1、Designation: E1502 16Standard Guide forUse of Fixed-Point Cells for Reference Temperatures1This standard is issued under the fixed designation E1502; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A numb
2、er in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.INTRODUCTIONDuring melting and freezing, pure material transforms from the solid state to the liquid state or fromthe liquid state to the solid state a
3、t a constant temperature. That constant temperature is referred to asa fixed point. Fixed points approached in the melting direction are referred to as melting points andfixed points approached in the freezing direction are referred to as freezing points. Fixed points ofhighly purified materials can
4、 serve as reference temperatures, and in fact, the InternationalTemperature Scale of 1990 (ITS-90)2relies on the melting and freezing points of some highly purifiedmetals as defining fixed points. Fixed points can be realized in commercially available systemsincorporating fixed-point cells. When the
5、 cells are properly made and used, they establish usefulreference temperatures for the calibration of thermometers and for other industrial and laboratorypurposes; with care, these fixed points can be realized with an uncertainty of a few millikelvins3orless.1. Scope1.1 This guide describes the esse
6、ntial features of fixed-pointcells and auxiliary apparatus, and the techniques required torealize fixed points in the temperature range from 29 to1085C.31.2 Design and construction requirements of fixed-pointcells are not addressed in this guide. Typical examples aregiven in Figs. 1 and 2.1.3 This g
7、uide is intended to describe good practice andestablish uniform procedures for the realization of fixed points.1.4 This guide emphasizes principles. The emphasis onprinciples is intended to aid the user in evaluating cells, inimproving technique for using cells, and in establishing pro-cedures for s
8、pecific applications.1.5 For the purposes of this guide, the use of fixed-pointcells for the accurate calibration of thermometers is restrictedto immersion-type thermometers that, when inserted into thereentrant well of the cell, (1) indicate the temperature only ofthe isothermal region of the well,
9、 and (2) do not significantlyalter the temperature of the isothermal region of the well byheat transfer.1.6 This guide does not address all of the details ofthermometer calibration.1.7 This guide is intended to complement special operatinginstructions supplied by manufacturers of fixed-point appara-
10、tus.1.8 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.9 The following hazard caveat pertains only to the testmethod portion, Section 7, of this guide. This standard does notpurport to address all of the safety concerns, if a
11、ny, associatedwith its use. It is the responsibility of the user of this standardto establish appropriate safety and health practices anddetermine the applicability of regulatory limitations prior touse.2. Referenced Documents2.1 ASTM Standards:4E344 Terminology Relating to Thermometry and Hydrom-et
12、ry1This guide is under the jurisdiction of ASTM Committee E20 on TemperatureMeasurement and is the direct responsibility of Subcommittee E20.07 on Funda-mentals in Thermometry.Current edition approved May 1, 2016. Published September 2016. Originallyapproved in 1992. Last previous edition approved i
13、n 2010 as E1502 10. DOI:10.1520/E1502-16.2Preston-Thomas, H., “The International Temperature Scale of 1990 (ITS-90),”Metrologia, Vol 27, No. 1, 1990, pp. 310. For errata see ibid, Vol 27, No. 2, 1990,p. 107.3In this guide, temperature intervals are expressed in kelvins (K) and millikel-vins (mK). Va
14、lues of temperature are expressed in degrees Celsius (C), ITS-90.4For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM webs
15、ite.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1E644 Test Methods for Testing Industrial Resistance Ther-mometers3. Terminology3.1 Definitions:3.1.1 reference temperature, na fixed, reproducibletemperature, to which a value is ass
16、igned, that can be used forthe calibration of thermometers or other purposes.3.1.2 Additional terms used in this guide are defined inTerminology E344.3.2 Definitions of Terms Specific to This Standard:3.2.1 first cryoscopic constant, A, na constant of propor-tionality between the freezing point depr
17、ession of, and concen-tration of impurities in, a sample of reference material, givenby the ratio of the molar heat of fusion of the pure material, L,to the product of the molar gas constant, R, and the square ofthe thermodynamic temperature of fusion, T, of the purematerial (freezing point):A 5LRT2
18、(1)3.2.2 fixed-point cell, na device that contains and protectsa sample of reference material in such a manner that the phasetransition of the material can establish a reference temperature.3.2.3 freeze, nan experiment or test run conducted with afixed-point cell while the reference material in the
