Every time I mention that RH has nothing to do with it I get laughed out of the room. I only ever use it as a practical example that most people understand (as above). 50% RH at a pretty standard room temperature is enough for most people to understand how easily plastic picks up moisture. The problem though is that it is an incorrect metric to use and the 3D printing community is a bunch of home enthusiasts who have great intentions but are using the wrong information to make their claims.

I had one guy just screaming about it at one point, he was doing the math and everything to "prove" that simply heating the air up is enough to dry the plastic because the increase in temperature results in a drastic decrease in RH and as such the "air pulls the moisture out of the plastic", except molecular diffusion occurs as a result of concentration gradients, and hotter air still has the same concentration.

My own experience with a very expensive equipment and testing methods begs to differ. I asked him what the actual amount of water in the air was when warm vs cold and he claimed it didn't matter, then stopped replying.
Of course this can only be demonstrated with a moisture analyzer for most materials, although a home user could print out test bars and do a mechanical strength test to see for themselves if they have some means to control the environment of two samples of plastic.

Heat is beneficial however, as you need heat to excite the water molecules and coerce them to leave the plastic. You can very well lower the moisture content of a material with simply forced hot air as well, which is an old method used in plastics. The problem is you will never lower the it below what the gradient between air and plastic will support, whether it's heated air or not.

This is the very reason why industrial drying units have dew point measurement, not RH. Dew point is effectively (though not technically) a reading of the absolute moisture content of the air. Typically drying occurs at a subzero dew point and this is achieved by actively drying the air. Vacuum drying on the other hand doesn't rely on that at all, which is why it is so cost effective.



I am glad someone here agrees with me about drying their filament. I still want to figure out and design an automated vacuum dryer. The professional units are very expensive for something that could be done for a couple hundred dollars. All you need is a means to heat the chamber for X minutes and then draw a vacuum for Y minutes, then turn on a light when it's done.