Thermistor Calibration for High Accuracy Measurements

thermistors as shown on ebay photoTechnicians and engineers often use thermistors to measure temperature in applications which require high accuracy. Thermistors operate by changing resistance as their temperature changes in a very predictable but non-linear way.

This characteristic allows them to provide higher accuracy than thermocouples or RTD’s. In order to ensure this high accuracy, thermistor calibration is an important consideration.

One challenge when using thermistors is calculating the temperature from the measured resistance value. To accomplish this, the Steinhart–Hart equation is used to convert a thermistor sensor’s resistance to temperature.

See the following project page on sourceforge to learn much more:

http://thermistor.sourceforge.net/.

it begins:

1 Abstract

The project offers support for NTC thermistor calculations. The Steinhart-Hart equation is a mathematical model for these thermistors that seems to fit for a wide range of temperatures with high precision. Software to calculate the characteristic Steinhart-Hart coefficients based on temperature-resistance tables for given thermistors as well as functions allowing conversion of temperature values to resistance and vice versa is provided.

2 Description

A model for the resistivity of a semiconductor as a function of the temperature was found by Steinhart and Hart 1968 ([1]). The Steinhart-Hart law describes the absolute temperature T (in Kelvins) as a function of the NTC thermistor’s resistivity (in Ω) according to the formula

Steinhart-Hart polynom
1/T = a0 + a1 · ln r + a3 · (ln r)3

The constants a0, a1 and a3, also called Steinhart-Hart coefficients, vary depending on the type of thermistor. To support developer when creating temperature measurement applications, thermistor manufacturer often supply these constants for their products. They also publicate tables where resistivity of thermistor products for a wider range of temperature values are listed.

This project provides software to

  • calculate temperature value for a given resistance of an NTC thermistor with given Steinhart-Hart coefficients,
  • calculate resistance value for a given temperature for an NTC thermistor with given Steinhart-Hart coefficients and
  • evaluate Steinhart-Hart coefficients for an NTC thermistor descibed by a temperature-resistance table.

Apart from the standard Steinhart-Hart equation other forms have been found. For application with lower CPU power a simplified form of the Steinhart-Hart equation can be used.

Simplified Steinhart-Hart polynom
1T = a0 + a1 · ln r

On the other hand a quadratic term can be inserted into the formula to increase accuracy giving the extended Steinhart-Hart equation

Extended Steinhart-Hart polynom
1/T = a0 + a1 · ln r + a2· (ln r)2 + a3 · (ln r)3

An introduction to thermistors and the Steinhart-Hart polynom can be found at Wikipedia [2].

https://en.m.wikipedia.org/wiki/Steinhart–Hart_equation.

When compared against other methods, Steinhart-Hart models will give you much more precise readings across the sensors’ temperature ranges, often within a few hundredths of a degree.

Although the Steinhart-Hart equation is not universally known, it is useful in data logging applications such as measuring lake water temperatures, solar hot water systems, and skin temperature measurement.

Many high quality data loggers such as the dataTaker DT8x, Grant SQ20xx and VersaLog VL-TH allow you to enter the coefficients to automatically derive temperature from measured thermistor resistance. As part of our free tech support, we at CAS DataLoggers often provide help in this area for customers who call in asking how to perform the conversion.

Thermistor manufacturers don’t always provide users with Steinhart–Hart coefficients for their sensors; they may simply provide resistance versus temperature tables. In the case of a manufacturer-provided table, it’s not immediately obvious how to derive the necessary coefficients. Or, the user may want to perform self-validation of thermistors by measuring the resistance at several known temperature points and use this data to derive the Steinhart-hart coefficients.

To speed up the process, there are several Steinhart-Hart calculators online which allow you to enter the temperature and resistance values and then generate the coefficients.

You’ll find a link to our own online calculator, along with an example table, at the end of this article.

NTC Thermistors Steinhart and Hart Equation
The Steinhart and Hart Equation is an empirical expression that has been determined to be the best mathematical expression for resistance temperature relationship of NTC thermistors and NTC probe assemblies.

https://www.ametherm.com/thermistor/ntc-thermistors-steinhart-and-hart-equation

Deriving Steinhart-Hart Coefficients for Thermistor Calibration:

In cases where the Steinhart–Hart coefficients are not provided by your thermistor manufacturer or if you are doing thermistor calibration, you can derive them yourself. First, you’ll need three accurate resistance values (either from a table or measured) at three known temperatures and then insert them into the formula to derive the A, B and C coefficients.

