That approach is what we’ve been referring to as “closed loop”, and I think it’s worth trying. A possible existence proof would be Flashforge’s ~$3K filament dryer (which also handles automatically regenerating the silica gel desiccant in situ) that they sell to enterprise customers. The main reason we hadn’t yet tried it is that silica gel’s efficacy goes down when it gets hot, and so the consensus opinion was that Mzip’s approach had the highest probability of success out-the-gate, since it avoided that possible pitfall. The two approach’s that might mitigate against decreased efficacy in higher closed-loop heat were to 1. just use a lot more desiccant, or 2. use molecular sieves, which are much more tolerant to the higher temperatures you’d be drying at. If you have the means to regenerate the molecular sieves, I’d go that direction. There were also some ideas about cooling the air in a closed loop system just prior to drying it by passing it over silica gel, but that’s added cost and complexity, and so far nobody stepped up to try it.
You raise a good point that I hadn’t previously considered, which is if you circulate the closed loop air slowly enough through the external column, then it probably gives off the heat in the carried air fast enough that it would work just fine. And, if not, just running it through a heat exchanger with lots of fins, and maybe a fan blowing over it, would be enough. I seem to recall that the ideas discussed in that one earlier thread with @IslandBill involved using refrigeration or a heat pump or something heavy duty along those lines, which was a bigger commitment that, in light of @MZip 's progress and results, are perhaps overkill compared to simply slowing down down the recirculation flow rate.
I just now did a quick search, and a simple yet sizable radiator is fairly inexpensive:
and applying a fan to it would cover your bets if you wanted guaranteed results.
You’d have more reason to be careful about constructing an airtight closed loop system, so that you don’t pull in additional moisture from the environment. With @Mzip’s existing design, it can be fairly leaky without much consequence, since it’s under positive pressure.
The simplest, least effort closed-loop I can think of, though, would be to buy Sunlu’s new E2 and load it with lots of molecular sieves inside and let the fans therein handle the intermixing and drying of the air:
At least notionally I think that’s what @KYZ_Design is suggesting above. @KYZ_Design , is that what you meant?
At least from the initial description, it sounds as though it’s designed to be more airtight than the S4:
Worth keeping an eye on as more details emerge after it’s January 8, 2025 release as to just how feasible (or not) that might be, depending on whether it’s always leaky to some degree or whether it really does have a heated airtight mode. However, based on the Sunlu’s current description, it sounds promising.
On the other hand, maybe the S1 is already airtight enough, given that you have to prop it open to vent moisture during the early phase of drying. If so, then it’s just a matter of whether you can find adequate space to array the molecular sieves.
Anyhow, so many ways to skin this cat that the more people who get involved and try things and share their results on what works and what doesn’t, the merrier it will be for all concerned.
Happy New Year! This has been a fun ride, and I look forward to learning even more in 2025. 
I’m missing something. Even starting at 50% RH if you heat 22C room air up to 50C it should be impossible to see an RH of 19% unless something in the container is actively outgassing moisture. I made a little check, and assuming my math is right it should be like 10% max.
That would be the filament?
When I put filament in the dryer and turn on the heat, the humidity starts rising after the spool starts heating. There is a pretty big humidity spike as the spool warms up. That’s the filament moisture coming off.
I haven’t kept watch on an empty filament dryer heating up since I’ve been focused on drying filament, but I would expect the RH % to go down as the dryer heats since hotter air can hold more water.
I’m not sure if that’s what you’re asking though?
I use food dehydrators which are constantly sucking in room air and heating it so I wouldn’t be able to use your method. The moist air is swept out and replace by heated room air, which has a lowered RH.
I was interested because you said you were using this as a termination condition and was trying to figure out if this was based in some physical phenomena that could be used as a rule (even if it only applied to your exact dryer) or was just heuristic that worked for you.
It’s just what is working for me. If you monitor weight during a dry the weight will asymptotically approach some value where the water stops leaving. That’s as good as you can get under those conditions.
