small probes with their tiny but stable sensors are essential to accurate temperature measurements in clinical-service and biomedical-research

laboratories, because the heat capacity and size of the

large mercury-in-glass thermometers would seriously

distort temperature within the small volumes of final reaction mixtures commonly used today.

Each of these papers, although contributed by authors of different expertise, backgrounds, and areas of

responsibility, supports the concept that temperature measurements for clinical laboratories within the United States (and elsewhere) can be improved considerably by the introduction of the gallium meltingpoint standard. For this goal to be achieved, a system must evolve within which the following functions and

responsibilities are discharged:

(1) National laboratories certify and distribute gal- lium melting-point cells. (2) Manufacturers use these certified gallium melt-' ing-point cells (or equivalents) in the production,

- The ranges of interest in the clinical laboratory are many and

varied; thus, any choice is neces.sarily arbitrary. We have cho.sen this to mean either 0-60 °C or 24-.S8 °C in this series of papers, although

0-10.5 °C is al.so given in a recently proposed ASTM Committee

E20.08.04 specification for thermistors for use in the clinical laboratory.

709
quality control, and calibration of the electronic ther- mometers and related heat equipment manufactured

for use in clinical laboratories.

(3) Clinical laboratory personnel use these certified gallium melting-point cells regularly to check and to maintain the calibration of these devices.

In the first paper, Mangum, who is a physicist in charge of the Medical Thermometry Project at the National Bureau of Standards (NBS), reviews the pri- mary defining fixed points of the International Practical Temperature Scale of 1968 (IPTS-68) and the in- terpolation role played by platinum resistance ther- mometry. The Scale is maintained and disseminated by

various national standards laboratories throughout the

world. In the United States this responsibility resides within the Heat Division of the Institute of Basic Standards of NBS. He tells how the Heat Division dis- seminates the Scale by calibration services and Stan- dard Reference Materials (SRM's) for use within our

industrial, scientific, and medical communities. The

historical and current investigational work on gallium

is reviewed, and he discusses why gallium is so well qualified as a secondary reference point for the IPTS68.In the second paper, Thornton, a professor of physics

at The George Washington University, presents his experimental data on small prototype gallium meltingpoint cells. As trace impurities in the gallium are decreased, the temperature of the melting point is shown

to change, becoming progressively higher and more

stable. An extremely reproducible melting point at 29.7714 ± 0.0014 °C is demonstrated with cells containing gallium of 99.99999% purity. The design of a

similar gallium cell, presently being issued by the NBS

Office of Standard Reference Materials as Clinical SRM

No. 1968, is given. In the third paper, Sostman—who is a temperature

physicist and vice president for product integrity of Yellow Springs Instrument Co., Inc., manufacturer of

electronic thermometers, thermistors, and other temperature-monitoring and controlling equipment

describes how the calibration needs of electronic ther- mometers are served by the gallium cell. He reviews types of sensors for electronic thermometers and the design considerations of a thermometer system for

clinical laboratory practice, and demonstrates that for the range from 29 to 31 °C a single-point calibration to gallium near the center of the range is adequate to en- sure calibration within ±0.01 °C. The precautions

necessary in making such a calibration by conventional means and the greater ease and certainty of calibration at the gallium point are given. He discusses the devel- opment of a gallium cell and the role of the gallium

reference point in YSI's metrology standards laboratory. In the fourth paper. Bowers and Inman—clinical chemist and instrument specialist, respectively—il- lustrate several uses of a gallium melting-point cell within a hospital clinical chemistry laboratory. The

uncertainty of temperature measurements traceable to IPTS-68 on using the gallium melting-point standard

is shown to be ±0.01 °C at 29.77 °C, in comparison to

prior uncertainty of ±0.03 °C to 0.05 °C at 30 °C and 37 °C for stirred-liquid comparison baths with SRM's 933 and 934 [c/. Clin. Chem. 20, 670 (1974)] as the reference thermometers. This certainty of calibration at 29.77 "C has been

exploited for measurement of the enzymic activity of

aspartate aminotransferase in serum by the IFCC ref- erence method. On the basis of this experience, it is suggested that the set-point of the reaction temperature

for reference methods in clinical enzymology be made

equal to this unique constant-of-nature: the gallium

melting-point standard at 29.7714 ± 0.0014 °C. Many novel applications of this stable temperature

reference point can be expected as this unique property

of gallium becomes more widely known. However, its most obvious primary and immediate role in the clinical laboratory will be to improve our ability to more accurately measure and control temperature.

710
The Gallium Melting-Point Standard: Its Role in Our Temperature

Measurement System

B. W. Mangum

The latest internationally-adopted temperature scale, the

International Practical Temperature Scale of 1968

(amended edition of 1975), is discussed in some detail and

a brief description is given of its evolution. The melting

point of high-purity gallium (stated to be at least 99.99999% pure) as a secondary temperature reference

point Is evaluated. I believe that this melting-point temperature of gallium should be adopted by the various medical professional societies and voluntary standards groups as the reaction temperature for enzyme reference methods in clinical enzymology. Gallium melting-point cells are available at the National Bureau of Standards as

Standard Reference Material No. 1968.

Although temperature measurements are probably

made more often by more people than any other type

of measurement, many are not cognizant of what temperature scale is being used, the basis of that scale, and

the difference between the thermodynamic and practical temperatures (to say nothing of the relevance of the meter reading). In this article, I shall discuss these points and give a brief history of the evolution of the temperature scale. I shall also discuss the utility of the melting point of gallium as a temperature fixed point,

its relationship to the temperature scale, and its great

potential in clinical laboratories. Temperature Scales

The unit of thermodynamic temperature is one of the seven base units of the metric system (7 ). Thermodynamic temperature is referred to as a fundamental

physical quantity and is given the symbol T (see Figure

1). This is the relevant quantity in all thermodynamic

calculations involving temperature. Its unit is the kelvin

(2), symbolized K, and this is defined as the fraction 1/273.16 of the thermodynamic temperature of the

triple point of water (3).

Nalional Bureau of Standards, WashiriKton, D.C. 2Q2M.

Received and accepted Jan. 24, 1977 (prereviewed).

Based on the historical development of temperature

scales, temperature is commonly expressed in terms of

its difference from that of the thermal state 0.01 K lower than the triple point of water. The temperature ex- pressed in this way is the Celsius thermodynamic temperature, symbol t, and is defined (4) by

t = T - 273.15 K

The degree Celsius, having the symbol °C, is the unit

of Celsius temperature and by definition it is equal in magnitude to the kelvin. A difference of temperature

may be expressed in either kelvins or degrees Celsius. Before the turn of the century, it was recognized by

the International Committee for Weights and Measures (CIPM) that an international practical temperature

scale was needed so that all temperature measurements

could be put on the same basis. The purpose would be

to overcome the practical difficulties of the direct re- alization of thermodynamic temperatures by gas ther- mometry and to unify the existing national temperature

scales. This led to the adoption of the International Temperature Scale of 1927 by the 7th General Confer- ence of Weights and Measures (5). This internation- ally-agreed-upon scale was intended to be easily and

accurately reproducible and to agree with thermodynamic temperatures.

The International Temperature Scale of 1927 was

revised in 1948 (6), and an amended edition of it was

adopted in 1960 by the 11th General Conference of Weights and Measures (7) under the new title "International Practical Temperature Scale of 1948 (amended

edition of I960)." It was recognized at that time that the

scale no longer represented the thermodynamic temperatures as closely as possible. In 1967, the 13th General Conference of Weights and Measures (8) gave the CIPM permission to introduce

a new temperature scale to replace the existing Inter- national Practical Temperature Scale of 1948 as soon

as the Advisory Committee on Thermometry (CCT) to

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Fig. 1. Guide to thermodynamic temperatures and to ttie International Practical Temperature Scale of 1968 (amended edition of 1975)

Not all of the secondary reference points are shown

712
Table 1. Defining Fixed Points of the IPTS-68'

Fixed points Triple point of equilibrium hydrogen*

Boiling point of equilibrium hydrogen at a pressure of 33 330.6 Pa (25/76 standard atmosphere)

Boiling point of equilibrium hydrogen

Boiling point of neon*^

Triple point of oxygen

Triple point of argon"* Condensation point of oxygen'^ ''

Triple point of water

Boiling point of water^

Freezing point of tin* Freezing point of zinc Freezing point of silver Freezing point of gold

Assigned value of Internalional Practical Temperature

r68(K) 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

/68(°C) -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

' Except for the triple points and the equilibrium hydrogen point at 17.042 K, the assigned values of temperature are for equilibrium states at a pressure of 101 325

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 ortho-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.

" The freezing point of tin may be used as an alternative to the boiling point of water.

the CIPM could make its recommendations for the new

scale. Following the recommendation of the CCT, the CIPM then agreed in 1968 [4] to adopt the new Inter- national Practical Temperature Scale of 1968 (IPTS68). The IPTS-68 extended the scale to lower temperatures to unify the existing national scales in the region 10 K to 90 K and brought the values measured on the

scale into agreement with thermodynamic temperatures

within the limits of the accuracy of measurement at that time. The IPTS-68 distinguishes between the International Practical Kelvin Temperature, symbol Trh, and the

International Practical Celsius Temperature, symbol

f68= Tea - 273.15 K

The units of ffi^ and Tfis are the same as for t and T,

respectively. In 1974, the CIPM proposed an amended version of the IPTS-68, and it was adopted by the 15th General Conference of Weights and Measures in 1975 (9). This

is only an amended version of the IPTS-68, not a re- placement. It differs from the 1968 version in that the argon triple point was introduced as an alternative to the oxygen boiling point; the reference function for the standard platinum thermometer is given in an improved

form; the criteria for the selection of thermocouples were changed; the values of some of the secondary ref- erence points have been changed; Table 7 of the original version of IPTS-68, which gave the estimated uncertainty of the assigned values of the defining fixed points, has been deleted; and some inconsistencies and deficiencies have been removed from and additional infor- mation added to the section on supplementary infor- mation. Any measured temperature, Teg, is unchanged

by this amended edition. Work is currently in progress toward extension of the IPTS-68 (amended edition of 1975) to lower temperatures and toward its possible revision.

Defining Fixed Points of the IPTS-68 (Amended

Edition of 1975)

The IPTS-68 (amended edition of 1975) is based on

the assigned values of the temperatures of 13 repro- ducible equilibrium states (defining fixed points) and

on standard instruments calibrated at those temperatures (see Figure 1). These equilibrium states and the

International Practical Temperatures assigned to them

are given in Table 1. Between these fixed-point temperatures, there are specified interpolation equations

that relate indications of the standard instruments to International Practical Temperatures.

Standard Interpolation Instruments The platinum resistance thermometer is the standard

instrument for the range 13.81 K (-259.34 °C) to 903.89 K (630.74 °C). Strain-free, annealed, pure platinum

whose resistance ratio is «(373.15 K)/ft(273.15 K) >

1.39250, where /?( Tfis) is the resistance at temperature

Trs, must be used as the resistor material. A reference

function and specified deviation equations provide the resistance-temperature relation below 0 °C, whereas

two polynomial equations provide the resistance-tem- perature relation for the range 0 °C to 630.74 °C.

713
Fig. 2. The Standard Platinum Resistance Thermometer

(SPRT)

In a, the overall SPRT is shown; in b, only the resistance element of the SPRT

is shown

The platinum-10% rhodium/platinum thermocouple

is the standard instrument for the range 903.89 K

(630.74 °C) to 1337.58 K (1064.43 °C). Its electromotive

force vs. temperature relation is given by a quadratic

equation. The purity of the platinum wire of the stan- dard thermocouple must be such that /?(373.15 K)/

/?(273.15 K) > 1.3920. Above 1337.58 K (1064.43 °C), the IPTS-68 is defined

in terms of the Planck radiation law, with 1337.58 K

(1064.43 °C) as the reference temperature and with a

specified value for the second radiation constant, C2, of Planck's law. For the benefit of those not familiar with the Standard Platinum Resistance Thermometer (SPRT) and its construction, one is shown in Figure 2. A brief description of it follows. The temperature sensing element is a four-terminal pure platinum wire resistor as described

above, which is mounted in a strain-free manner and is hermetically sealed in a tube filled with a dry 'gas containing enough oxygen to stabilize oxide impurities. In order to avoid contamination of the resistor and to minimize Peltier effects, a short length of the leads connected directly to the resistor should be of platinum.

The Standard Platinum Resistance Thermometer is very fragile, and either mechanical or thermal shock

will introduce defects. These can be removed by annealing. The thermometer should be annealed at a temperature that is greater them the intended operating

temperature and if the thermometer is to be calibrated,

it should finally be annealed at a temperature equal to

or slightly greater than 450 °C. In using the thermometer, care must be taken to avoid thermal radiation and

conduction, and, in addition. Joule heating due to the measuring current should be minimized. The latter can

be done by using zero-power resistance or by operating

at a measuring current that is the same as that used in the thermometer calibration, usually 1 mA. The reli- ability of a Standard Platinum Resistance Thermometer can be ascertained by the constancy of its resistance

at the triple point of water and by the constancy of its resistance ratio for other fixed points. Figure 3 shows

Fig. 3. Calibration of a standard platinum resistance thermometer

in a triple-point-of-water cell The ancillary equipment required for making the measurement is shown. Also shown in this figure are a gallium melting-point cell, a bottle of gallium stated

to be 99.99999-1- % pure, and a prototype of SBM 1968. So that the SPRT and

the triple-point-of-water cell could be displayed, the experimental arrangement shown here is not that used in actual measurements. In actual triple-point determinations, the triple-point cell is immersed in an ice bath and a black cloth covers the SPRT and the cell

measurements being made with a Standard Platinum

Resistance Thermometer in a triple-point-of-water cell. So that the SPRT and the triple point of water cell may

be displayed, the experimental arrangement shown here

is not that used in actual measurements. In actual tri- ple-point determinations, the triple-point cell is im- mersed in an ice bath and a black cloth covers the SPRT

and the cell. The cloth minimizes radiation getting into the cell. Secondary Fixed Points

In addition to the defining fixed points of the IPTS68 (amended edition of 1975) given in Table 1, there are

other, so-called secondary, reference points available

(9). I list some of these in Table 2, and they are shown

in Figure 1.

