1、Designation: E2488 09Standard Guide forthe Preparation and Evaluation of Liquid Baths Used forTemperature Calibration by Comparison1This standard is issued under the fixed designation E2488; the number immediately following the designation indicates the year oforiginal adoption or, in the case of re
2、vision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.INTRODUCTIONMany of the Standards and Test Methods under the jurisdiction of ASTM committee E20 onTemperature
3、Measurement make reference to the use of controlled temperature fluid baths for thecalibration of thermometers by the comparison method. In this method the thermometer under test ismeasured while immersed in an isothermal medium whose temperature is simultaneously determinedby a calibrated reference
4、 thermometer. The uncertainty of all such comparison calibrations dependsupon how well the isothermal conditions can be maintained. The bath temperature must be stable overtime and uniform within the working space at the operating temperatures. This guide provides basicinformation, options and instr
5、uctions that will enable the user to prepare and evaluate controlledtemperature baths for calibrations.1. Scope1.1 This guide is intended for use with controlled tempera-ture comparison baths that contain test fluids and operatewithin the temperature range of -100 C to 550 C.1.2 This guide describes
6、 the essential features of controlledtemperature fluid baths used for the purpose of thermometercalibration by the comparison method.1.3 This guide does not address the details on the design andconstruction of controlled-temperature fluid baths.1.4 This guide describes a method to define the working
7、space of a bath and evaluate the temperature variations withinthis space. Ideally, the working space will be as close aspossible to isothermal.1.5 This guide does not address fixed point baths, ice pointbaths or vapor baths.1.6 This guide does not address fluidized powder baths.1.7 This guide does n
8、ot address baths that are programmedto change temperature.1.8 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.9 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theres
9、ponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applicabil-ity of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E1 Specification for ASTM Liquid-in-Glass ThermometersE344 Terminology Relating to Thermom
10、etry and Hydrom-etryE644 Test Methods for Testing Industrial Resistance Ther-mometersE839 Test Methods for Sheathed Thermocouples andSheathed Thermocouple Material2.2 Other Documents:ITS-90 The International Temperature Scale of 19903NIST Monograph 126 Platinum Resistance Thermometry4NIST Monograph
11、150 Liquid-in-Glass Thermometry4NIST SP 250-22 Platinum Resistance Thermometer Cali-brations4NIST SP 250-23 Liquid-in-Glass Thermometer CalibrationService42.3 Military Standards:1This practice is under the jurisdiction ofASTM Committee E20 on TemperatureMeasurement and is the direct responsibility o
12、f Subcommittee E20.07 on Funda-mentals in Thermometry.Current edition approved Nov. 1, 2009. Published February 2010. DOI: 10.1520/E2488-09.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volum
13、e information, refer to the standards Document Summary page onthe ASTM website.3Preston-Thomas, H., METROLOGIA, Vol. 27, 1990, pp 3-10 and 107 (errata).Mangum, B. W., JOURNAL OF RESEARCH, National Institute of Standards andTechnology, Vol 95, 1990 , p. 69.4Available from National Institute of Standa
14、rds and Technology (NIST), 100Bureau Dr., Stop 1070, Gaithersburg, MD 20899-1070, http:/www.nist.gov.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.MIL-STD-202G Test Methods for Electronic and ElectricalComponent Parts53. Terminolog
15、y3.1 Standard terms used in this guide are defined in Termi-nology E344.3.2 Definitions of Terms Specific to This Standard:3.2.1 bath gradient error, nthe error caused by tempera-ture differences within the working space of the bath.3.2.2 immersion error, nan error caused by heat conduc-tion, radiat
16、ion or both between the temperature sensing portionof the sensor used in the bath and the environment external tothe measurement system. Immersion error is caused by anincorrect immersion length and the resulting incorrect thermalcontact of the temperature sensing portion of the sensor withthe mediu
17、m under measurement.3.2.3 isothermal, adjof, related to, or designating a regionof nominally uniform temperature.3.2.4 thermal stability, n, the degree of variability of thetemperatures within a specified working space over a specifiedtime interval.3.2.5 working space, nthe region within a controlle
18、dtemperature bath where the temperature uncertainty is main-tained within acceptable limits for the purpose of performingcalibrations by the comparison method.3.2.6 working temperature range, nthe minimum to maxi-mum temperature range for which the bath system providesadequate stability and uniformi
