NBS MONOGRAPH 37

lonal Bureau of Standards

library, jj.w. Bldg

SEP2 0 1961

nternational Practical Temperature Scale of 1948

Text Revision of 1960

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Dinctor

International Practical Temperature Scale of 1948

Text Revision of 1960

H. F. Stimson

Reprinted from the Journal of Research of the National Bureau of Standards—A. Physics and Chemistry, Vol. 6SA, No. 3, May-June 1961

National Bureau of Standards Monograph 37

Issued September 8, 1961

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 2S, D.C. - Price 10 cents

Contents

Page.

1. Introduction 2

2. Definition of the International Practical Temperature Scale of 1948 2

3. Recommendations 3

3.1. Standard resistance thermometer 3

3.2. Standard thermocouple 3

3.3 Pressure 3

3.4. Zero point of the scale and triple point of water 4

a. Zero point of the scale 4

b. Triple point of water 4

3.5. Oxygen point 4

3.6. Steam point 4

3.7. Sulfur point 5

3.8. Zinc point 5

3.9. Silver and gold points 5

4. Supplementary information 5

4.1 Resistance—temperature formulas 5

4.2. Secondary reference points 6

4.3. Relation between the International Practical Temperature Scale and the thermodynamic scale 6

iii
iIII
International Practical Temperature Scale of 1948

Text Revision of 1960

H. F. Stimson

(February 1, 1961)

The International Practical Temperature Scale of 1948 is a text revision of the Inter- national Temperature Scale of 1948, the numerical values of temperatures remaining the same. The adjective "Practical" was added to the name by the International Committee

on Weights and Measures. The scale continues to be based upon six fixed and reproducible

equilibrium temperatures to which values have been assigned, and upon the same inter- polation formulas relating temperatures to the indications of specified measuring instruments.

Some changes have been made in the text to make the scale more reproducible than its predecessor. The triple point of water, with the value 0.01 °C replaces the former ice point as a defining fixed point of the scale. It is also recommended that the zinc point,

with the value 419.505 °C, be used instead of the sulfur point. The recommendations

include new information that has become available since 1948.

An internationally accepted scale on which tem-

, peratures can be measured conveniently and accuIjrately is necessary for science and industry. As

' early as 1911 the directors of the national laboratories

1 1 of Germany, Great Britain, and the United States

1 1 agreed to undertake the unification of the tempera-

, ture scales in use in their respective countries. A

practical scale, named the International Temperature

Scale, was finally agreed upon, was recommended to the Seventh General Conference on Weights and

j Measures by its International Committee on Weights

I 'I and Measures, and was adopted in 1927.^ The General Conference on Weights and Measures

is the official international body now representing 36

nations that subscribe to the Treaty of the Meter.

I The General Conference normally meets every six years, and at those times may adopt recommenda-

: tions submitted by the International Committee.

The International Committee is the executive body

,

, elected by the General Conference. It consists of

|j 18 scientists, only one from any one nation, and it

|li normally meets every two years. The International

!| Committee now has six advisory committees of

specialists most of whom represent large national

i laboratories. The Advisory Committee on ThermomI etry was authorized in 1933 and first met in 1939.

ji In 1948 a revision of the International Tempera-

' ture Scale was prepared by the Advisory Committee

and proposed to the International Committee,

i The International Committee recommended this

I' revision to the Ninth General Conference which

I: adopted it.^ At this tinae the General Conference

also adopted the designation of degree Celsius in

place of degree Centigrade or Centesunal.^ The

revised scale was designed to conform as nearly as

' Comptes Rendus de la Septieme Conference Gfinfrale des Poids et Mesures,

p. 94 (1927).

' Comptes Rendus de la Neuvieme Conference Gfengrale des Folds et Mesures,

p. 89 (1948).

3 Comptes Rendus de la Neuvieme Conference G6n6rale des Poids et Mesures,

p. 64 (1948).

practicable to the thermodynamic scale as then

known, while incorporating certain refinements,

based on experience, to make the scale more uniform

and reproducible than its predecessor. In the

revision there were only three changes which affected

values of temperatures on the scale. One was to increase the value assigned to the silver point by

0.3 degree, merely to make the scale more uniform.

