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Temperature is one of the most measured variables in a myriad of fields ranging from scientific research to engineering and space science. It is a property of matter that can be expressed in different units, including the Fahrenheit scale and the Kelvin scale. The first is the most widely used unit in engineering and in countries like the United States and England, where the imperial system of units is still used.
On the other hand, the absolute temperature in Kelvin is essential in many branches of physics and chemistry, especially with regard to the study of ideal gases and thermodynamics. For this reason, knowing how to convert Kelvin to Fahrenheit is very useful, and this article teaches how to do it in different ways.
But before learning how to convert these two units of temperature, it is necessary to have an idea about what both scales are.
Fahrenheit scale
The Fahrenheit scale is a relative scale of temperature. This means that all temperatures expressed on this scale only indicate whether a temperature is lower or higher than a reference temperature, but does not actually indicate the temperature of a particular point.
Like any relative temperature scale, the Fahrenheit scale was established using two reference points: a low temperature that represents the “zero” of the scale, and a second temperature that defines the size of each Fahrenheit degree.
There are different versions, but they all seem to agree that Fahrenheit decided to establish the lowest temperature it could register as the zero of its scale, in order to avoid negative temperatures. This corresponded to a mixture of water, ice and ammonium chloride. This mixture has the particularity that it reaches a quite stable equilibrium temperature and also quite low, which is why Fahrenheit assigned it to zero on its scale at this point.
The next point of reference, according to some authors, was the freezing point of water, to which he assigned a temperature of 32 °F in order to divide the scale between 0 and 32 into 4 groups of 8 units each, something that is very common in the British imperial unit system. Using this scale, the normal boiling temperature of water occurs at 212°F.
The absolute temperature scale and the Kelvin (K)
Unlike the Fahrenheit scale, the Kelvin scale is an absolute temperature scale , which does not depend on any reference point. Zero on the Kelvin scale actually indicates the absence of thermal energy in a body. It was created by William Thompson (also known as Lord Kelvin, hence his name) in 1848, who assigned each unit on his scale the same size as the degree on the Celsius or centigrade scale. This makes converting from Kelvin to Celsius much easier, but makes converting to Fahrenheit a little less easy.
It should be noted that the temperature unit on the Kelvin scale is called Kelvin and its symbol is K ; It is not called a Kelvin degree nor is its symbol °K. This is precisely because it is an absolute and not a relative scale. For example, 273 K is read two hundred and seventy-three Kelvin and not two hundred and seventy-three “degrees” Kelvin.
Formula to convert Kelvin to Fahrenheit
The transformation from Kelvin to Fahrenheit is usually done indirectly, first transforming to Celsius and then to Fahrenheit. However, here we present a faster and more direct way that allows you to skip a step.
Using the reference temperatures shown above and knowing that the freezing point of water is at a temperature of 273.15 K (corresponding to 32 °F) and that the boiling point is at 373.15 K (corresponding to 212 °F), it can be deduced that each Kelvin unit is equal to 1.8 or 9/5 of a degree Fahrenheit. These observations are summarized in the following formula for converting Kelvin to Fahrenheit:
This formula is very simple, but it can be further simplified by dividing the fraction to get a decimal number. In that case, the equation remains:
In both formulas, °F represents the temperature in degrees Fahrenheit (the one you want to determine) and K represents the temperature in the Kelvin scale (the one you want to convert).
So converting K to °F is a simple two-step process:
Step 1: Multiply the temperature in Kelvin by 1.8
Step 2: The result of the multiplication is subtracted 459.67
Examples of conversion from Kelvin to Fahrenheit
Example 1:
It is desired to transform the average body temperature, which is 310.15 K, to degrees Fahrenheit. First substitute 310.15 in the formula where K appears.
Then the two operations are carried out:
Finally, 310.15 K equals 98.6°F.
Example 2:
Now, suppose we want to convert 150 K to Fahrenheit. As before, you start by substituting this temperature into the formula:
Then the two operations are carried out:
Finally, 150 K equals -189.67 °F.
The reverse conversion, from Fahrenheit to Kelvin
The reverse process is equally simple. To obtain the formula to transform Fahrenheit to Kelvin, all that is needed is to clear or solve the equation for K. The result is:
You can also avoid using the fraction in a similar way as before. The result is:
expert tip
When using these formulas, units should not be included as it can create confusion with the two temperatures. They should only be placed after the conversion is complete.
References
Brown, T. (2021). Chemistry: The Central Science (11th ed.). London, England: Pearson Education.
The mathematical treatment of the measurement results. (n.d.). Retrieved from https://espanol.libretexts.org/@go/page/1798
The measures. (2020, October 30). Retrieved from https://espanol.libretexts.org/@go/page/1796
The Editors of Encyclopaedia Britannica. (2018, October 25). Fahrenheit temperature scale | Definition, Formula, & Facts. Retrieved from https://www.britannica.com/science/Fahrenheit-temperature-scale
