Did you buy yours from Amazon, from Drierite, or from some other source? For future reference, I’m just wondering if you may have found a source with an even better price, including shipping, than either of those two.
Straight from Hammond/Drierite. It’s fairly expensive but it’s lab grade which drives up costs. I don’t know other sources of the chips. I don’t think you would need indicating Drierite at the low humidity levels it would see but I was also surprised at the dark growing into my silica gel column. Got that wrong. 
Here’s how I’m looking at that. I’m conservatively extrapolating Vision Miner’s apparent recommendation of using make-up air at -40C dewpoint to a prescription that the air in the drying chamber should not fall above -40C. If it does, then increase the flow rate of the make-up air to drive it back down. Now, to your point: what if that added flow rate means that the dryer’s heater can’t keep up? My answer to that is that generating heat is the easiest thing in this entire process. If need be, find a way to add more heat generating capacity so that it can keep up.
Now, in your case you’d probably pick a less demanding standard than -40C, since your main interest is PLA. You could measure your existing wattage useage and use that to judge how much headrom you might have left. Maybe your steady state is well below the 50w your S1 dryer is able to apply, in which case maybe your existing system would self-adjust the heat without any extra work on your end. Worst case, you might have to find a way to add more heat.
Maybe that simple kind of hysteresis approach would be sufficient. Or maybe a PID could manage it even more precisely.
I imagine you’re right that a chemical engineer, or somebody like that, could probably calculate a flow rate from initial conditions, but unless that’s something chatgpt can do 
maybe a control loop is good enough that one doesn’t have to be that smart.
The thing about higher flows is it can increase breakthrough. I was seeing a little of that in the test stand. Not from increasing flows since it was running flat out to counter all the flow restrictions, but just in running and having smaller desiccant containers. Drierite is supposed to grab water fast and hard but silica gel is a little slower and less strong. It’s a tradeoff with flow rate and column size to minimize breakthrough. But if you run a silica gel precolumn it’s a tradeoff of size with how much breakthrough into the Drierite you’ll allow at higher flows.
I haven’t used a Kill-O-Watt or other meter to see the effects of the air flow I’m using but the Sunlu (S2) has LEDs around the display that animate when the heater is on. With the setup as it is now, the flow rate I’m using isn’t enough to push the heater to its limits and once the spool heats up, the heater doesn’t run a whole lot. But no numbers on that. At higher flows, though, that could change especially at higher temperatures. When I was trying to improve the Sunlu S2 base, deficiencies in the base (too high of heat loss) were enough to make the Sunlu max out at 68C out of 70C it could hit before. (I improved the base and it’s what’s on the Sunlu now but it needs another revision to optimize it more.) The take home was that on the S2 there isn’t tons of headroom to work with.
Again, tradeoffs. If you want lowest RH you need lower flows through the desiccant, or bigger containers if you want high flows. Drierite looks tailor made for the drying you want to do and should work fine in a recirculating dryer. I think a column arrangement is better than a bed approach because of the way I’m seeing my column go to high humidity at the air inlet with the beads near the oulet seeing more dry air. Probably much more efficient.
With trying to hit the -40 dewpoint, first thing I’d do is get a few of those extended range hygrometers up and running and just see what you get out of a column of silica gel or even just drop a sensor into a silica gel container. I’d love to see those numbers. Drierite will easily hit the dryness you need but maybe silica gel would too?
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Testing what that calculator seems to indicate I’m running on just aquarium pump air at the same pump setting which should be higher flow with no desiccant column in line. It’s 9 hours in (I stopped it overnight but restarted early this morning) and chamber humidity is 35% (ambient is 43%). Doing similar feeding it dry air it would have been nearly finished (I stop at 19%) but certainly below 22% humidity.
That’s assuming this spool is similar to the others, but drying is noticeably slower and yet to see how low RH will go before it levels off. So in spite of what that calculator suggests dry air is making a difference in drying times. I assume the numbers are correct. The air has to be heated to temperature to hit the RH calculated value so until it heats up it’s higher humidity (if you could isolate a small air “packet”). It’s obviously carrying water into the chamber so it might be humidifying too and fighting itself to some extent.