Dissemination of the Temperature Scale

The IPTS-68 is maintained and disseminated by the various national standards laboratories and in this country the responsible institution is the National Bureau of Standards. The dissemination of the Scale

is accomplished through calibrations. Those national standards laboratories that perform such calibrations

are listed in Table 3. The National Bureau of Standards

calibrates thermometers either by making measure- ments at some or all of the defining fixed points, de- pending on the temperature range of interest, or by comparison with a Standard Platinum Resistance Thermometer. This ensures that the basis for temperature measurement, the IPTS-68, is the same every- where throughout the country. Those involved in tem714
Table 2. Secondary Reference Points

Internationat Practical Temperature

Equilibrium state Triple point of normal hydrogen^

Boiling point of normal hydrogen^

Triple point of neon

Triple point of nitrogen

Boiling point of nitrogen

Boiling point of argon

Sublimation point of carbon dioxide Freezing point of mercury

Ice point'' Triple point of phenoxybenzene (diphenyl ether)

Triple point of benzoic acid Freezing point of indium Freezing point of bismutfi Freezing point of cadmium

Freezing point of lead

Boiling point of mercury

Boiling point of sulfur Melting point of the copper-aluminum eutectic Freezing point of antimony

Freezing point of aluminum

Freezing point of copper

Freezing point of nickel Freezing point of cobalt Freezing point of palladium

Freezing point of platinum

Freezing point of rhodium

fVlelting point of aluminum oxide Freezing point of iridium

Ivlelting point of niobium

IVIelting point of molybdenum

Melting point of tungsten

r68(K) 13.956 20.397

24.561 63.146

77.344

87.294

194.674 234.314

273.15

300.02 395.52 429.784

544.592

594.258

600.652

629.81 717.824

821.41 903.905

933.61 1358.03 1728

1768

1827 2042

2236

2327 2720

2750

2896

3695

»68(°C) -259.194

-252.753

-248.589 -210.004

-195.806

-185.856

-78.476

-38.836

026.87

122.37 156.634 271.442

321.108

327.502

356.66 444.674

548.26 630.755

660.46

1084.88 1455

1495 1554

1769 1963 2054

2447

2477

2623 3422

' Normal hydrogen is a mixture of 75% ortholiydrogen 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.

perature measurements and those involved in devel- oping voluntary temperature standards should be aware

of the Scale on which their measurements are based.

Clinical Laboratory Needs and the National Bureau of Standards' Response

Discussions with professional organizations and

standards-setting groups involved with clinical laboratories have indicated (10-14) that there are some

critical temperature measurement and control problems

in the clinical laboratory, especially in the area of clinical enzymology. It is difficult for the user to measure the temperature of a reaction mixture accurately or to verify that the temperature control of such a mixture in, say, a cuvette is that which is required. There are many

different types of thermometers available for use in the biomedical temperature range, and they provide a wide

choice of stability, sensitivity, and ease of use, but their accuracy depends upon their calibration. At the level of accuracy required of temperature measurements in clinical enzymology (7,5), it is necessary periodically to

recalibrate the thermometer, or to check its calibration, against the International Practical Temperature Scale

of 1968. Recalibration is particularly important for many of the new electronic thermometers that depend

on sensitive resistive elements, thermistors, as sensors. The low cost, convenience, and high sensitivity of these thermometers make them attractive, but their complex

temperature characteristics—as well as the possibility of significant calibration drift—make it highly desirable

to have readily available a means of calibrating them. The College of American Pathologists has indicated

(16) that there is a need in the laboratories for temperature fixed points in the temperature range of biological interest. To be useful to the biomedical community, these temperature reference points should be

well-defined to ±0.002 °C and both stable and repro- ducible to ±0.005 °C. Such a set of accurately known

temperature-reference points, conveniently spaced

throughout the interval from -20 °C to 120 °C, is re- quired if calibration of thermometers is to be accurate. As can be seen in Tables 1 and 2, there is only one such

715
Table 3. National Standards Laboratories That

Calibrate Thermometers

National Standards Laboratory National Measurement Laboratory (NML)

National Research Council (NRC)

Physikalisch Technische Bundesanstalt

(PTB)

Country Australia Canada

Federal Republic of Germany

France German

Democratic

Republic

Italy Japan

Russia

United Kingdom

United States

Institute National de Metrologie (INM) Deutsches Amt fur Mass und Gewicht (DAMG)

Istituto di Metrologia "G. Colonnetti" (IMGC)

Ministry of International Trade and Industry's National Research Laboratory of Metrology (NRLM)

Physico-Technical and Radio-Technical Measurement Institute (PRMI) Mendeleev Institute (VNIIM)

National Physical Laboratory (NPL)

National Bureau of Standards (NBS)

fixed point between the triple point and the boiling

point of water. That solitary point is the secondary

reference point provided by the triple point of phenoxybenzene (diphenyl ether) at 26.87 ± 0.01 °C. There

are problems associated with realizing this point be- cause the material is an organic compound and, as is the

case for most organic materials, it is difficult to purify

and has a very small thermal conductivity. The result

is that the triple point of phenoxybenzene is not a very convenient reference point. Thus, there are no useful

fixed points in this temperature region of crucial im- portance to clinical laboratory scientists and physicians. In response to these clinical laboratory thermometry

needs, the National Bureau of Standards established

the Medical Thermometry Project. It is working to al- leviate the paucity of fixed points in the range of bio- medical interest by adequately documenting a series of such paints throughout the range. These fixed points

will be characterized regarding melting behavior, sample

size, sample design, and handling.

Gallium Melting Point as a Temperature Fixed

Point Appropriateness

Of several possible choices for secondary reference

points in the biomedical temperature range, the most

obvious one is that of the melting point of gallium. This

is because it is near 30 °C, in the range of crucial im- portance to the medical community, and because gal- lium can be obtained in highly pure form. The melting

point rather than the freezing point is suggested as the appropriate equilibrium state because there are some

minor problems associated with ascertaining the

freezing point. The problem is not the overall supercooling of the gallium, which can be circumvented, but

it may be related to local supercooling throughout the sample due to poor thermal conductivity (77, 18) and

to pressure effects (19). I refer the reader to the second paper of this series for more details on this. Recently, it has been proposed that the melting point

of gallium is suitable for and should be used as a clinical fixed point (20). In fact, it is highly suitable to serve as

a superior secondary fixed point of the International

Practical Temperature Scale, and it has been proposed

for that also (21 ). There are several obvious reasons for adopting the melting point of gallium as a secondary

reference point. Some of these are: gallium can be obtained in highly pure form (stated to be at least 99.99999% pure) and it is commercially available at that high purity; it is inexpensive; no complicated and ex- pensive equipment is required to obtain or realize the melting point, and the melting-point temperature can

be easily realized even by untrained personnel; the

melting-point temperature is very sharply defined, the constancy of the melt temperature (±0.0001 m °C or

better) is impressive, and a melt can be maintained for a fairly long time (hours) even with a small sample of gallium and with a relatively large temperature gradient

between it and its surroundings (see Figure 5 of the

following paper). With advantages such as these, it seems likely that the melting point of gallium will be adopted by the CIPM as a secondary reference point

and, furthermore, in the future (at, say, the next planned revision of the Scale in the mid-nineteeneighties) it could become a primary or defining fixed point for the International Practical Temperature

Scale. Gallium melting and freezing points have been in- vestigated for many years, and Table 4 lists the temperature values reported by the researchers for those

points (19, 20, 22-27). It has only been within the last decade, however, that the metal has become commercially available in high-purity form. Ths is due primarily

to the need for such highly pure gallium by the semi- conductor industry. Although 99.99999% pure gallium has been commercially available for only a few years, Roeser and Hoffman (23) of the National Bureau of Standards apparently had good success in purifying

their samples, because the temperature of the solidliquid equilibrium point that they reported in 1934 is in good agreement with the values obtained by recent workers (20, 25-27) (see Table 4). Because the melting-point temperature of gallium

(29.771.4 °C) is in the region of crucial importance to

clinical laboratories, and because gallium has the ad- vantages listed above, it should be of tremendous utility to the biomedical community as a temperature refer- ence point. Toward this end, about 25 g of high-purity

gallium under argon gas in a melting-point cell is available as a temperature reference point standard, Standard Reference Material (SRM) No. 1968, from the

Office of Standard Reference Materials of the National Bureau of Standards (NBS).

71R
Table 4. Published Temperatures of the Equilibrium State between the Solid and Liquid Phases of Gallium^

International Practical Temperature

Equilibrium Reported

stale Rel. ^ temperature C^C) r68(K) «68(°C) Melting point 19 29.752 302.893 29.743

Freezing point 22 29.755 302.896 29.746

Freezing point 23 29.780 ± 0.005 302.921 ± 0.005 29.771 ± 0.005

Melting point 24 29.75 302.90 29.75

Melting point 20 29.772 ± 0.002 302.922 ± 0.002 29.772 ± 0.002

Freezing point 25 29.7697 ± 0.0006 302.9197 ± 0.0006 29.7697 ± 0.0006

Melting point 26 29.770 ± 0.002 302.920 ± 0.002 29.770 ± 0.002

Melting point 27 29.77I4 ± O.OOI4 302.92I4 ± O.OOI4 29.77I4 ± O.OOI4

Melting point'' 29.772i ± 0.00025 302.922, ± 0.0025 29.772, ± 0.00025

' The values ol temperature are for equiiibrium states at a pressure of 101 325 Pa (1 standard atmosphere).

" Thornton, D. D., and Ivlangum. B. W., to be published.

Potential Applications of the Gallium Melting-Point

Temperature as a Temperature Fixed Point The International Federation of Clinical Chemists,

the American Association for Clinical Chemistry, and

the National Committee for Clinical Laboratory Standards have been trying to reach a consensus on a unique

reaction temperature for enzyme reference methods. The temperatures under consideration are 25, 30, and

37 °C. Until now, the choice has been arbitrary, with no agreement about the scientific evidence to justify se- lecting one instead of the others. ' As a result of the work

of the NBS on the melting point of gallium and its proposed adoption as a secondary reference point,

however, it seems to me that the arbitrariness of the choice of the temperature for the enzyme reference methods has now been removed. I suggest that the temperature of the melting point of gallium (29.7714 ±

0. 001,1 °C) be selected as the reaction temperature for reference methods in clinical enzymology. Because SRM

1968 is now available, there is a real possibility every

clinical laboratory having direct access to this reference temperature on the IPTS-68 for enzyme reactions conducted in those laboratories. The use of the melting-point temperature of gallium

as a temperature fixed point in the calibration of ther- mometers, performed by untrained personnel in the laboratory in which the thermometers are to be used,

is another potential application of great import. A series of temperature fixed points comprising the ice or triple point of water, the melting point of gallium, and others being developed throughout the range of biomedical

interest, will permit each and every laboratory to have

its own temperature standards laboratory. One will then have the capability of performing accurate thermometer

calibrations directly and easily vs. the IPTS-68. At the present time, the accuracy of a thermometer's calibration can be easily ascertained at two temperatures by means of the gallium fixed-point standard and the ice

' A lull (liscu.ssion of these problems appears in the Pniceedin/'s

(if Ihc Sc'cand Inlrnwiiiinai Symposium im Clinical Enzxmoliifix. N. W. Tiet/, A. Weinstock, and D. 0. Rodger.son, Eds.', AACC,

Washinffton, IXC, 197(), pp Tih-.'.M.—Ed.

point, but this is not sufficient to permit a calibration

to be made.

In summary, this brief review of our temperature- measurement system should lead to a better understanding of the temperature scales, how they have

evolved, and how they are maintained and disseminated by the National Bureau of Standards. The excellence

of the melting point of gallium as a secondary temperature fixed point has been abundantly demonstrated.

It is suggested that the temperature of the melting point

of pure gallium be adopted in clinical enzymology as the

reacticjn temperature for enzyme reference methods. We

hope that this suggestion will be approved by the vari- ous professional societies and the voluntary standards

groups. References

1. /,( Syslcme International d'l hiitt's. OFFILIB, -18 rue (!ay-l,ussac,

F. T.'ilKl.'i Paris, 1970 (Revised ediliun 197:1).

2. (\imptes rendus des seances de la Treizieme Conlerence (it-nerale des I'oids el Mesures, Resolution p 104 (19r>7-19(;K).

;!. Comples rendus des seances de la Treizieme Conlerence ( lenenile des Poids et Mesures, Resolution 4, p 104 (1907-19(18).

4. Comptes rendus de la Treizieme Conference (lenerale des Poids

et Mesures, Annexe 2, p Al (1967-1968); Coniite Consultalil' de Thermometrie, 8'' session, 1967, Annexe 18; and Mftrolof;ia .5, M.') (1969).

a. Comptes rendus des seances de la Sept ieme Conlerence Ciciierale des Poids et Mesures, Annexe 4, p 94 (1927).

6. Comptes rendus des seances de la Neuvieme Conference (lenerale des Poids et Mesures, Resolution 'A, pp 5.5, 6H. Resolution 6, p 64, Annexe 6, p 89 (1948).

7. Comptes rendus des seances de la Onzieme Conference Cienerale des Poids et Mesures, Resolution 12, p 87, Annexe 5, p 124 (1960).

8. Comptes rendus des seances de la Treizieme Conference Generale

des Poids et Mesures, Resolution 8, p 105 (1967).

9. Comptes rendus des seances de la Quinzieme Conference Generale

des Poids et Mesures, Resolution 7, p 105, Annexe 2, p AI (1975). Also, Mctrotofiia 12, 7 (1976).

10. Cross, R. E., Subcommittee on Enzyme Assay Conditions, National Committee for Clinical Laboratory Standards, private communication.

11. Burtis, C, Chairman, Subcommittee on Enzymes, American

A.ssociation for Clinical Chemistry, private communication.