Getting there is a chore, though, since it’s close to or is an exponential curve. As you approach the point where weight stops changing, you go through “where it barely changes”.
For my setup I found that drying really started slowing down and weight was barely changing around 19-20% RH indicated in the filament dryer at drying temperature. I left one spool on overnight since it hadn’t quite hit 19% and all it got down to was 16% after an additional 8 hours or so. Definitely diminishing returns. But the dry air I’m using to help drying is somewhere less than 10% RH (measured at 2% RH to the accuracy of the meter with my bead column that dries the air about half saturated/used up). If there was no more water coming off the filament, it should just purge down to the input air humidity at filament dryer temperature. So at the 19% where I pull my spools, there is still water coming off the spool. Just not so much and not so fast. But it’s still giving off water and the filament dryer humidity should keep on dropping over a very long time to the level of the purge air coming in.
You really only need to dry to where filament prints properly so I just picked 19% as my end humidity reading in the chamber when the chamber is at temperature. When the spool cools in the poly cereal boxes, it pulls on down to the minimum the hygrometers can read which is 10% RH (different hygrometers).
It’s kind of confusing but yep, water is still coming off but the filament is dry enough to print well.
I’m guessing you’ve compared drying time & eventual dryness reached in your contained dryer when using untreated vs pre-dried make-up air, but have you also tested against something that is just sweeping away all of the humid air like a food dehydrator? From your description it sounds like the RH in your dryer starts at some high number then slowly drops to 19%, wouldn’t you expect a dryer that always has 10.7% air to dry better and faster?
Yep, I have. Details are in the thread on filament drying. But there was a definite difference and dried air dried the filament faster to a significantly lower humidity in the filament dryer than I could hit with ambient air alone.
A food dehydrator will be the same as propping the door open or using the pump to purge with ambient air, just better circulation probably. All three are limited by the ambient humidity and can’t dry any more than that allows.
The way I do it, that higher humidity than yours slows drying too (comparatively) which is I think what you are getting at. Since the 10.7% you are measuring is lower than the 19% I dry to, the dehydrator must be superior. The mistake you are making is forgetting about ambient humidity and how it’s different for pretty much everyone. Mine is relatively high. A food dehydrator, like a propped-open filament dryer, or a room air purge are all limited by that higher ambient humidity. Others won’t hit your 10.7% RH value.
If your ambient humidity is low then a food dehydrator may work great for you. (Your case, apparently.) If your ambient humidity is high, not so much. (My case.)
I’ve never said dehydrators don’t work. A dehydrator won’t work for me.
For those trying to dry filament in higher humidity environments, doing something to reduce the humidity they are drying in can make the difference between drying and not drying, print problems or no print problems.
That’s the key thing and why I keep saying ambient humidity. It’s absolutely something that should be considered when recommending a drying method.
I explained that poorly, sorry. I was assuming your environment of 22C air at 50% RH. If you heat 22C/50%RH air to 50C, the RH drops to 10.7%.
So yes, the food dehydrator is limited by the ambient RH, but it has an intake fan and is warming the incoming air on electric coils. The RH in the cabinet is going to drop to 10.7% at 50C. In your dryer, from your description, the RH starts high and then eventually reaches 19%. It seems like always being 10.7% is better.
@krellboy You raise an interesting point. Like he said, there’s the possible difference in ambient humidity to consider, but even so I imagine @MZip could get to a lower RH% by increasing his flow rate–but then he’d be burning through desiccant faster, and at a fast enough speed maybe the heater on his S1 couldn’t keep up. But drying faster might offset burning through desiccant faster, at least to some degree. Maybe it’s the heater that’s the limiting factor here…? IIRC, @Mzip was most concerned about minimizing the amount of desiccant per dried filament roll, more than time spent drying per se. It might be that your priority is for speed over minimizing desiccant consumption. It’s not entirely obvious how one might trade-off against the other, without gathering some data points on a specific build.
It may be. I need to think about this some. There’s a definite advantage to changing out the air fairly quickly. And that change in RH by temperature is a physical thing that is true.
