These are cheap and relatively accurate, but you will have to open the bag or position it in the window to read. https://www.amazon.com/gp/product/B07GNMKYCZ/
These are bluetooth so you can check you phone and see the humidity without opening the bag.
https://www.amazon.com/gp/product/B07R586J37/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1)
Like the bluetooth, but 12 bucks. Otjhers look decent.
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 wonât be disappointed.
Oh man I forgot about this listing, thanks man
The first ones @DougDawson posted are the ones l use. Iâve 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 wouldnât have had so many. Plus experimenting is fun.
The brand escapes me but I got a bulk pack and they all read true.
Bad news is who knows how long those batteries have been on and they die quickly. Cheap enough fix just buy new batteries.
I have 17 of them so I must really like them 
I also use them to determine microclimate zones in my apartment. Any direction I look thereâs a sensor.
I use Dougâs system:
Buy a dozen of the little ones on the cheap from the Amazonians,
Put them all in a ball jar and run a âSalt Testâ (link below)
After 72 hours they should all read 75%
But they wonât 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 donât allow calibration adjustments.
In my experience youâll end up with over half that are usable ± 1% for less than the price of a single high-end hygrometer.
Cheers,
-Grouchy
Tried to buy the ones you posted, but they wouldnt ship to the States, so i got these, had good reviews and were cheap
Nice, they look the same. Hopefully they work fairly well bud.
When i pulled them out of the box, ALL read 43 % so looks good.
Yea those are the ones I have. No complaints except battery life
Checked this morning all 10 read 62 percent
They work so does the Boveda
How long was battery life ?
That will be hit and miss as there would be many factors at play
I must have had a bad batch to have registered such a wide range. Who knows how long mine might have sat in the Amazon warehouse
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.
Iâm good with a year life
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.
I keep planning to do this but havenât gotten around to it yet. Itâs been brewing for awhile 
Actually theyâre e13 batteries which are the same as lr44. Iâve done my research 
