The Salt Test
@BarrieButsers suggested doing a salt test with the new hygrometers to check their accuracy, hey, why not?
Setup
Basically a tablespoon and a teaspoon (15ml) of salt in a container plus a teaspoon (5ml) of water (any will do). Mix into a paste and place in a sealed container with the hygrometer and it should level out at 75%.
My ambient RH is about 32%
I went with a zip-lock sandwich bag. Two days later they have settled out at 63%.
Thinking about it, I wondered if I was getting shafted by the porosity of the plastic (they have micro-holes, sometimes a lot).
So I double bagged it in a large freezer bag. I also placed a hygrometer in the outer bag.
Results
Well guess what? The sandwich bag leaks like a sieve & the hygrometers in the inner bag are now at 72%, that’s within the prescribed +/- 3% so, a successful test.
Improvements
Use a gasketed snap-lid container instead of plastic zip-lock bags
Cheers
G