Another was to specify Planck's radiation formula

instead of Wien's formula so the scale would be

consistent with the thermodynamic scale above the

gold point. The third was to increase the value for

the second radiation constant to bring it nearer to the value derived from atomic constants. In 1954 the Advisory Committee proposed a resolution redefining the Kelvin thermodynamic scale by

assigning a value to the triple point of water. This

kind of definition was what Kelvin, in 1854, had said

"must be adopted ultunately." This resolution was recommended by the International Committee and

adopted by the Tenth General Conference.* As

soon as this resolution had been adopted it was

pointed out that it would be necessary to revise the

introduction of the text of the International

Temperature Scale of 1948 to conform with the action

just taken.

In preparing a tentative proposal for a new text

of the introduction it soon became evident that the

other three parts of the text would also profit by a

revision. For example, the triple point of water

could now be made one of the defining fixed points

of the scale and thus become the one defining fixed

point common to both the international and the

Kelvin scales. The Recommendations could include

new information that had become available since

1948. At the higher temperatures some new

determinations of differences between the international and thermodynamic scales could be included,

1 Comptes Rendus de la Dixleme Conference Genfirale des Poids et Measures,

p. 79 (1954).

601200—61 1
The values reported for these differences, however, were still not certain enough to warrant a change of

the scale itself. The new text, therefore, does not

change the value of any temperature on the 1948

scale by as much as the experimental error of

measurement.

In 1958 the tenative proposal was discussed in detail at sessions of the Advisory Committee in June, and many suggested changes were agreed upon.

It was proposed to the International Committee in

October. Minor corrections were made during the

next two years, and in 1960 the International

Committee gave the scale its new name. The

International Committee recommended this text revision to the Eleventh General Conference and it was adopted in October 1960.

A translation of the official text ^ follows.

1. Introduction

The Kelvin thermodynamic scale, on which temperatures are designated as °K and denoted by the

symbol T, is recognized as the fundamental scale to which all temperature measurements should ulti- mately be referable. The magnitude of the degree

Kelvin is now defined by the decision (Tenth General

Conference on Weights and Measures, 1954, Resolution 3) fixing the thermod3mamic temperature of the

triple point of water at exactly 273.16 °K.

The experimental difficulties inherent in the meas- urement of temperature on the thermodynamic scale

led to the adoption in 1927, by the Seventh General

Conference on Weights and Measures, of a practical

scale called the "International Temperature Scale".

This scale was intended to be conveniently and accurately reproducible and to agree as closely as practicable with the thermodynamic scale. The International Temperature Scale was revised

in 1948 to make it conform with the state of the

knowledge then available.

In May 1960 the International Committee on

Weights and Measures approved the new name, "International Practical Temperature Scale of 1948",

for the scale presented in this document. Inasmuch

as the numerical values of temperature on this scale are the same as in 1948, this scale is not a revision of

the scale of 1948 but merely a revision of its text.

2. Definition of the International Practical

Temperature Scale of 1948

Temperatures on the International Practical Temperature Scale of 1948 are expressed in degrees Celsius, designated by °C or °C (Int. 1948), and are denoted here by the symbol t or tint- The International Practical Temperature Scale is based on six reproducible temperatures (defining

fixed points), to which numerical values are assigned,

and on formulas establishing the relation between

temperature and the indications of instruments cali- brated by means of values assigned to the six defining

» Comptes Rendus de la Onzieme Conference Gfinfrale des Folds et Mesures

(1960).

ill

fixed points. These fixed points are defined by specified equilibrium states, each of which, except for the'^'

triple point of water, is under a pressure of 101 325

newtons/meter' (1 standard atmosphere).

The fixed points of the scale and the exact nu-f

merical values assigned to them are given in table 1. Table 1. Defining fixed points

Exact values assigned. The pressure is 1 standard atmosphere, except for the ' triple point of water. Temperature

°C{Int. 1948) Temperature of equilibrium between liquid oxygen

and its vapor (oxygen point) —182. 97

Temperature of equilibrium between ice, liquid water, and water vapor (triple point of water)-. +0. 01 Temperature of equilibrium between liquid water and its vapor (steam point) 100 Temperature of equilibrium between liquid sulfur and its vapor (sulfur point) 444. 6*

Temperature of equilibrium between solid silver and liquid silver (silver point) 960. 8 Temperature of equilibrium between solid gold

and liquid gold (gold point) 1063

*In place of the sulfur point, it is recommended to use the temperature of equi- librium between solid zinc and liquid zinc (zinc point) with the value 419.505 °0

