I have two Hygroset II Digital Hygrometers. The battery went dead in one of them, so I went onto Amazon and bought some cheap batteries at MYBATTERYSUPPLIER (4 for $2.25 total). After I replaced the battery, I calibrated it by doing the salt test. I tossed the 2nd hygrometer into the test also. The 2nd hygrometer with the good battery was off by +8, YIKES...! So my one humidor was running at 60rh rather than 68rh. I guess that's not too big of a mistake, but it sure taught me to start testing my hygrometers about every 6 months.