ASTM E2488-22

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E2488 − 09 (Reapproved 2014) E2488 − 22
Standard Guide for
the Preparation and Evaluation of Liquid Baths Used for
Temperature Calibration by Comparison
This standard is issued under the fixed designation E2488; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Many of the Standards and Test Methods under the jurisdiction of ASTM committee E20 on
Temperature Measurement make reference to the use of controlled temperature fluid baths for the
calibration of thermometers by the comparison method. In this method the thermometer under test is
measured while immersed in an isothermal medium whose temperature is simultaneously determined
by a calibrated reference thermometer. The uncertainty of all such comparison calibrations depends
upon how well the isothermal conditions can be maintained. The bath temperature must be stable over
time and uniform within the working space at the operating temperatures. This guide provides basic
information, options and instructions that will enable the user to prepare and evaluate controlled
temperature baths for calibrations.
1. Scope
1.1 This guide is intended for use with controlled temperature comparison baths that contain test fluids and operate within the
temperature range of –100°C–100 °C to 550°C.550 °C.
1.2 This guide describes the essential features of controlled temperature fluid baths used for the purpose of thermometer
calibration by the comparison method.
1.3 This guide does not address the details on the design and construction of controlled-temperature fluid baths.
1.4 This guide describes a method to define the working space of a bath and evaluate the temperature variations within this space.
Ideally, the working space will be as close as possible to isothermal.
1.5 This guide does not address fixed point baths, ice point baths, or vapor baths.
1.6 This guide does not address fluidized powder baths.
1.7 This guide does not address baths that are programmed to change temperature.
1.8 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
This guide is under the jurisdiction of ASTM Committee E20 on Temperature Measurement and is the direct responsibility of Subcommittee E20.07 on Fundamentals
in Thermometry.
Current edition approved Dec. 1, 2014Nov. 15, 2022. Published December 2014December 2022. Originally approved in 2009. Last previous edition approved in 20092014
as E2488 – 09.E2488 – 09 (2014). DOI: 10.1520/E2488-09R14.10.1520/E2488-22.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2488 − 22
1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.10 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E1 Specification for ASTM Liquid-in-Glass Thermometers
E344 Terminology Relating to Thermometry and Hydrometry
E644 Test Methods for Testing Industrial Resistance Thermometers
E839 Test Methods for Sheathed Thermocouples and Sheathed Thermocouple Cable
2.2 Other Documents:
ITS-90 The International Temperature Scale of 1990
NIST Monograph 126 Platinum Resistance Thermometry
NIST Monograph 150 Liquid-in-Glass Thermometry
NIST SP 250-22 Platinum Resistance Thermometer Calibrations
NIST SP 250-23 Liquid-in-Glass Thermometer Calibration Service
2.3 Military Standards:
MIL-STD-202G Test Methods for Electronic and Electrical Component Parts
3. Terminology
3.1 Standard terms used in this guide are defined in Terminology E344.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 bath gradient error, n—the error caused by temperature differences within the working space of the bath.
3.2.2 immersion error, n—an error caused by heat conduction, radiation or both between the temperature sensing portion of the
sensor used in the bath and the environment external to the measurement system. Immersion error is caused by an incorrect
immersion length and the resulting incorrect thermal contact of the temperature sensing portion of the sensor with the medium
under measurement.
3.2.3 isothermal, adj—of, related to, or designating a region of nominally uniform temperature.
3.2.4 thermal stability, n—the degree of variability of the temperatures within a specified working space over a specified time
interval.
3.2.5 working space, n—the region within a controlled temperature bath where the temperature uncertainty is maintained within
acceptable limits for the purpose of performing calibrations by the comparison method.
3.2.6 working temperature range, n—the minimum to maximum temperature range for which the bath system provides adequate
stability and uniformity.
4. Summary of Practice
4.1 This guide is intended to provide basic information that will enable the user to evaluate various controlled temperature bath
features and to enable the user to prepare and properly utilize such controlled temperature baths for calibration of thermometers
by the comparison method.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Preston-Thomas, H., METROLOGIA, Vol. 27, 1990, pp 3-10 and 107 (errata). Mangum, B. W., JOURNAL OF RESEARCH, National Institute of Standards and
Technology, Vol 95, 1990, p. 69.
