Calibration Oddity

[Eleven]

So I picked up 2 of these:



I did the salt test, 24 hours, both in the same baggie and they both showed 65%.

I thought well, they are cheap ($8 at walmart) so I went and got 2 more. Next time I put the 2 original ones back in a baggie and the 2 new ones in a second baggie.

This time after 24 hours the first set that had read 65% now both read 70% and the 2 new ones both read 65%.

I am beginning to think my method and mixture is causing the changes, cause they are uniform. If they all sucked, would the sets of 2 each give off exact same readings, and one of those sets did it 2 times.

The first time with the first set I left the salt/distilled mixture *barely* wet. The second time the mixture was much more fluid and slushy, but I could tell that the mixtures for each cap full were slightly different, and voila! I got 2 slightly different readings, but both hygrometers in each bag read the same!

I suspect all 4 are fine, I just changed the mixture slightly each time and got different readings each time. And each time the pairs read identical.

Anyone have experience with this?