# Why do we still need to know about the Rankine temperature scale?

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?

• I doubt even in the USA there are many who use Rankine. It's as useful as the Réaumur scale, in that some historical physics and chemistry research may have used them. BTW, C is 1.803e+12 furlongs/fortnight. See en.wikipedia.org/wiki/FFF_system – DrMoishe Pippik Jul 12 '18 at 1:41
• Speaking as a thermal engineer in the US, it is embarrassingly common here. I know of many of the largest and best known companies in the US do all their engineering in US customary units. Certainly not universal (I, for one, refuse to use it), but not at all uncommon. – ericksonla Jul 12 '18 at 2:10
• It's there for completeness' sake and costs all of 10 seconds to learn. Like learning Roman Numerals, it might come in handy some day. – Carl Witthoft Jul 12 '18 at 13:40
• @DrMoishePippik The trouble is that non-metric measures are common enough in the USA to cause confusion. NASA once crashed a satellite because the engineers accidentally mixed metric and imperial units. – matt_black Jul 12 '18 at 14:48
• @ericksonla Sometimes, I think that large US engineering companies use it just so they can blame their collaborators for the inevitable mistakes it causes! (I've been on the receiving end …) – alephzero Jul 12 '18 at 15:09

## 5 Answers

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:

(source: Wikipedia)

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.

• "This means Fahrenheit users who need to work with absolute temperatures will prefer to use the Rankine scale instead of the Kelvin scale." In my experience, that's almost never true. At best, that's an opinion. I expect that the significant majority of people who have a need to use an absolute-zero referenced temperature scale, who also use Fahrenheit for day-to-day temperatures, will use Kelvin, not Rankine. That is, of course, just my impression of what happens. We'd need to survey usage in order to determine the relative popularity. – Makyen Jul 12 '18 at 6:44
• You answer the question "What is Rankine?", which is never asked. The question is, does some branch or area actually use it in practice for day-to-day work. This answer only assumes that's true. OP (and me!) is not so sure. Could you point to some examples of its usage? – pipe Jul 12 '18 at 12:17
• Yes, it can be converted very easily, but how does that help if you still need Kelvins at the end because that's the SI unit? It sounds like you just assume that scientists use Rankine because of its simple relationship with Fahrenheit. If you could just show some example, or industry experience instead of just guessing, this answer would be good. – pipe Jul 12 '18 at 13:57
• @pipe It certainly is used by major companies like Boeing, but I can't offer you any proof of that statement without showing you company confidential documents. – alephzero Jul 12 '18 at 15:13
• The difference between Kelvin and Celsius is 273.15, not 273.16. The triple point of water is at 273.16, however. – md2perpe Jul 16 '18 at 21:01

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

1. When you're working with cryogenic temperatures and don't want to look at negative numbers. (Nice to have)
2. 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.