I pick up an inexpensive hygrometer yesterday, and set it in my bedroom to get a sort of baseline reading. It was registering at 45% relative humidity. While I'd be quite happy with that, I'd be surprised if the humidity was actually above 30%. Anyway, being a mechanical model, I expected the calibration to be off anyway, and it appears to have one of those little turn screws to recalibrate it, so no big deal.

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?