I'm doing the moist salt calibration method, and I'm encountering a weird issue, though. I think I still have another hour or two to go on it before I can say I've got a final reading, but the hygrometer's capping out at around 65%. 75% is what a properly calibrated hygrometer should read during this test, which means that mine's reading low, and that my room would actually be at 55% relative humidity, which seems completely out of line.

Also I think I was getting similar results in the basement, which has a dehumidifier running in it.

Anyway, one of the sites I read gave specific instructions to use a half cup of salt and a quarter cup of water. I didn't have that much salt so I estimated out about 1/4 cup of salt and 1/8 cup of water. Are the salt to water ratios really specific for this to work?

Also I'm using a large plastic freezer bag. Is that not airtight enough?

1) Readings here, by themselves, made more sense. I was getting a read of about 37-40% relative humidity.

2) That still means my bedroom at home is more humid than I thought, which is good because that's where I'm keeping the large podocarp I bought, although I'm still a bit surprised about this.

3) Assuming it IS off by 10 points, as per the calibration test, that means my dorm is actual 47-50% relative humidity and the bedroom is like, 60ish. Which I still think is high since

4) Humidity is about 10 points higher about a foot away from the window than two feet away, which makes a lot of sense since that's where I have my plants and the window acts like a giant condenser plate.