ASTM E2593-17(2023)

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.
Designation: E2593 − 17 (Reapproved 2023)
Standard Guide for
Accuracy Verification of Industrial Platinum Resistance
Thermometers
This standard is issued under the fixed designation E2593; 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.
1. Scope 2. Referenced Documents
1.1 This guide describes the techniques and apparatus re- 2.1 ASTM Standards:
quired for the accuracy verification of industrial platinum E344 Terminology Relating to Thermometry and Hydrom-
resistance thermometers constructed in accordance with Speci- etry
fication E1137/E1137M and the evaluation of calibration E563 Practice for Preparation and Use of an Ice-Point Bath
uncertainties. The procedures described apply over the range of as a Reference Temperature
-200 °C to 650 °C. E644 Test Methods for Testing Industrial Resistance Ther-
mometers
1.2 This guide does not intend to describe procedures
E1137/E1137M Specification for Industrial Platinum Resis-
necessary for the calibration of platinum resistance thermom-
tance Thermometers
eters used as calibration standards or Standard Platinum
E1502 Guide for Use of Fixed-Point Cells for Reference
Resistance Thermometers. Consequently, calibration of these
Temperatures
types of instruments is outside the scope of this guide.
E1750 Guide for Use of Water Triple Point Cells
1.3 Industrial platinum resistance thermometers are avail- E2623 Practice for Reporting Thermometer Calibrations
able in many styles and configurations. This guide does not E2488 Guide for the Preparation and Evaluation of Liquid
purport to determine the suitability of any particular design, Baths Used for Temperature Calibration by Comparison
style, or configuration for calibration over a desired tempera-
2.2 ANSI Publications:
ture range.
ANSI/NCSL Z540.2-1997 U.S. Guide to the Expression of
Uncertainty in Measurement
1.4 The evaluation of uncertainties is based upon current
ANSI/NCSL Z540.3-2006 Requirements for the Calibra-
international practices as described in JCGM 100:2008 “Evalu-
tions of Measuring and Test Equipment
ation of measurement data—Guide to the expression of uncer-
tainty in measurement” and ANSI/NCSL Z540.2-1997 “U.S.
2.3 Other Publication:
Guide to the Expression of Uncertainty in Measurement.”
JCGM 100:2008 Evaluation of measurement data—Guide to
the expression of uncertainty in measurement
1.5 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3. Terminology
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
3.1 Definitions—The definitions given in Terminology E344
mine the applicability of regulatory limitations prior to use.
shall be considered as applying to the terms used in this guide.
1.6 This international standard was developed in accor-
3.2 Definitions of Terms Specific to This Standard:
dance with internationally recognized principles on standard-
3.2.1 annealing, v—a heat treating process intended to
ization established in the Decision on Principles for the
stabilize resistance thermometers prior to calibration and use.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
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
This guide is under the jurisdiction of ASTM Committee E20 on Temperature the ASTM website.
Measurement and is the direct responsibility of Subcommittee E20.03 on Resistance Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
Thermometers. 4th Floor, New York, NY 10036, http://www.ansi.org.
Current edition approved July 1, 2023. Published July 2023. Originally approved JCGM 100:2008, Evaluation of measurement data—Guide to the expression of
in 2007. Last previous edition approved in 2017 as E2593 – 17. DOI: 10.1520/ uncertainty in measurement. Available from the BIPM, Sevres, France, http://
E2593-17R23. www.bipm.org/en/publications/guides/gum.html.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2593 − 17 (2023)
3.2.2 check standard, n—a thermometer similar in design to 3.2.15 test uncertainty ratio (TUR), n—the ratio of the
the unit under test, but of superior stability, which is included tolerance of the unit under test to the expanded calibration
in the calibration process for the purpose of quantifying the
uncertainty.
process variability.
3.2.16 uncertainty budget, n—an analysis tool used for
3.2.3 coverage factor, n—numerical factor used as a multi-
assembling and combining component uncertainties expected
plier of the combined standard uncertainty in order to obtain an
in a measurement process into an overall expected uncertainty.
expanded uncertainty.
3.2.17 unit under test (UUT), n—the platinum resistance
3.2.4 dielectric absorption, n—an effect in an insulator
thermometer to be calibrated.
caused by the polarization of positive and negative charges
within the insulator which manifests itself as an in-phase
4. Summary of Guide
current when the voltage is removed and the charges recom-
bine. 4.1 The UUT is calibrated by determining the electrical
resistance of its sensing element at one or more known
3.2.5 expanded uncertainty, U, n—quantity defining an
temperatures covering the temperature range of interest. The
interval about the result of a measurement that may be
known temperatures may be established by means of fixed-
expected to encompass a large fraction of the distribution of
point systems or by using a reference thermometer. Either an
values that could reasonably be attributed to the measurand.
