# Mollier diagram and reading the relative humidity when knowing the dry- and wet bulb temperatures

I initially put this one on the physics-side, but this might be a more appropriate place: https://physics.stackexchange.com/questions/666116/mollier-diagram-and-reading-the-relative-humidity-when-knowing-the-dry-and-wet

I had a thought-error when reading Mollier diagrams and thankfully we have the internet to assist, but even after finding the correct sources showing the answers, I can't get out of my thought-loop to understand why my thinking is wrong. I hope someone could in simple and very definite terms explain why my thinking is wrong.

So let's say that I have the dry bulb temperature of 20 Celsius and wet bulb temperature of 15 Celsius and I'd like to know the relative humidity.

My thought error was that I thought that I'd read this from the 100% humidity point at 15 Celsius, straight up along the absolute humidity lines. My thought here was that the absolute humidity is the same and the air's water content doesn't change no matter how much I spin my wet-bulb thermometer. So I'd have assumed from the image for the relative humidity to be about ~72% at 20 C. (red arrow)

However this source showed how it should be read and I was also able to test it online. So I should actually read it along the enthalpy lines, which gives me about 58% relative humidity at 20 C. (yellow arrow)

https://www.engineeringtoolbox.com/humidity-measurement-d_561.html

http://www.ringbell.co.uk/info/humid.htm


Now I'm a bit stuck: I can from one perspective understand that the enthalpy does not change and is constant, but I'm having a hard time to definitely understand why my initial assumption about the absolute humidity is the "wrong constant" in this case.

Could someone take it down to my level, and also possibly clarify if there is something I still seem to have misunderstood, or what should be good to recognize?

• Do you mind if I explain this with the ASHRAE psychometric chart? Sep 17, 2021 at 12:00
• Hi, not at all. Now I admit that it is some time since I last looked at these, and clearly I didn't understand it correctly then, but as I suppose it will be the same thing just from a "different angle". So I'm perfectly happy with it as long as I learn this. Sep 17, 2021 at 12:15
• Just given my final answer down below Sep 17, 2021 at 12:30
• hard to use a chart when I can't read the units or descriptions Sep 17, 2021 at 19:12