19、ty.4. Summary of Practice4.1 This guide is intended to provide basic information thatwill enable the user to evaluate various controlled temperaturebath features and to enable the user to prepare and properlyutilize such controlled temperature baths for calibration ofthermometers by the comparison m
20、ethod.5. Significance and Use5.1 The design of a controlled temperature bath will deter-mine what thermometers can be calibrated and to what extentan isothermal condition is achieved. The lack of thermalstability and uniformity of the bath are sources of error thatcontribute to the overall calibrati
21、on uncertainty.5.2 This guide describes a procedure for determining theeffective working space for a controlled temperature fluid bath.5.3 This guide describes a procedure for determining thethermal stability within a controlled temperature fluid bath.Overall thermal stability is composed of the bat
22、h performanceas specified by the manufacturer of the bath equipment and asa component of calibration uncertainty.5.4 This guide describes a procedure for determining thetemperature uniformity of the working space of the controlledtemperature fluid bath.6. Procedure6.1 Bath systemA controlled tempera
23、ture fluid bath sys-tem will incorporate most, if not all, of the following compo-nents: a fluid medium; a mechanical design that provides forcontainment and circulation of the fluid; a monitoring ther-mometer, a temperature control unit; and, elements that pro-vide for heating, cooling or both. The
24、re are many commerciallyavailable controlled temperature baths. These baths operatefrom as low as -100 C to as high as +550 C; although nosingle bath system is capable of operation over that entirerange. The design of each individual bath will create practicallimits for the working temperature range
25、. These limits aredetermined by considering the minimum and maximum tem-perature ratings for each of the components in the bath system.The user is advised to carefully review the bath manufacturersliterature to be certain that the bath system is suitable for theintended calibration temperature range
26、 and the types of ther-mometers to be tested. Figs. 1-4 represent various designs ofcontrolled temperature fluid bath systems. Fig. 4 shows a blockdiagram of a comparison calibration setup.5Available from Superintendent of Documents, U.S. Government PrintingOffice, Washington, DC 20401.FIG. 1 Altern
27、ative Designs of Top Stirred Comparison Bathswithout Controllers.E2488 0926.1.1 Fluid MediumThere are many types of fluid mediasuitable for use in liquid temperature comparison baths. Thephysical properties of the medium will establish the limits forthe safe operating temperature range as well as de
28、termine theoverall performance of the bath system. Fig. 5 provides apartial listing of common bath media that have been usedsuccessfully for liquid temperature comparison baths. Thisguide is not intended to restrict the user to only those fluidsshown in Fig. 5. It is advisable for the user to review
29、 carefullythe manufacturers literature on any alternative fluid to becertain that it complies with the safety considerations of6.1.1.1.6.1.1.1 Safety and Environmental Impact Considerations(See 1.8) It is strongly recommended that the Material SafetyData Sheet (MSDS) of any material used as a fluid
30、medium bereviewed and understood by the user before the material ishandled for the first time. The data sheets of all test fluidsshould be kept readily available during bath operation in caseof accidents or spills. Additionally, some producers of bathfluids provide a Global Warming Potential Index i
31、n theirspecifications that should be considered when choosing a bathfluid.6.1.1.1.1 Temperature LimitsFig. 5 provides minimumand maximum safe operating temperatures for several commonbath media. Flash point temperatures are also given for certainflammable media. Consult the manufacturers MSDS docu-m
32、ent for each bath fluid used.6.1.1.1.2 Flammability Fluids are easily ignited abovetheir flash point. Whenever possible, the bath fluid shall bemaintained below the specified flash point. Some fluids areflammable at room temperature so the user must exercisecaution to prevent the exposure of these f
33、luids to open flamesor sparks. As a general safety practice, a fire suppressionsystem (for example, extinguisher, blankets, hoods, lids, etc.)should always be readily available when operating a bath withflammable media.FIG. 2 Sample Design of Bottom-Stirred Comparison Bath withControllerFIG. 3 Sampl
34、e Design of Comparison Bath with Integral Heating,Cooling and ControllerE2488 0936.1.1.1.3 VentilationProper ventilation, such as exhausthoods or vents, is required to remove any fumes or vapors thatmay be toxic or otherwise harmful to the operators performingthe calibration.6.1.1.1.4 Toxicity Prote
