_STP_565-1974.pdf

EVOLUTION OF THE INTERNATIONAL PRACTICAL TEMPERATURE SCALE OF 1968 Sponsored by Subcommittee E20.07 on Fundamentals in Thermometry of ASTM Committee E-20 on Temperature Measurement ASTM SPECIAL TECHNICAL PUBLICATION 565 List price$7.00 04-565000-40 AMERICAN SOCIETY FOR TESTING AND MATERIALS 1916 Race Street,Philadelphia,Pa.19103 Copyright by ASTM Intl(all rights reserved);Sun Jan 3 21:10:37 EST 2016Downloaded/printed byUniversity of Washington(University of Washington)pursuant to License Agreement.No further reproductions authorized.()BY AMERICAN SOCIETY FOR TESTING AND MATERIALS 1974 Library of Congress Catalog Card Number:74-20090 NOTE The Society is not responsible,as a body,for the statements and opinions advanced in this publication.Printed in Baltimore,Md.November 1974 Copyright by ASTM Intl(all rights reserved);Sun Jan 3 21:10:37 EST 2016Downloaded/printed byUniversity of Washington(University of Washington)pursuant to License Agreement.No further reproductions authorized.Foreword This publication is a collection of papers gathered by ASTM Subcom-mittee E20.07 on Fundamentals in Thermometry.The papers recount the origin and history of the International Practical Temperature Scales,leading up to the most recent,IPTS-68.Copyright by ASTM Intl(all rights reserved);Sun Jan 3 21:10:37 EST 2016Downloaded/printed byUniversity of Washington(University of Washington)pursuant to License Agreement.No further reproductions authorized.Related ASTM Publications Supplement to the Bibliography and Abstracts on Thermostat Metals,STP 288C(1974),$4.25,04-288030-40 Manual on the Use of Thermocouples in Temperature Measurement,STP 470A(1974),$17.50,04-470010-40 The Theory and Properties of Thermocouple Elements,STP 492(1971),$7.25,04-492000-40 Copyright by ASTM Intl(all rights reserved);Sun Jan 3 21:10:37 EST 2016Downloaded/printed byUniversity of Washington(University of Washington)pursuant to License Agreement.No further reproductions authorized.Contents Introduction 1 The Origin and Present Status of the IPTS-68-H.PRESTON-THOMAS 3 A Compilation and Historical Review of Temperature Scale Differences-j.G.HUST 20 International Practical Temperature Scale of 1968-R.P.BENEDICT 50 The International Practical Temperature Scale of 1968-COMIT INTERNATIONAL DES POIDS ET MESURES 73 Copyright by ASTM Intl(all rights reserved);Sun Jan 3 21:10:37 EST 2016Downloaded/printed byUniversity of Washington(University of Washington)pursuant to License Agreement.No further reproductions authorized.Introduction STP565-EB/Nov.1974 The purpose of this publication is to bring together in one readily avail-able document of some degree of permanence,several of the major articles documenting the evolution of the International Practical Temperature Scale of 1968(IPTS-68),to summarize the reasons for the continuing revi-sion of temperature scales,and to assess their impact on the industry.The ideal temperature scale,which defines temperature,has not changed since it was first introduced in 1854 by Lord Kelvin.It is based on the gas laws which relate temperature ratios to pressure ratios in ideal gases.It has several important limitations:(1)no physical gas is ideal,or even nearly ideal except over a narrow range of temperature;(2)there is no device based on this scale which can be used satisfactorily to measure real temperatures;and(3)since it defines only temperature ratios,it must be tied to a fixed temperature in order to be useful.Hence,the International Practical Temperature Scales have been successively adopted,each repre-senting the best approximations available to the current technology,using practical instruments.These approximations include the selection and definition of benchmarks as well as practical laboratory instruments for interpolation between the benchmarks.New International Practical Temperature Scales were adopted in 1927,1948,1960,and 1968 as these benchmarks were redefined in the light of new technology.The present fixed point used to define the scale is the triple point of water at 273.16 K,adopted in 1PTS-48.The benchmarks which define 1PTS-68 are the carefully measured equilibrium temperatures of hydrogen,neon,oxygen,water,zinc,silver,and gold.Practical scale definition in segments defined by fixed points and inter-polated by physical instruments lead to,at best,complexity and compromise to avoid discontinuities in the scale and its derivatives at the transition points.These problems are minimized by reducing the number of inter-polating instruments used in defining the scale.One proposal for a future scale eliminates the Type S thermocouple by extending the range of the platinum resistance thermometer to the gold point.Ideally,a single inter-polation instrument would cover the whole range with good agreement with the thermodynamic scale,but such an instrument does not presently exist.It should be recognized that the differences between the International Practical Temperature Scale of 1948 and 1968 are subtle-within the exp