1、Designation: E 1502 98 (Reapproved 2003)e1Standard Guide forUse of Freezing-Point Cells for Reference Temperatures1This standard is issued under the fixed designation E 1502; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、 of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.e1NOTEUpdated caution note in Section 7.2.2 in November 2003.INTRODUCTIONDuring freezing, pure material transforms from the l
3、iquid state to the solid state at a constanttemperature known as the freezing point. The freezing points of highly purified materials can serve asreference temperatures, and in fact, the International Temperature Scale of 1990 (ITS-90)2relies onthe freezing points of some highly purified metals as d
4、efining fixed points. Freezing points can berealized in commercially available systems incorporating freezing-point cells. When the cells areproperly made and used, they establish useful reference temperatures for the calibration ofthermometers and for other industrial and laboratory purposes; with
5、care, the freezing points of highlypurified materials can be realized with an uncertainty of a few millikelvins3or less.1. Scope1.1 This guide describes the essential features of freezing-point cells and auxiliary apparatus, and the techniques requiredto realize freezing points in the temperature ra
6、nge from 29 to1085 C.31.2 Detailed design and construction are not addressed inthis guide.1.3 This guide is intended to describe good practice andestablish uniform procedures for the realization of freezingpoints.1.4 This guide emphasizes principles. The emphasis onprinciples is intended to aid the
7、user in evaluating cells, inimproving technique for using cells, and in establishing pro-cedures for specific applications.1.5 For the purposes of this guide, the use of freezing-pointcells for the accurate calibration of thermometers is restrictedto immersion-type thermometers that, when inserted i
8、nto thereentrant well of the cell, (1) indicate the temperature only ofthe isothermal region of the well, 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
9、 is intended to complement special operatinginstructions supplied by manufacturers of freezing-point appa-ratus.1.8 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 any, associatedwi
10、th 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:4E 344 Terminology Relating to Thermometry and Hydrom-etryE 644 Test Me
11、thods for Testing Industrial Resistance Ther-mometers3. Terminology3.1 Definitions:3.1.1 reference temperature, na fixed, reproducible tem-perature, to which a value is assigned, that can be used for thecalibration of thermometers or other purposes.1This guide is under the jurisdiction of ASTM Commi
12、ttee E20 on TemperatureMeasurement and is the direct responsibility of Subcommittee E20.07 on Funda-mentals in Thermometry.Current edition approved Nov. 1, 2003. Published November 2003. Originallyapproved in 1992. Last previous edition aprpoved in 1998 as E 1502 98.2Preston-Thomas, H., “The Interna
13、tional 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 millikelvins(mK). Values of temperature are expressed in degrees Celsius (C), ITS-90.4For referen
14、ced 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 website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West C
15、onshohocken, PA 19428-2959, United States.3.1.2 Additional terms used in this guide are defined inTerminology E 344.3.2 Definitions of Terms Specific to This Standard:3.2.1 first cryoscopic constant, A, na constant of propor-tionality between the freezing point depression of, and concen-tration of i
16、mpurities 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(1)3.2.2 freeze, nan experiment or
17、 test run conducted with afreezing-point cell while the reference material in the cellsolidifies.3.2.3 freezing curve, nthe entire time-temperature relationof the reference material in a freezing-point cell duringfreezing, including initial cooling, undercool, recalescence,freezing plateau, and fina
18、l cooling to complete solidification.3.2.3.1 DiscussionGraphic representations of freezingcurves are shown in Fig. 1 and Fig. 2.3.2.4 freezing plateau, nthe period during freezing inwhich the temperature does not change significantly.3.2.5 freezing-point cell, na device that contains andprotects a s
