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I use the super-absorbent polymer spheres/chunks soaked in 50/50 PG-water solution, then refill with distilled water, and occasionally more 50/50 to top up the PG because it eventually evaporates (but really really slowly). An example is the Humi-Care jelly jars, but you can make your own equivalents much cheaper with bulk super-absorbent polymer and any kind of plastic jars you like.
In the same humidors (well, Sterilite tubs actually), I also have multiple 69% Boveda packs. The way it works is, the polymer with PG and water emits and maintains approximately 69-70% RH, and the Bovedas do an excellent job of "trimming" the system, emitting when it's below that point and absorbing when it's above. I never have to recharge or even touch the Boveda packs, they stay at the proper plumpness level indefinitely, apparently because the "trimming" they do is very symmetric, so they emit/absorb about the same amount of vapor averaged over time.
The nice part is, recharging is easy and instant: just put more 50/50 or distilled in the jars. No waiting days or weeks for Boveda packs to very slowly recharge.
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