When we speak of drying filaments relative humidity often gets tossed around. Let’s take a look, a basic definition is the amount of water vapour in air compared to the maximum amount it can hold, expressed as a percent. Simple right?
Anyone make ice tea? It is easy to dissolve sugar in hot tea but icy cold tea is not so easy. Air is very similar, the hotter the air is the more water vapour it can carry. RH without temperature is meaningless. This brings us to absolute humidity, it is a measurement of weight of water in a given volume it is not a variable. (Lets assume you are not changing elevation for this and leave atmospheric pressure alone)
Some basic numbers:
100% RH @ 15ºC is 12.8 gm/m3
74% RH @ 20ºC is 12.8 gm/m3
42.3% RH @ 30ºC is 12.8 gm/m3
15.5% RH @ 50ºC is 12.8 gm/m3
If I was a marketing person I could spin that my drier in just a few hours will go from 100%RH to 16%, amazing, mine works faster and better than the competition. You save $ and time. The reality is there is no change, it does nothing.
If you are using a dryer check RH after the temps have stabilized. Then wait for the RH to decrease, desiccant can help to speed the process. I usually let filament sit for a while 8 hours usually, longer if necessary. I often run a small sample of filament before a print, I just extrude a bit and see how it behaves, nice clean smooth it is fine, if it pops and has a rough surface it needs drying.
I don’t know if this helps but perhaps it will dispel some of the marketing gimmicky. Just measuring RH at stable temps will demonstrate a real change not just a numbers game. You can also use a basic calculator to get absolute humidity, there are lots online, not necessary however.
My work space is quite dry, 21-25ºC around 20-25%RH around 4.5-5 gm/m3, for reference 60% RH at 23ºC is 12.3 gm/m3 this feels quite humid and my summer outside temps often hit this.