1、Designation: E1750 10Standard Guide forUse of Water Triple Point Cells1This standard is issued under the fixed designation E1750; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses in
2、dicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.INTRODUCTIONThe triple point of water is an important thermometric fixed point common to the definition of twotemperature scales of science and technology, the Kelvin Ther
3、modynamic Temperature Scale (KTTS)and the International Temperature Scale of 1990 (ITS-90). The ITS-90 was designed to be as close tothe KTTS as the experimental data available at the time of the adoption of the ITS-90 would permit.The temperatures (T) on the KTTS are defined by assigning the value
4、273.16 K to the triple point ofwater, thus defining the thermodynamic unit of temperature, kelvin (K), as 1/273.16 of thethermodynamic temperature of the triple point of water (1, 2).2The triple point of water, one of thefixed points used to define the ITS-90, is the temperature to which the resista
5、nce ratios W(T) = R(T)/R(273.16 K) of the standard platinum resistance thermometer (SPRT) calibrations are referred.The triple points of various materials (where three distinct phases, for example, their solid, liquid,and vapor phases, coexist in a state of thermal equilibrium) have fixed pressures
6、and temperatures andare highly reproducible. Of the ITS-90 fixed points, six are triple points. The water triple point is oneof the most accurately realizable of the defining fixed points of the ITS-90; under the best ofconditions, it can be realized with an expanded uncertainty (k=2) of less than 6
7、0.00005 K. Incomparison, it is difficult to prepare and use an ice bath with an expanded uncertainty (k=2) of lessthan 60.002 K (3).1. Scope1.1 This guide covers the nature of two commercial watertriple-point cells (types A and B, see Fig. 1) and provides amethod for preparing the cell to realize th
8、e water triple-pointand calibrate thermometers. Tests for assuring the integrity ofa qualified cell and of cells yet to be qualified are given.Precautions for handling the cell to avoid breakage are alsodescribed.1.2 The effect of hydrostatic pressure on the temperature ofa water triple-point cell i
9、s discussed.1.3 Procedures for adjusting the observed SPRT resistancereadings for the effects of self-heating and hydrostatic pressureare described in Appendix X1 and Appendix X2.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is therespon
10、sibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E344 Terminology Relating to Thermometry and Hydrom-etryE1594 Guide for Expression of Tempera
11、ture3. Terminology3.1 DefinitionsThe definitions given in TerminologyE344 apply to terms used in this guide.3.2 Definitions of Terms Specific to This Standard:3.2.1 inner melt, na thin continuous layer of water be-tween the thermometer well and the ice mantle of a watertriple-point cell.3.2.2 refere
12、nce temperature, nthe temperature of a phaseequilibrium state of a pure substance at a specified pressure, forexample, the assigned temperature of a fixed point.1This guide is under the jurisdiction of ASTM Committee E20 on TemperatureMeasurement and is the direct responsibility of Subcommittee E20.
13、07 on Funda-mentals in Thermometry.Current edition approved May 1, 2010. Published June 2010. Originallyapproved in 1995. Last previous edition approved in 2009 as E1750 - 09. DOI:10.1520/E1750-10.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3For re
14、ferenced 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, W
15、est Conshohocken, PA 19428-2959, United States.3.2.2.1 DiscussionAt an equilibrium state of three phasesof a substance, that is, at the triple point, both the temperatureand pressure are fixed.4. Significance and Use4.1 This guide describes a procedure for placing a watertriple-point cell in service
16、 and for using it as a referencetemperature in thermometer calibration.4.2 The reference temperature attained is that of a funda-mental state of pure water, the equilibrium between coexistingsolid, liquid, and vapor phases.4.3 The cell is subject to qualification but not to calibration.The cell may
17、be qualified as capable of representing thefundamental state (see 4.2) by comparison with a bank ofsimilar qualified cells of known history, and it may be soqualified and the qualification documented by its manufacturer.4.4 The temperature to be attributed to a qualified watertriple-point cell is ex
18、actly 273.16 K on the ITS-90, unlesscorrected for isotopic composition (refer to Appendix X3).4.5 Continued accuracy of a qualified cell depends uponsustained physical integrity. This may be verified by techniquesdescribed in Section 6.4.6 The commercially available triple point of water cellsdescri