SPRT or a PRT is recommended for use as the reference
3.2.5.1 Discussion—Typically, U is given at a coverage
thermometer. However, a liquid in glass (LIG) thermometer,
factor of 2, approximating to a 95.45 % confidence interval for
thermistor, or thermocouple may be acceptable, depending
a normal distribution.
upon the temperature of calibration, required accuracy, or other
3.2.6 hysteresis, n—property associated with the resistance
considerations.
of a thermometer whereby the value of resistance at a tempera-
ture is dependent upon previous exposure to different tempera-
4.2 The success of the calibration depends largely upon the
tures.
ability of the UUT to come to thermal equilibrium with the
3.2.7 normal distribution, n—a frequency distribution char- calibration temperature of interest (fixed point cell or compari-
acterized by a bell-shaped curve and defined by two param- son system) and upon accurate measurement of the sensing
eters: mean and standard deviation.
element resistance at that time. Instructions are included to
guide the user in achieving thermal equilibrium and proper
3.2.8 platinum resistance thermometer (PRT), n—a resis-
resistance measurement, including descriptions of apparatus
tance thermometer with the resistance element constructed
and instrumentation.
from platinum or platinum alloy.
3.2.9 rectangular distribution, n—a frequency distribution
4.3 Industrial platinum resistance thermometers are avail-
characterized by a rectangular-shaped curve and defined by
able in many styles and configurations. This guide includes
two parameters: mean and magnitude (semi-range).
limited instructions pertaining to the preparation of the UUT
into a configuration that facilitates proper calibration.
3.2.10 standard deviation of the mean, n—an estimate of the
standard deviation of the sampling distribution of means, based
4.4 Proper evaluation of calibration uncertainties is critical
on the data from one or more random samples.
for the result of a calibration to be useful. Therefore, a
3.2.10.1 Discussion—Numerically, it is equal to the stan-
considerable portion of this guide is devoted to uncertainty
dard deviation obtained (s) when divided by the square root of
budgets and the evaluation of uncertainties.
the size of the sample (n).
s
5. Significance and Use
Standard Deviation of the Mean 5 (1)
=n
5.1 This guide is intended to be used for verifying the
3.2.11 standard platinum resistance thermometer (SPRT),
resistance-temperature relationship of industrial platinum re-
n—a specialized platinum resistance thermometer constructed
sistance thermometers that are intended to satisfy the require-
in such a way that it fulfills the requirements of the ITS-90.
ments of Specification E1137/E1137M. It is intended to pro-
3.2.12 standard uncertainty, n—uncertainty of the result of
vide a consistent method for calibration and uncertainty
a measurement expressed as a standard deviation, designated evaluation while still allowing the user some flexibility in the
as S.
choice of apparatus and instrumentation. It is understood that
the limits of uncertainty obtained depend in large part upon the
3.2.13 Type A evaluation (of uncertainty), n—method of
apparatus and instrumentation used. Therefore, since this guide
evaluation of uncertainty by the statistical analysis of a series
is not prescriptive in approach, it provides detailed instruction
of observations.
in uncertainty evaluation to accommodate the variety of
3.2.14 Type B evaluation (of uncertainty), n—method of
apparatus and instrumentation that may be employed.
evaluation of uncertainty by means other than statistical
analysis of a series of observations.
5.2 This guide is intended primarily to satisfy applications
requiring compliance to Specification E1137/E1137M.
However, the techniques described may be appropriate for
Mangum, B. W., NIST Technical Note 1265, Guidelines for Realizing the
International Temperature Scale of 1990 (ITS-90). applications where more accurate calibrations are needed.
E2593 − 17 (2023)
5.3 Many applications require tolerances to be verified internal computation ability, performing both temperature and
using a minimum test uncertainty ratio (TUR). This standard statistical calculations. The use of DC offset compensation is
provides guidelines for evaluating uncertainties used to support
recommended. Caution must be exercised to ensure that the
TUR calculations.
excitation current is appropriate for the UUT and reference
thermometer to avoid excessive self-heating. Periodic calibra-
6. Sources of Error
tion is required.