(Int. 1948), The zinc point is more reproducible than the suhur point and the value which is assigned to it has been so chosen that its use leads to the same

values of temperature on the International Practical Temperature Scale as does

the use of the sulfur point. The procedures for interpolation lead to a division

of the scale into four parts.

a. From 0 °C to 630.5 °C (antimony point) the

temperature t is defined by the formula

where Rt is the resistance at temperature t of the i platinum wire resistor of a standard resistance

thermometer, and Rq is the resistance at 0 °C. The

constants Rq, A, and B are to be determined from

the values of at the triple point of water, at the

steam point, and at the sulfur point (or the zinc

point). The platinum wire of a standard resistance

thermometer shall be annealed and its purity shall be such that Rioq/Rq is not less than 1.3920.

b. From the oxygen point to 0 °C, the temperature t is defined by the formula

R,=Ro [l+At+Bt'+Cit-Uoo)f],

where Rq, A, and B are determined in the same manner as in a above, the constant C is to be determined from the value of Rt at the oxygen point, and

^100= 100 °C.

c. From 630.5 °C to the gold point the temperature t is defuied by the formula

where E is the electromotive force of a standard

thermocouple of platinum and platinum-rhodium

alloy, when one of the junctions is at 0 °C and the

other at the temperature t. The constants a, b, and c are to be determined from the values of E at 630.5 °C, at the silver point, and at the gold point.

The value of the electromotive force at 630.5 °C is to be determined by measuring this temperature

with a standard resistance thermometer.

2
i-J The wires of the standard thermocouple shall be

e'' annealed and the purity of the platinum wire shall

S|f,ibe such that the ratio i^ioo/-ffo is not less than 1.3920.

The platinum-rhodimn wire shall consist nominally • of 90 percent platinum and 10 percent rhodium by

,.! weight. When one junction of the thermocouple

is at 0 °C and the other is successively at 630.5 °C,

the silver point, and the gold point, the completed

thermocouple shall have electromotive forces such

, that

i; Z?Au=10 300 MV±50 /XV " £'Au-£'Ag=1183 MV+0.158 (E'au- 10300 /zv)±4 fiv

£'au-£'63o-5=4766 MV+0.631 (^au-10300 mv)±8 fiv.

,

[ d. Above the gold point the temperature t is defined by the formula

f

' J, '"'''[x(iA.+r„)]""\

I where Jt and Jau are the radiant energies per unit

! wavelength interval at wavelength X, emitted per

unit time per unit solid angle per unit area of a black-

( body at the temperature t and the gold point re- spectively, C2 is the second radiation constant with

i the value ^2= 0.014 38 meter-degrees, X is in meters,

! and ^0=273. 15 degrees.

3. Recommendations

The following recommendations are advisory

j

j rather than mandatory. The recommended apparaij j tUs, methods, and procedures represent good practice

j at the present time, but there is no intention of

Ij| retarding the development and use of improvements

i

i and refinements. Experience has shown these recomI

I mendations to be in the interest of uniformit}^ and

: reproducibility in the realization of the International

'

j Practical Temperature Scale defined in Section 2.

3.1. Standard Resistance Thermometer

,i A standard resistance thermometer should be so

[ designed and constructed that the wire of the

! platinum resistor is as nearly strainfree as practicable

I and will remain so during continued use. The

' platinum wire should be drawn from a fused ingot,

j not from forged sponge.

( Standard resistance thermometers have been

I made of wire with diameters between 0.05 and 0.5 mm, at least a short portion of each lead adjacent

:l to the resistor also being of platinum. The completed resistor of the thermometer should be annealed

I in air at a temperature higher than the highest

I temperature at which it is to be used, but in no case below 450 °C. There is reason to believe, furthermore, that better stability is obtained when the

[ tube protecting the completed resistor is filled with

li gas containing some oxygen.

Useful criteria which serve as safeguards against

inferior construction of the completed thermometer

and against errors in the calibrations at the fixed

points are that the value of the constant B is

( —0.5857 ±0.0010) XlO-^/deg' and that the value of

the constant 6Ms(-4.35±0.05)X10-'Vdeg^ Another

useful critei'ion of the adequacy of the annealing

and of the reliability of the thermometer is the

constancy of its resistance at some reference temperature. For example the resistance of a thermometer

at the triple point of water should not change by as much as the equivalent of 0.001 deg when the

thermometer is subjected to temperature cycles such

as are necessary for its calibration.

