[silkstring]

I managed to calibrate a Lifestyle Ambient synthetic hair large size hygrometer from Germany using a German technique of wrapping it up in a damp tea towel. This was supposed to bring it to 100% humidity. I was dubious but tried it out. The adjustment levers in this hygrometer are hard to locate at first, being accessed through the side vents. Another dificulty I experienced was in adjusting it quickly enough, because the hygrometer reacted immediately and dramatically to having the tea towel moved enough away to access the adjustment levers. However, I got it where I hoped it was at 100% which took about 20 minutes rather than the 12 hours for the salt test. To check it though, I ran a salt test the next night with "wet sand" consistency salt in a small jar lid about 3cm wide filled 3/4 full with salt and only enough water to get the wet sand consistency. The next morning it was almost dead on, reading what looked to be something like 74.99% RH. I don't know if this works for others, but it seems to have worked for me. I use the hygrometer for fragile musical instruments, so this is reasonably critical for me too.