6.1 Uncertainties are present in all calibrations. Errors arise 7.1.4 Reference Resistor—Reference resistors are specially
when the effects of uncertainties are underestimated or omitted.
designed and manufactured to be stable over long periods of
The predominant sources of uncertainty are described in
time. Typically, they have significant temperature coefficients
Section 12 and listed in Table 2.
of resistance and require maintenance in a temperature-
enclosed air or oil bath. Some have inductive and capacitive
7. Apparatus
characteristics that limit their suitability for use with AC
bridges. Periodic (yearly or semi-yearly) calibration is re-
7.1 Resistance Measuring Instruments—The choice of a
quired. Resistors (AC or DC) are required to match the type of
specific instrument to use for measuring the UUT and reference
measurement (AC or DC) system in use.
thermometer resistance will depend upon several factors. Some
of these factors are ease of use, compatibility with computer-
7.2 Reference Thermometers—The choice of a specific in-
ized data acquisition systems, method of balancing, computa-
strument to use as the reference thermometer will depend upon
tion ability, and so forth. All of the instruments listed are
several factors, including the uncertainty desired, temperature
commercially available in high precision designs and are
range of interest, compatibility with existing instrumentation
suitable for use. They require periodic linearity checks or
and apparatus, expertise of staff, cost limitations, and so forth.
periodic calibration. (Refer to Appendix X2 for detailed
All of the instruments listed are commercially available in
descriptions and schematics.) The uncertainty of the resistance
various levels of precision and stability and may be suitable for
measurements directly impacts the uncertainty of the tempera-
use. They all require calibration. The frequency of calibration
ture measurement as shown in Eq 2.
depends a great deal upon the manner in which they are used
Uncertainty
Ω
and the uncertainty required in use.
Uncertainty 5 (2)
t
Sensitivity
7.2.1 SPRT—SPRTs are the most accurate reference ther-
where:.
mometers available and are used in defining the ITS-90 from
approximately -260 °C to 962 °C. The SPRT sensing element
Uncertainty = equivalent temperature uncertainty at tem-
t
is made from nominally pure platinum and is supported
perature (t), °C,
Uncertainty = resistance uncertainty at temperature (t), Ω, essentially strain-free. These instruments are extremely deli-
Ω
and cate and are easily damaged by mechanical shock. They are
-1
Sensitivity = sensitivity at temperature (t), Ω °C
available sheathed in glass or metal and in long-stem and
capsule configurations. The design and materials of construc-
7.1.1 Bridge—Precision bridges with linearity specifications
tion limit the temperature range of a specific instrument type.
ranging from 10 ppm of range to 0.01 ppm of range and with
1 1 Some sheath materials can be damaged by use at high
6 ⁄2 to 9 ⁄2 digit resolution are available. These instruments are
temperatures in metal blocks or molten salt baths. Calibration
available in models using either alternating current (AC) or
on the ITS-90 is required.
direct current (DC) excitation. The linearity is typically based
upon resistive or inductive dividers and is generally quite 7.2.2 Secondary Reference PRT—Secondary Reference
stable over time. Modern bridges are convenient automatic PRTs are specially manufactured PRTs designed to be suitable
balancing instruments but manual balancing types are also calibration standards. These instruments are typically less
suitable. These instruments typically require external reference
delicate than SPRTs but have higher measurement uncertainties
resistors and do not perform temperature calculations.
and narrower usage ranges. They are typically sheathed in
7.1.2 Digital Thermometer Readout—Digital instruments
metal to allow immersion directly into metal furnaces or
designed specifically to measure resistance thermometers are
molten salt baths. Calibration on the ITS-90 is required.
available. Modern versions function essentially as automatic
7.3 Fixed Point Systems—Fixed point systems are required
potentiometers and reverse the current to minimize DC offset
in the ITS-90 calibration of SPRTs. Very low uncertainties are
errors caused by thermal emf and residual voltages inherent to
attainable with these systems, but their complex procedures
the electronic components and related circuitry. Precision
and design criteria may limit their application to other types of
instruments with linearity specifications ranging from 20 ppm
thermometers. However, certain adaptations are suitable for the
1 1
of indication to 1 ppm of indication and with 6 ⁄2 to 8 ⁄2 digit
calibration of industrial platinum resistance thermometers.
resolution are commercially available. Some models have
7.3.1 Triple Point of Water (TPW) Cell and Apparatus—The
extensive internal computation capability, performing both
triple point of water cell is a critical thermometric fixed point
temperature and statistical calculations. Periodic calibration is
for calibration and control of SPRTs. These devices can be
required.
7.1.3 Digital Multimeter (DMM)—Digital multimeters are useful in the calibration of industrial resistance thermometers
but typically are not used because of limited throughput
convenient direct indication instruments typically able to
indicate in resistance or voltage. Some models have extensive capabilities. For further information refer to Guide E1750.