3.2. Standard Thermocouple

Standard thermocouples have been made of wires

having a diameter between 0.35 and 0.65 mm.

Before calibration the wires of the thermocouple

should be carefully annealed to ensure the constancy

of the electromotive forces during use. For this it is necessary to heat the platinum wire to a temperature

of at least 1100 °C and the platinum-rhodium wire

to 1450 °C. If the annealing is done before the

wires are mounted in their insulators, the completed

thermocouple should be heated again to a temperature of at least 1100 °C, until the electromotive

force is stable and local inhomogeneities caused by

strains have disappeared. When the thermocouple

has been annealed sufficiently, its indications should

not vary with a change in the temperature gradient

along the wire; they should not vary, for example,

with the depth of immersion in an enclosure at uniform temperature.

The electromotive force of the thermocouple at 630.5 °C should be determined from measurements

at some uniform and constant temperature between

630.3 and 630.7 °C.

3.3. Pressure

In practice, pressures are determined with a mercury column. Pure mercury may be taken as having a mean density at 20 °C of 13 545.87 kg/m^

in a mercury column balancing one atmosphere. In

the practical determination of the standard at- mosphere the International Committee on Weights

and Measures recommends that the value of local

gravity be expressed in the Potsdam sj^stem until it sanctions the use of another system.

In the following sections on the oxygen point,

the steam point, and the sulfur point the formulas

for the equilibrium temperatures tp are given as polynomials in powers of (p/po-l), where p is the

equilibrium pressure and po is one standard at- mosphere. Tlie limits of accuracy are also given

for stated ranges of pressure. In practice, the

errors caused by using these formulas are less than

the errors caused by the instability of systems open

to the atmosphere. Greater stability and increased

accuracy can be realized in closed systems held at a constant pressure which is maintained within a few parts in a thousand of one atmosphere. Onlj'

the first power terms of (p/po-1) in the poljmomials

are then needed.

3
3.4. Zero Point of the Scale and Triple Point of Water

a. Zero Point of the Scale The zero point of the International Practical

Temperature Scale is defined as being the temperature exactly 0.01 deg below the triple point of water. Calculations show that the temperature of the former

"ice point", defined as the temperature of equilibrium

between ice and air-saturated water under a pressure

of one standard atmosphere, is 0 °C within 0.0001

deg.

It is difficult, however, to obtain the ice point

directly with an accuracy better than ±0.001 deg;

but when this accuracy suffices, the temperature

0 °C may be be realized by the use of a mixture of

finely divided ice and water saturated with air at 0 °C in a well insulated container such as a Dewar

flask. The equilibrium temperature t corresponding

to an ambient atmosphere pressure p and at a depth

h below the surface of the water, may be calculated

by the formula

t=0.01 (l-p/Po) °C-(0.7X10-« deg/mm)/i-.

b. Triple Point of Water The temperature of the triple point of water has

been realized in sealed glass cells which contain only

water of high purity; these cells have axial re-entrant

wells for thermometers. In such cells the triple-point

temperature is realized wherever ice is in equilibrium

with a liquid-vapor surface. At a depth of h below

the liquid-vapor surface the equilibrium temperature

between ice and liquid water is given by the formula

Z=0.01 "C-CO.TXIO-" deg/mm)h.

The recommended method of preparing a triplepoint cell for use is first to freeze a thick mantle of

ice around the well by cooling from within, and then

to melt enough of this mantle, also from within, to produce a new water-ice interface close to the well. After these cells have been prepared for use, the

temperatures measiired in the wells have been found

to rise by amounts ranging from 0.0001 to 0.0005

deg before becoming stable after from 1 to 3 days.

This initial change can probably be explained either

by the increase in the dimensions of the crystals of

ice or by the slow release of strains in the crystals.

A cell prepared in this manner and kept in an ice bath is capable of maintaining a temperature which

is constant within about 0.0001 deg for several

months. When cells from different sources have

been compared under these conditions, no differences

greater than 0.0002 deg have been reported.