12. .Jelfers, D. M., Chairman, Area Committee on Instrumentation.

National Committee for Clinical Laboratory Standards, private communication.

717
I.'l. I'atldii, M. M., Mfiiiber, Hoard of Directors, National Commillee

lor Ciinii-ai l,al)oratory Standards, and IVIember, AS'l'IVI Suticommillci' on Medical Thermometry, K 20.08, private communication.

14. Bowers, (!. N., -Jr., Chairman, Expert Panel on Knzymes, Standards Committee of the International Federation ol'Clinical Chemists,

private communication.

I.'l. Bowers, (i. N.,.Jr., Bergmeyer, H. U., and Moss, D. W., Provisional Recommendation (1974) on IFCC Methods for the Measurement of

Catalytic Concentration of Knzymes. Clin. Chim. Al ia 61, Fl 1 (1975). [Clin.Chcm. 22, ;!«4 (1 976)|.

"

Hi. Kilers, R. .1., a member of the Board of Governors of the College

of American Pathologists, and chairman of its Standards Committee,

private communication.

17. l^iwell, R. W., Woodman, M. J., and Tye, R. P., The anisotropic thermal conductivity of gallium. Br. J. Appl. Phys. 14, 4.'i5 (1963).

18. Pashaev, B. P., The change in heat conductivity of tin, bismuth, and gallium during melting. Fiz. Tverd. Tela 3, 416 (1961).

19. Richards, T. W. and Boyer, S., Further studies concerning gallium.

./ Am. Chcm. Sac. 43, 274 (1921).

20. Thornton, D. D., and Mangum, B. W., Clinical fixed points—the

melting [joint of gallium. Proces Verbaux du CCT, Ucjcument No.

<'(T/76-i:!,.June 1976.

21. Mangum, B. W., private communications to the chairman of Working (Injup 2 of the CCT.

22. Craig, W. M., and Drake, (!. W., Preparation and properties ol gallium and gallium trichloride. J. Am. Chcm. Sac. .">(), .'i84 ( 19.'i4). 23. Koe.ser, W. F., and Hoffman, .1. I., Freezing point ol gallium. J.

lies. NHS 13,67;i (19;i4). 24. Wenzl, H., and Mair, (!,, Absence of premelling phenomena in Ijure perfect gallium crystals revealed by Debye-Waller-Factor measurements using anomalous diffraction of x-rays. Z. /'/)\'.s. H 21,

9.'') (197.'')).

2.'). Lau, S., and Schlott, P., Investigation of representation of the

solidification temperature of gallium in the ASMW. Proces Verbaux du CCT, Document No. CCT/7fi-2:i, .June 1976.

2(). Sostman, H. K., Melting point of gallium as a temperature cali- bration standard. Rev. Sci. Inslrum. 48, 127 (1977).

27. Thorntim, D. D., Gallium melting point Standard: A determination of the liquid-solid equilibrium temperature of pure gallium on the International Practical Temperature .Scale of 1968. 1'lin. Chcm.

23, 719 (1977).

718
The Gallium Melting-Point Standard: A Determination of the LiquidSolid Equilibrium Temperature of Pure Gallium on the International

Practical Temperature Scale of 1968

Donald D. Thornton

The sharpness and reproducibility of the gallium melting

point were studied, and the melting temperature of gallium

in terms of IPTS-68 was determined. Small melting-point

cells designed for use with thermistors are described. Nine

gallium cells including three levels of purity were used in 68 separate determinations of the melting point. The

melting point of 99.99999% pure gallium in terms of IPTS-68 is found to be 29.7714 ± O.OOI4 °C; the melting range is less than 0.0005 °C and is reproducible to ±0.0004 °C.

Other papers in this series discuss the need for ac- curate thermometry in the clinical laboratory and the

role of temperature reference points in establishing a useful temperature scale of the required accuracy. Al- though the gallium point is, in itself, not a complete

solution to the problem of accurate thermometry in the

clinical laboratory, it represents significant improve- ment by providing a reference point at a particularly

important temperature.

The purpose of this paper is to document the gallium

melting point for use as a reference point. Consequently,

this paper will discuss briefly the general characteristics of melting and freezing points, describe the pertinent

properties of gallium, detail the considerations in the

design and construction of melting-point cells, describe

briefly the procedures followed in this characterization

of the melting point, and present the results of a large

number of melting-point determinations.

Melting Points and Freezing Points The transition of a pure metallic element from the

solid phase to the liquid phase at constant pressure is a "first-order" transition, which involves a latent heat

of fusion at a fixed temperature. Thus, if heat were applied at a fixed rate to a quantity of a pure metallic el- ement, the temperature of the sample would rise until

Heat Di vision, National Bureau o( Standards and The Ggot^g Washington University, Washington. D. C. Hcceived and accepted .Jan. 21, 1977 (prereviewed).

the solid-liquid transition was encountered. During the melting process, the temperature would remain constant, rising again only after the material was completely

melted. Upon removing heat from the liquid at a constant rate, the reverse would occur; the temperature

would fall until the solid began to form, remain constant during freezing, and fall again only after all the material had solidified. This highly idealized situation is indi- cated graphically in Figure la. Four properties of real materials change this description somewhat, even for very nearly pure samples.

First of all, melting or freezing generally occurs at a surface, the interface between the liquid and the solid phases. Because a solid always has a surface at which

melting may occur, the previous description of melting

is unaltered. A liquid, however, has no preferred locus

at which freezing may start. A small quantity of liquid must "nucleate," i.e., form a microcrystallite. For nucleation to occur, the temperature of at least some of the

liquid must be lowered below the liquid-solid transition temperature. Thus, in freezing we observe supercooling

of the liquid below the transition temperature until nucleation occurs. A period of recalescence follows, during which the latent heat released by the initial freezing of the microcrystallites warms the rest of the

material back to the liquid-solid equilibrium temperature. The second complicating property of real materials

is the finite thermal conductivity of the material; for heat to pass from one point in the sample to another,

there must be a temperature difference between the two

points. This temperature difference is sometimes re- ferred to as a "driving gradient," or just "gradient." In melting or freezing, such a gradient is needed between

the bath and the liquid-solid interface to remove or supply the latent heat. The size of this gradient deter- mines the rate at which melting or freezing may occur. The combined effects of supercooling and thermal

conductivity are illustrated in Figure 16, where the temperature plotted corresponds to that indicated by

719
Fig. 1. Schematic melting and freezing curves showing the temperature indicated by a thermometer in the re-entrant well as shown in (d)

(a) No impurities, no supercooling and no resistance to heat flow: (b) supercooling and finite tfiermal conductivity, but no Impurities; (c) supercooling, finite ttiermal conductivity, and a low content of impurities

a thermometer in a central well surrounded by solid during melting or freezing (Figure Id). A third material property affecting the melting or freezing of pure elements is the dependence of the liq- uid-solid equilibrium temperature on pressure. The

pressure on the material must be kept constant in order

for the liquid-solid transition to occur at a single, well-defined temperature. If the pressure were allowed

to vary freely, the transition would be "broadened," i.e., occur over a range of temperatures rather than at a

single temperature.

Finally, it is nearly impossible to completely remove

all impurities from any material. Only impurities that

are soluble in the liquid or the solid, or both, will affect the melting or freezing behavior significantly. A very minute quantity of an impurity may simply raise or lower the phase equilibrium temperature, but any sig- nificant quantity will broaden the transition as shown

in Figure Ic. In light of the above discussion, some of the reasons why the liquid-solid phase transitions of pure metals

generally make such excellent temperature reference

points are: the latent heat of fusion naturally maintains

the sample at the transition temperature for some time; the high thermal conductivity of most metals assures

that temperature gradients in the metal will be small; the metals can usually be highly refined; the degree of supercooling is normally quite small; and the pressure

dependence of the transition temperature is usually small enough to ignore.

Properties of Gallium Although gallium is not a common element, it has become available commercially in very pure form as a

result of its importance to the semiconductor industry.

The highest purity material that is readily available from commercial sources is 99.99999% (7N's),' in quantities ranging from a few grams to several kilo- grams. Gallium is a group III B metal, with atomic weight

69.72. Chemically it is similar to aluminum except that

it is not as reactive with the common acids and bases; and it oxidizes only slowly in dry air, though the pres- ence of moisture accelerates the process (/). Because many metals such as zinc, aluminum, and copper are very soluble in gallium, it is important in maintaining

purity to avoid contact with metal and metal-containing

materials. Gallium does not have the lowest melting point of the

metallic elements, but it does have the largest liquid range, from 29.77 °C to about 2400 °C. Over much of

this range, the vapor pressure is very low, less than 10~^ Pa (~10''' mmHg) below 900 °C. At normal pressures, gallium crystallizes in the pseudotetragonal form at 29.77 °C, with a density of 5.904 g/cm\ releasing a latent heat of fusion of 80.3 J/g. The density of the liquid is 6.095 g/cm-', so there is a 3.2% expansion on freezing.

This, combined with an ability to easily wet glass, makes

it inadvisable to store gallium in glass containers. The

melting point of gallium is quite pressure dependent,

decreasing 0.02 °C/MPa (1 MPa ^ 10 std. atmospheres)

(2). The heat capacities of liquid and solid gallium are approximately 0.375 J/g °C near the melting point; the thermal conductivities are about 29 W/m °C. The

thermal conductivity quoted is for polycrystalline

samples, because single crystals of gallium exhibit large anisotropy in thermal conductivity, as well as in electrical resistivity, thermal expansion, and the elastic constants (3).

Melting-Point Cells

In general practice, the melting- or freezing-point

cells used for calibrating thermometers involve large

samples with masses between 0.5 and 1 kg. Clearly, a 500-g sample of gallium would be very expensive, so we

were interested in determining the smallest quantity

that would provide a sharp, stable point during several hours. Because the need for recalibration is more

pressing for thermistors than for liquid-in-glass ther- mometers, we started with a thermometer well that was

designed to accept most electronic thermometer probes.

The material chosen for the thermometer well was

nylon, the idea being that the roughened surface of machined nylon might facilitate nucleation of the gal- lium. Thus, the metal would freeze from the well out-

' A purity "( (e.g.) 99.99999'!i. isofleii referred li>as "VN's" purity,

(if 99,999",, a.s ".'"iN's" purity, etc.

720
ward, once a layer of solid had formed. To what extent

nylon is better than other materials we do not know, but we have had good success in freezing the metal to it. Other materials—including glass, polyethylene, and

Teflon—were used for the outer portions of trial cells of different shapes and sizes. The thermal properties of the glass cells were good, but extreme care had to be taken to avoid breakage. Although most plastics are

sufficiently soft or flexible that breakage would not be

a problem, they conduct heat very poorly. Consequently, it was desirable to make the cell walls as thin

as possible. Because polyethylene is slightly permeable

to water vapor (and presumably to air) the chance of contamination with gallium oxide is probably high for

cells of this material. Polycarbonate cells can be made

with thin but vacuum-tight walls, which are also strong

enough to confine the expansion of the gallium without

breaking. If the 3.2% expansion were completely confined, the freezing point would be broadened by the

resulting pressure over as much as 2 °C. Teflon is soft enough to obviate this problem and is also vacuum tight. Figure 2 shows a typical gallium melting cell, available

as Standard Reference Material No. 1968. The polyethylene cells used for most of the determinations differ from the Teflon cells shown only in that the outer wall was made from polyethylene. Preliminary results in- dicated that in cells with less than 15 g of gallium, and

with annular spaces of less than 2 mm, or in which the

re-entrant well penetrated to within 2 mm of the bottom

of the cell, the indicated melting temperature was very

sensitive to the bath temperature. On the other hand,

samples of 20 g produced stable indicated melting

temperatures that were nearly independent of bath

temperature for 2 to 10 h, the duration depending on the bath temperature.

Samples

Because the main emphasis of this study was simply

to determine the suitability of the gallium liquid-solid

phase transition as a temperature reference point in

clinical laboratories, I made no attempt to survey a large number of suppliers or to obtain the highest purity

material available. Consequently, there is no systematic

study of the effects of various specific impurities or

possible variations in the isotopic content on the transition temperature. It was important to verify that

material of different purity from different sources re- produced the melting temperature with suitable precision, i.e., to within 0.005 °C. To this end, five samples

representing three grades of purity were obtained from two different sources. Two samples of 7N's pure gallium came from a single lot; two samples of 5N's pure gallium came from two different lots; and one sample of 3N's

pure gallium was also obtained.

All samples were received in polyethylene containers that had been sealed under argon. The gallium was

transferred to the melting-point cells in a dry argon atmosphere so that the cells were sealed in a dry, inert atmosphere. The cells used in the final melting-point determinations are indicated in Table 1.

9.9 cm

NYLON

TEFLON

^2.3 cm-^

Fig. 2. A cross-sectional drawing of a SRM No. 1968 gallium

melting-point cell The cells used in this study were the same except that the outer walls were of polyethylene and the base was nylon. Such a cell would contain from 20 to 25

g of gallium

The purities listed are those given by the supplier. No

analyses for the particular lot were supplied with these samples, although a "typical" spectroscopic analysis from the supplier of the 7N's material indicated 0.05 ppm (0.05 ng/g) of copper as the only impurity detected. At the end of this study, samples from cells G7902,

G5902, and G59A4 were submitted to the Analytical

Chemistry Division of the National Bureau of Stan- dards (NBS) for spectrochemical analysis. The report

of analysis listed no detectable impurities in the 7N's

material, indicating an upper limit of a few tenths of a microgram per gram for the common metals. In the 5N's

gallium from Lot No. 3792, 0.1 ^lg of silver and 0.5 jig of copper per gram (ppm) were reported. The only impurity detected in the 5N's gallium from Lot No. 1226 was

1 Mg of copper per gram. The report stated that the impurity figures were only approximate and that the

actual values could have been either larger or smaller by as much as a factor of two. In either case, it is clear both that the suppliers' grade of purity is substantially

correct, and that the handling procedures followed in this study have not significantly affected the purity of the samples. This latter conclusion is important, given the method

of constructing the melting-point cells. All parts were machined with carefully cleaned tools, then cleaned in

Table 1. Melting-Point Cells Used in This Study

Cell No. Purity Origin G7901,G7902 99.99999% Lot 081474 sample 1 G79A1,G79A2 99.99999% Lot 081474 sample 2 G59A4, G59A5 99.999% Lot 1226 G5901,G5902 99.999% Lot 3792 G3901 99.9%

721
a detergent solution to remove any grease acquired in handling. After they were rinsed thoroughly in distilled water and dried, the cells were assembled under an

argon atmosphere.