Available from National Institute of Standards and Technology (NIST), 100 Bureau Dr., Stop 1070, Gaithersburg, MD 20899-1070, http://www.nist.gov.
E2488 − 22
5. Significance and Use
5.1 The design of a controlled temperature bath will determine what thermometers can be calibrated and to what extent an
isothermal condition is achieved. The lack of thermal stability and uniformity of the bath are sources of error that contribute to
the overall calibration uncertainty.
5.2 This guide describes a procedure for determining the effective working space for a controlled temperature fluid bath.
5.3 This guide describes a procedure for determining the thermal stability within a controlled temperature fluid bath. Overall
thermal stability is composed of the bath performance as specified by the manufacturer of the bath equipment and as a component
of calibration uncertainty.
5.4 This guide describes a procedure for determining the temperature uniformity of the working space of the controlled
temperature fluid bath.
6. Procedure
6.1 Bath System—A controlled temperature fluid bath system will incorporate most, if not all, of the following components: a fluid
medium; a mechanical design that provides for containment and circulation of the fluid; a monitoring thermometer,thermometer;
a temperature control unit; and,and elements that provide for heating, cooling, or both. There are many commercially available
controlled temperature baths. These baths operate from as low as -100°C-100 °C to as high as +550°C;+550 °C, although no single
bath system is capable of operation over that entire range. The design of each individual bath will create practical limits for the
working temperature range. These limits are determined by considering the minimum and maximum temperature ratings for each
of the components in the bath system. The user is advised to carefully review the bath manufacturer’s literature to be certain that
the bath system is suitable for the intended calibration temperature range and the types of thermometers to be tested. Figs. 1-4
represent various designs of controlled temperature fluid bath systems. Fig. 4 shows a block diagram of a comparison calibration
setup.
6.1.1 Fluid Medium—There are many types of fluid media suitable for use in liquid temperature comparison baths. The physical
properties of the medium will establish the limits for the safe operating temperature range, as well as determine the overall
performance of the bath system. Fig. 5 provides a partial listing of common bath media that have been used successfully for liquid
temperature comparison baths. This guide is not intended to restrict the user to only those fluids shown in Fig. 5. It is advisable
for the user to review carefully the manufacturer’s literature on any alternative fluid to be certain that it complies with the safety
considerations of 6.1.1.1.
6.1.1.1 Safety and Environmental Impact Considerations—(See 1.81.9.) It is strongly recommended that the Material Safety Data
Sheet (MSDS) of any material used as a fluid medium be reviewed and understood by the user before the material is handled for
FIG. 1 Alternative Designs of Top Stirred Comparison Baths
Without Controllers.Controllers
E2488 − 22
FIG. 2 Sample Design of Bottom-Stirred Comparison Bath with
Controller
FIG. 3 Sample Design of Comparison Bath with Integral Heating,
Cooling and Controller
the first time. The data sheets of all test fluids should be kept readily available during bath operation in case of accidents or spills.
Additionally, some producers of bath fluids provide a Global Warming Potential Index in their specifications that should be
considered when choosing a bath fluid.
(1) Temperature Limits—Fig. 5 provides minimum and maximum safe operating temperatures for several common bath media.
Flash point temperatures are also given for certain flammable media. Consult the manufacturer’s MSDS document for each bath
fluid used.
(2) Flammability—Fluids are easily ignited above their flash point. Whenever possible, the bath fluid shall be maintained
below the specified flash point. Some fluids are flammable at room temperature so the user must exercise caution to prevent the
exposure of these fluids to open flames or sparks. As a general safety practice, a fire suppression system (for example, extinguisher,
blankets, hoods, lids, etc.) should always be readily available when operating a bath with flammable media.
(3) Ventilation—Proper ventilation, such as exhaust hoods or vents, is required to remove any fumes or vapors that may be
toxic or otherwise harmful to the operators performing the calibration.
(4) Toxicity—Protective clothing and shielding shall be required for operators who must handle fluids that are environmentally
hazardous or toxic. Proper disposal of excess fluids, spills, residues, or materials contaminated by the fluids shall be in accordance
with all regulatory policies.
