The most accurate temperature measurements ever made. Probably.

NPL Lecture by Michael de Podesta

It is now 25 years since the establishment of the International Temperature Scale of 1990. The scale has been extremely successful in enabling accurate and consistent temperature measurement around the world.

However, it has become clear that the thermodynamic temperature estimates on which ITS-90 is based were in error, even at temperatures close to the triple point of water.

The discovery and elucidation of this error is largely due to the development of acoustic thermometry.

Over the last decade, the development of combined microwave and acoustic resonators for the measurement of the Boltzmann constant has improved the state-of-the-art significantly and resulted in advances in theory, fabrication, and experimental techniques.

After reviewing some of these advances, we present new data on T – T90 at twenty temperatures in the range from 118 K to 303 K.

The differences agree well with other recent estimates, but our low uncertainty reveals previously unseen detail. These measurements probably constitute the most accurate measurements of temperature ever achieved.



Recorded: 16 June 2015

Speaker: Michael de Podesta

New references for high temperature measurements

Summary of work reported for high temperature measurements from NPL

As a culmination of an eight-year research programme an international collaboration has developed robust reference fixed points, studied their sensitivity to impurities and external conditions and finally measured their melting transition temperature.

This talk describes how 100+ measurements made by nine different NMIs have been combined to assign low-uncertainty thermodynamic temperatures to the melting transition of Re-C, Pt-C and Co-C metal-carbon eutectics.

At the simplest level, these fixed-points will provide new temperature references for the calibration of pyrometers at temperatures above the freezing point of silver (1234.93 K) and will thus reduce the uncertainties associated with high temperature measurement compared to those achievable using the International Temperature Scale of 1990 (ITS-90).

The thermodynamic temperatures of these fixed-points have been determined through direct measurement of the radiance of a blackbody cavity surrounded by the fixed-point material from Planck’s law and hence the Boltzmann Constant.

The evolving mise en pratique for the definition of the kelvin encourages the realisation and dissemination of thermodynamic temperature.

This may be directly – and the work described in this talk shows that filter radiometry is sufficiently mature for this, or it may be by providing fixed-points with reference thermodynamic temperatures that have associated uncertainties – and this talk outlines such temperatures.

Recorded: 16 June 2015

Speaker: Emma Woolliams

Last Updated: 10 Sep 2015


YouTube Video: video:

Further information

Related areas

The Significance of Thermowells

Consider relevant factors and standards

Thermowell Drawing
Thermowell Drawing
Courtesy ISA InTech

Online — In a May/June 2013 article in InTech Magazine Basics of thermowell design and selection”, author Ehren Kiker describes why thermowells are important and how their key parameters can be determined for a particular use situation.

The article begins:

“When planning for a temperature measurement application, a fair amount of consideration is typically given to sensor selection (e.g., thermocouple vs. RTD) and wiring of the output (e.g., transmitter vs. direct wiring), and how these factors will affect the measurement. Often, by comparison, relatively little consideration is given to the mechanical components of the sensor assembly, particularly the thermowell.

“Of all the components in a typical temperature assembly, a thermowell would seem to be the simplest and least critical. In reality, the thermowell is fundamentally important because it directly and significantly affects the life span of the sensor and accuracy of the measurement. It also protects the closed process, providing plant and personnel safety.”

Click here to read the full article now.


Ehren Kiker ( is a product manager with Endress+Hauser with more than 15 years of experience in process control instrumentation.

Intech MagazineInTech Magazine provides the most thought-provoking and authoritative coverage of automation technologies, applications, and strategies to enhance automation professionals’ on-the-job success.

There is much more online at:

About ISA

The International Society of Automation ( is a nonprofit professional association that sets the standard for those who apply engineering and technology to improve the management, safety, and cybersecurity of modern automation and control systems used across industry and critical infrastructure.\

Wake Frequency Calculator by TempSens Instrument

Plus two additional Calculators

Wake_Frequency_CalculatorAn online resource calculator for estimating the Wake Frequency of a thermowell in a flowing stream.

According to the website:

“TEMPSENS WAKE FREQUENCY CALCULATOR is easy to use and it also ensures that thermowell is designed within the dimensional limits of PTC19.3, 2010. This calculator establishes the practical design considerations for Thermowell installations in power and process piping, which also incorporates the latest theory in the areas of natural frequency, Strouhal frequency, in-line resonance and stress evaluation.”

The program is “FREE” and can be distributed to other users, this was developed for Tempsens internal use and then we have now a lot of customers requesting for a copy of this program. Please give your inputs for improvement in the program.


The same web page provides a Temperature Calculator to convert the Emf (mV)/Ohms generated by thermocouple & RTD to the temperature or vice versa as well as a SPRT Calculator to provide the ITS-90 coefficients for the set of resistance readings provided on the fixed point cell.

Thermocouple Junctions Are Not Voltage Sources!

by R. P. Reed, Ph.D, PEret

NOTE: The following is a brief overview of a special article written and published here by a noted authority on thermocouples. Dr. Ray P. Reed. Dr. Reed is a retired researcher from Sandia Laboratories in New Mexico, USA.

