Category: Science

If you measure something often enough, it becomes science

Silica Gel Beads: Regeneration Damage
The silica gel beads I’ve been using in the PolyDryer boxes start out a uniform yellow / light brown:

Polydryer Box desiccant tray – top view

The humidity indicating chemical seems to be methyl violet, described as changing from yellow to green when saturated, which has never happened here. For example, these beads, retrieved from random corners of the workbench, have been sitting in 40-ish %RH basement air for weeks:

Silica gel beads – 36pctRH ambient

The fragment just left of center looks greenish, but the rest are, at best, various shades of brown. This may be due to the (relatively) low humidity in the basement, but putting them under a damp sponge for a few hours didn’t change their color.

The most recent regeneration session started with an open cast-iron pan on an induction cooktop:

Silica gel beads – drying

The variety of browns comes from various amounts of adsorbed water in the PolyDryer boxes, but AFAICT there really isn’t much correlation between the humidity level and the amount of adsorbed water.

The drying process went like this:
- 650 g at start
- 50% power for 2 hr → 200 °F
- Covered the pan & turned it off overnight
- 623 g at start
- 50% power for 2 hr → 220 °F
- 612 g
- 50% power for 1 hr → 236 °F
- 610 g
- 30% power for 30 min → 205 °F
- 35% power for 30 min → 200 °F
- 609 g
So about four hours at 50% power would get all but the laser few grams of water out of the silica gel.

After all that, the beads looked about the same in a white bowl for cooling:

Silica gel beads – damaged indicator dye

Each regeneration cycle leaves more dark brown beads in the mix, which may be due to poor temperature control, and they do not return to their original yellow / pale brown shade.

Apparently cooking silica gel beads over 120 °C = 250 °F (various sources give various temperatures) can damage their structure or the methyl violet indicator; for sure some of those beads have been abused.

Unsurprisingly, the bead temperature rises as they dry out. Although the induction cooktop has a temperature control, we’ve found the setting doesn’t match the pan temperature and the overall control is poor. I could set the gas oven to 200 °F, but I’m certain it doesn’t control the temperature all that closely, either.

The original jug held 2 pounds = 907 g of beads. Add the 609 g from this session to the 350 g of allegedly dry beads in seven of the PolyDryer boxes: my regeneration hand is weak.
2026-01-07

PolyDryer Humidity: December

The measurements:

	2025-12-29
Filament	%RH	Weight – g	Wt gain – g	Gain %
PETG White	18	53.8	3.8	7.6%
PETG Black	18	53.8	3.8	7.6%
PETG Orange	22	52.3	2.3	4.6%
PETG Natural	22	53.4	3.4	6.8%
PETG-CF Blue	18	54.1	4.1	8.2%
PETG-CF Gray	23	54.1	4.1	8.2%
PETG-CF Black	18	53.8	3.8	7.6%
PETG Blue	10	52.9	2.9	5.8%
TPU Clear	18	54.4	4.4	8.8%
TPU Black	18	55.1	5.1	10.2%

I used to think there was some correlation between the indicated humidity and the amount of water adsorbed by the silica gel, with the humidity rising as the gel absorbed more water. That is obviously not the case.

AFAICT, I’d been reading the chart wrong:

Instead of the adsorption being a function of the equilibrium humidity, it’s the other way around. With the humidity held constant (by adding water vapor), the silica gel will adsorb thus-and-so percentage of its weight and equilibrate at that humidity. If the filament was an infinite reservoir of dampness, then the equilibrium humidity would indicate how much the silica gel had coped with.

At least I think that’s how it goes. I have been wrong before.

Anyhow, IMO the right way to proceed is to just replace the silica gel every month and be done with it.

Also true: the humidity meters aren’t particularly accurate at the low humidity values in those boxes.

2026-01-06

Dryer Vent Filter Snout: More Warping

I have unfairly maligned the TPU snout, because the PETG snout failed the same way:

Clothes Dryer Vent Filter Snout – warped PETG

Seen with the shock cord in place, it’s obvious that combining moderately high temperature with steady compression sufficed to bend the PETG enough to pop those tabs loose from the vent.

So the OpenSCAD model now produces a stiffening ring to be laser-cut from acrylic:

Clothes Dryer Vent Filter Snout – OpenSCAD stiffener

The whole snout builds as a single unit in the obvious orientation:

Clothes Dryer Vent Filter Snout – V2 – slicer

Because the part of the snout with the tabs is 7 mm tall, I glued a 4 mm acrylic ring to a 3 mm ring, with both of them glued to the snout:

Clothes Dryer Vent Filter Snout – acrylic gluing

That’s “natural” PETG, which I expected to be somewhat more transparent, but it’s definitely not a dealbreaker.

Mary will sew up another cheesecloth filter and we’ll see what happens to this setup.

As the saying goes, “Experience is what you get when you don’t get what you want.”

Fortunately, living in the future makes it easy to iterate on the design & implementation until experience produces what should have been obvious at the start.