It’s just so far not performing near as well with ambient air being pumped in. I thought I might have wasted my time by not actually running the numbers but it’s not looking like it. Using an aquarium pump may help purge the moist air out but there’s something about it where it’s not behaving like I would expect if RH really did drop that low at temperature. The water being carried in as humidity is slowing the process down.
How is the water being carried in as humidty? I thought you were feeding it under positive pressure air that was <10%RH. Or, do you mean the water in the humidity that re-encroaches when you have it switched off at night?
On the larger topic of recommended storage humidity, I find the Bambulab wiki advice seemingly contradictory:
On the one hand it says storing filament at an RH below 20% will keep it dry for “an extended period.” Well, ok, let’s say it’s 19.99999%. But then immediately afterward it says that if it’s kept in 20% RH, it will only remain dry for only approximately 2 to 7 days. What? 2 to 7 days doesn’t sound like much of an “extended period” to me. What if you want to keep it dry for a lot longer than that, say a year or longer? How low should the storage RH be to accomplish that? Or is that very question a non sequitur? If it’s even possible to get solid guidance on that, I sure wish it were more widely known. It’s partly for reasons such as this that I lean toward overkill and hopefully driving the storage RH as close to 0% as I can. Or perhaps a more solid target would be a storage dewpoint of -40C or lower, purely for lack of any other credible reference number and because it seems like it would be a nice, safe overkill number. Speaking only for myself, if I could get to that number and maintain it during filament storage, I’d feel comfortable calling it mission accomplished. Monitor it, of course, but comfortably done until it rises to something north of that. Still TBD as to what the trigger threshold should be to invoke another drying cycle. I don’t know how long it might take to figure that out, so I’d rather solve that on the back-end rather than do nothing at all and not even start drying filaments until I figure it out. I have filaments that are getting older, so I’d rather dry them to a conservative number sometime soon and store them at that number and then figure out an appropriate edge trigger afterward.
Or maybe what you’re measuring in RH is just a reflection of the moisture content in the outer loops of spool, and maybe there’s a higher moisture content trapped near the middle of the spool that takes time to find its way out and so doesn’t manifest until it sits in storage for a while.
Because that calculator kind of indicated that maybe the bigger effect was the pump air just sweeping the moist air out of the filament drying chamber since heating my current room air to PLA drying temperature of 55C calculates with that calculator to a 4% RH (IIRC). In order to test that before wasting peoples’ time and money if anyone was interested, I bypassed the desiccant column and am just pumping room air into the drying chamber.
But it’s looking like the spool is drying even slower than with the door propped open. The heater isn’t cycling a whole lot so I assume it’s getting to 53C (what I’m drying at) or near where the calculator predicts a 4% RH. I can’t see below 10% RH to know what the column is putting out when in line.
That’s why I say it’s probably all the water in the air at room temperature slowing this dry down. It was to test if it was just sweeping the moist air out that was important or if it was the dry air. What it’s looking like is it’s probably both but don’t know which is bigger. Where I wondered if dry air was important to drying is looking like it’s a key player in the results I’m seeing.
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I don’t know what target humidity to dry to is correct. I’m just going by the more common value I’ve seen mentioned of a target chamber humidity of 20%. I’m drying to 19% just to be sure it’s 20% or lower.
Once a spool is dried, if you put it in a container that’s impermeable and see the RH change then there could be a leak or the container is permeable. Or, it could be the filament wasn’t equilibrated fully and on sitting there was more water to absorb or release. But other than that, RH shouldn’t change unless there is a water source somewhere (ignoring temperature changes since RH also depends on temperature).
It’s opening and closing containers and loading and unloading filament that kill you. Each adds a little water back to the filament. If you use desiccant in your storage it’s important it gets dried below the humidity you dry your filament to or else it becomes a water source. That’s how to know which way the water would go. Dry to 20% RH and stick a packet of gel in there dried to 10% RH and the water will slowly move from filament to desiccant. Dry the silica gel to an RH% of 25% and it instead supplies water to the filament. Slow processes though and not practical for drying unless you just have lots of time. But keeping the desiccant dry is key to all this.
