Oasis Digital Hygrometer with case clip.
A ukulele, or any other stringed musical instrument made of wood, is at risk of cracking when the relative humidity drops below 40%. The Oasis® Digital Hygrometer accurately measures relative humidity and can help you decide when to use your Oasis® Humidifier. Because relative humidity varies greatly, even inside your home, we recommend that you keep your hygrometer close to your instrument.
To get an accurate humidity reading for the ukulele body, we recommend that you first lay the instrument case with the back to the floor and open the cover. Take a small piece of cloth and place it under the hygrometer to avoid scratching the top of your instrument. Put the hygrometer on the instrument top with the sensor at the edge of the sound hole. Close the case and allow 4-6 hours for the humidity to stabilize. Open the case and look at the reading within a few seconds and you get the humidity reading that existed just before you opened the case. Because the Oasis humidifier will somewhat distort the humidity reading, take readings with the Oasis humidifier in place and take readings without the Oasis humidifier. A well humidified instrument in its case will have relatively stable humidity readings, so you don’t have to measure the humidity all the time.
Honeywell Digital Hygrometer
Comfort zone highlights ideal room humidity. By using a room humidifier in your music room/place where you play, you will be able to keep that environment at a relative humidity above 40% throughout the winter months.
For more information, check out "A quick lesson in wood technology and humidity control" at http://www.bytowninstruments.com/pages/care-and-feeding