Water from most natural sources (normal water)

contains about 0.0148 mole percent deuterium, 0.20

mole percent O^*, and 0.04 mole percent O^''. A^ari- ations from this norm as large as 0.0015 mole percent

have been found in the deuterium content of natural

waters. An increase of 0.001 mole percent in the

deuterium content of water corresponds to an in- crease of 0.000 04 deg in the triple-point temperature. Waters from rivers that rise to the leeward

of mountain ranges or at the base of permaneni

glaciers may contain less than the normal amount

of deuterium, whereas waters from the surface Oi large lakes may contain more than the norma',

amount.

The isotopic composition at the water-ice inter- faces in the triple-point cells depends also on the natural differences in the proportions of the oxygen,

isotopes, on the process of distilling water, and on the procedure of freezing. The effects of these different

isotopic compositions on the temperatures realized

in triple-point cells are probably sufficiently small to be neglected.

3.5. Oxygen Point i The temperature of equilibrium between liquid

oxygen and its vapor is usually realized by the static; method. The platinum resistor of a standard

thermometer and the liquid oxygen in its container

are brought to the same temperature in a metal

block placed in a suitable cryostat. The metal,

block is usually immersed in a well-stirred bath of, liquid oxygen open to the atmosphere, but greater'

stability has been obtained by enclosing the metaK

block inside an evacuated envelope maintained at a. uniform temperature near the oxygen point. The,

oxygen vapor pressure is transmitted through a, tube leading out to a manometer. The entire,

length of this tube should be at temperatures above:

the saturation temperature of the oxygen.

Criteria that the equilibrium temperature has,

been realized are that the observed temperature,,

corrected to constant pressm'e at the free surface of j the oxygen, is independent of: Small variations in

the depth of immersion of the thermometer in thei metal block, the ratio of the volume of the liquid;

oxygen to the volume of the vapor, and small variations in the temperatme of the envelope.

The equilibrium temperature tp corresponding to a

pressm'e p at the surface of the liquid oxygen may be

found to an accuracy of a few thousandths of a degree over the range from ^ = 660 mm to p =860 mm

of mercury by means of the formula

t^ = [-lS2.97 +9.5S0(p/po-l)

-3.72{p/po-ir +2.2(p/po-in°C.

3.6. Steam Point

The temperature of equilibrium between liquid

water and its vapor is usually realized by the dynamic method with the thermometer placed within

the saturated vapor.' Open systems were formerly

used for the realization of the steam point but for

precise calibration it is preferable to use a closed

system in which a boiler and a manometer are con- nected to a manostat filled with air or, preferably,

helium.

The boUer should be constructed so as to avoid all contamination of the vapor. The thermometer

should be shielded against radiation from bodies that

are at temperatures different from the saturation

temperature.

4
The criteria that the equilibrium temperature has

J been reahzed are that the observed temperature,

j' corrected to a constant pressure, is independent of:

' The water used, the elapsed time, the variations in the heat input to the liquid water, and the depth of

immersion of the thermometer.

The equilibrium temperature tp, corresponding to a pressure p, may be found to within 0.001 deg

over the range between p = 660 mm and p=860 mm

'! of mercury by means of the formula

=1 [100+28.012 (2>/2?o-l)

-11.64 (p/^o- 1)^+7.1 (i>/^)o-l)^]°C.

A change in the proportion of deuterium in water

produces about one third as much change at the

boiling-point temperature as that produced at the

triple point, and in the same direction.

3.7. Sulfur Point

, The temperature of equilibrium between liquid

|i sulfur and its vapor is usually realized by the dynamic

method in an aluminum boiler similar in shape to that used for the steam point except that extra

shielding against radiation and larger spaces for free circulation of the vapor are needed.

The addition of 0.1 percent of arsenic and then of

0.1 percent of selenium to sulfur has been reported to

raise the normal boiling point by 0.02 deg and then

by 0.07 deg. These elements are common in sul- fur from volcanic somces. Commercial sidfur contains organic impm-ities which slowly decompose and

leave carbon when the sulfur is boiled. Carbon

itself probably has no observable effect on the boiling

point of sulfur, but it is preferable to remove the

S organic matter and carbon.

The criteria that the sulfur point has been real- ized are similar to those for the realization of the

steam point, except that it may take many hours to reach a constant temperature.

