"“…if friend do everything that the AASHTO accreditation needs you to do, if friend execute all of the needs of , friend will have a well-run laboratory."

Florida department of Transportation

Tim Ruelke, P.E., Director, Office of Materials

Document Library
News & Announcements
Newsletters Archives
Video Gallery


55+ years of endure
3,000+ PSP participants
23,000+ samples shipped every year
1,000 activities assessments every year
2,000+ accredited labs

Calibrating Thermometers: The scientific research Behind systematic Temperature Measurement, component 1: advent

ByMaria Knake, laboratory Assessment routine Manager

Posted: may 2012

A Promise is a PromiseI"ve said it myself: there is no calibration, we have no confidence in the measurements that we take. And also if a measure up isn’t meaningful, why also bother? Nevertheless, if you’ve read my vault articles, "The Anatomy the a Liquid-in-Glass Thermometer" and "Let’s obtain Digital," you understand that, approximately this point, I have (intentionally) avoided the topic of thermometer calibration. So, if calibration is such critical subject, why have actually I been avoiding it because that so long? The price is simple: I’ve to be procrastinating.

You are watching: What reference points are used in calibrating the scale of a thermometer?

As i promised in ~ the end of my last article, it’s time to lastly talk about thermometer calibration, or what I choose to speak to “the scientific research behind coherent temperature measurement.” If the cornerstone the any good measurement is calibration, then the cornerstone that any good calibration is the method(s) used to complete the calibration. End my next couple of posts, i will explain some that the details the thermometer calibration.

Units the TemperatureThroughout history, plenty of units and scales that temperature measurement have actually been arisen for miscellaneous applications around the world. In fact, historic accounts claim that over 35 different temperature scales had actually been occurred by the 18th century. One of the most well known scales was emerged by Daniel Gabriel Fahrenheit around 1714. Fahrenheit was the first scientist to usage mercury in a liquid-in-glass thermometer. He offered three solved points to create his temperature scale. The lowest point was the temperature of an ice and also salt water mixture (0 degrees), the midpoint to be a mixture of water and also ice (32 degrees), and the 3rd was person body temperature. After his death, the human body temperature fixed allude was replaced with the boiling suggest of water (212 degrees). Readily easily accessible and basic to use, Fahrenheit’s temperature scale easily gained acceptance and also is tho in usage today, generally in the unified States.

Anders Celsius emerged his temperature scale about 1740. Celsius used fixed points the were related to the freezing and also boiling points of water to construct his scale. Originally, the boiling allude of water was addressed at 0 degrees, and the freezing point at 100 degrees. The scale in between the two addressed points was divided into 100 equal increments. The range was later reversed, and also now the boiling allude is solved at 100 degrees, and the freezing allude at 0 degrees. The Celsius mechanism rapidly acquired popularity for its lull of use through a decimal-based counting system and is still supplied today almost everywhere the world.

In the 19th century, william Thomson (also recognized as mr Kelvin) arisen the Kelvin temperature scale, i beg your pardon is founded on the theoretical visibility of pure zero. Pure zero is the suggest at i m sorry a device no longer contains any kind of thermal energy and all molecular motion ceases. This temperature is designated by the fixed allude of 0 Kelvin. The is impossible for any type of temperature to be cooler than absolute zero, and also therefore an adverse numbers execute not exist ~ above the Kelvin scale. This range is the just one that is founded on the principals that thermodynamics and not top top material actions (such together the freezing and boiling the water). Because that this reason, the Kelvin was chosen as the official unit that temperature through the global System (SI) the Units.

Since most applications perform not call for temperature measurement anywhere near pure zero, the Kelvin scale is rather impractical for daily use. (Can you imagine if your local weather station reported the the temperature would certainly be 298 Kelvin today?) Therefore, for numerous applications, us still depend on dimensions from the Celsius or Fahrenheit temperature scales. It is quite easy to convert in between the Celsius and Kelvin scales, together the divisions of each are equal in magnitude. In other words, one Kelvin equates to exactly one level Celsius. To convert temperature from the Kelvin scale to the Celsius scale, just subtract 273.15.

I thought this was an alleged to be an short article aboutCalibration, notHistory!Okay, okay, perhaps I’m quiet procrastinating a little. However, the background behind the scientific research is an essential element in knowledge thermometer calibration.Seriously. Bear through me.