Thermometry

The objects of this study were to establish the

sharpness and reproducibility of the gallium fixed point

and to determine the value of the equilibrium temperature in terms of IPTS-68. Because these cells were

designed for use with thermistors, it was not feasible to measure the temperature of the cell directly with a standard platinum resistance thermometer. Thus, the

following procedure was followed. The resistances of

three thermistors were determined at 10 °C intervals from 0 to 60 °C. An analytic expression^ relating resis- tance to temperature was fitted to the data. These

thermistors were used in the gallium cells for several

series of melting-point determinations. Immediately

following each series of determinations, these thermistors were compared with a standard platinum resis- tance thermometer at several temperatures between 28 and 30 °C in a constant temperature bath of high ho- mogeneity. We estimate that this procedure resulted in an uncertainty of ±0.001 °C in the thermistor deter- minations of temperatures on the IPTS-68 (see below). The thermistors were bead-type thermistors, each

sealed in a glass envelope of diameter 0.31 mm and

nominal 25 °C resistance between 12 and 15 kfl. The

resistance of each thermistor was determined by using

a digital voltmeter to measure the voltage across the thermistor and the voltage across a 10-kS2 standard re- sistor when the thermistor and standard resistor were

connected in series with a constant-current source. These measurements were "four-lead" measurements,

with the current first in the normal direction and then

reversed. The constant-current source supplied a cur- rent of 10"*' A, which was independent of circuit resis- tance and stable to one part in 10''. The digital voltmeter had an input impedance of 10^ and displayed six digits with a 10~" V sensitivity. Thus, the limiting precision

of the measuring equipment corresponded to 0.0002 °C

in the determination of the temperature.

The International Practical Temperature Scale was

realized by a long-stem Standard Platinum Resistance Thermometer, the resistance of which was compared to that of a thermostated lOO-Q standard resistor, by use

of a Cutkosky AC Resistance Bridge (5). For the cali- bration of the thermistors, both the thermistors and the Standard Platinum Resistance Thermometer were im- mersed in a stirred constant-temperature bath in which

the temperature could be regulated to better than ±0.0005 °C. Each of these calibrations included a de- termination of the platinum resistance at the triple point of water.

-' 'I"he standard equation is known as a Steinhardt-Hardt lit: sec. for example: Steinhardt, .1. S., and Hart, S. R., Calibration curves for Iherniistors. Dccp-Si'a We.v. l.'i, 497 (UtfiH).

HEATING TIME IN HOURS

0 2 4 6 8

29 79

1 I 1

1 1 1

1

1

11 T 1

1 1

1

1

1

29,78

29 77 f ——

r

29 76

A

29.75

29.74

START

rFREEZE

29.73

1 1 1 1 1 1 1 1 1 1

1 1

1 16 14 12 10 £ 6 4 2 0 COOLING TIME IN HOURS

Fig. 3. Combined melting and freezing curves for 99.99999%

pure gallium

Because the gradients were slightly different the time scales for the two curves

are different. The supercooling was much deeper than shown, because the freeze was nucleated by a very cold rod, but the initial recalescence was as fast as shown

The major uncertainties in realizing the IPTS-68

temperatures on using the thermistors were: (a) cali- bration of the thermistors, ±0.0005 °C; (6) possible drift of thermistors between calibration, ±0.0002 °C; and (c) limiting precision of the measurement, ±0.0002 °C. An

additional uncertainty arises from the possibility of a difference in thermistor self-heating effects in the cal- ibration bath and the gallium cells; this may be as much

as ±0.0002 °C. Thus, as stated above, the overall uncertainty in the temperatures determined by the thermistors is about 0.001 °C.

Results

It is interesting to compare the melting and freezing behavior of gallium with the idealized behavior shown

in Figure 1. Figure 3 shows typical melting and freezing curves for 7N's gallium; clearly, the 7N's gallium melts

in a nearly ideal fashion, but the freezing curve exhibits

a deep supercooling, which was terminated by nucleating the freeze with a cold rod inserted into the thermometer well. The recalescence is extended and

irregular. The plateau is poorly defined, relatively brief, and well below the melting curve. This freezing behavior

results partly from the exaggerated supercooling and

the poor thermal conductivity of the metal. The pres- sure dependence of the equilibrium temperature may

also be a factor. If the growth of the solid were suffi- ciently irregular, regions of freezing liquid could be trapped so that the 3.2% expansion on freezing would

722
29 79

29 76 -

Table 2. Melting-Point Temperatures of Nine

Gallium Cells

29 73

2 4 6 8 HEATING TIME IN HOURS

Fig. 4. Melting curves ot 99.99999% ("7Ns"), 99.999%

("5N's"), and 99.9% ("3N's") pure gallium for about the same

gradient in each case is shown by —,

, and - • - ., respectively The shape of the 3N's curve is very sensitive to magnitude of the gradient

HEATIN6 TIME IHOURSI

Fig. 5. Melting curves of 99.99999% pure gallium for several

different gradients

result in strains that would broaden the transition as already discussed. In any case, it is clear that obtaining

reliable and well-defined freezing curves with small

quantities of pure material is at least difficult. Consequently, nearly all of the effort reported here deals with

the melting point. Figure 4 illustrates the effects of impurities by showing the melting curves of 7N's, 5N's, and 3N's gallium; Figure 5 illustrates the effects of

different bath temperatures during the melting of a sample of 7N's gallium. In the last two figures, the

Av melting

Cell lemp, '^C SD, °C G79A1 29.7716 ±0.0001 (4) G79A2 29.7716 ±0.0003 (6) G7901 29.7714 ±0.0003 (16) G7902 29.7713 ±0.0004 (12) G5901 29.7712 ±0.0002 (6) G5902 29.7712 ±0.0002 (8) G59A4 29.7703 ±0.0002 (6) G59A5 29.7701 ±0.0005 (5) G3901 29. 76''

This figure is approximate, because the melting point is not well defined at this purity. See Figure 4. Figures in parentheses are no. detns. averaged

samples were first preheated in the bath at about 29.70 °C. At time 0, the bath temperature was raised to the value indicated for each curve and regulated at that temperature for the duration of the melting period.

These curves exhibit the temperature dependence ex- pected for the melting of relatively pure material. In

fact, for all of the curves shown in Figure 5, the temperature variation on the melting plateau is of the same

order as the instrument resolution (i.e., ±0.0002 °C).

Also, it is important to note that the difference between

bath temperature and cell temperature has no effect other than to change the duration of the melt, which

shows that the thermometer well is sufficiently deep to ensure that the thermometer is indicating the temperature of the gallium. This was confirmed by withdrawing the thermistor during the melting period. The

temperature was found to be constant over about a centimeter, but after further withdrawal the temperature rose rapidly. Clearly, the necessary depth of in- sertion will be quite sensitive to the details of the ther- mometer probe, so this is an important consideration

in designing a melting-point cell. Both the length of cell and the diameter of the thermometer well may be adapted to various probes, although longer cells will, of

course, require more gallium.

In contrast to the melting behavior of the 7N's samples, the 5N's gallium takes somewhat longer to reach

a "constant" melting plateau; moreover, during the melting period, the cell temperature shows a constant

rise. This behavior is expected, given the higher content

of impurities. Nevertheless, if one were working at a precision of ±0.003 °C, it would not be possible to dis- tinguish the melting behavior of the 5N's gallium from

that of the 7N's material. Table 2 shows the results of 65 determinations of the melting points of nine different cells representing three degrees of impurity. Listed are the average melting

temperatures and the standard deviations, with the number of determinations that were averaged indicated parenthetically. Each determination represents the value obtained for the melting point with a given

thermistor. In some cases up to three thermistors were

723
used in a siiifile "melt," resulting in three determinations of that meltinf< point. Kvidently there is no sig- nil'itant variation in the melting temperatures for the

different cells containing 7N gallium. The melting

tem|)erature of the 5N's samples from Lot No. 3792 is only slightly lower than the average for the TN's samples; in contrast, the samples from Lot No. 1226 melt

nearly 0.001 °C lower. Presumably, this is the effect of the higher content of copper detected in the material

i'rom Ijot No. 1226. It appears, therefore, that the ob- served melting temperature is very sensitive to small concentrations of copper. Cell G59A5 was a glass cell with a nylon well. This cell was used in the preliminary

study and became contaminated with both oil and air as a result of the nearly inevitable cracking of the cell during freezing. In this case, the cracks were simply

covered with epoxy cement and the cell used despite the contamination. Thus, it appears that the presence of oil and oxide, if any of the latter formed, has little effect on

the melting curve, as would be expected if both were

insoluble in the metal. As regards the oxide, Roeser and

Hoffman (5 ) also noted that its presence seemed to have no noticeable effect on the melting point.

In placing an uncertainty on the temperature of the melting point, it is important to bear in mind that there

are two principal sources of this uncertainty. The first

is simply the uncertainty in determining a temperature on IPTS-68 by use of the thermistors. As previously

discussed, this uncertainty is about 0.001 °C. A second

source of uncertainty arises from not knowing how re- producible the melting point is. As a crude estimate of

this, we use the standard deviation in the melting

temperatures reported in Table 2; this is 0.0004 °C. Because these two uncertainties are nominally unrelated, we add them to obtain an estimate of the uncertainty of reproducing the gallium melting point on

II-'TS-68. Thus, we find the melting temperature of TN's pure gallium on IFTS-68 to be 29.771 , ± 0.001 , °C. It

is, however, quite possible that some of the scatter in the values of the melting point simi^ly reflects the random

errors arising from instrumentation and calibration. In

this case, the uncertainty would be somewhat less. With

the results presented here, we cannot resolve (his question.

In conclusion: I found the melting point of gallium to be nearly ideal as a temperature reference point for

clinical laboratories. The indicated melting temperature

of 29.771, °C changes by less than 0.000.5 °C during

several hours and reproduces from day to day and cell to cell with a precision of better than 0.001 °C. The cells themselves are simple and economical to construct, and

the melting procedure is very straightforward. It seems

likely that the gallium point may, in fact, be suitable as

either a secondary or primary reference point of the next

International Practical Temperature Scale.

References

1 . Sec. lor t'Xiimple, Shekn, I. A., Clinus, I. .S.. and Milyiircs a, 'I'. '1'..

77/ r ( Ih iiitslry of (Idllinni. Klscv'icr. Anislt'rdain. HHili.

2. l-losid, L., Melt in;i curves and |)()l\'m<M i)hic t r ansit iiHi oT gallium

al hij^h pressure. C. A', Ai tul. St i H 270, (19701.

:!. I'liwell, K. W., 'I'he electrical resistivity ol ^alliuni and some (it her anisotropic proper! ies of t his metal. Pntc- liny- Sue. {1, on (It in) A20i>,

.'|2.'"i ( IBfil).

4. Ciilkoskv. \i. 1)., An ac resistance thermometer bridfje. -7. A'r.s. NHS 74C, i."> (197(1).

.5. Roeser, W. F., and Hoffman, J. E., Freezing point of gallium. J. lies. NHS 13,673 (1934).

724
The Gallium Melting-Point Standard: Its Role in Manufacture and

Quality Control of Electronic Thermometers for the Clinical

Laboratory

Henry E. Sostman

I discuss the traceability of calibration of electronic ther- mometers to thermometric constants of nature or to the

National Bureau of Standards, from a manufacturer's basic standards through the manufacturing process to the user's laboratory. Useful electrical temperature sensors, their advantages, and means for resolving their disadvantages

are described. I summarize our development of a cell for realizing the melting phase equilibrium of pure gallium (at 29.770 °C) as a thermometer calibration fixed point, and enumerate its advantages in the routine calibration verification of electrical thermometers in the clinical chemistry

laboratory.

Electronic thermometers in which small and fast- responding thermistors are used as temperature sensors provide a valuable addition to the temperature-measuring capability of the clinical laboratory. The principal reason for this is that the thermistor thermometer's response to temperature is an electrical rather than

a visual analog of temperature, which can be interfaced

directly to a variety of important devices such as meters,

recorders, data-loggers, controllers, and computers

U).

My purpose here is to familiarize the reader with some of the properties of thermistors and other electrical temperature sensors; to show how a manufacturer

of electrical thermometers realizes, maintains, and

transfers the International Practical Temperature Scale (IPTS) to the user of an electrical thermometer; and to discuss the important role that frequent calibration at the gallium melting point plays in establishing and

maintaining the validity of that transfer.

Traceability

"Traceability" of calibration of a thermometer means

the ability to demonstrate an unbroken sequence of

Yellow Springs Instrument Co., Inc., Yellow Springs, Ohio

4.5387. Received and accepted .Jan. 24, 1977 (prereviewed).

steps between the basic truths of the IPTS, through the manufacturing calibration process, through final in- spection, to the thermometer in the laboratory of the

user. The basic truths of the IPTS are the temperatures

at which specific pure materials exhibit equilibrium

between two or three phases. These equilibrium points, also called fixed points, provide calibration environ- ments at precisely known and repeatable temperatures

that are physical constants of nature. The Metrology Standards Laboratory at Yellow

Springs Instrument Co. (YSI) maintains 10 phaseequilibrium fixed points that permits calibration against constants of nature for a wide range of thermometers. None of these thermometric fixed points falls within the 20 to 40 °C range of clinical importance. The gallium

point, at 29.770 °C (2), is the eleventh and latest fixed point to be added to this Laboratory's capability, and

is now used routinely to assure measurements near the midpoint of the clinical laboratory range {3}. Table 1 lists the complete series of fixed points maintained at YSI.