The equilibrium temperature tp, corresponding to a pressure p, may be found to an accuracy of about

0.001 deg in the range between i?=660 mm and

25=800 mm of mercury by means of the formula

fp=[444.6-t-69.010(2)/2?o-l)-27.48(2>/po-l)'

+ 19.14(2p/^o-l)']°C.

3.8. Zinc Point

Highly reproducible temperatures, closely related

to those which give the temperature of the liquidus cm-ve of an alloy, have been realized as plateau

temperatures on slow-rate freezing curves of high

purity (99.999 weight percent) zinc. The zinc is usually melted and frozen in high purity

artificial graphite (99.999 weight percent) crucibles

with axial thermom_eter wells, in simple block fur- naces. The crucibles should be about 5 cm in

diameter and deep enough to eliminate the effects of

heat conduction along the thermometer leads. When cooling has been started and solid has begim

to form on the crucible wall, the thermometer should

be removed, cooled to the room temperature, and

then reinserted in its well to induce a thin mantle of

solid zinc on the outside of the well. Another

technique, which has been used, is to remove the

thermometer when the temperature indicated by it

is 0.01 deg below the freezing point, and to insert a

silica rod for about 30 sec; the thermometer is then

replaced in its well. The plateau temperature is that

of equilibrium between the liquid zinc and the solid

zinc of the mantle while freezing is progressing slowly

inward from the outside of the crucible. The melts

are best made in an inert atmosphere to inhibit

oxidation of the graphite and zinc, yet there has been no evidence of the plateau temperatures being affected

by zinc oxide in the melt even after prolonged heating

in air. The plateau temperattu-es have been found

to increase 0.0043 deg per atmosphere.

A criterion of adequate purity of a sample is that

its melting range is not greater than about 0.001 deg.

Samples of zinc of this high purity, originating in

different countries, have given plateau temperatures

which were practically identical (within 0.0002 deg).

Samples with melting ranges of about 0.01 deg yielded

plateau temperatiu-es that were low by from 0.0004

deg to 0.0016 deg.

3.9. Silver and Gold Points

The temperature of equilibrium between solid and

liquid silver or between solid and liquid gold has been

realized in covered crucibles either of high purity

artificial graphite, or ceramic, or vitreous silica. The

dimensions of the crucibles should be such as to allow

for the considerable expansion of these metals on

melting and the crucibles should be deep enough to eliminate the effects of heat conduction along the

thermocouple wires. Silver must be protected from

oxygen while molten.

The crucible and its contents should be brought

to a uniform temperature a few degrees above the

melting point of the metal and then allowed to cool

slowly. A thermocouple, mounted in a protecting

tube of porcelain or other suitable material, with

insulators separating the two wires, is immersed in

the molten metal which is then allowed to freeze. Criteria that the equilibrium temperature has been

realized are: That the electromotive force of the

thermocouple is independent of small changes in the

depth of immersion during successive freezings, and

that the electromotive force remains essentially constant for a period of at least 5 min dm-ing a single

freezing.

For the range of the scale above 1063 °C, where

the Planck radiation formula is used, the gold-point

crucible should be modified to have a blackbody

cavity at the temperature of the freezing gold.

4. Supplementary Information

4.1. Resistance—Temperature Formulas

The interpolation formula for the range 0 to 630.5

°C as given in the definition of the scale (sec. 2, a),

5
may be written in the Callendar form

a V-n^o / \fioo / tit where

f^-i), and ^100= 100 °C.

The relations between the coefficients are

A=a(l+-~), a=A+Btm,

\ fioo/

A+Btm

The interpolation formula for the range 0°C to the oxygen point as given in the definition of the

scale (sec. 2b),

R,=R,[l+At+Bt'+C(t-tmW],

may be written in the Callendar-Van Dusen form

t--

Oioo ^ ^t\oo) J

The relations connecting A, B and a, b are the same

as those given above and the other relations are C=- al3 noo

/3= A+Bh

4.2. Secondary Reference Points

In addition to the defining fixed points of the

scale, given in table 1, certain other points may be

useful for reference purposes. Some of these and

their corresponding reported temperatures on the

International Practical Temperature Scale of 1948

are given in table 2. Except for the triple points,

each temperature is for a system in equilibrium under

a pressure of 1 standard atmosphere. The formulas

for the variation of temperature with pressure are intended for use over the range of pressures from

p =680 mm to ^ =780 mm of mercury.