2025-12-11
PolyDryer Humidity: White PETG

The white PETG filament started out at 39 %RH and 50 g of silica gel dragged it down to 23 %RH after a three days: still unusually high.

The beads weighed 54.6 g, a weight gain of 9 %, which is about as much as they’ll take. I replaced them with 50 g of new-from-the-bottle beads and the meter dropped to 14 %RH overnight.

Running the tiny fan for another day made no difference:

Polydryer Box desiccant tray – fan

Thereby confirming my suspicion that air circulation inside the box isn’t nearly as much of a problem as I expected.

So filament need not arrive bone-dry and, with enough surface area exposed to the air, silica gel beads can adsorb their limit of water vapor in a day or two.

2025-11-24

PolyDryer Humidity: November

The measurements:

	2025-11-04	2025-11-11				2025-11-19
Filament	%RH	%RH	Weight – g	Wt gain – g	Gain %	%RH
PETG White	39	39	27.4	2.4	9.6%	23
PETG Black	25	26	26.7	1.7	6.8%	14
PETG Orange	30	23	26.5	1.5	6.0%	23
PETG Blue	18	23	27.0	2.0	8.0%	10
PETG-CF Blue	25	25	26.5	1.5	6.0%	18
PETG-CF Black	22	22	26.3	1.3	5.2%	14
PETG-CF Gray	28	28	26.5	1.5	6.0%	18
Empty → PETG Clear	31	n/a	26.8	1.8	7.2%	18
TPU – Clear	28	29	26.3	1.3	5.2%	14
W empty → TPU – K	23	27	26.8	1.8	7.2%	18

All the boxes now have filament spools and 50 g of silica gel divided equally between the humidity meter and the tray in the bottom of the box:

Polydryer Box desiccant tray - installed — Polydryer Box desiccant tray – installed

The PETG White in the first row is the new spool loaded last month. I think the 39 %RH indicates the spools do not necessarily arrive bone-dry in their vacuum-sealed bags with a tiny desiccant packet.

Conversely, both the PETG Clear and TPU K filaments are new spools that seem reasonably dry out of their bags.

The auto-rewind spindle in the PETG Orange filament hasn’t been working quite right, so I opened the box a few times. It now has a new PETG-CF spindle.

2025-11-20

PolyDryer Box Desiccant Tray

Having used desiccant in tea bags inside the PolyDryer boxes with some success, I wanted to see what happens with more exposed surface area:

Polydryer Box desiccant tray – installed

The tray (jawbreaker boxes.py URL) is 2 mm chipboard with a quartet of additional notches fitting the protrusions in the bottom of the Polydryer box:

Polydryer Box desiccant tray – assembly

Although you’ll find plenty of printed trays, many with ingenious perforated lids, this was quick & easy:

Polydryer Box desiccant tray – cutting

They’re painfully prone to dumping their contents, despite the dividers which are intended to dissuade the beads from taking collective action and surging over the slightly higher outer walls. Fortunately, the dump occurs inside a sealed box and is entirely survivable.

Distributing 25 g of silica gel neatly fills the sections:

Polydryer Box desiccant tray – top view

Now it’s just a matter of time …

2025-11-17
Amazon Basics Alkaline AA Cell Failures

A few weeks ago, the house seemed unusually warm when I crawled out of bed. Checking the heat pump thermostat woke me right up:

Heat pump – battery critical

This, as they say, is not a nominal outcome.

A pair of AA alkaline cells powers the thermostat and, due to its wireless communication link to the heat pump’s air handler in the attic, it chews through two pairs a year. As you’d expect, it displays a “Battery Low” message for at least few days at the end of their lifetime, which was not the case for this failure.

After replacing the cells, the thermostat reported that, yes indeed, the house was much warmer than usual:

Heat pump – high temperature

A temperature monitor showed the heat had jammed on in the deep of the night:

Heat pump – runaway temperature

The heat pump exhaust temperature showed a similar event:

Heat pump – exhaust temperature

One of the AA cells showed about 1.3 V, but the other was around 0.25 V, suggesting an abrupt failure, rather than the normal gradual voltage decrease with plenty of time to replace the cells.

It’s reasonable to jam the heat on when the thermostat isn’t communicating, rather than let the house gradually freeze, but it did come as a surprise. I don’t know how the heat pump reacts to a battery failure during the cooling season; not refrigerating the house would be perfectly fine in most circumstances.

The Amazon Basics AA cells I’ve been using have worked as well as the Name Brand ones, so I was willing to write one off as happenstance.

However, during the recent Daylight Saving Time dance, I discovered the clock in Mary’s Long Arm Sewing Room had stopped, with an Amazon Basics AA alkaline cell from the same lot inside:

Failed clock AA cell

The date shows I’d replaced it in March, with the previous cell lasting an amazing 3-½ years. This one was completely dead, reading barely 0.1 V, after seven months. Mary hasn’t had a quilting project at the long-arm stage in recent months, so the clock may have been stopped for quite a while.

Perhaps something has gone badly wrong with Amazon’s battery supplier QC.

As the saying goes: Once is happenstance. Twice is coincidence. Three times is enemy action.

2025-11-04