Observation:
I use an Oasis humidifier and a soap-dish/sponge in each of my cases; Oasis in sound hole, soap dish behind headstock. I had been using a hygrometer only in my D40 case, as I figured that with similar case construction the readings should be about the same for all.
Then I decided to splurge on an second hygrometer for my DV52 case (Way to go, big spender)!
Surprise - considerable difference: 48% in D40, 42% in DV52. Same Oasis/soap dish in each.
So, let's first check the basics… Put both hygrometers side-by-side in the same case. Readings were within 2%. OK, close enough. Possibly case construction? I then returned both hygrometers to their respective cases, placing each in the same exact location - under the strings, next to the bridge.
Readings now almost identical! So it seems that when the meters are on storage compartment as opposed to on the body there is a considerable difference in cases.
Verrry interesting. So, placement seems to be important. Where do you guys measure humidity (in case)?