View Full Version : Take the calibrating hygrometer salt test with a grain of salt?

08-20-2015, 11:04 AM
In preparation for getting a new, solid-wood uke, I bought myself another hygrometer to keep tabs on the humidity levels in my uke case. Anybody who's calibrated a hygrometer has probably heard of the salt test where wet salt in a bottle cap placed in an airtight container will produce a humidity level of 75%. By placing your hygrometer in said container, you can tell whether it is high, low or right on.

I used the salt test to test my new hygrometer as well as the hygrometer I had in the case of my current uke. The readings for both hygrometers stabilized in about 10 hours but I monitored them for 36 hours. In the end, my old hygrometer held a steady 74% humidity for 24+ hours and my new hygrometer held a steady 76% humidity for 24+ hours. Great, these are exactly the type of results you'd want to see! The two hygrometers are only 2 degrees apart. All I had to remember was my old one is -1 degree from actual humidity and my new one is +1 from actual humidity right?

I took both hygrometers out of the airtight container and let them sit side by side on my table top to let them settle and acclimate to open air room humidity. The old one has now settled on 54% humidity and the new one has settled on 58% humidity. Now they are 4 degrees apart! A room hygrometer I keep in the living room measures 59% humidity so the newer hygrometer is probably the most accurate.

The lesson here is that a hygrometer that tests perfectly at 75% humidity using the salt test may not necessarily carry that same accuracy at lower levels of humidity. Once you've confirmed that it measures accurately as it should using the salt test, the hygrometer should be re-tested at the levels of humidity more suitable for your ukulele at 45-55%. Thus with my old hygrometer I need to shoot for readings of 50-60% +/- 1 humidity and my new one, 46-56% +/- 1 humidity even though they were virtually the same when comparing results of the salt test.