Don't waste money on a meter. Easy way to do it...
Calculate the volume of your box...
For example, the dryer box we make is 10 x 10 x 5" box = 500 cubic inches.
A 5g bucket is somewhere in the neighborhood of 1300cubic inches
That means that at 30C (86F), the max water content of your air (100% rh) is 30.4g/m3. (Let's use that temp for the sake of being conservative. Lower temps = less g/m3 of water)
So, that 500ci box (.00819 cu meters) can hold a maximum of .248976 g of H20 (.00819m3 x 30.4g/m3)
The 5g bucket (.0213m3) can hold approximately .64752g of H20
Your typical silica gel desiccant (the rechargeable kind) can adsorb up to 40% of it's weight. For the sake of being conservative, let's say it's 30%.
A 40g rechargeable desiccant pack @30% adsorbtion (desiccant doesn't ABsorb, it ADsorbs) can hold 12g of water.
That means 1 rechargeable pack can dry out a 500cu in box about 50 times before needing to be recharged.
Now, a couple of things to note. You will actually have less air in the box when you have your filament in it. You will also not likely 100% cycle the air out when you open/close it. You will also not be at 100%rh.
Also, you don't need to get sub 10% rh to keep your filament dry.
ABS, for example, should have less than 2% moisture content to be "dry" for extrusion.
So, 1kg spool of ABS, should, at most, have 20g of H20 in it. Let's say that you want to be well under the 2% and only want 1%. 10g is your max then.
Assuming your ABS is very dry (5g or less) when you start, we just saw that a 5g bucket can only have .65g of H20 in it at 86F and 100rh%. So then, you could store your filament in a 100%rh bucket and not get over 6g in your filament - still well below the desired 1% threshold.
Keep your box sealed, keep a 40g (or 2 if you want to be overkill) rechargeable desiccant box in it, and don't worry about it
