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Get one new Boveda at your desired RH. Get a gasketed airtight transparent plastic container, not too small or the Boveda will keep the RH inside a few points too high which will throw your calibration off (they claim it won't, but I've tested it multiple times, and it does), about the size of a shoebox or just a little bit smaller is good. Toss in the Boveda pack and one or more Western Caliber IV units (which are excellent), positioned so that you can read their displays through the container without having to open it. Wait 24 hours, then begin taking multiple RH readings from them, writing them down in a log. After you get a healthy set of samples, average the deviation from the target RH for each unit to arrive at how much to add or subtract to make it maximally accurate most of the time. Use the units' calibration function to add or subtract the right number of points to/from each unit, where needed.
Works perfectly and it's very easy. No salt, no mixing/ratio questions or uncertainty, and you're calibrating at your real-world storage RH, not at 75, an RH you'll never actually want to reach.
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