The Steinhart-Hart equation is commonly defined as:

thermistor calibration

where:

  • T is the temperature (given in kelvins)
  • R  is the resistance at T (given in ohms)
  • A, B, and C are the Steinhart–Hart Coefficients which differ according to your thermistor model/type and its particular temperature range
  • Ln is the natural logarithm

The equation is sometimes presented as containing a term, but this results in a lesser value than the other coefficients and is therefore not as useful for obtaining higher sensor accuracy.

To find the Steinhart–Hart coefficients, you need to know at least three operating points. For this, we use three values of resistance data for three known temperatures.

thermistor calibration

Steinhart-Hart Temperature Calculator

Thermistor resistance is  related to temperature in degrees Kelvin by the following formula:

1/T= A + B*ln(R/Rt) + C*ln(R/Rt)2 + D*ln(R/Rt)3

In the standard Steinhart-Hart equation the C parameter is set to zero.  However, some manufacturers use all 4 coefficients.  In the calculator below, you can specify whether to use this term or not, by just setting it to zero. 

Subtract 273.15 to convert Kelvin to Celsius. 

It’s wise to do a quick sanity check by putting in the coefficients and the same value for Rt and R.  If the result isn’t 25 C then there is a problem with the coefficients. 

http://www.daycounter.com/Calculators/Steinhart-Hart-Thermistor-Calculator.phtml

Steinhart-Hart Calculator – The Steinhart–Hart equation is a model of the resistance of a semiconductor at different temperatures.
Steinhart Equation

    where:
  • T is the temperature (in Kelvin)
  • R is the resistance at T (in ohms)
  • A, B, and C are the Steinhart-Hart coefficients which vary depending on the type and model of thermistor and the temperature range of interest. (The most general form of the applied equation contains a (ln(R))2 term, but this is frequently neglected because it is typically much smaller than the other coefficients, and is therefore not shown above.)

https://www.thermistor.com/calculators

 

Thermistor Glossary and Terminology

Thermistor Terminology

A glossary slightly modified from that given in a US government publication: MIL-PRF-23648D.
Note that the term being described is in bold typeface. (Note also, that this replaces a part of the information page here on thermistors – it has been slightly edited from the original)

A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance with a change in its temperature. The resistance is measured by passing a small, measured direct current (dc) through it and measuring the voltage drop produced.

The standard reference temperature is the thermistor body temperature at which nominal zero-power resistance is specified, usually 25 °C.

The zero-power resistance is the dc resistance value of a thermistor measured at a specified temperature with a power dissipation by the thermistor low enough that any further decrease in power will result in not more than 0.1 percent (or 1/10 of the specified measurement tolerance, whichever is smaller) change in resistance.

The resistance ratio characteristic identifies the ratio of the zero-power resistance of a thermistor measured at 25 °C to that resistance measured at 125 °C.

The zero-power temperature coefficient of resistance is the ratio at a specified temperature (T), of the rate of change of zero-power resistance with temperature to the zero-power resistance of the thermistor.

A NTC thermistor is one in which the zero-power resistance decreases with an increase in temperature.

A PTC thermistor is one in which the zero-power resistance increases with an increase in temperature.

The maximum operating temperature is the maximum body temperature at which the thermistor will operate for an extended period of time with acceptable stability of its characteristics. This temperature is the result of internal or external heating, or both, and should not exceed the maximum value specified.
.
The maximum power rating of a thermistor is the maximum power which a thermistor will dissipate for an extended period of time with acceptable stability of its characteristics.

The dissipation constant is the ratio, (in milliwatts per degree C) at a specified ambient temperature, of a change in power dissipation in a thermistor to the resultant body temperature change.

The thermal time constant of a thermistor is the time required for a thermistor to change 63.2 percent of the total difference between its initial and final body temperature when subjected to a step function

The resistance-temperature characteristic of a thermistor is the relationship between the zero-power resistance of a thermistor and its body temperature.

The temperature-wattage characteristic of a thermistor is the relationship at a specified ambient temperature between the thermistor temperature and the applied steady state wattage.

The current-time characteristic of a thermistor is the relationship at a specified ambient temperature between the current through a thermistor and time, upon application or interruption of voltage to it.

The stability of a thermistor is the ability of a thermistor to retain specified characteristics after being subjected to designated environmental or electrical test conditions.

Industrial temperature measurement | Basics and practice

Free Download From ABB

(Extract From the Introduction)

With this Handbook for industrial temperature measurements we are attempting to provide the technician with solutions to his wide variety of responsibilities. At the same time, it provides for those new to the field, insight into the basics of the most important measurement principles and their application limits in a clear and descriptive manner.