E2593 − 17 (2023)
7.3.2 Freezing-point Cell and Furnace—Metal-freezing 7.4.4 Dry-well Bath—Furnaces with built-in thermometer
point cells are used in the calibration of SPRTs and thermo- readouts can be used as the heat source for comparison
couples. These devices can be useful in the calibration of calibrations. Typically, these instruments are useful over the
industrial platinum resistance thermometers but typically are temperature range of -40 °C to 650 °C. The attainable uncer-
not used because of limited throughput capabilities. For further tainty is limited primarily by the temperature uniformity in the
information refer to Guide E1502. block and conduction losses up the stem of the reference
thermometer or UUT. For best results, the thermometer wells
7.3.3 Ice-point Bath—The ice point is a relatively simple to
should be deep and of the correct diameter to allow a slip fit of
realize fixed point that is useful in the calibration of resistance
the reference thermometer or UUT.
thermometers. The ice point bath can be used as a fixed point
with uncertainties attributed to the care of construction and
8. Preparation of UUT
maintenance. For further information refer to Practice E563.
8.1 Physical Configuration—UUTs that are not already
7.4 Comparison Apparatus—The choice of a specific com-
sheathed shall be assembled into protection tubes before
parison apparatus to use will depend primarily upon two
calibration. Closed-end glass or thin wall metal tubing of
factors: the temperature range of interest and the uncertainty
adequate length to allow sufficient immersion is recommended.
required. Secondary factors include ease of use, compatibility
A diameter that allows a slip fit without being too tight should
with computerized data acquisition systems or automation
be chosen. Ensure that the tube is clean and dry before
capability, flexibility, cost, and so forth. All of the apparatus
assembly. A thermally conductive filler material may be used
listed are commercially available in various levels of perfor-
within the sheath between the sensor and sheath to enhance
mance and are suitable for use. They may or may not require
thermal conductivity if desired. Ensure that the material will
periodic calibration.
not damage the sensor. The sensor lead wires are welded or
7.4.1 Liquid Bath—Liquid baths can be used as the heat
soldered to extension wires in 4-wire configuration (unless a
source for comparison calibrations. Typically, these instru-
2-wire or 3-wire calibration is specifically required) and the
ments are useful over the temperature range of -100 °C to
assembly inserted into the tube. If the connections are made
550 °C. The actual range of any one bath is limited by the
using solder, ensure that the solder is compatible with the
construction of the bath and the bath fluid. Bath fluids typically
temperature range over which the UUT will be calibrated.
have narrower temperature ranges than the baths themselves,
Additionally, if DC measurements are used, the connectors and
requiring changes in fluid or multiple baths to cover a typical
solder type should be chosen to minimize thermal emf. The
calibration range. The attainable uncertainty is limited primar-
insulation of the extension wires and the connection itself must
ily by the temperature uniformity and stability of the bath fluid.
also be suitable for the temperature range over which the
For further information refer to Guide E2488.
calibration will be performed. The assembled UUT should be
7.4.2 Liquid Nitrogen Comparison Bath—A liquid nitrogen
affixed to the tube at the point where the extension wires exit
comparison bath is essentially a high quality dewar with an
the tube to ensure that the UUT does not slide up the tube
equilibration block suspended in liquid nitrogen. Because
during calibration. If the UUT is to be calibrated below 0 °C,
liquid nitrogen will stratify within the dewar, large temperature
the tube should be dried internally and sealed to prevent water
gradients will exist without the use of an equilibration block.
vapor from condensing into the sheath.
Consequently, a block is required. Instrument-grade liquid
nitrogen is widely available and has a normal boiling point of
8.2 Annealing—Annealing is not recommended for routine
approximately -196.5 °C. Since the purity of the liquid nitro-
tolerance verification unless requested by the user or instructed
gen and the atmospheric pressure are unknown, the tempera-
otherwise. Before any annealing is undertaken, consult the
ture of the comparison bath must be established with a
manufacturer of the UUT or other technical expert knowledge-
reference thermometer. The attainable uncertainty is limited
able in the design and limitations of the UUT. (The stability of
primarily by the temperature uniformity in the block, the
the thermometer can be observed by cycling between the ice
conduction losses up the stem of the reference thermometer or
point and a maximum or minimum temperature.) An annealing
UUT, and the stability of the system due to changes in
procedure that can improve the performance of some UUTs
barometric pressure and other factors.
may prove useless or even detrimental to others. If annealing is
attempted, a record of the UUT resistance at the triple point of
7.4.3 Equilibration Block—Although not a comparison ap-
paratus per se, the equilibration block is utilized to enhance the water R or ice-point R (as applicable) at each step of
TPW 0
performance of a comparison bath. An equilibration block is a annealing is required to monitor UUT stability and the results
high thermal conductivity block suspended in the comparison of annealing.