He is a semi-retired, yet still a contributing member of the ASTM International Committee E20 on Temperature Measurement. He has written and presented many professional and peer-reviewed articles on temperature sensors, notably thermocouples in his long career.

His list of publications is on another page on this website,

This new article from R.P. Reed is published with his permission and is in downloadable format.

It is in Adobe PDF format and its size is about 310 kb.

Here’s a sample of the initial paragraph of the article:

“Thermocouples, based on the Seebeck effect, remain the simplest, most widely used, electrical sensor of temperature. Thermocouples consist only of thermoelectrically dissimilar conductor legs connected at junctions. The Seebeck emf occurs only in the legs. Therefore, commonplace calibration and thermometry errors relate to degraded thermoelements, not to junctions. A yet commonplace implicit Junction-Source Model incorrectly asserts that Seebeck emf occurs only in junctions. That erroneous concept hides problems that are commonplace in consequential thermometry.”

Link to a full Introduction to the article and the download link Link: junctions are not voltage sources!/

Precision & Accuracy in Measuring Surface Temperatures

A subject rife with errors, no matter how you look at it.

There’s a classic pair of books on temperature measurement that are hopelessly out of print. We are fortunate to have both in our little library. They are:

1. “Temperature Measurement in Engineering“, Volume I, by H.D Baker, E.A. Ryder and N.H. Ryder, John Wiley & Sons, Inc. New York and Chapman & Hall, Limited, London (Copyright 1953) Library of Congress Catalog Card Number 53-11565, and

2. “Temperature Measurement in Engineering“, Volume II, by H.D Baker, E.A. Ryder and N.H. Ryder, John Wiley & Sons, Inc. New York and London (Copyright 1961) Library of Congress Catalog Card Number 53-11565. Read more Precision & Accuracy in Measuring Surface Temperatures

New ASTM Standard For Digital Thermometers

ASTM E2877, Guide for Digital Contact Thermometers

Digital Display with Temperature 27 Deg. C by palomaironique
Image Courtesy of

W. Conshohocken PA, USA — A new ASTM International standard provides a variety of recommendations for the manufacture and selection of digital thermometers. ASTM E2877, Guide for Digital Contact Thermometers, was developed by Subcommittee E20.09 on Digital Contact Thermometers, part of ASTM International Committee E20 on Temperature Measurement.

Included in ASTM E2877 is a set of accuracy classes for digital thermometers. These classes pertain to the temperature interval from -200 °C through 500 °C, an interval important for many thermometry applications.

In order to qualify for a specific accuracy class, a thermometer must measure correctly to within a specified value over this interval or the subinterval in which the thermometer is capable of making measurements.

Digital thermometers that are used for measuring temperature in many laboratories and industrial applications are being increasingly seen as environmentally safe alternatives to mercury-in-glass thermometers, particularly since the U.S. Environmental Protection Agency’s efforts to phase out mercury thermometers are under way.

According to Christopher W. Meyer, a physicist at the National Institute of Standards and Technology, and an E20 member, the petroleum industry and others have used mercury thermometers for decades.

“These industries wish to convert to digital thermometers but until now there has been no ASTM standard for them,” says Meyer. “Also, there has been no set of defined accuracy classes that could help specify the type of thermometer needed for a given application. ASTM E2877 is necessary for instructing these industries in the basics of digital thermometers and for providing a standard that can be used in operation protocols.”

The new standard describes three types of sensors used in digital thermometers: platinum resistance sensors (PRTs or RTDs), thermistors and thermocouples (TCs).

“ASTM E2877 describes the various types of contact digital thermometers that are on the market and discusses the relative characteristics of each,” says Meyer. “It also defines a set of accuracy classes for digital thermometers that may be used to help specify the type of digital thermometer needed for an application. It will allow industries that have previously specified mercury thermometers in their protocols to use digital thermometers.”

All interested parties are invited to join in the standards developing activities of E20.09.

To purchase ASTM standards, visit and search by the standard designation, or contact ASTM Customer Relations (phone: 877-909-ASTM; ASTM International welcomes participation in the development of its standards. For more information on becoming an ASTM member, visit

For more news in this sector, visit or follow ASTM on Twitter @ASTMProductsRec.

ASTM Committee E20 Next Meeting: May 20-21, 2013, May Committee Week, Indianapolis, Ind.

Technical Contact: Christopher W. Meyer, National Institute of Standards and Technology, Gaithersburg, Md., Phone: 301-975-4825;

ASTM Staff Contact: Christine DeJong, Phone: 610-832-9736;

ASTM International, formerly known as the American Society for Testing and Materials (ASTM), is a globally recognized leader in the development and delivery of international voluntary consensus standards. Today, some 12,000 ASTM standards are used around the world to improve product quality, enhance safety, facilitate market access and trade, and build consumer confidence.

