It is difficult to measure temperature. The object to be measured is affected both by the environment and by the thermometer, which can heat or cool the surface. It is best to lower the thermometer into the material to be measured, but this is not always possible.

“Measuring the temperature of a surface sounds simple but is surprisingly difficult. When you put a thermometer against a surface, you cool the surface at the same time. It is difficult to reach very low measurement uncertainties when measuring surface temperature. If you have access to measure the inside of, for example, a pipe, it is much better, but often it is not possible”, says Patrik Broberg, a researcher at RISE who, among other things, works on developing calibration methods for surface temperature measurements.

Great need for quality-assured measurements

Although it is not optimal to measure surface temperature, the need for accurate and quality-assured measurements is great in many industries, for example in the steel industry, medicine, nuclear power, and transport. There are two main methods used in the industry, each with their own challenges. The traditional way is to put a thermometer against the surface, which requires the right technique and comes with the problem that the thermometer simultaneously cools the surface. Patrik Broberg is working on a project that will improve RISE's ability to calibrate equipment for this type of measurement. The principle of calibration is simple: the sensor to be calibrated is placed against a heated plate with a known and uniform surface temperature. The temperature of the thermometer is then compared with the temperature of the surface. At the National Laboratory for Temperature at RISE, there is a surface temperature calibrator that is widely used for calibrations from room temperature up to 600 degrees.

“But we get requests for calibration at higher temperatures than that. With the new equipment we are developing, we will be able to both increase the temperature and halve the measurement uncertainty, which is also requested by the industry”, says Patrik Broberg.

Pyrometers depend on the type of material

The second method is to use a pyrometer, which works in the same way as a thermal imaging camera but only measures one point. All materials radiate and by measuring the intensity of the radiation, the temperature of the material can be determined. Pyrometers are flexible and often good but depend greatly on the type of surface you want to measure. Metals and other surfaces that reflect a lot of light are difficult to measure with a pyrometer.

“When trying to measure the temperature with a pyrometer on a shiny metal surface, it is very much like trying to see the color of a mirror with the naked eye. Since a mirror reflects 95 percent of the light, this is not possible. It works in a similar way with pyrometers and many metals, such as aluminum. There are methods that can reduce the problem, such as painting surfaces with a color that does not reflect as much, but it is not always possible. Pyrometers are also sensitive to the angle you use when measuring. Trying to measure a pipe with a pyrometer can be difficult because of the angle”, says Patrik Broberg.

Patrik Broberg also does research on pyrometers. One example is developing methods to verify that pyrometers measure at the speed they promise. Some new pyrometers can measure 100,000 times per second, but there is a lack of methods to verify that the 100,000 times per second measurements are indeed correct.

But which of the two methods for measuring surface temperature is the best?

“It depends on the conditions. What instrument do you have access to? What material do you want to measure? If it is a shiny metal surface with good heat conduction, the contact method is best, if it is poor heat conduction and a surface with high emissivity, which radiates a lot, then perhaps a pyrometer is best. The pyrometer may also be best if the surface is difficult to access or too hot. But the most important thing in all measurements is that you have sufficient competence to perform the measurement”, says Patrik Broberg.

The transmission grid transports electricity produced in generation plants to the regional and local power grids. Svenska Kraftnät is responsible for the transmission grid, which includes roughly 17,000 kilometers of lines and approximately 200 substations throughout the country.

“We are constantly measuring the amount of electricity flowing into and out of the network to ensure correct billing. It’s important to maintain the different actor’s trust in the grid. Both the electricity producer and consumer want correct billing, and that requires accurate measurements”, says Anders Lindskog, measurement technology specialist at Svenska Kraftnät.

Accredited controls and continuous monitoring

Inspections of all energy measurement systems in the substations are carried out every six years by an accredited control body, according to legal requirements and regulations. Currently, RISE performs these inspections on site in the substations. The measuring instruments are verified and calibrated, and adjustments are made if any errors are detected.

“But even if the inspections carried out by RISE is of the highest quality, things can happen in the grid stations between the inspections. A component may break the next day, and then it is several years until the next inspection. To address this, we have collaborated with RISE for a long time to develop continuous monitoring of our measurements”, says Anders Lindskog.