19、bed in this standard are capable of achieving an expandeduncertainty (k=2) of between 60.1 mK and 60.05 mK,depending upon the method of preparation. Specified measure-ment procedures shall be followed to achieve these levels ofuncertainty.4.7 Commercially-available triple point of water cells ofunkn
20、own isotopic composition should be capable of achievingan expanded uncertainty (k=2) of no greater than 0.25 mK,depending upon the actual isotopic composition (3). Thesetypes of cells are acceptable for use at this larger value ofuncertainty.5. Apparatus5.1 The essential features of type A and type
21、B watertriple-point cells are shown in Fig. 1. A transparent glass flaskfree of soluble material is filled with pure, air-free water andthen is permanently sealed, air-free, at the vapor pressure of thewater. A reentrant well on the axis of the flask receivesthermometers that are to be exposed to th
22、e reference tempera-ture.5.2 For the lowest level of uncertainty, the water used as thereference medium shall be very pure and of known isotopiccomposition. Often it is distilled directly into the cell. Theisotopic composition of cells filled with “rain water” isexpected not to vary enough to cause
23、more than 0.05 mKdifference in their triple points. Extreme variations in isotopiccomposition, such as between ocean water and water from oldpolar ice, can affect the realized temperature by as much as0.25 mK (7). In cases where the isotopic composition isunknown, or if the cell has not been qualifi
24、ed by comparisonwith a cell of known isotopic composition, the larger value ofuncertainty (60.25 mK) should be assumed.5.3 For use, a portion of the water is frozen within the cellto form a mantle of ice that surrounds the well and controls itstemperature.5.4 The temperature of the triple point of w
25、ater realized ina cell is independent of the environment outside the cell;however, to reduce heat transfer and keep the ice mantle frommelting quickly, it is necessary to minimize heat flow betweenthe cell and its immediate environment. This may be done byimmersing the cell in an ice bath that maint
26、ains the full lengthof the outer cell wall at or near the melting point of ice.Alternatively, commercial automatic maintenance baths, builtspecifically for this purpose, are available. In such baths, thetriple point of water equilibrium of the cell, once established,can be maintained for many months
27、 of continual use. To avoidradiation heat transfer to the cell and to the thermometer, theouter surface of the maintenance bath is made opaque toradiation.FIG. 1 Configurations of two commonly used triple point of watercells, Type A and Type B, with ice mantle prepared formeasurement at the ice/wate
28、r equilibrium temperature. The cellsare used immersed in an ice bath or water bath controlled closeto 0.01C (see 5.4)E1750 1026. Assurance of Integrity6.1 The temperature attained within a water triple-point cellis an intrinsic property of the solid and liquid phases of waterunder its own vapor pres
29、sure. If the water triple-point condi-tions are satisfied, the temperature attained within the cell ismore reproducible than any measurements that can be made ofit.6.2 The accuracy of realization of the water triple-pointtemperature with a qualified cell depends on the physicalintegrity of the seal
30、and of the walls of the glass cell and ontheir ability to exclude environmental air and contaminants.6.3 Initial and continued physical integrity is confirmed bythe following procedures:6.3.1 Test for the Presence of Air:6.3.1.1 Remove all objects from the thermometer well.6.3.1.2 The solubility and
31、 the pressure of air at 101 325 Palower the ice/water equilibrium temperature 0.01C below thetriple-point temperature. Since air is more soluble in water atlower temperatures, the test for air shall be done at roomtemperature. The test is less definitive when performed on achilled cell.At room tempe
32、rature, with the cell initially uprightand the well opening upward, slowly invert the cell.As the axisof the cell passes through horizontal and as the water within thecell strikes the end of the cell, a sharp “glassy clink” soundshould be heard. The distinctive sound results from the suddencollapse
33、of water vapor and the “water hammer” striking theglass cell. The smaller the amount of air, the sharper the clinksound; a large amount of air cushions the water-hammer actionand the sound is duller.6.3.1.3 With a type A cell, continue to tilt the cell to makea McLeod-gauge type test until the vapor
34、 (water saturated air)bubble is entirely captured in the space provided in the handle.The vapor bubble should be compressed to a volume no largerthan about 0.03 cm3(4 mm diameter). It may even vanish as itis compressed by the weight of the water column. As in the tilttest, the bubble test is more de