(5) Chemical Stability—The bath fluid shall be chemically stable at the operating temperatures and inert to both the container
and the components or elements submitted to comparison testing. Warning—The salts or molten metals used for calibration at
high temperatures (above 260°C)260 °C) are particularly corrosive to many materials. Special care should be taken to determine
the compatibility of the materials used in construction of the thermometer. DISCUSSION: Chemical instability may change the
E2488 − 22
FIG. 4 Block Diagram of Comparison Calibration Setup
FIG. 5 Typical Bath Fluid Media and Useful Operating Temperature Ranges.Ranges
properties of a bath fluid in one or more of the following ways: (1) Safety—The flash point of the bath fluid may change over time
due to the chemical decomposition, breaking of chemical bonds, caused by repeated use at high temperatures. (2) Performance—A
bath fluid that is subject to polymerization when exposed to high temperatures for extended periods will become more viscous.
The increase in viscosity can degrade performance and will make maintenance and cleanup very difficult.
NOTE 1—Chemical instability may change the properties of a bath fluid in one or more of the following ways: (1) Safety—The flash point of the bath
E2488 − 22
fluid may change over time due to the chemical decomposition, breaking of chemical bonds, caused by repeated use at high temperatures. (2)
Performance—A bath fluid that is subject to polymerization when exposed to high temperatures for extended periods will become more viscous. The
increase in viscosity can degrade performance and will make maintenance and cleanup very difficult.
(6) Expansion of Fluids—The bath system must be designed to provide sufficient room for the expansion of fluids when heated
so that spills and overflows do not occur. It is also important to consider that fluids will contract when being refrigerated to very
low temperatures and then expand when allowed to return to room temperature.
(7) Cross Contamination Between Baths—Proper caution must be taken to avoid the mixing of test fluids when thermometers
are transferred from one adjacent bath to another during multiple calibrations. Depending upon the fluids and temperatures
involved, this can be a minor problem compromising bath performance, or it can be a major safety issue. For example, fluids at
temperatures above 100ºC100 ºC may react violently if water or a wet object is immersed into them. The introduction of water or
organic materials into a molten salt bath can also produce violent reactions.
6.1.1.2 Performance Considerations —Considerations—The physical properties of the fluid media will determine the overall
performance of the bath system.
(1) Fluid Viscosity—The fluid viscosity can vary greatly over a wide temperature range and this can lead to problems with
stirring, agitation, or the establishment of undesirable temperature gradients. In practice, a bath fluid with a viscosity of ten
centistokes, or less, usually provides good stirring and mixing action. When the viscosity of the fluid becomes 50 centistokes, or
greater, the stirring and mixing becomes less effective and the possibility of temperature gradients is increased. High viscosity also
leads to excessive fluid drag-out. These viscosity numbers are intended only as a general observation. The selection of a test fluid
should be based upon a careful consideration of many factors.
(2) Volatility—Fluids operated near their boiling points, or that have a high vapor pressure under normal laboratory
environments will present a problem in controlling the bath temperature. Evaporation from the surface of the liquid will produce
an undesirable temperature gradient because of the increased cooling effect at the surface. In addition, the loss of bath fluid over
time due to evaporation will cause the length of immersion of the thermometers to vary unless the bath design is such that it
replenishes the lost fluid.
(3) Moisture Condensation—Refrigerated baths can cause atmospheric moisture to condense on the surfaces above the fluid.
Precipitation of this moisture into the test fluid can seriously degrade the performance of the bath system.
(4) Dielectric Properties—The volume resistivity, dielectric strength and dielectric constant of the fluid are important
considerations whenever the thermometer or device under test has exposed conductors or electrical contacts wetted by the fluid.
In general, the dielectric strength of the fluids should be as high as practical when testing resistance thermometers. Fluids that
absorb moisture over time (for example, isopropyl alcohol) should be periodically checked and either replaced or treated to remove
the moisture.
