It gets complicated but I would argue desiccated air does indeed make a meaningful difference. The lower the humidity in the filament dryer, the faster and more thoroughly filament will dry.
What I’ve found is I can make my desiccant last longer by opening the filament dryer door with a prop until the RH in the drying chamber starts coming down. Then the dry air makes a big difference. During the initial spike you get when the spool is heating up, dry air is not as much a factor just because it gets swamped from all the water.
I use desiccated air and my filament dries to my target RH% in the dryer much faster than without dry air. Without dry air it may never reach my target RH. When I was doing my tests, ambient humidity was fairly high (around 40-50%) and filament would asymptote out and stop losing moisture before it was dry enough to print trouble free.
And about the moisture getting out of the center of a spool, the windings do slow the water movement some but not as much as you might expect. Even with a perfectly wrapped spool, the surface defects in the filament - cracks, pits, etc - are huge compared to the size of a water molecule. Millions of times bigger. Water molecules are 18pm on the long axis. Defects in filament are measured in microns if you check the micrographs.
Also, have you verified you are actually drying the filament by weight? If you’re seeing water effects then it’s a safe bet you have a fair amount of water. After drying, if you aren’t down by 2 to maybe 3 grams of water, your drying may be inefficient.
You can test water content in a spool by sealing it in a gallon ziplock freezer bag or poly cereal box with a hygrometer. The spool moisture will equilibrate with the air in the container and after 10-12 hours the RH you read will give you an idea of how much water content you have. It’s not the moisture content but is related to moisture content and that’s why you can get a go/no go idea. All of my spools peg the hygrometers in their poly boxes at the lowest reading they can display - 10%. They are cheap round hygrometers from Amazon but they give consistent results. I tried a spool of PETG HF straight from the shipping bag and the reading was like 42% RH IIRC. After drying it printed great and the reading was 10% when I stuck it in a poly container.
And looking at your other post, you’re going for multiple 8 hour drys and still getting poor results. I go 8 hours or so with dry air and get great results. I run with stock settings from Bambu and no moisture problems.
You really might be surprised what dry air can do for drying especially if you are having issues. Not everyone needs dry air but your experience seems to say you would.
It’s easy and cheap to try it out. A medium-sized aquarium pump, a container of desiccant beads, and some tubing and fittings and you can cobble up a test. You need to get the dry air into your filament dryer and it needs to be sealed up well enough to get the dry air to the filament. Sounds odd but the Sunlu S2 has a base held on by two screws that has a nice gap all the way around it so I used silicone seal to close the gap.
Also, dry air is more dense than humid so you want to bring the dry air in at the bottom. I think most dryers have holes near their tops to pass filament through but they are great for letting the moist air out.
I tried just using the aquarium pump with no desiccant in the line and it dried not that different from propping the door open. After the weights stopped changing (IIRC in the 30% RH range in the dryer), I put the desiccant back in line and RH started dropping again. I now just dry everything to a displayed RH of 19%. It works well for my printing. And that’s 19% RH hot - using the S2 RH display. When the spools cool down in the poly containers, the hygrometers always peg at 10% on their display.
The whole basis for the crazy things I say are in the thread “filament drying: preliminary results”. We covered a lot of stuff and there were some side discussions along the way but you can see how this was tested and verified and the actual numbers I may not remember so well. And big thanks to @NeverDie, @johnfcooley, and @IslandBill for their contributions.