ASTM’s leadership in international standards development is driven by the contributions of its members: more than 30,000 of the world’s top technical experts and business professionals representing 150 countries. Working in an open and transparent process and using ASTM’s advanced electronic infrastructure, ASTM members deliver the test methods, specifications, guides, and practices that support industries and governments worldwide.
Learn more about ASTM International at

Mercury Thermometers & Alternatives in the USA

Thermometer Image
Thermometer Image: Courtesy & mistermong

USA — The USA’s National Institute for Standards and Technology (NIST) began an active mercury-reduction campaign in 2007, and stopped calibrating Mercury (Chemical symbol: Hg) thermometers entirely on March 1, 2011.

A full range of thermometric calibration services continues for non-mercury devices according to the special NIST webpage at:

Mercury is a potent neurotoxin, and every thermometer that contains it is a potential environmental threat. In the 21st century, however, that is a risk that no one needs to take, and a worldwide effort is underway to deploy substitute devices (alternatives) in consumer, professional, and industrial applications.

New analog thermometers with safe filling materials are in production at several companies and recent ASTM standards have been developed to cover them. See ASTM Standards E1 ( and E2251 (

For more information on each, click each standard’s title (above).

Digital thermometry technologies are plentiful, trials versatile, and generally superior to modern variations on the mercury-in-glass design. Many of these digital devices have wider effective temperature ranges, and nearly all of them equilibrate about 10 times faster than Mercury-filled devices.

There are three classes of sensors that produce signals which can be converted into a digital temperature read-out: thermistorsplatinum resistance thermometers and thermocouples. For more information on each, click on the name on on their name.

Each sensor type used is digital thermometers uses a slightly different aspect of a well-characterized relationship between temperature and electrical resistance or induced voltage in certain materials.

The term “thermometer,” when used in the context of digital equipment, refers to electronic systems that capture signals from the sensors, convert them into temperatures using conversion methods compatible with ASTM and/or ITS-90 standards, and then display the result in some format.

The accuracy of digital thermometers thus depends on the sensor type used, the sensor’s quality, its calibration, and conformance to specified standards. Plus,  the conversion system’s electronics, it’s calibration and conversion technique used and the unit’s sensitivity to ambient temperature and other conditions result additional sources needing traceable calibration.

In many modern devices, these details can be transparent to the user and summary details are described in the unit’s ‘System” calibration certification.

If not, then it is the responsibility of the user to assure that all major components of the measuring system have certified, traceable calibration and then perform the required calculations to determine the system’s measurement capability and combined measurement uncertainty.

This is not always an easy task, but is quite straightforward, as described in the NIST Publication: NIST/SEMATECH Engineering Statistics Handbook (

Reference webpage: Selection of Alternatives to Liquid-in-Glass Thermometers

Promoting alternatives to mercury thermometers



Ref: ASTM E230 / E230M – Standard Specification and Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples

Platinum Resistance Thermometers (PRTs)

Standard Specification for Industrial Platinum Resistance Thermometers:
ASTM E1137 / E1137M – 08

Verification Methods for Alternative Thermometers

Background References & Links
(Laws and Regulations),

EPA’s Mercury home page — (

State Regs: 2005 Mercury Compendium

The Environmental Council of the States
50 F Street NW Suite 350,
Washington, DC 20001

Tel: +1  202-266-4920
Fax: +1 202-266-4937
Email: ecos (at) ecos (dot) org

The Pros & Cons of RTDs & Thermocouples – By Jim Pinto

Jim Pinto One useful resource on RTD and Thermocouple sensors was found at Jim Pinto’s Writings section of his website, borrowed from the original article that he wrote for Instruments & Control Magazine in June 2000, at:

In the article Jim takes particular care to describe the likely errors that can occur with less-than-perfect cold junction correction in some electronic thermocouple instruments (i.e. signal conditioners, displays, controllers and transmitters)

He prefaces his remarks by saying:

Resistance temperature detectors and thermocouples can be used for some of the same measurements, but each has strengths and weaknesses that must be carefully matched to the application at hand.

The original version of this article was published in the leading USA magazine – Instruments & Control Systems, June 2000

(Click here for a copy from the I&CS website:

Jim Pinto is an extraordinary person who has been very active in the ISA – the International Society for Automation, where he is Fellow of the Society. He founded Action Instruments and since about 1998 has shared his thoughts and experiences with the world through various media including, since about 2000, his website,  and his YouTube Channel, online at:

His personal profile is available on at: (credit for Jim Pinto’s Picture belongs to that page) and on Linkedin at: where many images and video recordings of Jim may be viewed.

NPL’s Beginner’s Guides to Temperature & Humidity & More…


The Guide to temperature is an introductory guide which aims to help first-time users who have no experience of how to measure temperature beyond what they may have picked up in school, college or everyday life.

You can visit the Comment Page on this provisional Guide by Richard Rushby online at the comment webpage for the Temperature guide at: Read more NPL’s Beginner’s Guides to Temperature & Humidity & More…