Humidity matters

[Dave Bagwill]

So: I have a guitar on my workbench, a new build, and I notice that, behind the bridge, the top is no longer domed or even flat. It was braced with 40' radius top braces.

I keep the shop at 40%-50% humidity according to my digital hygrometer.
I brought the hygrometer in the house and did the following test:
1. Got together a bottle cap from a large Pepse, some table salt, water, and a ziploc bag.
2. filled the cap with salt, and added a few drops of water to make a slurry, not enough to dissolve the salt.
3. put the cap and the hygrometer into the ziploc and sealed it, leaving some air in there so that the bag did not block the hygrometer.
4. Left it for 6 hours.

Repeated because I could not believe it.
But in fact, whereas the test should result in exactly a 75% humidity in the ziploc, my hygrometer reads -
55%. In both tests.

So, I built this guitar in perhaps 70% humidity without knowing it.
I don't really have a question here, but thought I'd share.

[Kevin in California]

I just noticed last night that my dred I built 6 years ago and sits on a stand in the house most of the time with the humidity varying from 35% to 65% (currently about 53% in the house) is now concave on the top just in front of the end block, and on the back, the same thing.
My crossover, which I'm keeping in a case until I can porefill and put the finish on it has all come back on the top except for that same area in front of the end block on the top, it is still low there. I keep the guitar inside of the case at 45%.

[Jay McClellan]

I had a similar problem with my first build because I trusted a cheap digital hygrometer. I had the top braced and once I figured out the hygrometer was way off (reading about 15% lower than reality) and put a dehumidifier in my shop, the top turned into a Pringles potato chip shape. Fortunately I hadn't glued up the box so I shaved off all the big braces and glued on new ones in the dry air after things had stabilized.

Since my cheap digital hygro had no way to calibrate it I bought a decent analog hygrometer plus a sling psychrometer to calibrate it. Just the psychrometer would be enough except it takes 3 minutes to get a reading and my arm gets tired swinging it that long, so it's nice to have the analog for a quick read each time I go out to the shop.