In addition to showing traceability of calibrations to constants of nature, it is also necessary to demonstrate

traceability to the National Bureau of Standards (NBS).

The YSI laboratory has two Standard Platinum Resistance Thermometers that meet the criteria for in- terpolation standards of the IPTS (4). These are transported to the NBS at regular intervals for cali- bration (5). They are then used to verify consonance between the thermometric fixed points maintained at the NBS and those at YSI. Resistance measurements

of these standard thermometers are related to the NBS

by calibrating the resistance-measuring bridges against standard resistors certified by the NBS.

The Metrology Standards Laboratory is in turn re- sponsible for maintaining and certifying the calibration of all standards of temperature measurement used in the production, test, calibration, and inspection of YSI

thermometers. For example, it is responsible for as725
Table 1. Thermometric Fixed Points IVIaintained in the Yellow Springs Instrument Co. Metrology

Standards Laboratory

Fixed point Carbon dioxide sublimation Carbon dioxide triple Mercury triple Water triple Gallium melt Benzoic acid triple Indium freeze

Tin freeze Lead freeze Zinc freeze Aluminum freeze

Temperature, IPTS-68,°C -78.476

-56.541^

-38.841^

0.01 29.770'' 122.37 156.634 231.9681 327.502

419.58

660.46

''' Not defining fixed points or secondary reference points on IPTS-68. The value

for gallium at 29.770 "C is from reference 2.

suring the accuracy of the thermometer probes that

control temperature within the specially-designed laminar-flow baths. Temperature gradients are known

to be less than 0.001 °C in such baths and absolute temperature accuracy with respect to the IPTS is es- tablished with the Standard Platinum Resistance Thermometer traceable to the NBS. These special liq- uid comparison baths are used as mentioned in this paper (a) to determine resistance networks of probes

at 0, 25, and 40 °C, in order to make them inter- changeable, and (6) to establish the final product per- formance at 25, 30, and 37 °C after calibration at the melting point of gallium.

Types of Temperature Transducer Used in Electronic Thermometers

Several electrical properties of materials vary with temperature and can be used as the temperaturesensing principle of electronic thermometers. I will discuss three types of sensors: (a) thermocouples that change voltage and ib) platinum and (c) semiconductor

thermistors; both of the last two change in resistance with change in temperature.

Thermocouples have been the mainstay of industrial temperature measurement for almost a century, al- though they are gradually being replaced in many applications by industrial platinum resistance thermometers. A thermocouple works in the following manner: when two conductors of dissimilar material are fused

together to form a junction (called the "measuring

junction") and the other ends are connected into a measuring system (called the "reference junction"), a small voltage is generated, which is related to the temperature difference between the two junctions (6, 7). Thermocouple pairs may be made of many materials and vary widely in temperature range and in linearity,

stability, and outputs, and so afford the user much

latitude in meeting the requirements of his work. Unfortunately, the electrical output is usually small. Also,

because the output is a function of the temperature

difference between the measuring and reference junctions, it is necessary to know the temperature of the

reference junction very accurately. Thermocouples can meet special needs in the clinical laboratory when very

small size (wires as small as 0.012 mm in diameter) and

very fast response (time constants in milliseconds) are necessary. Standardized thermoelectric pairs are designated by

letter identifications, and tables of thermoelectric

voltage vs. temperature difference from the reference junction temperature have been published by the NBS

(8) and others (6). Two popular thermocouples are type E and type T. A type E thermocouple made of chromel wire and constantan wire is usable over a range from —270 to +1000

°C. It has the highest output of any standardized ther- mocouple: 61 /uV per degree Celsius in the 20-40 °C

range. A type T thermocouple made from copper wire and constantan wire is usable over the range —270 to

-1-400 °C, and has an output of 41 ^W/°C over the 20-40 °C range (8). It has the best accuracy specification, because of the high purity of the copper wire. Platinum resistance thermometers depend upon the change in electrical resistance of pure platinum wire as

a result of change in temperature. The Standard Plat- inum Resistance Thermometer is the designated in- terpolation instrument over a large portion of the IPTS

(4). This laboratory standard, although an instrument

of great sensitivity and stability, is large, delicate, and

fragile, and requires complex and expensive auxiliary

equipment. In general, its use is limited to standards

laboratories. For the workaday purpose of many clini- cal-laboratory and industrial-control applications, smaller and more rugged platinum resistance ther- mometers have been developed, which retain the desirable attributes of platinum at some sacrifice in ulti- mate performance and are used in measurement and

control systems requiring accuracy and stability over

a wide temperature range in working environments

(.9).Thermistor resistance thermometers, like platinum

thermometers, exhibit a change in resistance with change in temperature, but they are made from semi- conductor materials instead of pure metal wire. The

usable range of thermistor thermometers is relatively narrow, but over this range they exhibit a very high

sensitivity (change in resistance/change in temperature). This large rate of change simplifies the design of readout instrumentation for narrow temperature

ranges. Thermistors are made in many shapes and sizes, of

several material formulations, and by several processes. Two types, however, are of primary importance in clinical temperature measurements: bead and disc thermistors (10, 11). Bead thermistors may be thought of as single drops

of material that are bridged across a pair of conducting

wires and then furnace-fired into a bead of ceramic

material. The bead diameter is typically 0.5 mm, but

726
may he as small as 0.1 mm. A typical permissible temperature range is —50 to +300 °C. Tolerances on resis- tance and the slope of the curve of resistance vs. temperature are typically wide, because the properties of

the finished thermistor depend on the original material and the details of process, and no adjustment can be made during manufacture.

Disc thermistors are made from materials that generally are similar to those used in bead thermistors, but

are stamped into pellets in a pill press, are sintered in furnaces, fitted with connecting wires, adjusted to

specifications, and overcoated, typically with epoxy. The operating range may be —80 to +150 °C. Tolerances on resistance and sensitivity can be controlled within

close limits in manufacturing; thus disc thermistors are

closely interchangeable, while bead thermistors are not. The long-term stability of discs is not as good as that of the best beads.

Thermistor Sensors in Practicable Measurement

Systems

For instruments intended for use over narrow temperature spans, such as those commonly used in clinical chemistry, the high sensitivity of thermistors is a compelling reason to use them as sensors. However,

thermistors exhibit some less-desirable characteristics that must be mitigated or eliminated by design and

calibration. In the discussion of these techniques, the example used will be a commercially-available temperature instrument designed for measuring temperature in spectrophotometer cuvettes.' The Cuvette Thermometer has five ranges, a survey range of 20 to 40 °C and four high-resolution ranges, 24 to 26 °C, 29 to 31 °C, 31 to 33 °C, and 36 to 38 °C. The specified accuracy of the high-resolution ranges is ±0.05 °C within 0.1 °C of center scale and ±0.1 °C over the rest of the scale, with readability of 0.01 °C and repeatability of 0.005 °C. Thermistor probes for the instrument are made to be interchangeable within these guaranteed limits by at- tention to the following:

/; Corrections to assure linearity and interchangeability. Figure 1 shows the curve of electrical resistance

vs. temperature for the disc thermistor used as sensor

for the Cuvette Thermometer. The complete curve is for the survey range of 20 to 40 °C; the shaded portions

are the four high-resolution ranges. It is obvious that the

slope of the line is different at the lower and upper end

of the temperature range and also that the slope of each

high-resolution range is different from that of the others.Disc thermistors are adjusted in manufacture to an

interchangeability of ±0.1 °C; bead thermistors cannot

be adjusted, but must be selected for interchangeability.

' YSl Model 45CU Cuvette Thermometer and Probes 4501 (bead

thermistor) and 4.502 (disc thermistor). Temperature transducers must work in realizable instrument systems; therefore of necessity the discussion in this paper will be limited to this electronic ther- mometer and its two probes.

5 l—J—I

'—'—I'—I—I 1—I—I—I—I—I—I—I—I—I—I 1 20 25 30 35 40

°C

Fig. 1. Relationship between temperature and resistance ot a

typical disc thermistor over the range 20-40 °C, showing

characteristic nonlinearity. The range of the curve is the survey range of the Cuvette Thermometer. The four shaded portions are the four high-resolution ranges.

Neither type can be obtained with interchangeability

sufficient to satisfy the final specifications for the Cuvette Thermometer. The problems of nonlinearity and

of interchangeability are solved simultaneously by a

resistive network built into the probe plug. Figure 2a shows a schematic of this network and Figure 26 shows

the physical location of the resistors. In the schematic, the resistor labeled is the

thermistor, and R\, Ro, and Ra are fixed resistors chosen

to adjust the linearity and the interchangeability of the thermistors. In manufacture, each thermistor probe

(without the network) is measured at 0, 25, and 40 °C

in specially-designed laminar-flow baths whose temperature control and accuracy was discussed under

Traceability.

On the basis of these measured values, R-z. and R-.i (which are to an extent interdependent) are calculated by a computer program to adjust the network to simulate the characteristics of a hypothetical nominal

thermistor. The network makes the four high-resolution ranges exactly linear; and compensates for all but 0.2 °C

of the nonlinearity of the survey range, which is compensated by use of a slightly nonlinear scale on that range only. The differences in slope of the four highresolution ranges are corrected by use of the range-selection switch to slightly change the amplifier gain, so

that all four high-resolution ranges can be displayed on

the same scale.

2: Correction for instability. Thermistors are less

727
THIS TYPICAL CALIBRATION SHIFT COMPRISES

^TRANSLATION

\ AND ROTATION

A^

Fig. 2a. A resistance network for adjustment of linearity and

interchangeability

Fixed precision resistors, selected by a computer program, adjust the thermistor Rj to nominal calibration and correct for the non-linearity shown in Fig. 1

Fig. 2b. Physical location of adjustment resistor network

The plug has three electrical contacts, separated by insulation. The resistors R,. R^, and R3 are shown as they are connected to the several conductors and

are enclosed within the plastic handle of the plug. The cable extends from the handle to the thermistor Rj.

stable over long periods of time than are platinum re- sistance thermometers, although they may be more

stable than base-metal thermocouples. To study the

stability of disc thermistors, five YSI probes made with

disc thermistors were sent to the NBS for calibration

at —40, 0, +40, and +70 °C, and the resistances were

reported. These probes have subsequently been recal- ibrated at the NBS seven times between 1962 and 1975. The differences in resistance between 1962 and subse- quent calibrations were converted to temperature. Table 2, line 1, shows the change from the 1962 initial calibration at +40 °C found upon each subsequent

calibration; line 2 shows the change at 0 °C. It will be

seen that the change at +40 °C is usually not the same

as the change at 0 °C. Each shift contains two components: a rotational shift, or change in sensitivity, and

a translational shift, or change in calibration that is the same at any temperature. Figure 3 clarifies the meaning

of these terms. It is obvious that if the instrument has

a means for adjusting the meter reading at one point,

and the user has available one precise temperature

standard at which to calibrate, the thermistor probe

may be subjected to that precise temperature, and the adjustment may be used to eliminate the effect of

translational shift. However, a calibration at one temperature cannot eliminate rotational shift. Using the data of Table 2, one can eliminate the

translational shift by subtracting Line 2 from Line 1 for each probe. This is the arithmetical analog of making

THIS COIl^PONENT CAN BE REMOVED BY I -POINT CALIBRATION

THIS COMPONENT CAN NOT

Fig, 3. Types of thermistor shift with age and use

an instrument calibration adjustment to shift the position of the pointer relative to the scale. The remaining

shift, shown on Line 3, is the rotational shift between

0 °C and 40 °C, which cannot be eliminated by making

a single calibration adjustment. However analysis of data at other temperatures shows that this rotational

shift is essentially linear over this range, so that the

rotational shift over the range from 20 to 40 °C will be

V2 of the value shown on Line 3. Furthermore if a single calibration adjustment is made so that the instrument

reading is correct at the middle of the 20 to 40 °C range, the calibration at 20 and 40 °C will be correct within ± '/> the 20-40 °C error, or ±'/4 of the value shown on Line 3. The value for the irreducible ± error over the range 20

to 40 °C for each of these probes is shown on Line 4. This is an important statement, because it shows that

errors due to thermistor shift can be reduced, by a sin- gle-point calibration near mid-scale, to insignificant errors over the entire 20-40 °C range.

3: Heat-extraction errors via connecting wires are analogous to stem-loss errors in liquid-in-glass ther- mometers, and are common not only to all thermistor thermometers, but to all electrical thermometers. This problem is resolved only by user understanding and

care. When a sensor, attached to connecting wires, is im- mersed into a temperature environment, heat flows from the warmer to the cooler end of the wires. The

temperature depression of the sensing tip depends on

(a) the temperature difference of the bath-to-ambient

interface, (b) the distance from the sensing tip to that

interface, and (c) the thermal conductivity and sectional area of the connecting wires. In one probe for the Cuvette Thermometer, a bead thermistor has connecting

wires of 0.025 mm platinum-iridium, whose thermal

728
Table 2. Historical Shifts in Five YSI Thermistor Probes Calibrated at the NBS^

Date Temp,

Line calibrated °C Probe 1 Probe 2 Probe 3 Probe 4 Probe 5

1 Jan. 1964 40 -0.027 -0.021 -0.023 -0.025 -0.017

2 0 -0.007 -0.006 -0.006 -0.005 -0.005

3 0.020 0.015 0.017 0.020 0.012 4 ±0.005 ±0.004 ±0.004 ±0.005 ±0.003

1 Dec. 1964 40 -0.004 -0.017 -0.017 +0.002 -0.006

2 0 -0.003 -0.015 -0.028 -0.001 -0.000

3 0.001 0.002 0.011 0.001 0.006 4 ±0.000 ±0.001 ±0.003 ±0.000 ±0.002

Jan. 1966 40

0 +0.001 -0.004

0.005 ±0.001

-0.004

-0.003

0.001 ±0.000

-0.029 -0.003

0.026 ±0.007

+0.002

-0.003

0,005 ±0.001

-0.002 -0.003

0.001 ±0.000

Jan. 1976 40

0 -0.020

-0.004

0.016 ±0.004

-0.025 -0.006

0.019 ±0.005

-0.025

-0.018

0.007 ±0.002

-0.014 -0 003

0.011 ±0.003

Dec. 1976 40

0 +0.023 -0.002

0.025 ±0.006

+0.029 -0.002

0.031 ±0.008

+0.002 -0.012

0.014 ±0.004

+0.012

-0.002

0.014 ±0.004

+0.008

-0.001

0.009 ±0.002

Mar. 1970 40

0 -0.008

-0.001

0.007 ±0.002

-0.016

-0.002

0.014 ±0,004

-0.018

-0,010

0.008 ±0.002

-0.008

-0.001

0.007 ±0.002

-0,014

-0,003

0,011 ±0,003

1 Nov, 1975 40 -0.018 -0.016 -0.018 -0.018 -0.024

2 0 -0.025 -0.025 -0.024 -0.020 -0.027

3 0.007 0.009 0.006 0.002 0.003 4 ±0.002 ±0.002 ±0.002 ±0.001 ±0.001

" The initial calibration was made in January, 1962. Values shown below are differences in °C from the values of the initial calibration. Note: All values in Lines 4 have been rounded off to the nearest 0,001 °C.

conductivity is 30 W/(m-K). In another, larger and more

mechanically rugged probe, a disc thermistor has con- necting wires of 0.21 mm copper, whose thermal conductivity is 410 W/m-K. Figure 4 shows the errors due

to insufficient immersion for each of these probes. Stem-loss errors are relatively easy to evaluate in any

given real situation. The sensor is gradually moved into the medium and the indicated temperature noted. When further immersion gives no continued change in indicated temperature, the immersion is sufficient. Such

errors, where they_pccur in cuvette temperature measurements, can often be decreased or eliminated by

coiling a few centimeters of wire inside the sample

chamber.

