Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Category: Science
If you measure something often enough, it becomes science
Having accumulated a bunch of used activated alumina desiccant, I figured now was a good time to try regenerating it. Industrial applications use dry gas and very high temperatures, but perhaps holding it over 100 °C for a few hours will suffice for my purposes.
After an hour the surface temperature was around 150 °F, so I covered the pan with a water-cooled lid to see if any vapor condensed on it:
Alumina regeneration – lid cooling
It did, indeed, so I alternated covering and exposing the pan, which was likely a waste of my time, until the alumina dried enough that the lid didn’t collect any condensation. The whole process took just under four hours with the cooktop set to its maximum of 460 °F for most of the time.
The beads then cooled to room temperature in a covered dish:
Alumina regeneration – final cooling
The beads weighed 626 g at the start of the adventure and sweated down to 593 g, parting with 33 g = 1.2 oz of water in the process for a loss of 5.6%. I have no idea how dry they are now, but they’re an ounce drier than before.
The indicator cards show the humidity is maybe a little over 10 %RH:
PolyDryer – TPU base – 10pctRH cards
The meter jammed in the other end of the box splits the difference at 15 %RH:
PolyDryer – TPU base – 15pctRH TP
Put the box atop the improved PolyDryer, set it for the recommended 12 hours with “two bars” of oomph (which may roughly correspond to the temperature), and fire it up.
The OEM meter occasionally glitches to 10 %RH:
PolyDryer – TPU dry 1200h – 10pctRH glitch OEM
That type of humidity meter apparently reports values from 10 %RH upward, so this seems like the kind of glitch where the reading jams at one end of the range due to the sensor opening up / shorting / misbehaving. It does not correlate with any nearby electrical activity due to fans / heaters / 3D printers / whatever.
A little under eight hours later, it shows 17 %RH:
PolyDryer – TPU dry 0425h – 17pctRH OEM
Although it still has glitches to 10 %RH.
The cards look about the same, although I could be persuaded the 10% spots look ever so slightly more blue:
PolyDryer – TPU dry 0425h – 10pctRH cards
The meter in the back shows it’s toasty in there:
PolyDryer – TPU dry 0425h – 10pctRH TP
A psychrometric chart shows heating air from 66 °F & 15 %RH to 117 °F will put it at 3 %RH without removing any water vapor. This is far below the level my cheap “instrumentation” can measure, but it does suggest the meters should bottom out, regardless of whatever the silica gel is doing.
Allowing six hours to cool down & stabilize after the PolyDryer turns off in the middle of the night (because for science does not include all-nighters) shows a rebound to 26 %RH on the OEM meter:
PolyDryer – TPU dry 0000h – 26pctRH OEM
The cards remain unchanged:
PolyDryer – TPU dry 0000h – 10pctRH cards
The meter in the back again splits the difference at 16 %RH:
PolyDryer – TPU dry 0000h – 16pctRH TP
I pulled the larger meter and both cards out of the box.
After sitting undisturbed for a day, the OEM meter in the box stabilized at 10 %RH:
PolyDryer – TPU post dry – 10pctRH OEM
The card agrees, to the best of its limited resolution:
PolyDryer – TPU post dry – 10pctRH card
The silica gel weighs 25.0 g, exactly what it did when I loaded the meter case. I think the scale’s 0.1 g resolution exceeds its accuracy, but even if the silica gel weighed 25.2 g ≅ 0.8 % water the humidity would be under 5 %RH.
As far as I can tell:
The filament on the spool isn’t outgassing water vapor
The air in the TPU box remains under 15-ish %RH at normal basement temperature
Running a PolyDryer cycle at 15-ish %RH doesn’t stuff any more water vapor in the silica gel
Cheap humidity meters lack accuracy around 15-ish %RH
Humidity meters take longer than you think to stabilize
Humidity indicating cards may be as good as you (well, I) need
A pair of PolyDryer boxes has been holding black and gray PETG-CF for a while:
PolyDryer – PETG-CF – 32 pctRH Black 31 pctRF Gray
A few days ago I slipped humidity indicator cards into the boxes:
The black PETG-CF card suggests 30 to 40 %RH:
PolyDryer – PETG-CF – 32 pctRH Black test card
Yes, I dropped that card into the box upside-down.
