SI Units – Temperature

The kelvin (K) is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 ×10⁻²³ when expressed in the unit J K⁻¹, which is equal to kg m² s⁻² K⁻¹, where the kilogram, meter and second are defined in terms of h, c and ∆ν_Cs. The temperature 0 K is commonly referred to as "absolute zero." On the widely used Celsius temperature scale, water freezes at 0 °C and boils at about 100 °C. One Celsius degree is an interval of 1 K, and zero degrees Celsius is 273.15 K. An interval of one Celsius degree corresponds to an interval of 1.8 Fahrenheit degrees on the Fahrenheit temperature scale.

The standard temperature at the triple point of water is provided by a special cell, an evacuated glass cylinder containing pure water. When the cell is cooled enough so that a mantle of ice forms around the reentrant well, the temperature at the interface of solid, liquid, and vapor is 273.16 K. Thermometers to be calibrated are placed in the reentrant well.

Celsius Temperature Poem

Activity:  Estimate the Celsius temperature in each scene (ice crystals, tulip flowers, park bench, and swimming pool). Recite and reflect on the Celsius Temperature Poem. Use the hints provided by the poem.

• 30 °C is hot
• 20 °C is nice
• 10 °C is cold
• 0 °C is ice

Resources for Students and Teachers

• Kelvin: Introduction – Temperature is one of the most important and ubiquitous measurements in human life. (NIST)
• How Do You Measure Air Temperature Accurately – For purposes of safety, accuracy and convenience, air temperature is most often measured using electronic thermometers, which detect the changes in the resistance to the flow of electrical current through a metal.
• Absolutely Hot –  Analyze a common chemical reaction. Explore heat and energy and learn how they are measured. (Jason Learning)
• Kelvin Temperatures and Very Cold Things! – To keep track of some of the coldest things in the universe, scientists use the Kelvin temperature scale. (NASA)
• Building a Thermometer – Build an instrument to measure water temperature. (GLOBE Program)
• Thermometer Model – A thermometer is a device used to measure temperature. Explore the interactive model. Review thermometers types. (JavaLab)

League of SI Superheroes – Dr. Kelvin

This comic book-style video animation series has been developed to help middle school students learn about the 7 SI base measurement units. With the ability to speed up or slow down particles, Dr. Kelvin can measure any temperature. The kelvin temperature scale begins at absolute zero, the coldest possible temperature and the point at which even atoms would stand perfectly still.

Navigate to more SI base unit information:

• SI Units
• Length – meter (m)
• Time – second (s)
• Amount of substance – mole (mole)
• Electric current – ampere (A)
• Temperature – kelvin (K)
• Luminous intensity – candela (cd)
• Mass – kilogram (kg)

Resources

• Everyday Estimation
• Metric Publications
• SI Units
• SI Education & Training
• Unit Conversion