35、ctive clothing and shielding shallbe required for operators who must handle fluids that areenvironmentally hazardous or toxic. Proper disposal of excessFIG. 4 Block Diagram of Comparison Calibration SetupFIG. 5 Typical Bath Fluid Media and Useful Operating temperature ranges.E2488 094fluids, spills,
36、 residues or materials contaminated by the fluidsshall be in accordance with all regulatory policies.6.1.1.1.5 Chemical StabilityThe bath fluid shall be chemi-cally stable at the operating temperatures and inert to both thecontainer and the components or elements submitted to com-parison testing. Wa
37、rningThe salts or molten metals used forcalibration at high temperatures (above 260 C) are particularlycorrosive to many materials. Special care should be taken todetermine the compatibility of the materials used in construc-tion of the thermometer. DISCUSSION: Chemical instabilitymay change the pro
38、perties of a bath fluid in one or more of thefollowing ways: 1) Safety - The flash point of the bath fluidmay change over time due to the chemical decomposition,breaking of chemical bonds, caused by repeated use at hightemperatures. 2) Performance - A bath fluid that is subject topolymerization when
39、 exposed to high temperatures for ex-tended periods will become more viscous. The increase inviscosity can degrade performance and will make maintenanceand cleanup very difficult.6.1.1.1.6 Expansion of FluidsThe bath system must bedesigned to provide sufficient room for the expansion of fluidswhen h
40、eated so that spills and overflows do not occur. It is alsoimportant to consider that fluids will contract when beingrefrigerated to very low temperatures and then expand whenallowed to return to room temperature.6.1.1.1.7 Cross Contamination Between Baths Propercaution must be taken to avoid the mi
41、xing of test fluids whenthermometers are transferred from one adjacent bath to anotherduring multiple calibrations. Depending upon the fluids andtemperatures involved, this can be a minor problem compro-mising bath performance, or it can be a major safety issue. Forexample, fluids at temperatures ab
42、ove 100 C may reactviolently if water or a wet object is immersed into them. Theintroduction of water or organic materials into a molten saltbath can also produce violent6.1.1.2 Performance ConsiderationsThe physical proper-ties of the fluid media will determine the overall performanceof the bath sy
43、stem.6.1.1.2.1 Fluid ViscosityThe fluid viscosity can varygreatly over a wide temperature range and this can lead toproblems with stirring, agitation, or the establishment ofundesirable temperature gradients. In practice, a bath fluid witha viscosity of 10 centistokes, or less, usually provides good
44、stirring and mixing action. When the viscosity of the fluidbecomes 50 centistokes, or greater, the stirring and mixingbecomes less effective and the possibility of temperaturegradients is increased. High viscosity also leads to excessivefluid drag-out. These viscosity numbers are intended only as ag
45、eneral observation. The selection of a test fluid should bebased upon a careful consideration of many factors.6.1.1.2.2 Volatility Fluids operated near their boilingpoints, or that have a high vapor pressure under normallaboratory environments will present a problem in controllingthe bath temperatur
46、e. Evaporation from the surface of theliquid will produce an undesirable temperature gradient be-cause of the increased cooling effect at the surface. In addition,the loss of bath fluid over time due to evaporation will causethe length of immersion of the thermometers to vary unless thebath design i
47、s such that it replenishes the lost fluid.6.1.1.2.3 Moisture CondensationRefrigerated baths cancause atmospheric moisture to condense on the surfaces abovethe fluid. Precipitation of this moisture into the test fluid canseriously degrade the performance of the bath system.6.1.1.2.4 Dielectric Proper
48、ties The volume resistivity,dielectric strength and dielectric constant of the fluid areimportant considerations whenever the thermometer or deviceunder test has exposed conductors or electrical contacts wettedby the fluid. In general, the dielectric strength of the fluidsshould be as high as practi
49、cal when testing resistance thermom-eters. Fluids that absorb moisture over time (for example.,isopropyl alcohol) should be periodically checked and eitherreplaced or treated to remove the moisture.6.1.1.2.5 Thermal Conductivity The thermal conductivityof the bath fluid medium should be relatively high. This willkeep bath temperature gradients within the fluid as small aspossible, and will also subject the device under test to a moreuniform temperature over its immersed surface.6.1.1.2.6 Specific HeatIt is not always possible to selectfluids t
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