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Temperature Scales

by Ron Kurtus (revised 13 November 2014)

A temperature scale is a way to indicate or measure temperature relative to a starting point (0 or zero) and a unit of measurement. This is usually done by defining some physical occurrences at given temperatures—such as the freezing and boiling points of water—and defining them as 0 and 100 respectively. Then the unit can be determined by dividing the range into 100 units or degrees.

The major temperature scales used are the Fahrenheit, Celsius, and Kelvin scales.

The first major temperature scale was the Fahrenheit scale, which is still used in the United States. Its temperature unit is somewhat awkwardly defined. Afterwards, the centigrade or Celsius scale was established. Celsius is considered metric or SI and is used throughout most of the world. The Kelvin scale is a variation of the Celsius scale that starts at absolute zero and is used in scientific measurements and in many heat-related calculations.

You can use a formula to convert a temperature in one scale to another.

Questions you may have include:

How was the Fahrenheit scale determined?

What are the Celsius and Kelvin scales?

How do you convert between scales?

This lesson will answer those questions. Useful tool: Units Conversion

Fahrenheit

Around 1700, a German scientist Gabriel Fahrenheit invented the thermometer by putting water in a thin glass tube. The warmer the temperature, the higher the water went up the tube.

To be able to measure temperature with numbers, Fahrenheit had to determine a unit of measurement. Just as the measurement of weight and length was based on multiples of an arbitrary unit of measurement (the pound, gram, inch, meter), the measurement of temperature is based on an arbitrary unit of measurementthe degree.

Fahrenheit defines units

Fahrenheit used his body temperature as 100° F (100 degrees Fahrenheit) and the freezing temperature of saturated salt water as 0° F. He marked those levels on his thermometer and divided the scale into 100 parts for each degree.

Poor choices

The choices of his body temperature for 100° F and the freezing temperature of salt water for 0° F were unfortunate.

Fahrenheit's metabolism was higher than most people, so 100° F for him resulted in 98.6° F as the body temperature for the average person.

Fahrenheit designated the freezing temperature of water saturated with salt as 0° F. But that certainly is not the coldest temperature you can experience in winter weather. It also makes the freezing point of water an awkward 32° F. Since ocean water is not saturated with salt, it freezes at 28° F.

U.S. still uses it

What is more amazing than this poor selection of temperature units is the fact that countries such as Great Britain and the United States embraced that system of measurement. England has since gone to the the Celsius scale, while the U.S. still stays with the Fahrenheit scale.

Metric temperature units

The metric or SI (System International) units of temperature are Celsius and Kelvin.

Celsius

About 20 years after Fahrenheit invented the thermometer, Swedish professor Anders Celsius defined a better scale for measuring temperature. He proposed using the boiling point of water as 100° C and the freezing point of water as 0° C. This made a lot more sense, and it was called the centigrade system. (Centi- means hundred and centigrade means divided into 100 units.)

The centigrade scale was used until the 1960s, when the scientific community renamed it the Celsius system in honor of the inventor. Some people still call it the centigrade scale.

Europe and most of the world measures temperature in Celsius units.

Kelvin

For scientific work with the energy of molecules, it is good to have a starting point where the energy level is at a lowest possible state. This point is called absolute zero.

The Kelvin scale was determined based on the Celsius scale, but with a starting point at absolute zero. Temperatures in the Kelvin scale are 273 degrees less than in the Celsius scale. Thus absolute zero is -273° C and the boiling point of water 100° C is 373 K or 373 kelvins.

Note: There is no degree sign (°) when using the Kelvin scale. Don't ask me why. That is just the way they wanted to do it.

There is also the Rankine scale, which also starts at absolute zero but is based on the Fahrenheit scale. Since most scientific work is done based on the Kelvin scale, the Rankine scale for absolute temperatures is seldom used.

Converting between systems

Because Fahrenheit is used in the United States and Celsius is used almost everywhere else, you may have to convert degrees from one system to another. A simple way of doing that is by using the following conversion application:

Temperature Converter

Enter a number in one box and then Click Here.

Use the reset button to clear the numbers.

°F °C Click Here

If you have to learn to make the calculation by hand, the following equations are used:

Celsius to Fahrenheit

The formula to convert Celsius to Fahrenheit is:

F = 9C/5 + 32

In other words, if C = 100° C (boiling point of water), then

F = (9 x 100)/5 + 32 = 212° F

Fahrenheit to Celsius

The formula to convert Fahrenheit to Celsius is:

C = 5(F - 32)/9

In other words, if F = 50° F , then

C = 5*(50 - 32)/9 = 5*(18)/9 = 10° C

Celsius to Kelvin

Converting from degrees Celsius to Kelvin is simple.

K = C + 273

Thus, if C = 10° C, the Kelvin temperature would be 283 K.

Summary

Temperature is designated as a multiple of a unit of temperature. There are three major temperature scales used, The United States uses the Fahrenheit system of temperature units. The metric unit of temperature is Celsius, which is used throughout most of the world. A scientific unit is the Kelvin, used in many heat-related calculations.

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