19、ample of reference material in such a manner thatthe freezing point of the material can establish a referencetemperature.3.2.6 freezing range, nthe range of temperature overwhich most of the reference material in a freezing-point cellsolidifies.3.2.6.1 DiscussionThe freezing range is indicated graph
20、i-cally in Fig. 1.3.2.7 nucleation, nthe formation of crystal nuclei in liquidin the supercooled state.3.2.8 recalescence, nthe sudden increase in temperatureof reference material in the supercooled state upon nucleationand crystal growth, due to the release of latent heat of fusion ofthe reference
21、material.3.2.9 reference material, nthe material in a freezing-pointcell that melts and freezes during use, the freezing point ofwhich can establish a reference temperature.3.2.10 supercooled state, nthe meta-stable state of refer-ence material in which the temperature of the liquid phase isbelow th
22、e freezing point.3.2.11 undercool, nthe temperature depression below thefreezing point of reference material in the supercooled state.4. Summary of Guide4.1 A freezing-point cell is used for thermometer calibrationby establishing and sustaining a reference material at thefreezing point, to which a v
23、alue of temperature has beenassigned. The thermometer to be calibrated is inserted into areentrant well in the cell; the well itself is surrounded by thefreezing reference material.4.2 The cell is heated to melt the reference material. Thetemperature of the surrounding environment is then reduced to
24、about 1 K below the freezing point so that the referencematerial cools. Following the undercool, nucleation, and re-calescence, the well temperature becomes constant during thefreezing plateau. After a time, depending on the rate of heatloss from the cell, the amount of reference material, and thepu
25、rity of the reference material, the temperature starts todecrease and eventually all of the material becomes solidified.4.3 Since the temperature in the reentrant well remainsconstant during the freezing plateau, one or more test ther-mometers may be calibrated by inserting them singly into thewell.
26、 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-eters. The duration of the plateau may be lengthened bypreheating the test thermometers.4.4 Measurements are made also duri
27、ng each freeze with adedicated monitoring thermometer. These measurements, to-gether with other special test measurements, provide qualifi-cation test data (see 6.4 and 7.5).5. Significance and Use5.1 A pure material has a well-defined freezing behavior,and its freezing point, a characteristic of th
28、e material, can serveA = Stabilized temperature of cell before freezing, typically about 1 K abovefreezing point.B = Freezing point of cell.C = Temperature of cell surroundings during freezing, typically about 1 Kbelow freezing point.D = Maximum undercool.E = Onset of recalescence.F = Freezing plate
29、au.G = Total freezing time.H = Freezing range.FIG. 1 Structure of a Typical Freezing CurveFIG. 2 Freezing Curve of a Sample of Highly Purified TinE 1502 98 (2003)e12as a reproducible reference temperature for the calibration ofthermometers. The freezing points of some highly purifiedmetals have been
30、 designated as defining fixed points onITS-90. The freezing points of other materials have beendetermined carefully enough that they can serve as secondaryreference points (see Table 1 and Table 2). This guide presentsinformation on the freezing process as it relates to establishinga reference tempe
31、rature.5.2 Freezing-point cells provide users with a means ofrealizing freezing points. If the cells are appropriately designedand constructed, if they contain material of adequate purity,and if they are properly used, they can establish referencetemperatures with uncertainties of a few millikelvins
32、 or less.This guide describes some of the design and use consider-ations.5.3 Freezing-point cells can be constructed and operatedless stringently than required for millikelvin uncertainty, yetstill provide reliable, durable, easy-to-use fixed points for avariety of industrial calibration and heat tr
33、eatment purposes.For example, 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 be commensurate with that of freezing points (see 6.2.10).5.4 The test procedure described in thi
34、s guide producesqualification test data as an essential part of the procedure.These data furnish the basis for quality control of the freezing-point procedure; they provide for evaluation of results, theyassure continuing reliability of the method, and they yieldinsight into the cause of test result
