This seems cool, but I wasn’t a big fan of the part where the guy put his fingers right up near the 300 degrees nozzle. 
Regarding silica gel desiccant, if I dry it at 140C and put it in a sealed container with a hygrometer, the ambient air inside the container might get as low as 2% RH.
At the other end of the spectrum, when it is nearly saturated, it might get as high as 40% RH.
So, plainly there’s a continuium inbetween as the desiccant picks up more and more moisture.
So, lately, after drying a spool, I’ll put it in a sealed cereal container with a hygrometer and no desiccant just to see what the RH levels out to when it cools off. Afterward, I may have some not still fully dry desiccant (perhaps leftover from protecting some other spool), and I have to decide whether to re-use it again before re-drying it. So, how I decide is this: if the RH in a sealed container at ambient of that desiccant is lower than the RH of the sealed filament alone, I can use it. My guess is that if the steady state RH of the desiccant is already above that of the steady state filament by itself, then it would probably be donating moisture to the filament, and not the other way around.
I haven’t actually tested this to confirm whether or not it’s true, but common sense seems to indicate it would be true. What do you think?
I suppose in your case, if the desiccant has an RH < 20%, it would still be worthwhile (though obviously the lower the better)? In this case I’m just referring to packaged or unpackaged desiccant that cohabitates with the filament, not desiccant in a column used in a filament dryer.
The sealed container test works great. They are small in volume especially with a spool in there displacing air. The small volume of air means very little water can have a big effect on humidity. In general, whatever you put in there will dominate a humidity measurement.
So if you put silica gel in a container with a hygrometer, the RH will move to the RH characteristic for whatever water content is in the beads. If the beads carry more moisture the humidity will be higher, less water and it will be lower. It’s a physical characteristic of the silica gel beads and manufacturers generate humidity/moisture content curves that show that relationship. If you specify a humidity, you’ve specified a moisture content because of that water/humidity relationship and you can find lots of those curves are available on the web.
Filament is the same way but we don’t have many published moisture content / humidity curves. Thing is, even if we don’t know those curves there is a relationship very similar to silica gel.
If you test silica gel and filament separately, and their RH readings are different, water will move from higher RH/moisture content to lower to establish equilibrium.
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Clarification - if you put them in the same container then the one that would have a higher equilibrium humidity would donate water to the air to where the air would move to that equilibrium humidity value.
This is done all the time in the museum and archival storage fields. Companies sell silica gel preconditioned to moisture contents that will produce equilibrium humidities that help preserve documents and artifacts.
Humidity higher than equilibrium for the filament moisture content will just allow the filament to absorb water until it’s in equilibrium with the new high humidity value.
This is why I commented before that people who leave silica gel sitting out are letting it absorb moisture from the air to try to equilibrate to ambient humidity. To then take that silica gel and store it with filament just provides a ready reservoir of water to hydrate the filament with. Silica gel can never dry to lower humidity than its equilibrium value.
When the silica gel is used and already has moisture you get into the situation where it doesn’t have so much water capacity in reserve to hold water that the filament might release. It may scavenge some water but the humidity will rise quicker than if there was lots of remaining capacity to hold water.
Used/partially hydrated silica gel will still scavenge water as long as its equilibrium humidity is lower than the filament’s. You just need to keep a closer eye on it because the humidity will rise faster as water infiltrates and it can become ineffective sooner and need regeneration.
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Tangential, but this gizmo’s approach is interesting: Speculation about new printer announcements - #2883 by NoahKatz
That looks interesting. I think it could work. Only thing is like you said is it a deep dry or surface dry. They soaked the filament for a fair while so I don’t know their results are biased to just surface moisture. Using their PLA numbers it looks like they got the equivalent of a little over 1.5g of water off a kg spool which is in the ball park but maybe a little low from what I was seeing with PLA and that was with them loading up with extra water.
Grain of salt. I haven’t seen it and have some questions. But it should work. The question is how well.
And the inline dryers are stuck with the feed rate you use. That sets residence time in the dryer. The faster you print the less time water has to get out.
Also, it’s something else to set and check if you use different filaments with different drying parameters.
I don’t know. It could be good.