Vapor

Going through my father’s old technical books I found the slim reference volume Thermodynamic Charts: Also Special Tables for Turbine Calculations. The original copyright is 1939 (under the pithier title Vapor Charts), though this edition is from 1959. It’s a beautiful object with its oxblood cover and gold lettering, but the binding is cracked, and there are more easily accessed reference materials now. I’ll take it to Half Price books, but that’s kind of like taking a old feral animal to the pound. I’m just making someone else endure the pain of actually disposing of it.

Cover of the hardback book "Thermodynamic Charts"

Thermodynamic Charts contains introductory text, some description, and numeric tables in back, but the bulk of the volume is devoted to dense graphs of the thermodynamic properties of steam, ammonia, and freon. The chart for a given substance may span multiple pages. The one for water is stitched together across twenty one pages, and an overview in front shows how they are connected, like the frontispiece of a detailed road atlas.

Index chart for a collection of graphs

At first glance these charts appear graphomaniacally dense, but a worked example in the back shows you how to read them. Say you want to know the enthalpy, entropy, and volume of a pound of steam having at absolute pressure of 1400 pounds per square inch and a temperature of 900º Fahrenheit. You use the guide in front to find the right page. On that page you find the intersection of the 1400 psi and 900º F lines in the curvilinear grid. Volume can be read off the corresponding point on the horizontal axis, and enthalpy can be read off the vertical. Entropy is read off another gently curving grid that cuts roughly perpendicular to temperature.

Pages of curvilenear graphs

The information these tables contain is current, since the thermodynamic properties of water will remain invariant throughout the lifetime of the universe, but you’d never use them today, despite their elegance, because it’d be easier to just look the information up on a computer. My thermodynamics is rusty, but I’m confident that they are just plots of an equation or two. In fact, if we allow that someone out there knows how to plug values into the proper equations, these charts could be fully recreated from a handful of empirically observed constants. Thermodynamic Charts is lovely to look at, but it could probably be boiled down to a few numbers scribbled on the back of a napkin.

There is a branch of information theory that attempts to put this notion of essential content on a more precise footing by defining the Kolmogorov complexity of an object. This can be thought of as the smallest amount of information you need to store in order to recreate a passage of text, picture, thermodynamics reference book, or what have you. This was awfully abstract when Andrey Kolmogorov and others proposed it back in the 1960s, but nowadays when MPEGs and megabytes are part of everyone’s daily vocabulary and lack of disk space a common complaint, it’s become a lot easier to grasp.

A computer is, among other things, a device whose representation of an object can be compressed down to near the theoretical limit elucidated by that thing’s Kolmogorov constant. (Unsurprisingly, since that’s pretty much the definition of the latter.) This marks a great advance in our ability to manage quantity of information, but the existence of a book like Thermodynamic Charts demonstrates that quantity is not all there is. Back in 1939 turbine engineers could have carried around an index card with thermodynamic constants in their pockets and ground through the arithmetic every time they wanted to know a particular temperature and volume, but they didn’t because that would have been a pain. Better to do all the calculating up front and then record it in graph form that, once you get used to squinting, is actually quite easy to read. This representation takes up more room, but in the absence of electronic calculators, it is much faster. (Sadly, the Kolmogorov constant is blind to oxblood covers and gold lettering.) Software engineers call this trading space for speed, and it turns out that everyone’s been doing it all along.

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