The gray PETG-CF card shows similar results:
PolyDryer – PETG-CF – 31 pctRF Gray test card
The desiccant in the black PETG-CF box weighed 80.9 g, a gain of 5.9 g = 10.8%. The chart suggests that corresponds to 35 to 40 %RH:
Desiccant adsorption vs humidity
The gray PETG-CF box had 102.0 g of desiccant. I apparently loaded 25 g in the meter container and 70 g in seven tea bags, but I don’t trust those numbers enough to go any further.
Unlike the black PETG box mismatch, these black PETG-CF numbers seem plausible. The results may depend on allowing far more time for the filament + air to equilibrate with the desiccant tucked in its containers than the days I’ve been giving it.
An adjacent pair of PolyDryer boxes have black and orange PETG filament:
PolyDryer – PETG – 27 pctRH Black 25 pctRH Orange
They’ve been sitting closed up for a week or so, with only 25 g of activated alumina in the desiccant holder (no tea bags with additional desiccant) pulling moisture out of their air and, presumably, filament.
The desiccant from the black filament weighed 29.0 g, showing it pulled 4.0 g of water out of the air, 16% of its original weight.
The “aluminum oxide” curve shows 16% adsorption should correspond to more than 50% RH, so the numbers don’t quite match up. On the other paw, I don’t know how much I can trust the meter accuracy.
I replaced the desiccant with 25 g of silica gel, tucked a humidity indicating card into the box, and snapped it closed again. The orange PETG box also got an indicating card so I can compare results.
Mary found the wrench I made five years ago in the bottom of her tool bucket:
Hose Valve Knob – five years later
Having moved away from the garden with all the valves that wrench turned, it can now go into the 3D Printed Sample Box for use in the unlikely event I ever give another talk on the subject.
I’d design it differently these days, what with BOSL2 in my sails, but it got the job done.
A critter made off with our battered plastic rain gauge, so I set up an Ambient Weather WS-5000 station to tell Mary how much rain her garden was getting. I added the Official Bird Spike Ring around the rain gauge to keep birds off, but robins began perching atop the anemometer while surveying the yard and crapping on the insolation photocell.
After a few false starts, the anemometer now has its own spikes:
Weather station with additional spikes
It’s a snugly fitting TPU ring:
Weather Station Spikes – build test piece
The spikes are Chromel A themocouple wire, because a spool of the stuff didn’t scamper out of the way when I opened the Big Box o’ Specialty Wire. As you can tell from the picture, it’s very stiff (which is good for spikes) and hard to straighten (which is bad for looking cool).
The shape in the middle is a hole diameter test piece. Next time around, I’ll use thicker 14 AWG copper wire:
Weather station spikes – test piece
The test piece showed I lack good control over the TPU extrusion parameters on the Makergear M2, as holes smaller than about 2 mm vanish, even though the block’s outside dimensions are spot on. This application wasn’t too critical, so I sharpened the wire ends and stabbed them into the middle of the perimeter threads encircling the hole.
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It just barely clears the curved air guide inside:
PolyDryer airlock plate – tiny fan installed
The tea bags full of desiccant allow some wind between them and the filament in the spool, but I obviously must re-think that setup. There’s enough clearance for what should be reasonable circulation, so i defined it to be good enough for now.
The box of TPU started at 25 %RH, dropped to 22 %RH overnight, then returned to 25 %RH the next day:
PolyDryer TPU – 25 pct RH
Now that I’m watching more often, I’ve seen the meter glitch to 10% for a few seconds:
PolyDryer TPU – 10 pct RH glitch
A humidity indicator card suggests the air is under 20 %RH:
PolyDryer TPU – humidity indicator card
It may be the filament can outgas water vapor as rapidly as the desiccant can remove it, but I expected the fan to make at least a little difference.
The Smell of Molten Projects in the Morning 