The financial consequences can be substantial if something is wrong. Small errors such as 0.1 percent can result in hundreds of thousands SEK in billing error per month for a larger measuring point.

“The legal requirements are 0.5 percent measurement accuracy, but we always strive to exceed the legal requirements, not only to avoid erroneous debits but also to build and maintain the market´s trust in the grid”, says Anders Lindskog.

First project in 2013

Modern measuring instruments makes it possible to measure more than just electricity in and electricity out, and to collect the measurements in real time. The first joint research project between RISE and Svenska Kraftnät to develop methods to utilize these measurements started already in 2013.

“We have now implemented the methods for continuous monitoring that we developed for daily use. This not only ensures the quality of the measurements but also aids in interpreting what they mean, enabling informed decisions based on the measurements. With continuous monitoring, we can identify errors when they occur, errors that could otherwise go unnoticed until a physical inspection, up to six years later, says Niklas Eriksson, senior researcher at RISE and continues:

“We can spot and fix a variety of minor errors that previously could go undetected. For instance, a blown fuse might cause a minor error, which would have been difficult to detect even during a physical inspection.

Anders Lindskog at Svenska Kraftnät sees many advantages in collaborating with RISE.

“While the accredited inspection is undeniably independent, it’s also important in our other collaborations that RISE operates as an independent party. RISE also has the expertise and knowledge we require to develop methods and working processes”, he says.

Can we measure too much?

“Measurements cost money, but every time we start to measure more accurately, we always find something to develop and improve. We have only just begun to see the benefits of this”, says Anders Lindskog.

The technology is based on using materials and colors that reflect sunlight and emits thermal infrared radiation in a wavelength range that allows the heat to be dissipated through the atmosphere and out into the cold space. It is particularly effective in areas with a lot of sunlight and little cloudiness, such as deserts, where low temperatures can be achieved even in the middle of the day.

"The technology is particularly attractive in warmer countries, but even in Sweden we have problems with phenomena such as the heat island effect, which makes cities warm. Replacing black roofs with white ones can be one way to counteract that", says Anne Andersson, researcher at RISE, and continues:

"We also receive many enquiries from Swedish companies that are interested in developing their materials with these properties in mind."

Cools its surroundings

When two objects of different temperatures come into contact, heat is transferred between them. The hotter object gets colder, and the colder object gets hotter. In this way, buildings can be cooled when, for example, the roof gets colder. The effect can be observed on some mornings when frost forms on the car's windscreen, even if it is not freezing, when the surface has become colder than the surroundings. The more sunlight that is reflected and heat that is emitted, the more the material can cool its surroundings. Through measurements, a solar reflective index can be determined to quantify the effect of different materials and colors.

"All colors and materials reflect, but some more than others. White color that reflects a lot of sunlight and at the same time allows heat to be emitted is the best choice. At the same time, the material and color need to work for the building from other aspects as well, therefore it is important to characterize different materials to find the most optimal solutions", says Anne Andersson.

European project develops the measurements

Homes, office buildings, industries, shopping centers and data centers can all benefit from the technology as a cost-effective cooling system. And even the cooling effect only results in a few degrees colder, it can be an important piece of the puzzle to help combat climate change. However, despite the obvious advantages, there is still a lack of methods to evaluate and compare the performance of different materials and colors under real conditions. Anne Andersson participates in the European research project PaRaMetriC (Metrological framework for passive radiative cooling technologies), where she performs and develops measurements on these materials and colors.

"This is nothing new, it is no coincidence that buildings are white in warmer countries. But to develop the technology and be able to choose the most optimal materials and colors, we need methods to compare, which requires us to develop reliable measurements", says Anne Andersson.

In its Circular Economy Action Plan, the EU aims to double the share of recycled materials used in the Union between 2020 and 2030. The stated aim is to reduce the extraction of so-called virgin materials and the production of new raw materials. But, so far, the transition is slow, not least because of low acceptance among end consumers, gaps in technology and system development, and uncertainty linked to recycled raw materials. 

Low pace of transition stems from uncertainty 

In 2021, the proportion of recycled material corresponded to 11.7% of the materials used in the EU, an increase of only 1% since 2010. 

