Yup. Put them in jars with boveda packs for a day or two. Most are within a point maybe two. Close enough for what we look for. Couple years back I found a place one eBay that sold these. Great buy not able to calibrate but solid readings you can use a silver sharpie to add plus or minus after they sit in a jar with boveda packs day or two. EDIT TO ADD I DONT KNOW WHY THE CYRILLIC LANGUAGE CAME UP IN THE LINK ??? THEY ARE $12 for 6. I bought a couple batches.
Read the description and you won’t be disappointed.
The first ones @DougDawson posted are the ones l use. I’ve gotten a few packs of them over the years and gave a couple to a friend, for the most part they were pretty accurate within 1%. Out of the 40 or so ive had, I think I only ever saw 3 or 4 that were off and even those were only about 2 to 3% different than the rest
I used them to monitor my reptile/amphibian cages too or I probably wouldn’t have had so many. Plus experimenting is fun.
I use Doug’s system:
Buy a dozen of the little ones on the cheap from the Amazonians,
Put them all in a ball jar and run a “Salt Test” (link below)
After 72 hours they should all read 75%
But they won’t all read 75%, so you can keep the good ones and steal the batteries out of the bad ones before pitching them since the cheapos don’t allow calibration adjustments.
In my experience you’ll end up with over half that are usable ± 1% for less than the price of a single high-end hygrometer.
They die slowly. Like the display fades. I think mine are 2 years old and half have died. The others you can still read if you look at them at just the right angle.
Almost worth just buying new ones than changing the batteries.
Buy the batteries in bulk on Amazon, I bought some LR44s for a timer a couple of months ago, they are only $10 for a hundred, they might not last as long as big name brands but for the money they are good.