Calibration of Thermistor Thermometers

A thermistor thermometer, like any other ther- mometer, is calibrated by exposing it to an accurately

known temperature and noting the indication. In most

electronic thermometers the meter pointer position can

be adjusted while the probe is at a known temperature,

to correct the indication. Thus, the accuracy of cali- bration is limited by the ability of the operator to adjust

to the calibration mark and the accuracy of the cali- bration temperature. Calibration temperature envi- ronments are achieved by one of two means:

1: Stirred-liquid comparison baths allow comparison

of a thermistor probe with a certified reference ther- mometer, which may be a Standard Platinum Resis- tance Thermometer or a precision mercury-in-glass

thermometer (12, 13). Many journal articles and special publications adequately cover the techniques for such comparison methods (14-16); for example, the National Committee for Clinical Laboratory Standards, a vol- untary national consensus standards organization, has

rec- tly published a Tentative Standard (No. TSI-2)

enti. jd "Standard for Temperature Calibration of Water F ths. Instruments, and Temperature Sensors"

729
(17). This standard was prepared to "provide the necessary background information and methodology for the performance of practical temperature calibrations using

Standard Reference Material (SRM) thermometers

available from the NBS, Washington, D. C, and from

commercial sources." By observing proper precautions, some of which are peculiar to the use of mercury-in-glass

thermometers {18), it is possible to make comparison

calibrations between these NBS reference thermometers (SRM 988 and 934) and thermistor probes that are traceable and accurate to IPTS-68 to ±0.0.3 °C at 25, 30, and 37 °C {12), but one cannot go below these levels of

certainty, owing to certification limits and scale resolution. The SRM 933 and 934 thermometers were an im- portant interim step toward accurate and traceable

calibration of clinical laboratory thermometers. However, it must be realized that they are not primary

temperature standards, but rather only a means of

transferring temperature standards; that is, they do not represent facts of nature, but simply transfer to the

user's laboratory the standards that are based on the

facts of nature as established at the NBS. Therefore all mercury-in-glass thermometers require periodic veri- fication against the ice point or some other more pri- mary temperature standard to assure their constancy

and accuracy {12-16).''

2: Fixed-point temperature standards. The temperatures at which pure materials exist in two- or three-phase equilibrium are, on the other hand, fun- damental constants of nature. First-order phase transitions (those that involve latent heat) occur at unique

temperatures and can provide stable and primary en- vironments for thermometer calibration {19). The IPTS

defining fixed points and secondary reference points have already been discussed in Mangum's paper of this

series (20) and some have been listed in my Table 1. However, these phase equilibria of pure materials are

so fundamental to high-accuracy thermometry that a

brief review is in place here. The thermal condition in which pure water can exist simultaneously in its liquid and solid form at its own

- The equipment, skill, and time needed to make such comparisons

is often grossly underestimated. To make comparison calibrations which approach ±0.01 °C, numerous sources of error listed in refer- ences 1 1-1.5 must be considered. For example, the following are fre- quently overlooked: ( 1 ) he-bath verification of scale at 0 °C. It is not

generally appreciated how difficult it is to use an ice bath correctly. Errors due to failure to maintain a homogeneous slurry, to assure air saturation, and to keep if fresh (an ice bath immediately begins to dissolve COj from the air) may result in icepoint depressions as large as 0.01 °C. (2) Time constant errors. The response to temperature change of bead and disc thermistors is many times faster than that of mercury-in-glass thermometers. In a temperature bath in which

the bath-medium temperature fluctuates randomly or cyclically, the thermistor probe may follow the changes while the SRM thermometers integrate them, and temporal differences of indication may be observed. One solution is to hang in the baf n integrating block of copper or aluminum, with holes filled with le bath medium into which the reference thermometer and the ttiermistor can both be

inserted to their proper immersion depths.

1.25 * ' - -J - - J 1 g 0 0.1 02 0.3 04 0 5 DEPRESSION OF INDICATION , °C

Fig. 4. Immersion errors for two types of thermistor probe and

lead wire

Ttiermistor connecting wires will cause errors by extracting tieat unless im- mersion depth is adequate- ( Top curve: 450 1 probe (bead); bottom curve: 4502

probe (disc))

vapor pressure, the "triple point," is defined as 273.16 K and 0.01 °C. It can be maintained for days or longer

and realized to within 0.0001 °C once the initial strain-free equilibrium is reached. The thermal condition in which pure water can exist simultaneously in liquid and solid form, the ice point, is defined as —0.01 °C below its triple point; and that in which it can exist simultaneously in its liquid and vapor form at (1 stan- dard atmosphere of ambient pressure), the boiling point,

is defined as 373.1,5 K or 100 °C {4).

Other temperatures are defined by the liquid-solid

equilibria of high-purity metals such as tin (231.9681

°C) and zinc (419.58 °C). In suitable but relatively

simple equipment such phase equilibria may be maintained for many hours with temperature plateaus constant to 0.001 °C {4).

Gallium Phase-Equilibrium Studies and

Applications ^

In 1973, in an attempt to discover such a fixed-point

temperature standard in a range useful in clinical laboratories, we undertook a study of the liquid-solid

equilibrium temperature of pure gallium. We sought to determine what that temperature is, and to describe techniques for maintaining it for long enough to permit

practicable thermometer calibration. Other workers

have reported similar efforts; Mangum's paper in this

series contains a list of references {20). We have published our work elsewhere (2) and will only summarize

some of its key features before discussing its application

to clinical thermometers. We obtained gallium metal from two sources. In each sample the purity of metal was stated by the manufacturer to be 99.9999% ("6N") pure gallium. Purity of a metal influences the temperature at which it melts, and

also the sharpness of the melt plateau. These effects of

increasing purity of gallium are discussed by Thornton

in a prior paper in this series (21 ).

730
Table 3. Accuracies Found for 20 Disc Thermistor Probes before and after Calibration at the Gallium

Point ^

Col. 1 29.770 °C Col. 2 Col. 3 Col. 4 Col. 5 Probe No. (gallium) Error, °C 25.000 30.000 °C 37.000 °C

-| 29.760 —Q Q1Q OU.tJUU Q 7 A 1 A

2 29.770 Q OA QQc; OA Ann 07 AAC

3 29.765 —Q QQ5 OA QQ^ 07 AAC

4 29.770 Q 24 995 •^7 nnc; 5 29.775 + 0 005 25 000 '^7 nnn

g 29.775 +0 005 24 995 0\J .\J\J\J •^7 nn^

7 29.770 Q 25 000 '^7 nnn

3 29.765 —Q QQ5 24 995 '^n nnn 07 nn^^ g 29.790 +0.020 25 000 ^n nnn 37 000

10 29.775 + 0.005 25 000 QA AAA ou .uuu 37 000

1 1 OQ 77(^ u.uuo OC AAA QA AAA 07 AAA 0 / .uuu 12 29.770 0 25.000 30.000 37.000

13 29.775 +0.005 25.000 30.000 37.000

14 29.770 0 24.995 30.000 37.005

15 29.770 0 25.000 30.000 37.000

16 29.775 0 24.995 30.000 37.005

17 29.765 -0.005 24.995 30.000 37.005

18 29.765 -0.005 24.995 30.000 37.010

19 29.760 -0.010 25.000 30.000 37.000

20 29.765 -0.005 24.995 30.000 37.005

Note: Each of 20 probes were plugged into the readout instrument and the instrument zero adjusted according to the instruction manual. The probes were then used to measure the gallium point, and the indicated temperature and error are shown in columns 1 and 2. respectively. The instrument was then adjusted to show

the gallium point correctly for each probe, and the probes were subjected to 25, 30, and 37 °C in stirred baths. The indications obtained are shown in columns

3, 4, and 5.

teau durations of 7.5, 12, and 26 h at the same bath el- evations.

Quality Control of Thermistor Thermometers

Smaller gallium melting-point cells containing 40 g

of 99.9999% pure metal were constructed with central

wells of 3.5 mm inside diameter for the insertion of thermistor probes, either bead thermistors (Model

4501) or disc thermistors (Model 4502). Using these

cells, we tested 20 probes, representing a production run

of Model 4502, for conformity to specification. A YSI

Model 45CU Cuvette Thermometer was operated in accord with the Instruction Manual on the 29-31 °C

high-resolution range. Each probe was inserted in turn

While the gallium point can be realized by chilling the metal to

its solid pha.se and then heating to slightly above the melt temperature

in a stirred liquid bath, some skill and awareness of possible problems

is nevertheless required. It is an attractive idea to contemplate an automated device for routine daily use by lajjoratory personnel that would remove these problems. A relatively straightforward device can

be imagined in which the cell is mounted in a block that is heated and

cooled either as a function of elapsed time or of thermal profile, to provide the thermal cycle require^! to realize the melt platejja without operator attention. It should be possible to turn on such a device with

a timer, or at the beginning of the working day, and have available all day a fundamental temperature reference at 29.77 °C, accurate within

a few thousandths of a degree. We have such a device under devel- opment and it will be available, it is hoped, in the fall of 1977.

To establish the melt temperature for 99.9999% pure

material, we made repeated measurements with our NBS-traceable Standard Platinum Resistance Thermometers. Two large polypropylene cells were constructed, each containing 500 g of gallium from a dif- ferent source. These large cells were necessary to obtain

sufficient immersion depth and mass of metal around

the temperature-sensing element of the Standard

Platinum Resistance Thermometers. The melt equi- librium temperature of each cell was measured five times with each of our two standard thermometers. The

calibration of each thermometer was verified at the

triple point of water before and after each set of measurements. The mean and standard deviation for 10 measurements (five with each of two thermometers) for each of the two cells were (a) 29.7695 ± 0.0012 °C and

(6) 27.7696 ± 0.0009 °C. We therefore reported (2) that the melting equilibrium temperature of gallium of this purity is 29.770 °C, with an uncertainty of ±0.002 °C on IPTS-68. An atmospheric pressure effect of 0.0015 ±

0.0005 °C per standard atmosphere (101 kN/m2) was

determined during these measurements. The duration of the constant-temperature melt pla- teau of these large cells was found to be 19, 31, and 68 h when the water bath was set 0.3, 0.2, and 0.1 °C, re- spectively, above the melt temperature. Small cells containing 40 g of gallium of the same purity had pla731
into the gallium cell at its melting equilibrium point and

the meter indication noted. Column 1 of Table 3 shows

the results obtained. It will be noted that while the in- strument's accuracy specification is ±0.1 °C at 29.77 °C

(±0.05 °C within 0.1 °C of center scale, which for this range is 30 °C) the twenty probes in one electrical thermometer displayed errors at 29.77 °C of only —0.010 °C and +0.020 °C or less. Accuracy at 25, 30, and 37 °C after Adjustment to Gallium

Kach probe in turn was inserted into one of the small

4()-g gallium cells maintained at its melting equilibrium

point. The Model 4.'5CU Cuvette Thermometer was

adjusted, using its zero-adjustment knob, to display

exactly 29.77 °C on its indicating meter. Following this adjustment at the gallium point it may be stated that

the instrument is traceable to IPTS-68 at this one point

by the work previously described (2). Kach probe, while connected to the calibrated system, was then placed in sequence into stirred calibration baths at 25, 30, and 37 °C and the indications noted

(bath control and accuracy described under Traceability). These indications are shown in Columns 3, 4, and

5 of Table 3. It will be seen that errors at 30 °C could not be detected, while at 25 °C the largest error noted was

-0.005 °C and at 37 °C the largest error noted was

+0.010 °C.

In summary, thermistors, because of their high-re- sistance change per degree Celsius near ambient temperature, provide a rapidly responding and relatively stable temperature transducer for electronic ther- mometers. By frequent calibration at the gallium

melting equilibrium temperature in an appropriate cell, the user of electronic thermometers with thermistor probes can calibrate them at 29.77 °C to ±0.01 °C, with

traceability to IPTS-68. This calibration to a constant

of nature represents standardization at the center of the range of temperatures of interest within the clinical laboratory.

References

I. Trolander, H. W., Current state of electrical thermometry for bi- ological applications. In Temperature, Its Measurement and Control

in Science and Industry, 4, H. H. Plumb, Ed. Instrument Society of America, 400 Stanwix St., Pittsburgh, Pa. 15222, 1972, p 203.5.