bath within which the PRTs and reference instrument are
8.3 Immersion Length Test—If the minimum immersion
inserted. The block should be cylindrical and contain enough
length of the UUT is unknown, it must be determined in
holes to hold the reference thermometer, check standard, and
accordance with Section 7 of Test Methods E644.
several UUTs. Additionally, the block should be of sufficient
depth to completely cover the sensitive portions of all ther- 8.4 Insulation Resistance Test—The insulation resistance
mometers involved. The block material must be chemically should be tested in accordance with Section 5 of Test Methods
compatible with the bath fluid. Recommended materials in- E644 using the criteria of Section 9 of Specification E1137/
clude oxygen-free copper, low oxygen copper, and aluminum. E1137M.
E2593 − 17 (2023)
9. Procedure the bath fluid will cause a decrease in the fluid depth as the
temperature is reduced.
9.1 The number, location, and sequence of temperature
points required for UUT calibration depends upon the uncer- 9.6 Temperature Measurement—The specific steps required
tainty required, the suitability of the mathematical model, and to obtain a temperature measurement depend upon the type of
the hysteresis exhibited by the UUT. Thus, the specific cali- reference thermometer and readout instrument employed. The
bration points and sequence are best determined through following steps provide a general outline. Allow sufficient time
experimentation. Once determined for a specific design of for the system to stabilize and equilibrate. This is easily
UUT, the measurement strategy can be used in subsequent observed if the readout instrument has graphing capabilities or
calibrations provided the results remain satisfactory. It is is connected to a computer system with graphing capabilities.
recommended that the redundant points be included in an effort Otherwise, the readout indication shall be observed until
to reveal hysteresis or stability problems. If hysteresis and stability is achieved. Once a steady state has been achieved,
instability are small compared to the overall tolerance, the perform several individual temperature measurements using
redundant points may be omitted. Refer to Table 1 for the reference thermometer and calculate the mean, standard
recommended points and sequence. Also, it is immaterial if deviation, and standard deviation of the mean (sample size ≥
these measurements are performed in fixed-point systems or by 36 is recommended). The mean represents the measured value.
comparison. If several UUTs are to be calibrated per run, The standard deviation is used to compute the standard
comparison calibration is usually more efficient. The following deviation of the mean as shown in Eq 1. The standard deviation
procedure assumes concurrent calibration of several UUTs by of the mean represents the measurement noise (or precision of
comparison. If fixed-point systems are being used at one or measurement, item 12.2.1d in Table 3). If the values obtained
more temperature points, each UUT must be calibrated at that are within the uncertainty limits allowed, proceed with mea-
temperature point individually and the procedure shall be surements of the UUTs. (Some readout instruments allow
adjusted accordingly. simultaneous measurement of the reference and UUTs. If this
is the type of instrument being used, steps 9.6 – 9.8 are
9.2 Connection of the UUTs—If a direct resistance measure-
combined with the statistics calculated in real time.)
ment scheme is being used, connect the UUTs to the measure-
ment system. Use a 4-wire configuration (unless a 2-wire or 9.7 Measurement of UUTs—Measure the resistance of the
3-wire calibration is specifically required) and observe polarity. check standard and each UUT. As with the measurement of the
If a potentiometric measurement scheme is being used, connect reference thermometer, these measurements should consist of
the UUT current leads in series to the current supply and the several individual measurements. Calculate the mean, standard
voltage leads to the switch system, potentiometer or digital deviation, and standard deviation of the mean. The mean
multimeter DMM input, observing polarity. Refer to Appendix represents the measured value. The standard deviation is used
X2 for guidance if necessary. to compute the standard deviation of the mean as shown in Eq
1. The standard deviation of the mean represents the measure-
9.3 Connection of the Check Standard—Connect the check
ment noise (or precision of measurement, item 12.2.3b in Table
standard to the measurement system in the same manner as the
3). If the values obtained are within the uncertainty limits
UUTs.
allowed, proceed with a second (closure) measurement of the
9.4 Connection of the Reference Thermometer—Connect the
temperature. The number of UUTs measured between refer-
reference thermometer to the R input of the measurement
X
ence thermometer measurements depends on the stability of the
instrument and, if applicable, the reference resistor to the R
S
calibration medium and the speed of the measurement system.
input. A single instrument may be used to measure the UUTs,
Refer to Appendix X1 for guidance on PRTs not in a 4-wire
the reference thermometer, and the check standard, if appli-
configuration.
cable. Refer to Appendix X2 for guidance if necessary.
9.5 Insertion into Comparison Bath—Insert the reference
TABLE 2 Uncertainty Summary
thermometer, check standard, and UUTs into the com
...