Polydryer Humidity: Another Month of Data

The 25 g of silica gel in each Polydryer box produced these results after a month:

	8 Sept 2025			11 Sept	23 Sept
Filament	%RH	Wt – g	Wt gain – g	%RH	%RH
PETG White	25	27.6	2.6	15	21
PETG Black	22	27.3	2.3	15	20
PETG Orange	21	27.2	2.2	21	23
PETG Blue	19	27.3	2.3	14	15
PETG-CF Blue	24	27.4	2.4	21	22
PETG-CF Black	21	27.3	2.3	15	19
PETG-CF Gray	27	27.1	2.1	24	26
TPU	25	27.4	2.4	22	24
Empty 1	51	no gel	n/a	27	30
Empty 2	35	27.9	2.9	19	28

The humidity levels seem higher than before, with a bit under 10% weight gain.

The two “Empty” boxes show the difference between ambient basement humidity and letting 25 g of silica gel work on the box for a month. Comparing the latter’s weight gain with the other boxes shows occupying (much of) the interior with (relatively) dry filament reduces the desiccant’s workload.

The beads in the “Empty 2” box were definitely darker after soaking up an entire box full of 50 %RH air:

The meter reads 37%, rather than 35%, due to being out of the box for a few minutes.

They’re the darker swirl in the pan of beads:

That’s an accumulation of beads from a few months, not just what you see in the table.

I used an induction cooktop to heat the cast-iron pan. Some fiddling with the cooktop’s constant-temperature mode got the beads to 200 °F with a 460 °F setting in about an hour. Setting the cooktop to 50% in constant-power mode worked better, as the beads reached 220 °F in an hour and 230 °F after another hour.

The bead weights at various stages:

• Start = 531 g
• +1 hr at constant temperature = 491 g
• + 1 hr at 50% constant power = 483 g
• + 1 hr ditto = 480 g

The 41 g weight loss is 8.5% of the dry weight, roughly what you’d expect from the humidity readings.

After reloading the meters with 25 g of alumina beads, the 11 Sept humidity readings are slightly lower and the 23 Sept readings are roughly comparable.