35、 discrepancies. The testprocedure is applicable to the most demanding uses offreezing-point cells for precise thermometer calibration; it maynot be appropriate or cost-effective for all applications. It isexpected that the user of this guide will adapt the procedure tospecific needs.6. Principles6.1
36、 Freezing Process: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 realmaterial from liquid to solid, as heat is released in semi-equilibrium freezing, exhibits a complex
37、 time-temperaturerelation (freezing curve) as shown in Figs. 1 and 2.6.1.2 The deposition of the solid phase from the liquid phaserequires the presence of liquid in the supercooled state,nucleation, and crystal growth. Nucleation may begin sponta-neously in the meta-stable supercooled liquid, or it
38、may beinduced artificially. As crystals nucleate and grow, the liberatedlatent heat of fusion produces recalescence.6.1.3 The undercool 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 theund
39、ercool can depend on the initial temperature, the coolingrate, and the purity of the material.6.1.4 Following recalescence, the temperature remains rela-tively constant for a while during the freezing plateau. Thetemperature associated with the freezing plateau is the tem-perature to which a value i
40、s assigned as the freezing point ofthe material.6.1.5 As freezing progresses, some trace impurities in thefreezing material tend to be swept in front of the advancingliquid-solid interface and concentrated in the remaining liquid.Since impurities usually depress the freezing point of thereference ma
41、terial, the temperature of the material decreasesever more rapidly until all of the material is solid.6.1.6 The effect of small concentrations of impurities maybe estimated from an approximation rule: the temperaturedifference between the start of freezing and midpoint offreezing (when half the mate
42、rial is solid) equals the tempera-ture difference between the freezing point of the ideally pureTABLE 1 Characteristics of Pure Freezing-Point Reference MaterialsMaterial Freezing Point, ITS-90, CTypicalUndercool, KPressure Coefficient at Freezing Point First CryoscopicConstant, K1mK/Pa mK/m (of liq
43、uid)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 . . .Cadmium 321.069 0.050.5 + 61 + 4.8 0.0021Lead 327.462 0.15 + 79 + 8.2 0.0016ZincA419.527 0.050.1 + 43 + 2.7 0.0018Antimony 630.630 20 + 8 + 0.5 0.0029AluminumA6
44、60.323 0.41.5 + 70 + 1.6 0.0015SilverA961.78 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.TABLE 2 Estimated Achievable Uncertainties in Freezing-PointCellsAMaterialsLaboratoryPrimary, mK Indust
45、rial, mKGalliumB0.1 1Indium 1 10Tin 1 10Cadmium 2 10Lead 2 10Zinc 1 10Antimony 10 50Aluminum 2 20Silver 2 40Gold . . . . . .Copper 10 50AValues for cells of good design, construction, and material purity used withcareful technique. Cells of lesser quality may not approach these values.BRealized as m
46、elting point.E 1502 98 (2003)e13material and the freezing point (at the start of freezing) of thereal reference material (see 8.6.2). The product of this tem-perature difference and the first cryoscopic constant gives anestimate of the mole fraction impurity concentration in thereference material. C
47、onversely, if the impurity concentration isknown, then the temperature difference can be estimated.6.1.7 The change in temperature during the freezing plateaudue to a change in pressure is generally less than 0.1 K/Pa(Table 1). Thus, normal changes in atmospheric pressure havelittle effect on the fr
48、eezing point, but the effect of the pressureof a head of dense liquid reference material may be significant.The freezing point is usually taken to be the temperature duringthe freezing plateau at a pressure of 101 325 Pa.6.2 Freezing-Point Cells:6.2.1 The usual freezing-point apparatus consists of a
49、freezing-point cell containing the reference material, a meansto melt the reference material and allow the material to freezeslowly and uniformly, with provision for exposing one or moretest thermometers to the freezing point. A typical cell andauxiliary furnace are shown in Figs. 3 and 4. Control equip-ment is not shown.6.2.2 The freezing-point apparatus must be able to maintaina freezing plateau of useful duration and must include enoughreference material to establish an isothermal region and depthof immersion suitable for the intended use. Typic
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