“Several companies are not ready to take decisions that would have a real impact,” says Peter Stigson, Research and Business Developer at RISE. 

There are many reasons for this, including uncertainty about traceability and quality, as well as long-term availability. 

“It’s true that there is a lot of variation in the availability of recycled products right now,” says Jönsson. “But remember, in the long run, the opposite is true: access to virgin material will be similar to the situation with oil. Other raw materials, such as cotton, which requires an unreasonable amount of water and area to cultivate, are becoming increasingly problematic. In other cases, we will have a type of material competition. For example, what should we use Swedish forests for? Clothes, paper, house, energy...? We will not be able to do everything, but must find ways to instead recycle and reuse what is what we can.” 

One example is the critical metals and minerals that will be needed in the energy transition. Here, RISE has carried assessments on the long-term availability: 

“These must also enter the recycling cycle. Less than one percent is recycled today.” 

Traceability challenging but important 

And equally important is our common approach to recycling. Peter Stigson has headed up expert groups with a focus on increased traceability and circular economy from a system perspective. An interesting question was how the sometimes problematic history of the original material should be viewed. Take cobalt as an example, which may have been mined in a reprehensible way, many people feel that it should be recycled since it is already in circulation. But can recycling really erase a sense of guilt? And how should the history of a material be communicated? 

“Our group of experts concluded that you can’t just ignore the origin of a material – it must be regulated and communicated in some way. But this is complex, particularly when it comes to metals that are melted down and mixed time after time. The way the issue of responsibility and information should be viewed is more akin to philosophy, ethics, and morality. The important thing is that everyone, not least consumers, understands the problem and accepts the solution.” 

Christina Jönsson: 

“A circular economy requires trade-offs, for example, when it comes to the properties and sometimes even the quality of recycled materials.” 

Customers must accept new ways of thinking 

A clear example is the concept of mass balance, which means that companies mix a certain amount of recycled material into a production run. This means that individual products, such as a bag, may not contain the amount of recycled material claimed by the company, but the series itself does over time. 

“This is a concept that some find difficult to accept,” says Stigson. “If someone buys a backpack that is stated to consist of 50 percent recycled, the person expects that particular bag to contain that percentage. But on a larger scale, it doesn’t work that way, and if the major manufacturers are to accept recycled material, they need to be able to set targets over time.” 

Long list of policies now becoming regulations 

A lot of changes will be noticeable in the future. On the one hand, numerous policies at both EU level and international level have now been adopted and have started to trickle down as regulations. 

“But we are also starting to reach a point where we are achieving both a technical and regulatory level and becoming more willing to make investments,” says Jönsson. “There has previously been a lack of technologies, operators, traceability systems, logistics systems... Now, politics is also pushing us in that direction and processes and technology are starting to become more commonplace.”

Temperature has always played a central role in human life, but despite this it was not until the 18th century that common temperature scales began to be developed, giving us common references to describe temperature.

Philo and Galileo

The first devices to measure temperature, thermoscopes, could be used to compare temperatures or observe temperature changes. However, these thermoscopes did not give an accurate indication of how hot or cold it actually was. One of the first thermoscopes was developed around 2,000 years ago by the Greek engineer Philo of Byzantium. Philo's thermoscope consisted of a hollow sphere filled with air and water, connected to an open water container. When the sphere was heated by the sunlight, the volume of the water in the sphere increased, which pushed out the air and in turn increased the volume of the water in the container.

This principle of the thermoscope was developed in the 17th century by Galileo Galilei and Santorio Santorio, among others. The thermoscope was gradually equipped with scales, including by Robert Fludd in 1638. However, each scientist or instrument maker had his own scale and it was difficult or impossible to compare readings from different instruments.

Universal temperature scales

In the 18th century, thermometry developed, and several scientists proposed universal temperature scales. Fahrenheit created his temperature scale in 1724, where the freezing point of water was set at 32 degrees and the boiling point at 212 degrees. Fahrenheit also invented the mercury thermometer in 1714.

The Swede Anders Celsius set the boiling point to 0 °C and the freezing point to 100 °C for his temperature scale. After his death, the scale was turned so that 0 °C corresponded to the freezing point of water. In our everyday life, it is this scale that we use to measure temperature.

