ASTM E2593-07

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: E 2593 – 07
Standard Guide for
Accuracy Verification of Industrial Platinum Resistance
Thermometers
This standard is issued under the fixed designation E 2593; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 644 Test Methods for Testing Industrial Resistance Ther-
mometers
1.1 This guide describes the techniques and apparatus re-
E 1137/E 1137M Specification for Industrial Platinum Re-
quired for the accuracy verification of industrial platinum
sistance Thermometers
resistance thermometers constructed in accordance with Speci-
E 1502 GuideforUseofFreezing-PointCellsforReference
fication E 1137/E 1137M and the evaluation of calibration
Temperatures
uncertainties.Theproceduresdescribedapplyovertherangeof
E 1750 Guide for Use of Water Triple Point Cells
–200 °C to 650 °C.
2.2 ANSI Publication:
1.2 This guide is not intended to describe the procedures
ANSI/NCSL Z540-2-1997 U.S. Guide to the Expression of
necessary for the calibration of platinum resistance thermom-
Uncertainty in Measurement
eters intended to be used as calibration standards or Standard
2.3 Other Publication:
Platinum Resistance Thermometers. Consequently, calibration
ISO/TAG4/WG3 GuidetotheEvaluationofUncertaintyin
of these types of instruments is outside the scope of this guide.
Measurement
1.3 Industrial platinum resistance thermometers are avail-
able in many styles and configurations. This guide does not
3. Terminology
purport to determine the suitability of any particular design,
3.1 Definitions—The definitions given in Terminology
style, or configuration for calibration over a desired tempera-
E 344 shall be considered as applying to the terms used in this
ture range.
guide.
1.4 The evaluation of uncertainties is based upon current
3.2 Definitions of Terms Specific to This Standard:
international practices as described in ISO/TAG 4/WG 3
3.2.1 annealing, v—a heat treating process intended to
“Guide to the Evaluation of Uncertainty in Measurement” and
stabilize the resistance thermometers prior to calibration and
ANSI/NCSL Z540-2-1997 “U.S. Guide to the Expression of
use.
Uncertainty in Measurement”.
3.2.2 check standard, n—a thermometer of similar design
1.5 This standard does not purport to address all of the
but superior stability than the UUTs which is included in the
safety concerns, if any, associated with its use. It is the
calibration process for the purpose of quantifying the process
responsibility of the user of this standard to establish appro-
variability.
priate safety and health practices and determine the applica-
3.2.3 coverage factor, n—numerical factor used as a mul-
bility of regulatory limitations prior to use.
tiplier of the combined standard uncertainty in order to obtain
2. Referenced Documents an expanded uncertainty.
2 3.2.4 dielectric absorption, n—an effect in an insulator
2.1 ASTM Standards:
caused by the polarization of positive and negative charges
E 344 Terminology Relating to Thermometry and Hydrom-
within the insulator which manifests itself as an in-phase
etry
current when the voltage is removed and the charges recom-
E 563 Practice for Preparation and Use of an Ice-Point Bath
bine.
as a Reference Temperature
3.2.5 expanded uncertainty, n—quantity defining an inter-
val about the result of a measurement that may be expected to
This guide is under the jurisdiction of ASTM Committee E20 on Temperature
encompass a large fraction of the distribution of values that
MeasurementandisthedirectresponsibilityofSubcommitteeE20.03onResistance
could reasonably be attributed to the measurand, designated as
Thermometers.
U.
Current edition approved Dec. 1, 2007. Published January 2008.
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 Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
the ASTM website. 4th Floor, New York, NY 10036, http://www.ansi.org.
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E2593–07
3.2.5.1 Discussion—Normally, U is given at a coverage 4.2 The success of the calibration depends largely upon the
factor of 2, approximating to a 95 % confidence interval. ability of the UUT to come to thermal equilibrium with the
3.2.6 hysteresis, n—property associated with the resistance calibration temperature of interest (fixed point cell or compari-
of a thermometer whereby the value of resistance at a tempera- son system) and of accurate measurement of the resistance of
ture is dependant upon previous exposure to different tempera- the sensing element at that time. Instructions are included to
tures. guide the user in achieving thermal equilibrium and proper
3.2.7 normal distribution, n—a frequency distribution char- resistance measurement, including descriptions of apparatus
acterized by a bell shaped curve and defined by two param- and instrumentation.
eters: mean and standard deviation. 4.3 Industrial platinum resistance thermometers are avail-
3.2.8 platinum resistance thermometer (PRT), n—a resis- able in many styles and configurations. This guide includes
tance thermometer with the resistance element constructed limited instructions pertaining to preparation of the UUTinto a
from platinum or platinum alloy. configuration that facilitates proper calibration.