The basic themes include material science and measurement technology, applications, signal processing and fieldbus communication.

A practice oriented selection of appropriate temperature sensor designs for the process field is presented as well as therequired communication capability of the meter locations.

The factory at Alzenau, Germany, a part of ABB, is the Global Center of Competencefor Temperature, with numerous local experts on hand in the most important industrialsectors, is responsible for activities worldwide in this sector.

125 years of temperature measurement technology equates to experience and competence. At the same time, it forms an important basis for continued innovation.

In close cooperation with our customers and users, our application engineers create conceptsto meet the measurement requirements.

Our Sector-Teams support the customer, planner and user in the preparation of professional solutions.

Free download available online at: https://library.e.abb.com/public/6bfb8fc893ac4d0da0a806ce8cd73996/03_TEMP_EN_E.pdf

Author Team:
Karl Ehinger, Dieter Flach, Lothar Gellrich, Eberhard Horlebein, Dr. Ralf Huck, Henning Ilgner, Thomas Kayser, Harald Müller, Helga Schädlich, Andreas Schüssler, Ulrich Staab,

ABB Automation Products GmbH

Many thanks to the publishing group at ControlEngineering-Europe for alerting us to this new online resource (http://www.controlengeurope.com/article/140944/Handbook-aims-to-simplify-industrial-temperature-measurement.aspx)

Mercury Thermometer Alternatives by NIST

Promoting alternatives

no mercuryOnline —  The USA’s National Institute for Science & Technology (NIST) is not only  the nation’s National Metrology Institute (NMI), it also serves additional roles, including cooperating with other government agencies to safeguard people from harm due to sensors or practices that could be hazardous.

About 20 years ago the use of mercury-filled sensors, such as barometers, hygrometers and liquid-in-glass thermometers were recognized as sources of long-term hazards to man and nearly all animals.

The Federal Drug Administration (FDA) and Environmental Protection Agency (EPA) began efforts to ban the use of mercury in such devices and NIST has been in the forefront of the effort, along with volunteer organizations like ASTM International.

NIST has published a series of webpages that describe the issues related to mercury filled thermometers and considered several alternatives, some of which, in this Editor’s opinion are long overdue.

The rest of this article is copied from the December 22, 2016 NIST webpage: https://www.nist.gov/pml/sensor-science/thermodynamic-metrology/mercury-thermometer-alternatives-promoting-alternatives that begins the NIST series of information pages to help users understand some of the alternatives to mercury-filled  Liquid-in-Glass thermometers.

In effect these new temperature sensor alternatives bring many testing and measuring practices into the modern world of both sensor and display technologies, providing durability, precision and traceability along with digital options, in many cases.

Mercury-filled thermometers have historically served numerous industries as reliable temperature standards. Increased regulation and the high cost of cleaning up mercury spills have encouraged the use of alternative types of thermometers.

To support the use of alternative thermometers, the NIST Temperature and Humidity Group provides guidance documents, training, and technical consultation to other government agencies and standards-developing organizations.

Replacement of mercury thermometers with suitable alternatives will reduce releases of mercury into the environment and will reduce costs incurred to clean up mercury spills.

Historically, healthcare and regulated testing laboratories have relied greatly on NIST-calibrated mercury-in-glass thermometers as stable reference standards of temperature.

The use of mercury thermometers has been virtually eliminated in routine hospital use, but a wide variety of regulations and test methods continue to specify mercury thermometers.

Mercury thermometers have several intrinsic advantages:

  • they are stable for long periods,
  • failure is usually visually apparent, and
  • they require little training or maintenance.

 

However, mercury is a powerful neurotoxin, and the cost of cleaning a mercury spill in industry is many thousands of dollars. Furthermore, many states restrict the sale of mercury thermometers.

In 2008, the NIST Temperature and Humidity Group worked with several organizations to reduce or eliminate the use of mercury thermometers.

Environmental Protection Agency (EPA):  the EPA hosted meetings in the Spring of 2008 to discuss strategies to eliminate the use of mercury thermometers in EPA regulations and laboratories. NIST provided technical guidance documents, presentations, and technical advice as experts in temperature measurements.

Clinical Laboratory and Standards Institute (CLSI):  NIST Temperature and Humidity Group staff have worked with CLSI staff to update standards calling for the use of mercury-in-glass SRM thermometers, enabling laboratories to use other thermometer types with NIST traceability.

