1、Designation: E2488 09 (Reapproved 2014)Standard 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,
2、 in the case of revision, 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
3、E20 onTemperature 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 ca
4、librated reference 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,
5、 options and instructions 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 100C to 550C.1.2 This
6、guide describes 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 def
7、ine the workingspace 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 Th
8、is guide does not 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 us
9、e. 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:2E1 Specification for ASTM Liquid-in-Glass ThermometersE344 Terminology Rela
10、ting to Thermometry and Hydrom-etryE644 Test Methods for Testing Industrial Resistance Ther-mometersE839 Test Methods for Sheathed Thermocouples andSheathed Thermocouple Cable2.2 Other Documents:ITS-90 The International Temperature Scale of 19903NIST Monograph 126 Platinum Resistance Thermometry4NIS
11、T Monograph 150 Liquid-in-Glass Thermometry4NIST SP 250-22 Platinum Resistance Thermometer Calibra-tions41This 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 appro
12、ved Dec. 1, 2014. Published December 2014. Originallyapproved in 2009. Last previous edition approved in 2009 as E2488 09. DOI:10.1520/E2488-09R14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandard
13、s volume 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
14、 Standards and Technology (NIST), 100Bureau Dr., Stop 1070, Gaithersburg, MD 20899-1070, http:/www.nist.gov.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1NIST SP 250-23 Liquid-in-Glass Thermometer CalibrationService42.3 Military Sta
15、ndards:MIL-STD-202G Test Methods for Electronic and ElectricalComponent Parts53. Terminology3.1 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 wo
16、rking space of the bath.3.2.2 immersion error, nan error caused by heatconduction, radiation or both between the temperature sensingportion of the sensor used in the bath and the environmentexternal to the measurement system. Immersion error is causedby an incorrect immersion length and the resultin
17、g incorrectthermal contact of the temperature sensing portion of thesensor with the medium under measurement.3.2.3 isothermal, adjof, related to, or designating a regionof nominally uniform temperature.3.2.4 thermal stability, nthe degree of variability of thetemperatures within a specified working
18、space over a specifiedtime interval.3.2.5 working space, nthe region within a controlledtemperature 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-
19、mum temperature range for which the bath system providesadequate stability and uniformity.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 properlyutili
20、ze such controlled temperature baths for calibration ofthermometers by the comparison method.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
21、and uniformity of the bath are sources of error thatcontribute to the overall calibration 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 with
22、in a controlled temperature fluid bath.Overall thermal stability is composed of the bath 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
23、of the controlledtemperature fluid bath.6. Procedure6.1 Bath SystemA controlled temperature 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 monitoringthermometer, a te
24、mperature control unit; and, elements thatprovide for heating, cooling or both. There are many commer-cially available controlled temperature baths. These bathsoperate from as low as -100C to as high as +550C; althoughno single bath system is capable of operation over that entirerange. The design of
25、 each individual bath will create practicallimits for the working temperature range. 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 cer
26、tain that the bath system is suitable for theintended calibration temperature range 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.6.1.1 Fluid MediumThere are
27、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 determine theoverall performance of the bath system. Fig. 5 provides apartial listing of common bath med
28、ia 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 carefullythe manufacturers literature on any alternative fluid to becertain that it complies with the
29、 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 medium bereviewed and understood by the user before the material ishandled for the first time. The da
30、ta 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 in theirspecifications that should be considered when choosing a bathfluid.(1) Temperature LimitsFig.