2. Sostman, H. E., Melting point of gallium as a temperature cali- bration standard. Reu. Sci^ Instrum. 48, 127 (1977).

3. ASTM Subcommittee E20.08.04, Draft Revision of November 1976

of Praposed specifications for thermistor type sensors to be used for

clinical laboratory temperature measurements. American Soc. for Testing and Materials, 1916 Race St., Philadelphia, Pa. 19103.

4. Preston-Thomas, H., The International Practical Temperature

Scale of 1968, amended edition of 1975. Metrologia 12, 7 (1976).

5. Riddle, J. L., Furukawa, G. T., and Plumb, H. H., Platinum resis- tance thermometry. Nat. Bur. Stand. Monograph 126, Supt. of Documents, U. S. Govt. Printing Office, Washington, D. C. 20402.

6. ASTM Special Technical Publication STP 470, Manual on the Ihc

of Thermocouples in Temperature Measurement, American Society

for Testing and Materials, 1916 Race St., Philadelphia, Pa. 1910.S.

7. Baker, H. Dean, Ryder, E. A., and Baker, N. H., Temperature Measurement in Engineering, Wiley & Sons, New York, N. Y., 1966.

8. Powell, R. L., Hall, W. J., Hyinck, C. L. Jr., et al.. Thermocouple

Reference Tables based on the IPTS-68. Nat. Bur. Stand. Monograph

!'25, Supt. of Documents, LI. S. Govt. Printing Office, Washington,

I). C. 20402.

9. Carr, K. R., An evaluation of industrial platinum resistance ther- mometry. In ref. / , p971.

10. Sapoff, M., Thermistors for biomedical use. In ref. /, p 2109.

11. Sachse, H. B., Semiconducting Temperature Sensors and Their

Applications, Wiley-Interscience, New York, N. Y., 1975.

12. Maiigum. B. W., and Wi.se, .1. A., De.scriplion and use of precision Ihermomelers for Ihe clinical laboratory, SRM 993 and SRM 9:i4, NHS Special I'ublicatum 26(1- IH. Nal. Bur. Stds.. Washington. I). C. 2(l2:i4. \cf. Clin. Chem. 20, 670 ( 1974)|.

13. Mangum, B. W., Standard Reference Materials 93.3 and 934: The

National Bureau of Standards precision thermometers for the clinical laboratory. Clin. Chem. 20, 670 (1974).

14. Ween, S., Correct application of liquid-in-glass thermometers for accurate temperature measurements in the clinical laboratory. Clin. Chem.22, 1112 (1976).

15. Swindells, J. F., Calibration of liquid-in-glass thermometers. Nat.

Bur. Stand. Monograph 90, Supt. of Documents, Govt. Printing Office, Washington, D. C. 20402.

16. Wise, J. A., Liquid-in-glass thermometers. Nat. Bur. Stand. Monograph 150, Supt. of Documents, Govt. Printing Office, Washington, D. C. 20402.

17. NCCLS Standard for Temperature Calibration of Water Baths, Instruments and Temperature .Sensors, Tentative Standard TSI-2. National Committee for Clinical Laboratory Standards, 771 E. Lancaster Ave., Villanova, Pa. 19085 (August 1975).

18. Sostman, H. E., Notes on the calibration of Model 45CU with

liquid-in-glass thermometers. Yellow Springs Instrument Co., Inc., Yellow Springs, Ohio 45387 (1976).

19. McLaren, E. H., The freezing points of high purity metals as precision temperature standards. In Temperature. Its Measurement

and Control in Science and Industry, 3, F. G. Brickwedde, Ed., Reinhold, New York, N. Y., p 185, (1962).

20. Mangum, B. W., The gallium melting point standard: Its role in our temperature measurement system. Clin. Chem. 23, 711 (1977).

21. Thornton, D. D., The gallium melting-point standard: A deter- mination of the liquid-solid equilibrium of pure gallium on the In- ternational Practical Temperature Scale of 1968. Clin. Chem. 23, 719

(1977).

732
The Gallium Melting-Point Standard: Its Application and Evaluation

for Temperature Measurements in the Clinical Laboratory

George N. Bowers, Jr. and Stanford R. Inman

We are impressed with the ease and certainty of calibrating electronic thermometers with thermistor probes to ±0.01 °C at the gallium melting point, 29.7714 °C. The IFCC

reference method for measuring aspartate aminotransferase activity in serum was run at the reaction temperature of 29.77 14 °C. By constantly referencing to gallium as an integral part of the assay procedure, we determined

the absolute reaction temperature to IPTS-68 (International Practical Temperature Scale of 1968) to ±0.02 °C. This unique temperature calibration standard near the center

of the range of temperatures commonly used in the clinical laboratory is a valuable addition and can be expected to improve the accuracy of measurements, especially in clinical enzymology.

Over the past two decades, we have been concerned

with the accuracy and control of temperature(s) used

in clinical laboratory testing, particularly the reaction temperature(s) in clinical enzymology (J-4). As ex- plained in a preceding paper (5), temperature measurements, to be considered accurate, must be traceable

either directly or by interpolation to the defining fixed points or secondary reference points of the International Practical Temperature Scale of 1968 (IPTS-68).

In this laboratory, traceability to IPTS-68 was achieved

before 1974 by sending a suitable mercury-in-glass

thermometer to the National Bureau of Standards (NBS) for calibration (6'), since 1974 by use of the clinical Standard Reference Materials 933 and 934 (7). In September 1976 we started an evaluation of a small prototype gallium melting-point cell,' which was de- signed specifically to aid in the calibration of small

Department of Pathology, Hartford Hospital. Hartford. Conn.

06115.

G. N. B. Jr. Is director of the Clinical Chemistry Laboratory; S. R.

I. is an instrumentation specialist.

' NBS prototype gallium melting-point cell, identified as G79A2.

with an assigned value of 29.771 1 ± 0.0014 °C (see third paper of this series by Thornton, ref. ^i). Received and accepted Jan. l(i, 1977 (prereviewed).

thermistor probes of electronic thermometers (8). This paper describes some of our studies with this prototype

gallium cell, especially as they relate to the accurate determination of the final reaction temperature for reference methods in clinical enzymology.

Calibration of an Electronic Thermometer and Its Probes

Prior "in-house" calibrations for two identical elec- tronic thermometers in various combinations with four bead-thermistor type probes,- as judged from comparisons in stirred liquid baths with use of SRM's 933 and 934 mercury-in-glass thermometers as the reference thermometers, had given an uncertainty at 25, 30, and

37°Cof±0.03°C(7).

Table 1 shows our "in-house" calibrations for the same two electronic thermometers in various combinations with the same four bead-thermistor probes, but with use of the liquid-solid equilibrium temperature of the NBS prototype gallium cell. In adjusting the elec- tronic thermometers, we exactly followed the instruction manual and then switched to the 29 to 31 °C high- accuracy range. With the bead thermistor of the probe

deeply immersed within the well of the gallium cell, the

fixed point of the gallium melt was then judged visually from the meter's dial. The IPTS-48 dial readings of the Yellow Springs Instrument Co. thermometers were

converted to IPTS-68 by making a correction of —0.0093

°C.'' As Table 1 shows, probes No. 157 and 178 seemed

to calibrate nearest to 29.77 °C (IPTS-68) with both

electronic thermometers. Note that the calibrations of

all probes, whether performed by one of us (G.N.B.)

-Model 45CU Cuvette Thermometers (No. LSf) and 146) with bead-thermistor probe 4,501 (No. 157, 1.59, 178, and 130); Yellow

Springs Instrument Co., Inc., Yellow Springs, Ohio 45.!87.

' Calculated for 29.77 °C IPTS-68 from a table found in the Yellow

Springs Instrument Co. instruction manual from data for corrections

at 29 and ,30 °C.

733
after only a few hours' experience with the gallium cell or three months later by S.R.I, after much greater ex- perience, were repeatable to within 0 to ±0.01 °C. Furthermore, no readings of the probes at the 29.7714 °C gallium reference point were found to be beyond the manufacturer's claims of "readability of ±0.01 °C with

absolute accuracy of ±0.05 °C with NBS traceability." The complex corrections due to the nonlinearity of response that are inherent in most thermistors must be understood and clearly defined for the average user

before the certainty of calibrations at the melting point

of gallium, 29.7714 °C, can be used to judge accuracy as one moves to more distant temperatures, i.e., 25 or 37

°C. As Sostman has suggested in the preceding paper

(9), the uncertainty at 30 °C is also ±0.01 °C, once the

calibration at 29.77 °C is made. The move to another, more-distant temperature requires switching to another "high-accuracy range" setting, and therefore increases the uncertainty; yet, surprisingly, Sostman's data at 25 and 37 °C show no probes to be beyond ±0.01% (see Table 3 of ref. 9). As good as these results are, the fact remains, nevertheless, that one-point calibration of thermistors can never be as sure as two- or three-point

checks. The need within the clinical laboratory community for other, more distant fixed points, particularly

in the range of body temperature from 36 to 38 °C (or even to 40 °C) is all too obvious, not only for calibrations of fever thermometers but also for critical tests such as blood gases and pH determinations.

Stability of Measuring System

The combination of a Yellow Springs Instrument Co.

electronic thermometer and a strip-chart recorder'* is used in our laboratory to measure temperature in the

1.0 to 3.0 ml volumes of the final reaction mixtures used

for enzyme assays. We sought to understand the in- herent sensitivity and stability of this system by ex- panding and recording the signals given with the probes

in the gallium cell. Representative recorder tracings of temperature response vs. time for the various component parts of this "expanded" system are given in Figure

1. The recorder in these studies was adjusted to equal

0.10 °C full scale (250 mm) when the high-accuracy range switch of the Yellow Springs Instrument Co.

electronic thermometer No. 135 was set to the 29-31 °C

position. Each 1 mm of chart space was therefore equal

to 0.0004 °C, giving an expansion of 50-fold over the dial scale. The noise and drift of the various components

listed in the legend of Figure 1 were studied. These re- corder tracings reveal that the resolution of the gallium

melting-point temperature (Figure IC) in this measuring system is limited by the noise associated with the

electronic thermometer's circuitry (Figure IB). By es- timating a center line between the extremes of the noise amplitudes at the beginning and at the end of a halfTable 1. Calibration of Two Electronic Thermometers and Four Probes

Thermometer No. 146

with probes no.

178 230

Mode/operator/month Temp, °C

Visual^/C.N.B./Sept. 29.76 29.75 29.77 29.73

Visual''/S.R.I./Dec. 29.765 29.755 29.77o 29.725

Thermometer No. 135

with probes no.

157 159 178 230

Mode/operator/month Temp, °C

Visual^/G.N.B./Sept. 29.76 29.75 29.77 29.73

Visual^/S.R.I./Dec. 29.77o 29.755 29.775 29.73o

set to Recorder'^/S.R.I./Dec. 29.7714 29.7515 29.78I5 29.7315

Only to 0.01 °C.

" Estimated to nearest 0.005 °C.

' Estimated from chart to nearest 0.0005 °C with probe No. 157 set to 29.77I4.

hour's tracings, we could show that the drift in each

tracing (A, B, and C) was less than ±0.001 °C/h.

Using this stable expanded system again with elec- tronic thermometer No. 135, we restudied the four probes to document more carefully any differences be- tween them. Probe No. 157 was arbitrarily set equal to 29.77I4 °C. Note how closely the data from these ex- panded studies paralleled, both in sign and in magnitude, those obtained by the simpler direct visual readings of Table 1. The sensitivity and stability of this expanded system

to gallium has also been used in turn to study the cycling

characteristics and the accuracy of the set-point of the temperature control block on an automated enzyme

analyzer.'' As shown in D of Figure 1, the thermoelectric heat pump controlling this block completes a heat/cool

cycle every 12-15 s, with amplitudes of ±0.003 °C

around an exceedingly stable set-point. As shown in E

and F of Figure 1, we have used the gallium fixed point

to adjust the block to exactly 29.77 °C. As shown in Table 2, repeated checks of this control block's set-point

temperature during several weeks have shown no

change from 29.77 °C beyond ±0.01 °C.'' Other studies to determine the exact temperature of the reaction mixture within the cell in the KA-150 vs. the temperatures of its spectrophotometric metalwalled cuvette and the control block are in progress in order to understand and control the various sources of

Model 7100 BM (Hewlett-Packard Co., Palo Alto, Calif. 94.'?0:!), with the I'ollDwint; specifications: linearity 0.2'K. min, dead band 0. 1'S,, max. response time 0.0.5 full-scale, accuracy 0.2'». full-scale.

Model KA-1.50 Kinetic Analyzer; Perkin-Elmer Corp., Norwalk.

Co- n. 068.56.

'' Neither Figure I nor Table 2 is intended to show that the temperature of the final reaction mixture in the K A- 1.50 cell is known or

controlled to such narrow limits.

734
KA 150 BLOCK

il.w.A y.Miii.A.i.ii J 0.005 °

29.77J 0,001°

BLOCK ,29,771 J I 0.01 °c

5 MINUTES

Fig. 1 . Recordings of temperature vs. time in a study of electronic thermometers and probes calibrated with the gallium melting

point standard

The recorder s full scale (250 mm) was made equal to 0. 1 "C. (A) Recorder alone

with input shorted out (pen width, about 0.7 mm = 0.0003 "C). (S) Recorder plus electronic thermometer No 135 (set to 10 with 1 .0 adjust control). (C| Complete system with probe No. 157 placed into the NBS gallium cell No. G79A2

at 29.7714 °C. (D) KA-150 thermoelectrically controlled block. (£) Setting mid-point of block cycles to 29.77 °C with aid of gallium, (f) Same as E. but

full-scale = 2 °C between 29 and 31 "C.

heat additions and losses. Frequent calibration of those

tiny thermistor sensors at the gallium melting point

assures us that the work is accurate and traceable to IPTS-68. Thus, in addition to serving as an accuracy

standard for calibration purposes, a gallium cell provides an extremely useful temperature fixed-point from

which to make pragmatic comparison of electronic noise

levels, sensitivity, and stability on our equipment.