Was printing spool holders to go in the cereal boxes and using an undried roll of basic black that had been in the AMS for a while. I need to change the desiccant in my AMS and the black was starting to act up with the last three prints looking like the one on the left. (Adhesion issues where the back corner was lifting a little.) When that spool ran out I swapped in one that I had dried using the air dryer and got the print on the right. Still need to change out desiccant in the AMS though. It just ticked up to “3” a few days ago.
It’s a curious thing that the filament moisture manifested in only just that one corner and seemingly nowhere else. The picture seemed to be overcompressed (maybe that’s something the Bambu forum does by default when new pictures get uploaded), so I couldn’t see anything more than fuzzie pixels in that corner when I zoomed in. Nonetheless, I believe you. It’s the same type of thing I observe too when filament that’s too moist gets printed.
I don’t think it was that moisture was just in an area. It was losing adhesion to the build plate in the corner and the print was lifting a little to where the nozzle was dragging on the print and leaving that gnarly area. I had three prints get that bad top layer finish in the same place which for whatever reason is to the back of the build plate away from the door. I’ve had it do this before when the black filament is a little wet. New rolls have fixed it as well as replacing the desiccant in the AMS. In this case a dried roll fixed it and I will be replacing the AMS desiccant before firing it up today.
Also, got the answer on just using an aquarium pump to provide air exchange - it turns out desiccant is a big factor. Without it humidity in the Sunlu was decreasing but slowly. Took around 12 hours overall to reduuce the humidity in the drying chamber to 33%. It took it 3.5 hours to hit 33% from 34% with a weight loss in that time of about 1/3g. I hooked the column back in the line and humidity started down again and hit 19% in the Sunlu in close to another 3.5 hours and lost slightly over a half gram. Drying was stopped at 19% chamber humidity in the Sunlu.
The moisture in room air with 45% humidity still let the spool lose about 1.5g but dry air brought it down even more and the change came much faster even at the tail end of drying. This mimics the door propped open test. It gets the first water ok but is slower and RH levels out without hitting low values when ambient humidity is high enough.
That calculator said the air would be about 4% RH just by heating it but that water in the air slows filament drying and blocks hitting low RH. That’s really the last experiment I can think to do at this point and everything points to this thing actually working. If you count the last roll it’s now dried 16 spools of filament, the exit air still has the exit hygrometer pegged at 10% (as low as these hygrometers can read for those new to this). The indicator color change is visible in about the bottom third of the reservoir so it looks like 20 or even 30 spools can be dried on a charge of silica gel. Can’t swear on the 30 spools worth but it looks like that’s where it’s headed.
At this point I’m just going to finish out my inventory of open spools and get this dryer posted. It’s not perfect. It does need to be sealed up somehow. It costs a bit to build. But once built it just hums along helping an otherwise marginal at best Sunlu S2 shine.
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I was thinking about your storage questions and it is confusing guidance. I think the reason is they oversimplified the advice. The way to maybe make mote sense of it is to consider water sources.
In those poly cereal boxes or similar sealed and impervious containers (to the extent they are sealed and impervious) the only water sources are filament and any desiccant. You should be able to store filament for extended periods without it picking up water as long as the boxes are sealed and impermeable. Once RH stabilizes in the cereal box after putting in filament and any desiccant, it really shouldn’t change.
But if storing filament in the open at 20% humidity, there is an “infinite” supply of 20% RH air and filament can absorb that water. Under those conditions, the longer the filament is in that environment the more water it will absorb and print quality will degrade over time.
About the 20% vs 19.999% thing, I think that’s further convoluted. They had to pick a number that would give good prints but also discuss how long you can store filament. They can only count on people having an AMS at best so are probably talking about open storage. Without all the details though it does sound contradictory.
That black print is a spool/hygrometer/desiccant holder for my cereal boxes and with a dry spool, fresh desiccant, and the hygrometer, all the hygrometers are going to 10% indicated RH within a few minutes of closing the boxes and sealing them. They should stay that way for a while but since the filament was only dried to 19% RH and the desiccant was apparently much dryer, water will still move from filament to desiccant. As the desiccant picks up water, the humidity in the container will increase very slowly. There’s no additional water being added. It will just move around. In this case the filament is the water source and things will percolate until RH stabilizes and no net water is moving around.