Kelvin scale

The confusion with different scales led physicists to seek a scale based on the basic physics of temperature. This resulted in the Kelvin scale, which was introduced by Lord Kelvin (1824-1907). The scale sets the zero point at the lowest possible temperature for matter (-273.15°C) and the kelvin has been the official SI unit for thermodynamic temperature since 1954.

The steps on the Kelvin scale and the Celsius scale are the same, which means that the relationship between temperature expressed in degrees Celsius and Kelvin is:

t = T - 273.15

where t is the temperature in degrees Celsius and T is the temperature in Kelvin.

The definition of a kelvin

In 1954, the kelvin was defined as 1/273.16 of the thermodynamic temperature at the triple point of water. The triple point is the specific temperature and pressure where water can exist in all three states simultaneously - solid (ice), liquid (water) and gas (steam). As this temperature differs depending on the composition of the water, an internationally agreed composition of water called Vienna Standard Mean Ocean Water is used.

Since 2018, the unit is defined based on Boltzmann constant, which relates thermodynamic energy in a substance to its temperature.

Thermodynamic temperature

In everyday life, we often think of temperature as a comparison - how hot or cold something feels compared to something else. However, thermodynamic temperature is a measure of an object's internal energy, which includes the average kinetic energy of the object's atoms and molecules. Simply put, you can say that the more the atoms move, the higher the thermodynamic temperature. According to classical physics, motion ceases at absolute zero (but according to quantum theory, there is still random motion, so-called "zero-point motion", even at absolute zero, due to Heisenberg's uncertainty principle).

Because it is very difficult to measure the internal energy directly, scientists instead measure its effect as it moves as heat between objects. It is this effect that we use when we compare whether something is cold or hot. When heat no longer flows between the objects, that is, when they are in thermal equilibrium, that is their thermodynamic temperature.

How is kelvin realised?

Internal energy and temperature are different but directly related. To connect energy and temperature, Boltzmann's constant is used:

E = kT

where E is the material's kinetic energy, k is Boltzmann's constant and T is the material's temperature.

Realisation of an SI unit, that is, "making" the unit in the real world from its definition, is done through established and approved methods called mises en pratique. There are several mises en pratique to realise the kelvin, all of which are relatively complex, such as measuring the speed of sound in an ideal gas (the speed of sound depends on the temperature of the gas, which allows you to calculate the temperature from the speed). Although the triple point of water no longer defines the kelvin, the method is still used as a practical realisation with sufficiently low measurement uncertainty for calibrating thermometers.

The temperature scale ITS-90

Because of the difficulties in realising kelvin, practically useful temperature scales have long been used for calibrating thermometers. The first version came in 1889. Today there are two temperature scales, PLTS-2000 for very low temperatures (0.9 mK to 1 K) and ITS-90 for other temperatures. It is ITS-90 that we use for calibrations at RISE. The triple point of water is a good reference point to use for calibrations, but it is only one point. ITS-90 therefore contains 17 different fix points which are based on phase transitions (freezing or melting) or triple points in several different substances. At these points, the temperature is known with very low measurement uncertainty. This makes it possible to calibrate thermometers at many different temperatures.

Some of the fix points:

• Triple point of hydrogen = 13.8033 K (-259.3467 °C)
• Triple point of oxygen = 54.3584 K (-218.7916 °C)
• Triple point of mercury = 234.3156 K (-38.8344 °C)
• Freezing point of tin = 505.078 K (231.928 °C)
• Freezing point of aluminum = 933.473 K (660.323 °C)
• Freezing point of gold = 1337.33 K (1064.18 °C)

How is a thermometer calibrated?

The principle is simple. The reading from a reference thermometer is compared with the reading of the thermometer to be calibrated. The reference thermometer is usually a so-called platinum resistance thermometer, but at high temperatures a radiation thermometer is used which measures the radiation from the object.

The reference thermometer is in turn calibrated against ITS-90. When calibrating against ITS-90, fix point cells are used: prepared containers with the correct pressure and composition of the substance, which are heated or cooled. When, for example, gold changes from liquid to solid form, i.e. freezes, the temperature is known to be 1337.33 K. The reading of the reference thermometer is then compared with the temperature of the fix point cell.

The video shows a simplified view of calibrating a thermometer.