Reaction Temperature and Analytical Variability

The profound effect that unrecognized reaction temperature changes can have on analytical variability

in clinical enzymology has recently been re-emphasized

in the clinical chemical literature (10, 11). Results of a

large survey conducted by the Center for Disease Control (CDC) show that there is great intralaboratory

variability in measuring the activity of aspartate amiTable 2. Stability of KA-150 Control Block during

Six Weeks as Judged with Visual Reading

1976 Reading

12/3 29.775

12/6 29.775 12/7 29.775 12/14 29.775 12/23 29.77o 12/30 29.77o 1977

1/4 29.775 1/18 29.77o

1/21 29.77o 1/25 29.77o

notransferase (EC 2.6.1.1) in serum, even when an

identical reaction temperature is said to have been used

(72). Our review of these data for this enzyme shows

that the mean activity reported at 37 °C divided by the mean at 30 °C [x U/liter at 37 °C/x U/liter at 30 °C]

gives a "temperature factor" ranging from 1.38 to 1.44

for each of the four high-purity stable samples of as- partate aminotransferase. However, intensive study of these same samples at both 30 and 37 °C, in order to define in greater detail the temperature/activity rela- tionship, gave a "temperature factor" of 1.52 in three

laboratories in which special attention was given to controlling the reaction temperatures as accurately as

possible.'' This 1.52 "temperature factor" found on these

high-purity, well-characterized reference materials was

in agreement with the value calculated from the Ar- rhenius plots published by Rej et al. (73). This "apparent" lowering of the "temperature factors" suggests

that the reaction temperatures might well be somewhat

different from the 30 or 37 °C reported. It is well rec- ognized that many other methodologic factors besides temperature (e.g., substrate concn.) could also contribute to these differences. However, to decrease the

analytical variability of aspartate aminotransferase measurements in routine daily assays to <±2% due to temperature considerations alone, a goal for reaction temperature accuracy and control must be set at ±0.1 °C at both 30 and 37'°C. Recommendations for reference methods in clinical enzymology deliberately seek to minimize temperature

as a source of serious analytical variability. However,

it can be shown that even the tight specifications for temperature as proposed by the Expert Panel on En- zymes of the International Federation of Clinical Chemistry (IFCC) can contribute an uncertainty of about ±0.5% to the analytical variability of the final measurement (4). Figure 2 shows activity of aspartate aminotransferase as a function of reaction temperature

near 30 °C for the well-characterized aspartate aminotransferase from erythrocyte cytoplasm described by

Rej et al. (73). We determined this slope by measure- ments at 25 and 30 °C, and using the provisional rec- ommendations for the IFCC Reference Method for as- partate aminotransferase (14). In Figure 2 the wider

vertical bar centered on 30 °C graphically depicts the accuracy limits for the reaction temperature recom- mended for reference methods by the IFCC's Expert Panel on Enzymes (4). This specification for accuracy

to IPTS-68 at the 30 °C set-point is given as ±0.05 °C. Thus two laboratories might vary in set-points by 0.1 °C yet still be within specifications. As shown, these extremes of the temperature limits around 30 °C in- tersect the slope of the plot of activity vs. temperature

to give a potential variability (A of the top set of hori- zontal lines) between two laboratories which could reach

" Personal communication and data from: I, Dr. Carl Biirtis, Center

for Disease Control; 2, Dr. Robert Rej. New York State Health De- partment Laboratories; and 3, Ms. M. L. Kelley, Clinical Cbemislrv

Laboratory. Hartford Hospital.

735
t X /.

REACTION TEITERATURE (°C) Fig. 2. Change in enzyme activity vs. reaction temperature

The shaded vertical bars at 30 ± 0.05 °C and 29.77 ± 0.01 °C repfesent written specifications for accuracy and precision of reaction temperature for IFCC and

for gallium, respectively. Tlie resulting maximum variability in activity owing to

tliese temperature specifications is represented by ^ (8/ 1 000) and 8(1.6/1 000). Note tfiat eacfi enzyme's activity/temperature plot varies, as sfiown by ttie plot

for alkaline phosphatase (dotted) and isocitric dehydrogenase {dash)

as high as eight parts per 1000, or about 1%. Also in Figure 2, the dotted line for alkaline phosphatase (EC

3.1.3.1) and the dashed line for isocitrate dehydrogenase

(EC 1.1.1.42) show that variability in activity due to temperature changes alone may be as low as five parts per 1000 (in the case of alkaline phosphatase; see ref. 3) or as high as 20 parts per 1000 (in the case of isocitrate dehydrogenase; see ref. 2), depending upon the enzyme

being studied. The reaction-temperature accuracy and precision

specifications for reference methods were written by the members of the IFCC Expert Panel in 1972, when tra- ceability to IPTS-68 at 30 °C was difficult to achieve (4). Despite the introduction in 1974 of certified Clinical Standard Reference Materials Nos. 933 and 934, de- signed in part to help improve clinical enzymology by

providing calibrations at 25, 30, and 37 °C certified to ±0.03 °C, it is still extremely difficult to be certain that the reaction temperature is indeed 30.0 °C within the

1.0 to 3.0 ml of the final reaction volumes. This is mainly

because of the size and heat capacity of mercury-in-glass

thermometers, which preclude their direct use in small volumes of reaction mixtures. As important as these SRM's 933 and 934 are, it is difficult to anticipate any

large network of laboratories achieving these IFCC

temperature specifications by use of mercury-in-glass

thermometry alone. Fortunately, the introduction of the gallium melt- ing-point standard and the availability of relatively stable and sensitive electronic thermometers with

thermistor probes, greatly simplifies the problem of making accurate temperature measurements in small

final reaction volumes. Furthermore, when the reaction temperature is made to correspond exactly to the gal- lium melting point, great simplification occurs. For

example, the narrower set of vertical lines centered on

29.77 °C in Figure 2 represents our experience of the

past few months and vividly illustrate the fact that point

calibration to gallium is easily achieved to ±0.01 "C. Therefore the temperature variability (B of the lower

pair of horizontal lines) as related to written specifica- tions can be reduced from 8 parts per 1000 to 1.6 parts per 1000 by this one move alone. The ease and certainty of operationally achieving the temperature of 29.77 ± 0.01 "C by use of the gallium

melting-point standard, suggests that even a large

network of laboratories, either nationally or internationally, can be pulled together easily and quickly to achieve remarkable accuracy of reaction temperature

at 29.77 °C. The slight decrease in total enzymatic activity at 29.77 vs. 30.00 "C is negligible in comparison

to the advantages gained by point calibration to this unique constant-of-nature. A Reference Method Run at 29.77 °C

We have experimentally improved the certainty of our reaction temperature measurements for the IFCC

Reference Method for aspartate aminotransferase (14) by pragmatically using a gallium melting-point cell as an integral and concurrent part of the method. The

gallium cell is placed in a water bath, which is initially controlled to about 0.15 "C above 29.77 "C. The bath

circulates water, first to a jacketed rectangular quartz

cuvette^ and then (in series) to the heat exchanger of the spectrophotometer's seunple compartment.^ The

probe of the electronic thermometer/recorder system

is repeatedly moved between the gallium cell and the enzyme reaction mixture in the cuvette as the temperature is continuously recorded with full-scale expansion

equal to 2 °C between 29 and 31 "C, as shown in Figure

3. The recovery time—i.e., the time required for the

slightly cooled 1 ml of reaction mixture to equilibrate

after transfer from test tubes in the bath through cooler ambient air in a syringe and then into the jacketed

spectrophotometric cuvette—is about 2 min. However,

to avoid interference with the carefully timed spectro- photometric observations, which should start 60 s after

initiation of the reaction and then be made every 30 s for 300 s, the thermistor probe is withdrawn when the

operator observes that the temperature curve is rapidly

ascending and then slowing as equilibration starts. During the period of the 10 spectrophotometric readings, the probe is placed back into the gallium reference

cell and it is again returned to the reaction mixture after the last spectrophotometric reading is taken. From this

final portion of the recorder tracing of the reaction temperature we graphically determine the temperature

to the nearest 0.01 "C by comparison to the position of

the adjacent "gallium" line. From time to time the water bath is re-adjusted ever so slightly to bring this final reaction temperature to rest as close as possible to that

8 Constant-temperature jacketed quartz cells No. 160 or 160B

(Hellma Cells, Inc., Box 544, Borough Hall Station, Jamaica, N. Y.

11424).

^ Gary Model 16 spectrophotometer, equipped with heat exchanger

for sample compartment (Varian Associates, Palo Alto, Calif. 94303).

736
IMCING OF REACTION TEMPERATURE

TIME IN MINUTES

-TYPICAL COURSE DURING

SPECTROPHOTOHETRIC

Fig. 3. Recording of temperature vs. time when gallium melting-point all at 29.77 14 °C is used as a part of the IFCC refer- ence method for aspartate aminotransferase

of the gallium melting-point equilibrium temperature. Measurements of reaction temperature by this tech- nique are not made beyond ±0.05 °C of the gallium

reference point. We believe the absolute reaction temperature traceable to IPTS-68 within this reaction mixture is known to ±0.02 °C by this technique of concurrently referencing to gallium.

In conclusion, the ease of calibrating electronic thermometers and probes with the gallium meltingpoint standard at 29.7714 °C is indeed remarkable. Furthermore, this calibration at 29.7714 °C is accurate and traceable to IPTS-68 to ±0.01 °C. Similar accuracy through easy traceability of temperature to IPTS-68

awaits others who recognize and make use of the gallium

melting-point standard. The gallium liquid/solid

equilibration temperature is truly a fundamental constant of nature, well suited for use in clinical laboratory. The acceptance of the gallium melting-point temperature as the reaction temperature for the measurement

of enzymatic activity by reference methods in clinical enzymology will help to decrease interlaboratory vari- ability of results. These improved measurements and control of the

reaction temperature will in turn provide the basis for

providing a sounder body of factual information upon

which to develop national or international agreements

concerning reference materials and methods in clinical enzymology.

References

1. Bowers, G. N., Jr., Durant, J. R., Beckett, R. S., and Tennant, R., An accurate simplification of the serum glutamic oxaloacetic and

pyruvic transaminases. Method, observations and normal values. Am.

J. Clin. Pathol. 30, 1 (1958).

2. Bowers, G. N., Jr., Measurement of isocitric dehydrogenase activity

in body fluids. Clin. Chem. 5, 509 (1959).

3. Bowers, G. N., Jr., and McComb, R. B., A continuous spectro- photometric method for measuring the activity of serum alkaline phosphatase. Clin. Chem. 12, 70 (1966).

4. Bowers, G. N., Jr., Bergmeyer, H. U., and Moss, D. W., Provisional recommendations (1974) on IFCC methods for the measurement of

catalytic concentration of enzymes. Clin. Chim. Acta 61, F11-F24

(following p 238) (June, 1975). [Also Clin. Chem. 22, 384 (1976).]

5. Mangum, B. W., The gallium melting-point standard. Its role in our temperature measuring system. Clin. Chem. 23, 711 (1977).

6. Swindells, J. F., Calibration of liquid-in-glass thermometers. National Bureau of Standards Monograph 90, 1965, available from

Supt. of Documents, U.S. Govt. Printing Office, Washington, D.C.

20402.

7. Mangum, B. W., Standard Reference Materials 933 and 934: The

National Bureau of Standards' precision thermometers for the clinical laboratory. Clin. Chem. 20,670 (1974).

8. Thornton, D. D., The gallium melting-point standard. A deter- mination of the liquid-solid equilibrium temperature of pure gallium on the International Practical Temperature Scale of 1968. Clin. Chem.

23,719 (1977).

9. Sostman, H. E., The gallium melting-point standard. Its role in manufacture and quality control of electronic thermometers for the

clinical laboratory. Clin. Chem. 23, 729 (1977).

10. Tiffany, T. O., Thayer, P. C, Coelho, C. M., and Manning, G. B., A propagation of error analysis of the enzyme activity expression. A

model for determining the total system random error of a kinetic enzyme analyzer. Clin. Chem. 22, 1438 (1976).

11. Bowie, L., Esters, F., Bolin, J., and Gochman, N., Development

of an aqueous temperature-indicating technique and its application

to clinical laboratory instrumentation. Clin. Chem. 22, 449 (1976).

12. Cooperative Enzyme Experiment—1976, Report of the aspartate aminotransferase survey results as presented at the Manufacturers

Assistance Meeting held at the Center for Disease Control, Atlanta, Ga. 30333, on November 11 and 12, 1976.

13. Rej, R., Vanderlinde, R. E., and Fasce, C. P., Jr., An L-aspartate:2-oxoglutarate aminotransferase reference material from human

erythrocytes: Preparation and characterization. Clin. Chem. 18, 374

(1972).

14. Expert Panel on Enzymes, Committee on Standards of IFCC.

Provisional recommendations on EFCC methods for the measurement

of catalytic concentrations of enzymes. Part 2. Aspartate aminotransferase. Measurement of the catalytic concentration in human

serum. Clin. Chim. Acta 70, F19-F42 [foUows p 336 (July, 1976)]. To

appear in Clin. Chem. 23 (1977).

737
NBS-114A {REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

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1. PUBLICATION OR REPORT NO. NBS SP 481

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

The Gallium Meltlng-Polnt Standard

5. Publication Date June 1977

6. Performing Organization Code

7. AUTHOR(S) B. W. Mangum and D. D. Thornton

8. Performing Organ. Report No.

9. PERFORMING ukuANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

lU. Project/ 1 ask/ Work Unit J\o.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Itonbers 77-600017

16. ABSTRACT (A 200-word or less [actual summary of most sigpilicant information. H document includes a significant

bibliography or literature survey, mention it here.)

This Special Publication contains a series of papers published in

Clinical Chemistry concerning our temperature measurement system, the

gallixam melting-point as a thermometric fixed-point, how the gallium

melting-point fits into the measurement system, and applications of the

gallium melting-point standard.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Clinincal laboratory; fixed-point; galliim; melting-point standard; temperature;

thermometric fixed-point •

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