Temperature Scales and Conversions

Temperature measurement is fundamental to industrial process control. While Celsius and Fahrenheit are common in everyday use, industry professionals must understand multiple temperature scales, their relationships, and proper conversion methods. Each scale - Kelvin, Celsius, Fahrenheit, and Rankine - has specific applications and reference points that make it valuable in different industrial contexts.
Rankine will not be discussed here.

Kelvin, Celsius and Fahrenheit scales

1. Kelvin

The kelvin (K) is the base unit of temperature in the International System of Units (SI). It is named after William Thomson, 1st Baron Kelvin, a British physicist who proposed the absolute temperature scale in 1848.

Kelvin is the SI unit of temperature.

What makes the Kelvin scale unique:

• Absolute Zero: The Kelvin scale starts at absolute zero (0 K), which is the theoretical temperature at which all molecular motion ceases. This is approximately -273.15 degrees Celsius.
• Same Size Units as Celsius: The kelvin has the same unit size as the degree Celsius. This means that a change of 1 kelvin is equal to a change of 1 degree Celsius.
• Conversion: To convert from Celsius to Kelvin, you simply add 273.15. For example, 25 degrees Celsius is equal to 298.15 kelvin (25 + 273.15 = 298.15).

2. Celsius

Celsius is a temperature scale that most of us are familiar with. It's used in everyday life in most parts of the world, from checking the weather forecast to setting the thermostat.

What makes the celsius scale unique:

• Absolute Zero:Different from kelvin the celsius don't start from zero. Celsius absolut zero is -273.15 degrees Celsius (°C). while 0 °C is the freezing point of water, absolute zero is a whopping 273.15 degrees colder
• Simplicity and Familiarity:The Celsius scale is based on the freezing and boiling points of water, two very common and easily observable phenomena. This makes it intuitive and easy to understand for everyday use.
• Decimal-Based:The Celsius scale is divided into 100 equal parts between the freezing and boiling points of water. This decimal-based system makes it easy to work with and perform calculations.
• Scientific Use:While Kelvin is the absolute temperature scale used in many scientific calculations, Celsius is still widely used in many scientific fields due to its practical applications and its direct relationship with Kelvin (a simple addition or subtraction converts between the two).

3. Fahrenheit

Fahrenheit is a temperature scale that, while not as widely used globally as Celsius, still holds a prominent place, particularly in the United States. It's a scale with a rich history and some unique characteristics.

What is Fahrenheit?
The Fahrenheit scale defines the freezing point of water at 32 degrees Fahrenheit (°F) and the boiling point at 212 °F. This means there are 180 degrees between these two points.


Why is Fahrenheit used
The primary reason Fahrenheit is still used, especially in the United States, is simply due to historical.
For people who have grown up using Fahrenheit, it's the scale they're familiar with. Temperature readings in Fahrenheit are part of their everyday experience, from weather reports to oven settings.
In some specific industries in the US, Fahrenheit might still be the preferred or customary scale. This can be due to established practices, equipment calibrated in Fahrenheit, or simply resistance to change

Conversion formulas and relationships

See my temperature conversion calculator here

Absolute zero concept

Absolute zero represents the theoretical lowest possible temperature in the universe, where all molecular motion ceases except for the small vibrations due to quantum effects. It equals -273.15°C, 0 K, or -459.67°F.

Practical Applications:

• Understanding temperature limits in cryogenic processes
• Designing cooling systems for superconducting devices
• Calibrating temperature measurement instruments
• Calculating theoretical efficiencies in thermodynamic cycles
• Reference point for many scientific calculations

International Temperature Scale (ITS-90)

The International Temperature Scale of 1990 (ITS-90) represents the current international standard for temperature measurement. It was designed to closely approximate thermodynamic temperature while being highly reproducible and practical to realize in laboratories worldwide.

The International Temperature Scale of 1990 (ITS-90), established by the International Committee for Weights and Measures (CIPM), provides a standard for calibrating instruments used to measure temperatures on the Kelvin and Celsius scales.

The International Temperature Scale of 1990 (ITS-90) is a document that specifies how to calibrate thermometers in a way that closely approximates the thermodynamic temperature scale. It was adopted by the International Committee for Weights and Measures (CIPM) in 1989 and is used worldwide.