I've had the worst time with my PETG filament, test prints literally exploding during the print and finished prints being very brittle. The most likely explanation is that the PETG is too moist, it's supposedly very sensitive to that. My friend Christina got me some Lock & Lock boxes, they are nice and quite airtight and fit four spools very nicely. So, it's time to dry things up!
My first drying attempt was a Drybag™, which I think I put in on January 12th. It didn't make a noticeable difference. The next attempt, based on the recommendation for drying wet phones, was rice. I used a wild&long rice mix, starting 2021-01-24. It didn't do much either. Next up, baking soda, starting 2021-01-30. Nothing really noticeable.
Sure, I could try with actual commercial desiccant bags, but where's the fun in that? Also, they're somewhat expensive. So based on a suggestion from the almighty Internetz, I got some silicate cat litter. It's quite cheap, so I figured why not?
On 2021-02-09, I just put two ramekins with cat litter in. This time, to get a more precise measurement, I also dropped in the little temperature and humidity logger I found at the local measurement instruments store (yes, we have a measurement instruments store in my neighbourhood). The humidity changed drastically - but to more humid! It ended up at ~60%. In this chart, the X marks are where the humidity sensor picked up movement. You can see it matches drops in humidity when I take out the box and open it on the evening of the 9th and on the 12th.
On 2021-02-11, since there was a nice frost going on, I figured the frosty air would be very low humidity and would allow the silicate to evaporate off some water. I left some outside overnight and put it back in in the morning, this time in a little custom box I had printed in the meanwhile. Not any better. You can see a brief dip in humidity and temperature, but it quickly goes back up.
My next attempt was to bake off the humidity. Not wanting to use devices that also get used for food, in case there are other things in the litter, I used Deathtrap, our Danish-style toaster. I poured some litter into a container made of aluminium foil and set it to toast outside, stirring a couple of times. I could sit inside and watch the steam come off, but after half an hour I could see no more steam. I stirred it again and let it cool down to merely warm, then just plopped the foil container into the filament box (which had been standing open, letting it get back to the ~30% humidity in the room).
I did this twice, the second batch letting it have over an hour of baking, and boy did this make a difference! You can see where I take out the box and open it in the morning, and it drops from 63% to 33%. Then there's a temperature peak when I put in first batch of still-warm litter, and the humidity drops to about 15% relative (relative to the now warm air, of course), and evens out at 24%. In the late afternoon, I put in the second, longer-baked litter at 18:00, after letting it cool off for a bit. Bam! Single-digit humidity!
I left the humidity logger in there for several days after, and the results were curious. The box stands in a closet in my bedroom, and I always sleep with my door tilted open a bit, for coolness and fresh air. You'd expect the humidity to change smoothly along with the temperature when it's just left to its own devices, but that wasn't quite the case - or at least the measurements didn't follow that pattern. See how in the first day, when the temperature starts to drop, the humidity almost doubles over 45 minutes, then stays at almost exactly 10% for most of the night, only to drop off much faster than the temperature rises come morning. And then this effects slowly disappears over the following days. I would love to hear an explanation for this!
I can also confirm that the Prusa doesn't like when it's this cold. When I tried to start a print one of these cold mornings, it refused, claiming there was an error in the thermistor! Being metal, the bed had gotten even colder, and the temperature readings started at 11ºC, even though the room was warmer. Here's what it looked like a bit later, I was able to print shortly after this.
