There's an old saying that a man who has a clock always knows what time it is, but a man who has more than one is never really sure.
I ran into the humidistat version of that today. I used to not care what the humidity was in the house. It would occasionally get so dry that I'd need Chapstick or something similar, but otherwise I didn't really worry about it.
Getting a quality violin changed that. So I bought a whole-house humidifier, and finally got it set up today. Pierre (Fiddlerman) had recommended a digital humidistat a week or so ago, so I got one of those, too.
The digital one said it was 35-37% - rather low. When I turned on the humidifier for the first time, it said 47%. I moved it some (the first place I measured was towards the back of the house, the humidifier is more towards the front, and it went up to 52%. The digital one said that same area was 41%.
So I dug out the two cases I have that have humidistats in them. One agreed with the humidifier, one agreed with the digital humidistat.
But I found a way out, which is why I'm posting. https://planetcalc.com/2167/ is a converter from absolute to relative humidity, and vice versa.
I got the temperature, (relative) humidity, and barometric pressure from weather.com. That gave me an absolute humidity. Then I used that number to calculate the relative humidity at the (somewhat warmer) temperature inside the house. Within a couple of percent, it agreed with the digital humidistat, so that's probably what I'll trust now.
Just in case anyone found that explanation confusing, I put in:
51% relative humidity (from weather.com)
20 degrees Centigrade for air temperature (also from weather.com. If you get your weather in degrees F, you'll have translate. Search google for 'degrees f to c'.)
1007.8 mm Hg for barometric pressure (also from weather.com)
Calculation precision 3 (the default, and probably as high as was justified.)
That gave me .009
I then went down to the section below it (absolute to relative) and put in
Absolute humidity 0.009
Air temperature 24 (inside my house)
Barometric pressure 1007.8 mm Hg (same as above, unless you have the Big Bad Wolf trying to blow your house down.)
That gave me 41% relative humidity, which matched the digital humidistat.
Hope this helps somebody.
Update: You can pretty much ignore the air pressure. I finally got it to react, but I had to use insane numbers (like 15,000 mm Hg, 15 times normal atmospheric pressure) to get it to react. If you're planning to keep your violin in a pressurized scuba tank, you may need to take this into consideration. For everybody else, anything halfway near 1000 will give you sane results.