I am learning process principles in chemical engineering and I was taught various temperature units like Fahrenheit, Celsius, Kelvin, and Rankine. I see the first three being used quite commonly, but I rarely see the fourth being used. Where is the Rankine scale used then?
Rankine is commonly used in the aerospace industry in the United States.
Rankine is to Fahrenheit what Kelvin is for Celsius. So when people in the United States were creating programs and using equations that needed an absolute temperature, they used Rankine before Celsius became dominate for scientific calculations. The reason people still sometimes use it in the aerospace industry is that there are a lot of programs that were developed using Rankine, so in order to be compatible with those old programs, it's often simpler to just use Rankine in the new programs too.
Thus, as an intern at NASA, I was writing simulations that used Rankine in 2009.
Most of us in the world use the Celsius scale to measure temperature for day-to-day purposes. The Kelvin scale has been designed in such a way, it is not only an absolute temperature scale, but also 1°C change is equal to a 1K change. This makes conversion from Celsius to Kelvin pretty easy, involving just the addition or subtration of a certain constant (in this case, it turns out to be 273.16).
However, our friends who live in the United States prefer to use the Fahrenheit scale. The problem is, converting Fahrenheit to Kelvin is not as easy as it is for people using Celsius.
So what do they do? Here steps the Rankine scale. The Rankine scale is sort of like the Kelvin analog for Fahrenheit users. Rankine is an absolute temperature scale, and has the property of having a 1°R change equal to a 1°F change. This means Fahrenheit users who need to work with absolute temperatures will find it easier to use the Rankine scale instead of the Kelvin scale. Fahrenheit can be easily converted to Rankine by:
$$R = F + 459.67$$
This indeed is the case, as engineering systems using the Fahrenheit scale use the Rankine scale for absolute temperatures:
The Rankine scale (/ˈræŋkɪn/) is an absolute scale of thermodynamic temperature named after the Glasgow University engineer and physicist William John Macquorn Rankine, who proposed it in 1859. (The Kelvin scale was first proposed in 1848.) It may be used in engineering systems where heat computations are done using degrees Fahrenheit.
I am currently studying mechanical engineering in the US, and I have used Rankine.
It is used similarly to Kelvin. For example, in my thermodynamics class we used it to analyze various heat engines. Tables are available with properties of gases and steam using such units as BTU/R.
I can tell you that it is somewhat of a pain to use, because it is often coupled with units such as BTU or Horsepower, which don't line up with the rest of the customary system.
Mostly I use Rankine as a "way station" when converting between Fahrenheit and Kelvin, which often happens when working with manufacturing facilities in the USA. Because the Rankine and Kelvin scales have no additive offset between them, the conversion route Fahrenheit <--> Rankine <--> Kelvin requires less mathematics than Fahrenheit <--> Celsius <--> Kelvin.
There are two kinds of situations where you want an absolute temperature scale
- When you're working with cryogenic temperatures and don't want to look at negative numbers. (Nice to have)
- When you're doing multiplication or division with temperatures. (Must have)
You probably won't run into Rankine much for #1, but if you're otherwise working in Fahrenheit and run into situation #2 it's a lot more convenient to convert to Rankine (simple addition) than to convert to Kelvin. And Fahrenheit is still around, so it's good to know about Rankine.