[exmadscientist]

It's the weird units. Students don't like weird units.

I remember I once, many years ago now, had a chemistry professor give temperatures in Fahrenheit for a thermodynamics problem, which to solve needed absolute temperatures in Kelvin. On a final exam. The class was not happy! (I didn't care, I both know the conversion and have a calculator that can do it.) He just wrote a table on the board with five entries: the boiling point of water in °F and °C, and the freezing point of water in all three systems.

Can you imagine that the students got even more upset at this? He eventually gave in and just did the conversion for everyone. I learned more than one thing that day.

Now I'm in industry and completely unfazed by this kind of thing. Sometimes shit's in Fahrenheit, that's just the way it goes, you just have to deal with it. Don't sweat the easy stuff!

[chaxor]

The Stanford lecture by Robert Channing for one of the first courses in chemical engineering (ChE 101?) on youtube is fun for this.

He goes through how many of the units are defined and how it has had an effect on various projects throughout the years.

One of the best ones is about how the size of the space shuttles' tanks were constrained. They had to be able to fit on a train, so things had constraints relating to the distance between railroad tracks. Those railroad track sizes could be traced back to car axel sizes, and therefore previous horse-carriage axels. These carriages of course had the Romans before them, but all of these had axels defined by the width of two horses.

So ultimately the size constraints on the space shuttle arose from the distance between two horses asses. Kind of fun.

[mjevans]

So strange, I remember the Fahrenheit (32, 212) and Celsius (0, 100) numbers, but the precise Kelvin number (which is hard-required for all the good physics equations I've forgotten), that is how much lower than 0C absolute zero is, I only vaguely remember as 272 something. However that's just a well known constant. Very reasonable to include it in the reference dataset.

[Dylan16807]

273 something.

For a good while it was defined as exactly 273.15, but as of a couple years ago it's now based on a specific number of Joules.

[exmadscientist]

The difference between Kelvin and Celsius is still the same. But the Kelvin scale itself has shifted a bit, taking the other scales with it.

[Dylan16807]

Well, okay, they changed Kelvin and Celsius at the same time, so that 273.15/273.16 are preserved, with Celsius based on Kelvin instead of the other way round.

But when you're dealing with the freezing point of water and the old version of Celsius for simplicity, then those numbers aren't as correct as they used to be.

[exmadscientist]

They're not far off, especially near the triple point of water: https://www.bipm.org/documents/20126/2619564/Estimates_Diffe...

See also section 2 of the more-or-less authoritative parent document "Mise en pratique for the definition of the kelvin in the SI", available at https://www.bipm.org/documents/20126/41489682/SI-App2-kelvin...

[hef19898]

Units-of-measure are fun, especially in accounting and procurement.