3.2.9 rectangular distribution, n—a frequency distribution 4.4 Proper evaluation of calibration uncertainties is critical
characterizedbyarectangularshapedcurveanddefinedbytwo for the result of a calibration to be useful. Therefore, a
parameters: mean and magnitude (semi-range). considerable portion of this guide is devoted to uncertainty
3.2.10 standarddeviationofthemean,n—anestimateofthe budgets and the evaluation of uncertainties.
standarddeviationofthesamplingdistributionofmeans,based
5. Significance and Use
on the data from one or more random samples.
3.2.10.1 Discussion—Numerically, it is equal to the stan-
5.1 This guide is intended to be used by laboratories
dard deviation obtained (s) when divided by the square root of
interested in the calibration of industrial platinum resistance
the size of the sample (n).
thermometers that are intended to satisfy the requirements of
Specification E 1137/E 1137M. It is intended to provide a
s
Standard Deviation of the Mean 5 (1)
consistent method for calibration and uncertainty evaluation
n
=
while still allowing the user some flexibility in choice of
3.2.10.2 standard platinum resistance thermometer (SPRT),
apparatus and instrumentation. It is understood that the limits
n—a specialized platinum resistance thermometer constructed
of uncertainty obtained depend in a large part upon the
in such a way that it fulfills the requirements of the ITS-90.
apparatusandinstrumentationused.Therefore,sincethisguide
3.2.10.3 standard uncertainty, n—uncertainty of the result
is not prescriptive in approach, it provides detailed instruction
of a measurement expressed as a standard deviation, desig-
in uncertainty evaluation to accommodate the variety of
nated as S.
apparatus and instrumentation that may be employed.
3.2.10.4 Type A evaluation (of uncertainty), n—method of
5.2 This guide is intended primarily to satisfy applications
evaluation of uncertainty by the statistical analysis of a series
requiring compliance to Specification E 1137/E 1137M. How-
of observations.
ever, the techniques described may be appropriate in applica-
3.2.10.5 Type B evaluation (of uncertainty), n—method of
tions where higher accuracy calibrations are needed.
evaluation of uncertainty by means other than the statistical
analysis of a series of observations.
6. Sources of Error
3.2.10.6 test uncertainty ratio (TUR), n—the ratio of the
6.1 Uncertainties are present in all calibrations. Errors arise
tolerance of the unit under test to the expanded calibration
whentheeffectsofuncertaintiesareunderestimatedoromitted.
uncertainty.
The predominant sources of uncertainty are described in
3.2.10.7 uncertainty budget, n—an analysis tool used for
Section 12 and tabulated in Table 2.
assembling and combining component uncertainties expected
in a measurement process into an overall expected uncertainty.
7. Apparatus
3.2.10.8 unit under test (UUT), n—the platinum resistance
thermometer to be calibrated. 7.1 Resistance Measuring Instruments—The choice of a
specificinstrumenttouseformeasuringtheUUTandreference
4. Summary of Guide
thermometerresistancewilldependuponseveralfactors.Some
4.1 The UUT is calibrated by determining the electrical of the factors are ease of use, compatibility with computerized
resistance of its sensing element at one or more known
data acquisition systems, method of balancing, computation
temperatures covering the temperature range of interest. The ability, etc. All of the instruments listed are commercially
known temperatures may be established by means of fixed-
available in high precision designs and are suitable for use.
point systems or by using a reference thermometer. Either an
They require periodic linearity checks or periodic calibration.
SPRT or a PRT is recommended for use as the reference
(Refer to Appendix X2 for detailed descriptions and schemat-
thermometer. However, liquid in glass (LIG) thermometers,
ics.) The accuracy of the resistance measurements directly
thermistors, or thermocouples may be acceptable depending impacts the accuracy of the temperature measurement as
uponthetemperatureofcalibration,requiredaccuracy,orother
shown in Eq 2.
considerations.