Dry air absolutely works but it would save dealing with regenerating beads if a dehydrator does the same thing in even the same amount of time.
The open and flow through air using the room as the reservoir complicates using humidity as a measure of “doneness”, though. Humidity changes/differences between inlet and outlet would be low and the contribution of a couple or few grams of water won’t make a huge difference in the room. Because of the comparatively high air flow all you would see would probably be heated ambient humidity from when you put the spool in to taking it out.
On the other hand, with dry air I get very consistent results. The dehydrator would see pretty consistent results that could be even better. You could probably just go by time.
But I think the dehydrator loses access to information about moisture content. By drying to consistent 19% (usually), there will be uniformity between filaments for things that depend on moisture content, good and bad.
I see what you’re getting at. Are you running a dehydrator and how/procedure? What kind of print results are you seeing?
Even with a faster flow rate that might interfere with checking for doneness, you could maybe still check for “doneness” by occasionally choking off the flow altogether and then watching what happens to the interior humidity after a period of time. I do like what you’ve discovered about doneness. It’s nice not having to weigh the spools but just check an RH% number instead.
I don’t know how important that is though. Not enough experience to say. As long as spools are in long enough to be “dry” but might need to work out a procedure guaranteed to go to a low enough moisture level.
About choking flow to test, that’s an interesting thought.
What would be good would be an in-depth look with weights and times and looking at moisture contents. But if it’s working for people that’s hard to argue with.
I admire your parsimonious approach.
I have a restless tendency to want to try all possibilities, whereas I’d probably be better off just settling on the first thing I come across that works.
This is interesting though and I would like to see a systematic look into it because it could be a better way. I’d love to not fiddle with silica gel beads. I already did my time looking into drying though, 
and have a procedure that works well for me. But I have stuff backed up now and don’t have the time to look into drying again, at least not for a while.
I can see some questions to nail down procedures. I am pretty sure some have posted here with drying/moisture issues after drying with a dehydrator but maybe they didn’t go long enough or there were other issues. It would be good to put it all to bed.
Questions as I see it at least are how well it actually dries filament, how to know when done or if just going by time is good enough or how to verify drying, and is uniformity in drying anything of importance?
One way to know how drying went is the hygrometer in a container with filament but that’s fairly slow. That works but few do it even to troubleshoot other drying issues.
Those using dehydrators now can answer some stuff right off. Their procedures and results would be good to hear. It could be a really good thing.
I actually made my own DIY dryer once out of PETG in my early years of printing. To say the less It didn’t go well…….
I’m pretty sure that one is important, as I attribute this rather bizarre banding to my own non-uniform drying of the filament (from back in the day, before I knew anything):
That’s pretty cool. It depends on how much variation there would be in the dried spools using a dehydrator. There may not be much. It would be good to know more about the process/details and how well it works for people, too.
It was dried in a Print Dry Pro3, which IMHO is not a well designed machine, and therefore overpriced for what it is. Hot air comes out the center of the machine, so the side of the filament spool facing the center is heated to a higher temperature than the other side, which is what led to the non-uniform heating/drying that I think happened in this instance. The one and only virtue of that machine is that it can heat to 85C. Well, more like claims it can heat to 85C–in truth, not really.
Anyhow, I wouldn’t recommend that machine to anyone. Just memorializing how I believe that banded result came about. The cause-effect relationship may be applicable to some other types/brands/models of machines also. https://www.printdry.com/product/printdry-filament-dryer-pro3/
Been a bit busy. One thing that might be happening is that the sensors used in your dryer are designed to have the substrate at the measured air temperature, not the ambient temperature. So if they are mounted on a PCB which is shielded from the hot air except for a small aperture and exposed to ambient on the other the die temperature might be 20C lower than the hot air temperature. The physics of the sensor is probably directly measuring RH but still they rely on temperature compensation for accuracy. Thinking about it more, the measuring element being at the wrong temperature might also be a significant error source independent of the temperature compensation.
If you run your unit empty does the measured RH track with the expected drop as the air is heated?