The global Temperature ScaleBy the turn of the 20th century, scientists started to realize the there was a have to standardize the approaches used to define temperature ~ above an international level. In 1927, the first International Temperature scale was arisen by the general Conference that Weights and Measures. This range is often referred to together the ITS-27. The ITS-27 defined specific fixed clues on the Celsius scale. These resolved points were based on the intrinsic physics properties of details pure materials, such as the boiling allude of oxygen. The ITS-27 had actually six referral points. This range was polished in 1948, in ~ which time the Kelvin was adopted for use through the international temperature scale. The scale was polished again in 1968, 1976, and also finally in 1990. The ITS-90 consists of 17 fixed referral points which are used to define the modern-day international Temperature Scale. The scale proceeds to be polished every 20 years or so, together measurement technology improves. We are likely, therefore, to see an additional update to the scale sometime in ~ the next few years.

Interestingly, the freezing point of water, or ice point, i beg your pardon is generally known to be 0.0 levels Celsius (273.15 Kelvin), is not characterized on the ITS-90. Instead, the triple suggest of water, or 0.1 degrees Celsius (273.25 Kelvin), is used. At this temperature, water have the right to exist in solid, liquid, and also gas phases simultaneously. The triple suggest of water is just one of the most valuable fixed points on the ITS-90, with an accuracy of plus 0.0°C or minus 0.00015°C. The ice point of water, typically accomplished by usage of a water and also ice bath, has an accuracy the is greater than plus or minus 0.002°C under the ideal of circumstances.

The ITS-90 and also Thermometer CalibrationPrimary standards laboratories, such as the national Institute of criter and an innovation (NIST), use fixed-point cells to calibrate thermometers, together as conventional platinum resistance thermometers (SPRTs), in accordance v the ITS-90 scale. These fixed-point cells space highly-engineered solution that room stabilized at the preferred fixed temperature point out by usage of thermodynamic principals and also known material behaviors.

Calibration using ITS-90 resolved points is not perfect, however, and a small degree that measurement suspicion is intrinsic to success of among the fixed points on the ITS-90 scale. Because that example, the is assumed the the metals and gasses provided in the fixed point cells room one hundred percent pure. However, ensuring this level that purity down to every last part per million is impossible. Secondly, the ITS-90 scale is based on the presumption that the gasses used obey the ideal gas laws without exception. This is seldom the case, and adds a small degree of hesitation to the process. However, calibrations detailed by NIST using fixed point cells are very accurate by most standards. Because that instance, the measurement hesitation for a NIST calibration on a high-quality SPRT in ~ the triple point of water is 0.00006 degrees Celsius, or 0.06 milikelvins (abbreviated mK).


Fixed suggest cells are available commercially, and also are provided as recommendation standards by part calibration agencies. These high-dollar items, while not fairly as exact as those preserved by a primary standards laboratory, deserve to still provide a luxury calibration and also a short measurement suspicion that much exceeds what is required in most trial and error laboratories. ASTM D1502,Standard overview for use of Fixed-Point cells for referral Purposes, provides further guidance top top the usage of fixed-point cells for thermometer calibration.

Comparison MethodsThermometer calibration by fixed-point cells is not just costly, but it is additionally excessive for many temperature measurement applications used in building and construction materials laboratories. An ext commonly, thermometers space calibrated by use of a comparison method in i m sorry the thermometer gift calibrated (a.k.a. “test thermometer”) is contrasted to a recommendation thermometer at specific temperatures within the range of use. In most comparison methods, a stirred fluid bath or isothermal dried block is provided as the measure up medium. Dimensions are taken v both a reference maker and the test thermometer, and the two outcomes are compared.

Comparison techniques are based on theZeroth regulation of Thermodynamics, which states that “If two systems room in heat equilibrium v a 3rd system, climate they space in equilibrium with each other.” by virtue of this law, it can be determined that if a recommendation thermometer is the very same temperature as a measure up medium, then a second thermometer put in the tool will equilibrate to the same temperature together the referral thermometer. Both stirred fluid baths and dry blocks run under this straightforward concept.

Stirred liquid BathsStirred liquid baths are by much the most common medium provided to carry out thermometer calibration. When using this type of device, the reference thermometer and the test thermometer are placed directly into the bath tool or into a fitted test sleeve that is immersed in the check medium. The bath is provided with a stirring device to ensure uniformity the the liquid’s temperature. At colder temperatures (below 1 degree Celsius), alcohol (such as ethanol or methanol) is commonly used together the bathtub medium. Water is supplied at temperature from 0.5 levels to roughly 95 levels Celsius. Oil baths are typically used indigenous room temperature to approximately 300 degrees Celsius. For temperatures up to approximately 600 degrees, salt baths are common.