4.3. Relation Between the International Practical

Temperature Scale and the Thermodynamic

Scale

When the International Temperature Scale was

adopted in 1927 it was in as close accord with the

thermodynamic scale as was practicable with, the

knowledge then available. It was recognized, however, that further research would increase our knowledge of the actual differences between values

of temperature on the two scales. When it is

-78.5

-38. 87

O.OOO 26.88

32. 38

660.1

Table 2. Secondary reference points Under a pressure of 1 standard atmosphere, except for the triple points.

[ Temperature

°C(Int. mS)

Temperature of eqiiiUbrium between solid carbon dioxide and its vapor - <p=[-78.5 to -|-12.12(p/po-l)-6.4(7)/po-l)2]°C Temperature of equilibrium between solid mercury and liquid mer- cury Temperature of equiUbrium between ice and air-saturated water Temperature of triple point of phenoxybenzene (diphenyl-ether) Temperature of transition of sodium sulphate decahydrate Temperature of triple point of benzoic acid 122. 36 Temperature of equilibrium between solid indium and liquid indium 166. 61 Temperature of equilibrium between Uquid naphthalene and its vapor. 218.

0

«p[ = 218.0+44.4(p/p„-l)-19(p/po-l)2]°C Temperature of equilibrium between sohd tin and liquid tin 231. 91 Temperature of equilibrium between liquid benzophenone and its vapor -_- - _ 305.9 <p+[305.9+48.8(p/p„-l)-21(p/p„-l)2]°C Temperature of equilibrium between soUd cadmium and liquid cad- mium 321.03 Temperature of equiUbrium between solid lead and liquid lead 327.

3

Temperature of equilibrium between liquid mercury and its vapor 356. 58

<, = [356.,'i8+55.552(p/p„-l)-23.03(p/p„-l)2+14.0(p/p„-l)3]°C Temperature of equiUbrium between soUd aluminum and liquid alu- minum

Temperature of equilibrium between solid copper and liquid copper

(in a reducing atmosphere) 1083 Temperature of equiUbrium between solid niclsel and liquid nickel. . _ 1453 Temperature of equiUbrium between soUd cobalt and Uquid cobalt... 1492 Temperature of equiUbrium between solid palladium and Uquid pal- ladium 1552 Temperature of equilibrium between solid platinum and Uquid plat- inum 1769 Temperature of equilibrium between soUd rhodium and Uquid rho- dium 1960 Temperature of equiUbrium between solid iridium and Uquid iridium. 2443 Temperature of melting tungsten 3380

desired to know the value of a temperature on the

thermodynamic scale, the usual procedure is to obtain the value on the International Practical Temperature Scale and then to convert it to the thermodynamic scale by adding the appropriate difference

between the scales. These differences, however,

have to be determined by experiment. They are

difficult to determine accurately because they are small compared with their Kelvin temperature

Some of these differences obtained in various parts

of the scale are given below in order to show the

present state of our information about the agTeement

of the two scales. On account of the uncertainties in these differences

it seems preferable not to modify the values of

temperature on the International Practical Temperature Scale now but to further improve our knowledge of the differences between the scales

When it is desired, it will be possible to improve the means of determining temperatures on the Inter- national Practical Temperature Scale without changing significantly the values of temperature. This

procedure will avoid the confusion which would

result from too frequent changes in values of

temperature.

In the range from 0 °C to the sulfur point, intercomparisons of two nitrogen gas thermometers with

standard resistance thermometers were reported in

1939 from The Massachusetts Institute of Technology. The differences found between the thermodynamic Celsius temperature ttn (definition of 1954)

and the temperature tint (1948 scale) have been

formulated as follows:

ler

t Ih tint

^100 r-o.ooeonY-^-i)

(0.041 06-7.363X10-Meg-^«)J deg.
This relation gives 99.994 °C (therm.) for the steam

point and 444.70 °C (therm.) for the sulfm' point.

The results obtained with the two gas thermometers

differed by 0.005 deg at the steam point and 0.05

deg at the sulfur point. In 1958 the PhysikalischTechnische Bundesanstalt reported the value 444.66

°C (therm.) for the sulfur point.