(5) Thermal Conductivity—The thermal conductivity of the bath fluid medium should be relatively high. This will keep bath
temperature gradients within the fluid as small as possible, and will also subject the device under test to a more uniform
temperature over its immersed surface.
(6) Specific Heat—It is not always possible to select fluids that have ideal properties for each bath system or range of
temperatures that will be encountered. Some compromises in the selection of bath fluids must be expected. However, when all
other factors are essentially equal, it is advisable to select the fluid with the highest specific heat. A fluid with a higher specific
heat means that for a given amount of heat loss or gain to the bath system, a smaller change in the temperature of the bath fluid
will result.
6.1.2 Mechanical Design—The mechanical design of the bath and the materials employed in its construction determine the limits
for the working temperature range as well as the overall stability and uniformity of the bath. A poorly designed bath will not
provide the desired levels of uncertainty needed for precision calibration. However, the performance of a marginal bath system may
be improved to acceptable levels of uncertainty by the use of an equalizing block immersed in the bath fluid.
6.1.2.1 Container—The material used to contain the fluid in a bath system must be compatible with the medium selected. Most
commercial baths are constructed of a borosilicate glass or a stainless steel. These materials provide chemical resistance to a wide
range of fluids and operating temperatures. Nevertheless, it is advisable that the user checks the chemical compatibility of the
desired test fluid with the container materials and to avoid all circumstances where chemical reaction or corrosion can exist.
6.1.2.2 Insulation and Non-metallic Materials—The types and temperature ratings of insulation materials used in the construction
of the bath system will establish the limits for the working temperature range. Gaskets, seals, and plastic insulating materials used
in bath construction will become brittle or cracked when exposed to temperatures below their minimum temperature rating. These
materials will also be susceptible to swelling, deformation, or melting when exposed to temperatures that are above their high
temperature rating. Materials that may be damaged by excessive temperature exposures can lead to fluid leaks, loss of thermal
insulating properties or loss of electrical insulating properties.
6.1.2.3 Circulation or Mixing—Stirring and circulation of the fluids is very important in order to minimize gradients within the
E2488 − 22
bath container. There are baths designed to provide a laminar flow of the bath fluid—examples of this type may be found in Test
Methods E644 and Method E839—however, these baths are usually used for thermal response-time testing and not used for
calibration of thermometers by the comparison method. Many commercially available calibration baths are designed to provide
both vertical and horizontal mixing of the bath fluid to minimize gradients within the media. Circulating pumps or impellers
mounted on motorized shafts typically provide the mixing action. There are limits to the amount of stirring that is desirable for
a given bath design. Too much agitation of a liquid can lead to foaming or frothing as air mixes into the medium. Fluid separation
(cavitation) may occur if the fluid flow rate is too high. Such turbulent liquid flow can result in temperature instability. Too little
stirring will obviously lead to the establishment of gradients and thus poor uniformity.
6.1.2.4 Heating and Cooling Systems—Commercial baths may contain heating elements, cooling elements, or both, depending
upon the desired range of operating temperatures. Heating or cooling elements, or both, should be carefully selected so that the
amount of heat transfer to the bath fluid is ideal. Too much heat transfer to or from the fluid medium will result in excessive
temperature variation over time (for example, poor stability of bath temperature). If the cooling elements are built into the bath,
then the user must be aware of the upper and lower temperature limits imposed by the refrigerant gases or liquids used in the
system. Widely separated heating and cooling elements within a bath container can lead to significant temperature gradients.
Modern, commercially available, precision laboratory baths offer integral heating and cooling elements or surfaces (see Fig. 3).
Such integrated elements or surfaces give the appearance of a single thermal energy source and this has the effects of reducing
temperature gradients and improving bath stability.
6.1.2.5 Heat Capacity—The volume of the bath relative to the volume of thermometers or other devices immersed in the bath must
be sufficient so that the work introduced does not significantly affect the temperature of the fluid. Classically, a bath with a volume
equal to 1000 times the volume displaced by the units under test has been considered ideal. This may not always be practical to
achieve for a variety of reasons, but the test operator must be aware of the limitations imposed on the bath system and factor this
into uncertainty reporting. The depth of bath container must also be large enough to accommodate the minimum immersion depth
of t
...