31、5 provides minimum andmaximum safe operating temperatures for several commonbath media. Flash point temperatures are also given for certainflammable media. Consult the manufacturers MSDS docu-ment for each bath fluid used.(2) FlammabilityFluids are easily ignited above theirflash point. Whenever pos
32、sible, the bath fluid shall be main-tained below the specified flash point. Some fluids are flam-mable at room temperature so the user must exercise caution toprevent the exposure of these fluids to open flames or sparks.5Available from Superintendent of Documents, U.S. Government PrintingOffice, Wa
33、shington, DC 20401.E2488 09 (2014)2As a general safety practice, a fire suppression system (forexample, extinguisher, blankets, hoods, lids, etc.) should al-ways be readily available when operating a bath with flam-mable media.(3) VentilationProper ventilation, such as exhaust hoodsor vents, is requ
34、ired to remove any fumes or vapors that maybe toxic or otherwise harmful to the operators performing thecalibration.(4) ToxicityProtective clothing and shielding shall berequired for operators who must handle fluids that are envi-ronmentally hazardous or toxic. Proper disposal of excessfluids, spill
35、s, residues or materials contaminated by the fluidsshall be in accordance with all regulatory policies.(5) Chemical StabilityThe bath fluid shall be chemicallystable at the operating temperatures and inert to both thecontainer and the components or elements submitted to com-parison testing. WarningT
36、he salts or molten metals used forcalibration at high temperatures (above 260C) 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 properties
37、 of a bath fluid in one or more of thefollowing ways: (1) SafetyThe 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) PerformanceA bath fluid that is subject topolymerization when exposed to
38、 high temperatures for ex-tended periods will become more viscous. The increase inviscosity can degrade performance and will make maintenanceand cleanup very difficult.(6) Expansion of FluidsThe bath system must be de-signed to provide sufficient room for the expansion of fluidswhen heated so that s
39、pills 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.(7) Cross Contamination Between BathsProper cautionmust be taken to avoid the mixing of test fluids wh
40、enthermometers are transferred from one adjacent bath to anotherduring multiple calibrations. Depending upon the fluids andFIG. 1 Alternative Designs of Top Stirred Comparison BathsWithout Controllers.FIG. 2 Sample Design of Bottom-Stirred Comparison Bath withControllerE2488 09 (2014)3temperatures i
41、nvolved, this can be a minor problem compro-mising bath performance, or it can be a major safety issue. Forexample, fluids at temperatures above 100C may react vio-lently if water or a wet object is immersed into them. Theintroduction of water or organic materials into a molten saltbath can also pro
42、duce violent reactions.6.1.1.2 Performance Considerations The physical proper-ties of the fluid media will determine the overall performanceof the bath system.(1) Fluid ViscosityThe fluid viscosity can vary greatlyover a wide temperature range and this can lead to problemswith stirring, agitation, o
43、r the establishment of undesirabletemperature gradients. In practice, a bath fluid with a viscosityof ten centistokes, or less, usually provides good stirring andmixing action. When the viscosity of the fluid becomes 50centistokes, or greater, the stirring and mixing becomes lesseffective and the po
44、ssibility of temperature gradients is in-creased. High viscosity also leads to excessive fluid drag-out.These viscosity numbers are intended only as a generalobservation. The selection of a test fluid should be based upona careful consideration of many factors.(2) VolatilityFluids operated near thei
45、r boiling points, orthat have a high vapor pressure under normal laboratoryenvironments will present a problem in controlling the bathtemperature. Evaporation from the surface of the liquid willproduce an undesirable temperature gradient because of theincreased cooling effect at the surface. In addi
46、tion, the loss ofbath fluid over time due to evaporation will cause the length ofimmersion of the thermometers to vary unless the bath designis such that it replenishes the lost fluid.(3) Moisture CondensationRefrigerated baths can causeatmospheric moisture to condense on the surfaces above thefluid
47、. Precipitation of this moisture into the test fluid canseriously degrade the performance of the bath system.(4) Dielectric PropertiesThe volume resistivity, dielec-tric strength and dielectric constant of the fluid are importantFIG. 3 Sample Design of Comparison Bath with Integral Heating,Cooling a
48、nd ControllerFIG. 4 Block Diagram of Comparison Calibration SetupE2488 09 (2014)4considerations whenever the thermometer or device under testhas exposed conductors or electrical contacts wetted by thefluid. In general, the dielectric strength of the fluids should beas high as practical when testing
49、resistance thermometers.Fluids that absorb moisture over time (for example, isopropylalcohol) should be periodically checked and either replaced ortreated to remove the moisture.(5) Thermal ConductivityThe thermal conductivity ofthe bath fluid medium should be relatively high. This will keepbath temperature gradients within the fluid as small as possible,and will also subject the device under test to a more uniformtemperature over its immersed surface.(6) Specific HeatIt is not always possible to select fluidsthat have ideal properties for ea
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