Accuracy
V
Accuracy 5 (2)
t
Sensitivity
Mangum, B. W., NIST Technical Note 1265, Guidelines for Realizing the
where:
International Temperature Scale of 1990 (ITS-90).
E2593–07
capsule configurations. The design and materials of construc-
Accuracy = temperature accuracy at temperature (t), °C,
t
tion limit the temperature range of a specific instrument type.
Accuracy = resistance accuracy at temperature (t), V, and
V
-1
Some sheath materials can be damaged by use at high
Sensitivity = sensitivity at temperature (t), V °C
temperatures in metal blocks or molten salt baths. Calibration
7.1.1 Bridge—Precision bridges are available with linearity
on the ITS-90 is required.
specifications ranging from 10 ppm of range to 0.01 ppm of
1 1
7.2.2 Secondary Reference PRT—Secondary Reference
range and with 6 ⁄2 to 9 ⁄2 digit resolution. These instruments
PRTs are specially manufactured PRTs designed to be suitable
are available in models using either AC or DC excitation. The
as calibration standards. These instruments are typically less
linearity is typically based upon resistive or inductive dividers
delicatethanSPRTsbuthavehighermeasurementuncertainties
and is generally quite stable over time. Modern bridges are
and narrower usage ranges. They are typically sheathed in
convenient automatic balancing instruments but manual bal-
metal to allow immersion directly into metal furnaces or
ancing types are suitable as well. These instruments typically
molten salt baths. Calibration on the ITS-90 is required.
require external reference resistors and do not perform tem-
perature calculations. 7.3 Fixed Point Systems—Fixed point systems are required
in the ITS-90 calibration of SPRTs. Very low uncertainties are
7.1.2 Digital Thermometer Readout—Digital instruments
attainable with these systems, but their complex procedures
are available that are designed specifically to measure resis-
and design criteria may limit their application to other types of
tance thermometers. Modern versions function essentially as
thermometers.However,certainadaptationsaresuitableforthe
automatic potentiometers and reverse the current to eliminate
calibration of industrial platinum resistance thermometers.
spurious thermal emf. Precision instruments are commercially
7.3.1 TPW Cell and Apparatus—The triple point of water
available with linearity specifications ranging from 20 ppm of
1 1
indication to 1 ppm of indication and with 6 ⁄2 to 8 ⁄2 digit cell is a critical thermometric fixed point for calibration and
controlofSPRTs.Thesedevicescanbeusefulinthecalibration
resolution. Some models have extensive internal computation
capability, performing both temperature and statistical calcu- of industrial resistance thermometers but typically are not used
because of limited throughput capabilities. For further infor-
lations. Periodic calibration is required.
mation refer to Guide E 1750.
7.1.3 Digital Multimeter (DMM)—Digital multimeters are
7.3.2 Freeze-Point Cell and Furnace—Metal freeze point
convenient direct indication instruments typically able to
indicate in resistance or voltage. Some models have extensive cells are used in the calibration of SPRTs and thermocouples.
These devices can be useful in the calibration of industrial
internal computation ability, performing both temperature and
statistical calculations. The use of DC offset compensation is platinum resistance thermometers but typically are not used
because of limited throughput capabilities. For further infor-
recommended. Caution must be exercised to ensure that the
excitation current is appropriate for the UUT and reference mation refer to Guide E 1502.
thermometer to avoid excessive self-heating. Periodic calibra-
7.3.3 Ice-Point Bath—The ice point is a relatively simple to
tion is required.
realize fixed point that is useful in the calibration of resistance
thermometers. The ice point bath can be used as a fixed point
7.1.4 Reference Resistor—Reference resistors are specially
with uncertainties attributed to the care of construction and
manufactured resistors designed to be stable over long periods
maintenance. For further information refer to Practice E 563.
of time. Typically, they have significant temperature coeffi-
cients of resistance and require maintenance in a temperature 7.4 Comparison Apparatus—The choice of a specific com-
enclosed air or oil bath. Some have inductive and capacitive
parison apparatus to use will depend primarily upon two
characteristics that limit their suitability for use with AC factors: The temperature range of interest and the uncertainty
bridges.Periodiccalibration(yearlyorsemiyearly)isrequired.
required. Secondary factors include ease of use, compatibility
Resistors (AC or DC) are required to match the type of with computerized data acquisition systems or automation
measurement (AC or DC) system in use. capability, flex
...