Just the other day, Brian from Strand Brothers Heating & Air Conditioning was at our house to inspect and tune up our HVAC system in preparation for the icy cold winter ahead. We got to talking about our knowledge wheels and he told me guys in his line of business have a wheel they use. It sounded interesting, but he said they're rather expensive.
Well, blow me down and call me "Shorty"! Look what I found on the wall of my sewing room! It's the type of wheel Brian told me about! This one is in like-new condition and appears to be from the 1950s or so. The very tiny copyright notice says 1948. This wheel is nine inches in diameter.
According to The Internet, where I found a training manual:
"Psychrometry is an impressive word which is defined as the measurement of the moisture content of air. In broader terms it is the science and practices associated with atmospheric air mixtures, their control, and the effect on materials and human comfort. This can be accomplished through use of psychrometric tables or a psychrometric chart. The tables are somewhat more accurate, but the chart is accurate enough for all practical purposes and is much easier to use."
I couldn't begin to tell you how to use this thing, but it seems that if you know the dry bulb temperature of something, you're halfway there. On the back the crazy, curvy chart isn't coated with anything, so you can make notes with a pencil -- to a reasonable degree!