Centers for Disease Control and Prevention (CDC):  Control of temperature is critical to proper storage of vaccines, in order to preserve safety and efficacy. At CDC’s invitation, the NIST Temperature and Humidity Group gave a presentation at the May, 2008 “Vaccine University” that CDC sponsors. Over 60 participants learned how traceable temperature measurement and control can be achieved with modern electronic thermometers.

These activities build on support provided in 2007 to the Food and Drug Administration (steam processing of food) and ASTM committee D2 on petroleum.

In an environment of increased regulatory and economic pressures to discontinue the use of mercury thermometers, NIST has provided timely and critically important technical advice to other federal agencies and thermometer users, ensuring that important industrial and health-care temperature measurements are performed efficiently and accurately.

Major accomplishments:

  • Guidance document published on how to identify alternatives to mercury liquid-in-glass thermometers.
  • Technical support provided to other government agencies and to developers of documentary standards.

 

Links to other NIST webpages:

 

Selected Publications & Related Links

 

Questions about Mercury Thermometer Alternatives?

TI Temperature Measurement Videos

TI, or Texas Instruments, is one of the world’s most prolific and largest makers of temperature sensors. They make all kinds but their sensors are mostly in the form of Integrated Circuit semiconductors.

TI also does an exceptional job in educating users how their devices work and how they can be interfaced and incorporated in measurement systems. Especially useful are the videos showing how some of their other integrated circuit modules can be used with external temperature sensors, like Thermocouples, RTDs and Thermistors.

Here’s an example of an interesting one:

Developed through TI’s expertise in MEMS technology, the TMP006 is the first of a new class of ultra-small, low power, and low cost passive infrared temperature sensors. It has 90% lower power consumption and is more than 95% smaller than existing solutions, making contactless temperature measurement possible in completely new markets and applications.

Check out their Video Channel on YouTube, especially the long list of videos already published about “Temperature Measurement”. It very straightforward; just go to: https://www.youtube.com/user/texasinstruments/search?query=%22temperature+measurement%22

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 OpenClipArt.org

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 www.astm.org and search by the standard designation, or contact ASTM Customer Relations (phone: 877-909-ASTM; sales@astm.org). ASTM International welcomes participation in the development of its standards. For more information on becoming an ASTM member, visit www.astm.org/JOIN.

For more news in this sector, visit www.astm.org/sn-consumer 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; cmeyer@nist.gov

ASTM Staff Contact: Christine DeJong, Phone: 610-832-9736; cdejong@astm.org

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 www.astm.org/ABOUT/overview.html.

FREE Online Lectures on Industrial Instrumentation

A 40 Hour Video Lecture Series

by: Prof. Alok Barua
Department of Electrical Engineering
IIT Kharagpur, India

We have featured several educational video lectures on Temperature Sensors in Prof. Barua’s online course, (Numbers 8, 9 and 7 on Thermocouples, RTDs and Thermistors, respectively) but realize the depth and extent of just this one component of the NPTEL Courses available online for free is something special!

Below is a list of the courses available via YouTube and The NPTEL Website in this the Electrical Engineering :: Industrial Instrumentation Series along with his first video lecture in the Series.

Forty Lectures online,  just in this series and more than 4,000 available at no cost except your time and willingness to learn! Incredible!

Here’s the list of the 40, approximately one hour, lectures in this course. Note several others, such as “Signal Conditioning” and “Problems on Temperature Sensors” have a bearing on the use of temperature sensors. The number in brackets represent the lecture time as [hh:mm:ss]

1 – Introduction to Industrial Instrumentation [59:56]
Read more FREE Online Lectures on Industrial Instrumentation

NPTEL Lecture 7 – Thermistors

Lecture Series on Industrial Instrumentation

NPTEL Lecture 7 – Thermistors (Duration: 60 minutes)
by Prof.Alok Barua
Department of Electrical Engineering
IIT Kharagpur, India.

For more details on NPTEL visit http://nptel.iitm.ac.in

Category: Education
Tags: Thermistor, resistance
Also see this video on YouTube at: www.youtube.com/watch?v=5_MpULxwijg

NPTEL Lecture 17 – Problems on Temperature Sensors

Lecture Series on Industrial Instrumentation by Prof.Alok Barua, Department of Electrical Engineering, IIT Kharagpur.

For more details about NPTEL visit http://nptel.iitm.ac.in
Category: Education
Tags: Problems & Solutions for RTDs , Thermistors and Thermocouples on Temperature Sensors

View directly or get the video yourself at YouTube: http://www.youtube.com/watch?v=ksJYAt9YupY