19、cell solidi-fies.3.2.4 freezing curve, nthe entire time-temperature relationof the reference material in a fixed-point cell during freezing,including initial cooling, undercool, recalescence, freezingplateau, and final cooling to complete solidification.3.2.4.1 DiscussionGraphic representations of f
20、reezingcurves are shown in Figs. 3 and 4.3.2.5 freezing plateau, nthe time period during freezingwhen the temperature does not change significantly.3.2.6 freezing range, nthe range of temperature overwhich most of the reference material in a fixed-point cellsolidifies.3.2.6.1 DiscussionThe freezing
21、range is indicated graphi-cally in Fig. 3.3.2.7 melt, nan experiment or test run conducted with afixed-point cell while the reference material in the cell liquifies.3.2.8 melting curve, nthe entire time-temperature relationof the reference material in a fixed-point cell during melting,including init
22、ial heating, melting plateau, and final heating tocomplete liquification.3.2.8.1 DiscussionGraphic representations of meltingcurves are shown in Figs. 5 and 6.3.2.9 melting plateau, nthe period during melting inwhich the temperature does not change significantly.3.2.10 melting range, nthe range of t
23、emperature overwhich most of the reference material in a fixed-point cell melts.3.2.11 nucleation, nthe formation of crystal nuclei inliquid in the supercooled state.3.2.12 recalescence, nthe sudden increase in temperatureof reference material in the supercooled state upon nucleationand crystal grow
24、th, due to the release of latent heat of fusion ofthe reference material.3.2.13 reference material, nthe material in a fixed-pointcell that melts and freezes during use, the fixed point of whichcan establish a reference temperature.3.2.14 supercooled state, nthe meta-stable state of refer-ence mater
25、ial in which the temperature of the liquid phase isbelow the freezing point.3.2.15 undercool, nthe temperature depression below thefixed point of reference material in the supercooled state.4. Summary of Guide4.1 A fixed-point cell is used for thermometer calibration byestablishing and sustaining a
26、reference material at either themelting or freezing point, to which a value of temperature hasbeen assigned. The thermometer to be calibrated is insertedinto a reentrant well in the cell; the well itself is surrounded bythe melting or freezing reference material.4.2 For freezing point realizations,
27、the cell is heated to meltthe reference material. The temperature of the surroundingenvironment is then reduced to about 1 K below the freezingpoint so that the reference material cools. Following theundercool, nucleation, and recalescence, the well temperaturebecomes constant during the freezing pl
28、ateau. After a time,depending on the rate of heat loss from the cell, the amount ofreference material, and the purity of the reference material, thetemperature starts to decrease and eventually all of the materialbecomes solidified.FIG. 1 Examples of Fixed-Point CellsE1502 1624.3 For melting point r
29、ealizations, the cell is heated toapproximately 1 K below the melting point. The temperature ofthe surrounding environment is then increased to about 1 Kabove the melting point so that the reference material beginsmelting. Following stabilization, the well temperature becomesconstant during the melt
30、ing plateau.After a time, depending onthe rate of heat gain by the cell, the amount of referencematerial, and the purity of the reference material, the tempera-ture starts to increase and eventually all of the materialbecomes molten.4.4 Since the temperature in the reentrant well remainsconstant dur
31、ing the phase transition plateau, one or more testthermometers may be calibrated by inserting them singly intothe well. In some cases the plateau can be sustained for manyhours, and even under routine industrial conditions, the plateaumay be readily sustained long enough to test several thermom-eter
32、s. The duration of the plateau may be lengthened bypreheating the test thermometers.4.5 Measurements are also made during each plateau with adedicated monitoring thermometer. These measurements, to-gether with other special test measurements, provide qualifi-cation test data (see 6.5 and 7.5).NOTE 1
33、This example shows an insulated furnace body and twoalternative types of furnace cores. The core on the left is a three-zoneshielded type. The core on the right employs a heat pipe to reducetemperature gradients.FIG. 2 Example of Fixed-Point FurnaceA = Stabilized temperature of cell before freezing,
34、 typicallyabout 1 K above freezing point.B = Freezing point of cell.C = Temperature of cell surroundings during freezing, typi-cally about 1 K below freezing point.D = Maximum undercool.E = Onset of recalescence.F = Freezing plateau.G = Total freezing time.H = Freezing range.FIG. 3 Structure of a Ty