A stirred liquid bath does wonderful job of giving a continuous temperature environment for thermometer calibration. However, also in the ideal baths, temperature gradients might still exist, and also therefore that is important that both the reference and test thermometers be immersed to the very same horizontal position during calibration to mitigate errors. Liquid baths have tendency to attain thermal equilibrium slowly, and also it deserve to be time-consuming come calibrate a thermometer in ~ multiple temperature points utilizing a single stirred bath. Nonetheless, these tools are still the preferred method of thermometer calibration for plenty of applications. When used correctly, stirred liquid baths can carry out a high-quality calibration at a reasonable cost.

Dry BlocksDry blocks, also called drywells or temperature block calibrators, go not end up being commonplace till the 1980s. This devices generally consist of a metal block supplied with a temperature regulator that is offered to maintain a consistent temperature. The reference thermometer and also test thermometer are placed into bored holes in the test block. These tools are usually noted with a protective covering that houses the temperature regulator and also the block into one compact unit. While dried blocks are generally considered to carry out a calibration that is inferior to the of stirred fluid baths, lock do carry out a calibration system which is both very portable and also convenient. In addition, dried blocks tend to accomplish thermal equilibrium faster than stirred liquid baths, and can therefore expedite the calibration process.

Because they room still fairly new to the metrology world, the design, stability, and accuracy of dried blocks is not well regulated. Furthermore, the impact of style variability top top the top quality of these tools is not universally understood. The purity of the materials used, dimensions, and also other engineering facets can considerably influence the top quality of dried block calibrators. Dried block tools should be scrutinized closely before use come ensure that they room of the proper dimensions and also design. A calibration overview published through the european Association of nationwide Metrology institutes (EURAMET) entitled“Calibration the Temperature Block Calibrators” (Guide 13)provides extr guidance top top the makeup and also dimensional requirements of dried blocks, and it is recommended that this guide be made use of to for sure the top quality of dry blocks provided for thermometer calibration.

Other comparison MethodsWhile unusual in construction materials applications, a hold of other devices besides stirred liquid baths and dry block are occasionally used because that thermometer calibration through comparison. One such machine is thewet well, which is a hybrid device consisting of a portable block comprise a little amount the liquid. Another somewhat rarely calibration instrument, thefluidized powder bath, includes a powered material such as aluminum oxide. A uniform flow of gas is blown through the powder bathtub to maintain temperature equilibrium. In addition,tube furnacesare periodically used at very high temperature where various other comparison methods are not suitable. The typical theme with any type of thermometer comparison maker that is supplied is the it must provide a method for the referral thermometer and also test thermometer to be kept at the same temperature.

What’s Next?Now that we’ve explored the history, background, and two basic techniques the thermometer calibration, it’s time to reallydig into the details that the subject. In mine the next few postsI will proceed to discover the scientific research behind coherent temperature measurement. Some of the topics that will certainly be extended include:

generally used calibration procedures and also methodologies Thermometer calibration “dos and also don’ts” Ice allude and steam point verification What come look because that on a calibration record Hysteresis (what is it and why must you care?) Uncertainty and traceability Water triple-point cell Visual inspection of liquid-in-glass thermometers replacing mercury thermometers through safer choices Choosing a recommendation thermometer

Sure, we could talk around all of this now, yet I think I’d prefer to procrastinate just a small bit longer. Ns promise that it will be worth the wait.

See more: Which Types Of Organisms Developed First Due To The Early Environmental Conditions On Earth?


ASTM International, “ASTM E1502, Standard guide for usage of Fixed suggest Cells for recommendation Temperatures,”Book the Standards, Volume 14.03, 2010. ASTM International, “ASTM E220, traditional Test an approach for Calibration of Thermocouples by compare Techniques,”Book of Standards, Volume 14.03, 2007(a). Calibration of Temperature Block Calibrators (EURAMET/cg-13/v.01),”European association of nationwide Metrology Institutes, July 2007. “Temperature Calibration through Isotech Block Baths,”Isothermal an innovation Limited, pine tree Grove, Southport, Merseyside, England, 1999. Preston-Thomas, H., “The global Temperature scale of 1990 (ITS-90),”Metrologia, Springer-Verlag, Volume 27, 1990. Strouse, G.F., “Standard Platinum Resistance Thermometer Calibrations native the Ar TP to the Ag FP,” unique Publication 819, nationwide Institute the Standards and also Technology, Gaithersburg, Maryland, January 2008. Sostmann, Henry E., “Fundamentals the Thermometry part I: The Absolute, or Thermometeric Kelvin, Temperature Scale,”The Encyclopedia that ThermometryIsotech journal of Thermometry>, Vol. 1, No. 1, first Quarter, 1990, p. 1.

Printer trusted Version