35、finitive when the cell is at roomtemperature (see 6.3.1.2). Since type B cells do not have aspace to capture the vapor, the amount of air in the cell isestimated by comparing the sharpness of the clink sound withthat of a type A cell.6.3.2 Test for the Presence of Water Soluble Impurities:6.3.2.1 Wh
36、en ice is slowly formed around the thermometerwell, impurities are rejected into the remaining unfrozen water.Therefore, the impurity concentration of the unfrozen waterincreases as the ice mantle thickens. The ice is purer than theunfrozen water. Consequently, the inner melt (see section7.1.3) that
37、 is formed from the ice mantle is purer than theunfrozen water outside of the mantle.6.3.2.2 Prepare a relatively thick ice mantle, according toSection 7, by maintaining the dry ice level full for about 20minutes. Make certain that the ice does not bridge to the cellwall (see 7.1.9).6.3.2.3 Prepare
38、an inner melt according to 7.1.13. Using anSPRT, make measurements on the cell and determine thezero-power resistance according to Section 8 and AppendixX1.6.3.2.4 After 6.3.2.3, remove the SPRT. Gently invert thewater triple-point cell and then return it to the upright positionseveral times to exch
39、ange the unfrozen water on the outside ofthe ice mantle with the inner melt water. (WarningWheninverting the cell, do not allow the floating ice mantle toseverely strike the bottom of the water triple-point cell.)6.3.2.5 Reinsert the pre-chilled SPRT used in 6.3.2.3 intothe well. Make measurements o
40、n the cell and determine thezero-power resistance, according to Section 8 and AppendixX1.6.3.2.6 Typically, for high quality water triple-point cells,the results of 6.3.2.3 and 6.3.2.5 will not differ by more than60.03 mK.6.4 Any cell that had previously been qualified by compari-son with cells of k
41、nown integrity (as in 4.3), that has notthereafter been modified, and which currently passes the testsof 6.3.1 and 6.3.2, is qualified as a water triple-point cell.6.5 Any cell that fails to pass the tests of 6.3.1 and 6.3.2,even though previously qualified, is no longer qualified for useas a water
42、triple-point cell.7. Realization of the Water Triple-Point Temperature7.1 The ice mantle that is required to realize the triple-pointtemperature of water can be prepared in a number of ways.They produce essentially the same result.Acommon procedureis as follows:7.1.1 Empty the well of any solids or
43、liquids. Wipe the wellclean and dry, and seal the well opening with a rubber stopper.7.1.2 If the triple point of water cell has not already beentested for the presence of air, perform the tests indicated in6.3.1 for presence of air.7.1.3 To obtain an ice mantle of fairly uniform thicknessthat exten
44、ds to the top, immerse the cell completely in an icebath, and chill the cell to near 0C.7.1.4 Remove the cell from the bath and mount it upright ona plastic foam cushion. Wipe the cell dry around the rubberstopper before removing the rubber stopper.7.1.5 Remove the rubber stopper and place about 1 c
45、m3ofdry alcohol in the well to serve as a heat-transfer medium whileforming an ice mantle around the well within the sealed cell.7.1.6 Place a small amount of crushed dry ice at the bottomof the well, maintaining the height of the dry ice at about 1 cmfor a period of 2 to 3 min. In repeated use of t
46、he cell, the icemantle melts mostly at the bottom; hence, it is desirable thatthe ice mantle be thicker at the bottom. Crushed dry ice may beprepared from a block or by expansion from a siphon-tube tankof liquid CO2.7.1.7 At the interface of the well, the water is initiallysupercooled, and the well
47、becomes abruptly coated with fineneedles of ice frozen from the supercooled water.7.1.8 After a layer of ice forms around the bottom of thewell, fill the well with crushed dry ice up to the vapor/liquidinterface.7.1.9 Replenish the dry ice as it sublimes, maintaining thewell filled to the liquid sur
48、face, until a continuous ice mantle asthick as desired forms on the surface of the well within thewater (usually 4 to 8 mm thick). The mantle will appear thickerthan its actual thickness because of the lenticular shape of thecell and the refractive index of water. The actual thickness maybe best est
49、imated by viewing from the bottom of the cell whileit is inverted or by immersing the cell in a large glass containerE1750 103of water. (WarningDuring preparation, the mantle shouldnever be allowed to grow at any place to completely bridge thespace between the well and the inner wall of the cell, as theexpansion of the ice may break the cell. In particular, ifbridging occurs at the surface of the water at the top of the cellunder the vapor space, melt the ice bridge by warming the celllocally with heat from the hand, while gently shaking the cell.)7.1.10 When the mantle att