35、pical Freezing CurveFIG. 4 Freezing Curve of Sample of Highly Purified TinE1502 1635. Significance and Use5.1 A pure material has a well defined phase transitionbehavior, and the phase transition plateau, a characteristic ofthe material, can serve as a reproducible reference temperaturefor the calib
36、ration of thermometers. The melting or freezingpoints of some highly purified metals have been designated asdefining fixed points on ITS-90. The fixed points of othermaterials have been determined carefully enough that they canserve as secondary reference points (see Tables 1 and 2). Thisguide prese
37、nts information on the phase transition process as itrelates to establishing a reference temperature.5.2 Fixed-point cells provide users with a means of realizingmelting and freezing points. If the cells are appropriatelydesigned and constructed, if they contain material of adequatepurity, and if th
38、ey are properly used, they can establishreference temperatures with uncertainties of a few millikelvinsor less. This guide describes some of the design and useconsiderations.5.3 Fixed-point cells can be constructed and operated lessstringently than required for millikelvin uncertainty, yet stillprov
39、ide reliable, durable, easy-to-use fixed points for a varietyof industrial calibration and heat treatment purposes. Forexample, any freezing-point cell can be operated, oftenadvantageously, as a melting-point cell. Such use may result inreduced accuracy, but under special conditions, the accuracymay
40、 be commensurate with that of freezing points (see 6.3.10).5.4 The test procedure described in this guide producesqualification test data as an essential part of the procedure.These data furnish the basis for quality control of the fixed-point procedure. They provide for evaluation of results, assur
41、econtinuing reliability of the method, and yield insight into thecause of test result discrepancies. The test procedure isapplicable to the most demanding uses of fixed-point cells forprecise thermometer calibration; it may not be appropriate orcost-effective for all applications. It is expected tha
42、t the user ofthis guide will adapt the procedure to specific needs.6. Principles6.1 Freezing Point Realization:6.1.1 Ideally pure material at a given pressure has a uniquetemperature when its solid and liquid phases are in perfectthermal equilibrium. In contrast, the phase transition of a realmateri
43、al from liquid to solid, as heat is released in semi-equilibrium freezing, exhibits a complex time-temperaturerelation (freezing curve) as shown in Figs. 3 and 4.6.1.2 The deposition of the solid phase from the liquid phaserequires the presence of liquid in the supercooled state,nucleation, and crys
44、tal growth. Nucleation may begin sponta-neously in the meta-stable supercooled liquid, or it may beA = Stabilized temperature of cell before melting, typicallyabout 1 K below melting point.B = Melting point of cell.C = Temperature of cell surroundings during melting, typi-cally about 1 K above melti
45、ng point.D = Onset of melting.E = Melting plateau.F = Total melting time.G = Melting range.FIG. 5 Structure of Typical Melting CurveFIG. 6 Melting Curve of Sample of Highly Purified TinTABLE 1 Characteristics of Pure Fixed-Point Reference MaterialsMaterial Fixed point, ITS-90, CTypicalUndercool, KPr
46、essure Coefficient at fixed point First CryoscopicConstant, K1nK/Pa mK/m (of liquid)GalliumA,B29.7646 76 20 1.2 0.0073IndiumA156.5985 0.1 + 49 + 3.3 0.0021TinA231.928 25 + 33 + 2.2 0.0033Bismuth 271.403 0.19 34 3.4 . . .ZincA419.527 0.050.1 + 43 + 2.7 0.0018AluminumA660.323 0.41.5 + 70 + 1.6 0.0015S
47、ilverA961.78 13 + 60 + 5.4 0.00089GoldA1064.18 13 + 61 + 10.0 0.00083CopperA1084.62 12 + 33 + 2.6 0.00086ADefining fixed point for ITS-90.BRealized as melting point.E1502 164induced artificially.As crystals nucleate and grow, the liberatedlatent heat of fusion produces recalescence.6.1.3 The underco
48、ol of materials may range from as little as0.05 K, for some materials such as zinc, to more than 20 K fortin and other materials (see Table 1). The magnitude of theundercool can depend on the initial temperature, the coolingrate, and the purity of the material.6.1.4 Following recalescence, the tempe
49、rature remains rela-tively constant for a while during the freezing plateau. Thetemperature associated with the freezing plateau is the freezingpoint of the material.6.1.5 As freezing progresses, trace impurities in the freezingmaterial tend to be swept in front of the advancing liquid-solidinterface and concentrated in the remaining liquid. Sinceimpurities usually depress the freezing point of the referencematerial, the temperature of the material decreases ever morerapidly until all of the material is solid.6.1.6 The effect of low concentra