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Yes, I can confirm. With my marginally more accurate hygrometers, I typically get readings of 5% RH plus or minus inside the same kind of plastic cereal storage container after I dry a spool in my blast oven and pack it into the plastic cereal storage container with, say, 300g of fresh silica gel desiccant, even with the ambient make-up air during the drying process having a dewpoint of 68F, like it is at this very moment.
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I also get about the same reading without the desiccant, so it’s the spool of filament that dominates. Or, so it appears.
In contrast, with just the desiccant and no spool of filament, I get a lower reading.
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I’m using WiseDry 50g silica gel packs here but same difference. Works like a champ and very satisfying to see the humidity drop like that and better, my prints using the dried filaments are printing better. They’ve almost always been pretty good but I’ve had water issues in black and dark blue PLA before that all this should eliminate.
I started spool #17 this morning and output is still at 10%. I need it to start getting full of water to see how it behaves as the desiccant gets loaded up but it’s still behaving pretty similar to how it did when I first turned it on. I think the reservoir is “big enough” for sure. Times to hit 19% humidity in the filament dryer may be getting longer but some of these spools have been open for while and were stored in ziplock bags with desiccant. No hygrometers and no drying so no idea what their starting water is without comparing weights.
I stopped weighing for now and am just drying to 19% in the dryer. I think that’s all I need to do now to have well-dried PLA spools and it makes it easier to just crank through the last of my open spools to see when it stops drying or the output hygrometer starts indicating above 10%.
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I should add that most of my measurements are on spools of filament that have cardboard spools, and it may well be that in this kind of configuration, it’s that cardboard may well be the dominant actor in affecting the sealed container measurements. I can’t yet claim it with certainty, but early preliminary measurements seem to indicate that, fresh from the sealed factory packaging, it’s the cardboard that holds far more water than the plastic filament, at least for lower temperature plastics like PLA, PETG, and maybe even ABS as well.
Also, this is using fresh from the package silica gel (I’m using mostly dry-and-dry sachets at the moment), and although the orange indicators seem to imply that it’s dry, maybe it nonetheless could be even dryer if I were to bake it before using it. Maybe doing that would make it a bit more “grabby”. Or maybe not. Maybe this is as good as it gets. There’s so much contradictory information floating on the internet about this particular point that I probably can’t really know for sure until just do the experiment.
Anyhow, regardless, all this is below what Bambu somewhat vaguely advised in its wiki, so many I’m splitting hairs needlessly. I just want to finally get to the bottom of “what the heck is really going on?” as best we can, once and for all.
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Are you thinking about building one? If so I can post the 3mf here before the model page is ready on MW. The formatting is a pain and I’ve got lots of photos to try to explain it all.
Not yet. Right now I’m gathering parts to try something related, but a little different first. However, I’m inspired by your efforts, and I’m learning from your experience. For instance: leaks. Because of that possibility, I want some kind of confirmation at the end of the stack that I’m getting usable flow. So…I wish I could find a suitable air flow indicator for a low flow application. How low flow? Well, that gets back to your earlier question, and I don’t know. So, for now, I’m just going to wing it with this:
which, ironically, means I guess my minimal flow rate, for this entirely arbitrary reason, will have to be 0.1L/minute or greater.
Your idea of some kind of bubble indicator would be viable. I could put one of those on the very tail end of it as proof that air is flowing out of the very end, as a way of measuring final emission from the filament dryer. That would give me full dynamic range down to even very, very low flow rates. And it’s simple. Simple is good. So, we’ll see how it goes. Might need to add a check valve to ensure fluid doesn’t siphon backward when things cool down.
Most likely the inlets and outlets of the various parts will be different diameters, so this will most likely be a huge hack attack fitting it all together with adapters and reducers. Meh, it’s either that or think harder and longer, and I’m doing too much of that already, so… onward and hopefully forward I go, as I muddle through this. A first pass is better than no pass.