In the range from the oxygen point to 0 °C,

investigations at the Physikalisch-Technische Reichsanstalt, reported in 1932, and at the University of

Leiden, reported in 1935, give a set of values which

indicate that the differences t a—tint have a maximum of about +0.04 deg at about —80 °C. Below — 100 °C some of the reported differences have

opposite signs. These differences are of the order of

magnitude of the possible uncertainties in the gas

thermometer measurements. For the oxygen point,

the results published since 1927 by four laboratories

have been recalculated on the basis of the value

2^0=273. 15 °K (adopted in 1954). These values are

90.191 °K from the Physikalisch-Technische Reichsanstalt (1932), 90.17 °K from the Tohoku University, Sendai, Japan (1935), 90.160 °K from the University of Leiden (1940), and 90.150 °K from the

Pennsylvania State University (1953). The

average of these four results is 90.168 °Iv or -182.982 °C (therm.).

The International Practical Temperature Scale

is not defined below the oxygen point.

In the neighborhood of 1000 °C new determinations

of the thermodynamic temperature of the silver point and gold point have been made in recent years

in Germany and Japan. At the PhvsikalischTechnische Bundesanstalt (1958) 962.16 °C (therm.) was obtained for the silver point and 1064.76 °C

(therm.) for the gold point. At the Tokyo Institute

of Technology (1958), the values 961.20 °C (therm.)

and 1063.73 °C (therm.) were obtained for these

points; these last two values being little different

from those reported by the same laboratory in 1956,

namely 961.28 °C (therm.) and 1063.69 °C (therm.)

In the range above the gold point the Planck

radiation formula is used. The Planck formula is consistent with the thermodynamic scale and hence

it would give true values of Kelvin temperature if

the correct values were known for tlie Kelvin temperature of the gold point and for the second radiation constant C2. An analysis of variance of data on atomic constants

by The California Institute of Technology, published

in 1955, gave the value 0.014 388 8 meter-degree

for C2. A similar analysis from The Johns Hopkins

University, published in 1957, gave the value

0.014 388 6 meter-degree for CV

The international practical Kelvin temperatures

are obtained by adding the value of 7*0= 273. 15 deg

to the international practical Celsius temperatures,

defined above. Values of the thermodynamic Celsius

temperatures are obtained by subtracting To from

the thermodynamic Kelvin temperatures. Table 3 gives the recommended designations; the arrows

point from the defined temperatures to the temperatures derived by changing the origin.

Table 3.

International Practical Scales

Celsius Absolute

International

Practical Temperature

tin!

Names

Symbols

Designations

International

Practical Kelvin Temperature

°0 (Int. 1948) degrees Celsius

international practical 3 948

°K (Int. 1948) degrees Kelvin

international practical 1948 Thermodynamic Scales

Celsius Absolute

Thermodynamic Celsius Temperature

t= T- To

Names

Symbols

Designations

Thermodynamic Kelvin Temperature

°C (therm.) 1 °K

degrees Celsius degrees Kelvin

tliermodynamic 1 (To=273.15 deg) Note. For the international practical temperature, the subscript "int" after t may be omitted if there is no possibility of confusion. Washington, D.C.

7
National Bureau of Standards Monographs

Selected Publications of Interest

7. Precise Measurement of Heat of Combustion with a Bomb Calorimeter 25

10. The "1958 He* Scale of Temperatures" 20?f 20. Ideal Gas Thermodynamic Fucntions and Isotope Exchange Functions for the Diatomic Hydrides, Deuterides, and Tritides $2. 75

21. Specific Heats and Enthalpies of Technical Solids at Low Temperatures. A Compilation from the Literature 2Qi

27. Bibliography of Temperature Measurement, January 1953 to June 1960 15^

29. Thermal Expansion of Technical Solids at Low Temperatures. A Compilation

from the Literature 20f5 30. Corrected Optical Pjrometer Readings 55fS NBS Monographs are available by purchase from the Superintendent of Documents, Government Printing Office, Washington 25, D.C.

8 U.S. GOVERNMENT PRINTING OFFICE: 1961
THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical

vcork. In general, each section carries out specialized research, development, and engineering in the field indicated by its title. A brief description of the activities, and of the resultant publications, appears on the

inside of the front cover. WASHINGTON, D.C.

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Chemistry. Inorganic Chemistry.

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• Office of Weights and Measures. BOULDER, COLO.

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