I've been having some trouble getting reliable Relative Humidity readings using analogue as well as digital meters. Last week I came across this weird - at least for me - metering method. It consists of 2 thermometers. One is dry and one is wet (the bulb being inside a little water-filled container).
You read the temperature on the dry side (e.g. 26) and subtract the reading on the wet side (e.g. 22.5 Centigrades). You then take the difference, e.g. 3.5 degrees, and look up the intersecting value in a table where the dry value horizontal line intersects with the difference value vertical line (e.g. 3.5). The value at the intersection point gives you the RH% value (73%).
Sounds complicated, hence I'm including a couple of pics.
Anyone has experience with this device, knows the physics of its workings, or has knowledge about its accuracy?