1、Designation: E 1750 02Standard Guide forUse of Water Triple Point Cells1This standard is issued under the fixed designation E 1750; 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
2、indicates the year of last reapproval. Asuperscript epsilon (e) 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 T
3、hermodynamic 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 val
4、ue 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 resi
5、stance 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 pressur
6、es 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 tha
7、n 60.00005 K. Incomparison, it is difficult to prepare and use an ice bath with an expanded uncertainty (k=2) of lessthan 60.002 K (3).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) ha
8、ve fixed pressures 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 (
9、k=2) of less than 60.00005 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
10、 cell to realize the 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
11、triple-point cell is 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 u
12、se. It is theresponsibility 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:1This guide is under the jurisdiction of ASTM Committee E20 on TemperatureM
13、easurement and is the direct responsibility of Subcommittee E20.07 on Funda-mentals in Thermometry.Current edition approved May 10, 2002. Published June 2002. Originallypublished as E 1750 95.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.1Copyright A
14、STM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.E 344 Terminology Relating to Thermometry and Hydrom-etry3E 1594 Guide for Expression of Temperature33. Terminology3.1 DefinitionsThe definitions given in TerminologyE 344 apply to terms used in th
15、is 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 reference temperature, nthe temperature of a phaseequilibrium state of a pure substance at a specified pr
16、essure, forexample, the assigned temperature of a fixed point.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
17、cell in service 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 calibratio
18、n.The cell may 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-p
19、oint cell is exactly 273.16 K on the ITS-90.4.5 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 cellsdescribed in this standard are capable of achieving an e
20、xpandeduncertainty (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.5. Apparatus5.1 The essential features of type A and type B watertriple-point cells are shown in Fig. 1. A tran
21、sparent 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 the reference tempera-ture.5.2 The water used as the re
22、ference medium shall be verypure. Often it is distilled directly into the cell. The isotopiccomposition of cells filled with “rain water” is expected not tovary enough to cause more than 0.05 mK difference in theirtriple points. Extreme variations in isotopic composition, suchas between ocean water
23、and water from old polar ice, can affectthe realized temperature by as much as 0.25 mK.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 water realized ina cell is indepe
24、ndent 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 maintains the full lengthof the outer
25、 cell wall at or near the melting point of ice.3Annual Book of ASTM Standards, Vol 14.03.FIG. 1 Configurations of two commonly used triple point of watercells, Type A and Type B, with ice mantle prepared formeasurement at the ice/water equilibrium temperature. The cellsare used immersed in an ice ba
26、th or water bath controlled closeto 0.01C (see 5.4)E1750022Alternatively, 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 of continual use. T
27、o avoidradiation heat transfer to the cell and to the thermometer, theouter surface of the maintenance bath is made opaque toradiation.6. 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 v
28、apor pressure. 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
29、the seal 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 solub
30、ility and 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 r
31、oom temperature, 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 sudden
32、collapse 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-gage type test until t
33、he vapor (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
34、 more definitive 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.
35、3.2.1 When 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
36、.3) that 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
37、Prepare 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
38、 to exchange 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 measur
39、ements on 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 ce
40、lls of known 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
41、a water 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 so
42、lids or 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 thicknessth
43、at extends 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 a
44、bout 1 cm3ofdry 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
45、use of the 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 t
46、he well becomes abruptly coated with fineneedles of ice frozen from the supercooled water.E17500237.1.8 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 t
47、o the liquid surface, 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 thicknes
48、s maybe best estimated by viewing from the bottom of the cell whileit is inverted or by immersing the cell in a large glass containerof 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 c
49、ell, 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 attains nearly the desired thicknessor after maintaining the dry ice level in the well at the watersurface for about 20 min, return the cell to the ice bath with theentrance to the thermometer well slightly above the ice bathsurface and allow the dry ice to sublime completely. Byallowing the dry ic
