I like the statement in a video that I watched lately: “At the factory, all the incompetent people in the thermometer department are sent to the hygrometer department.”
You may have been thinking of buying or have bought a hygrometer to measure the humidity around your wooden instrument since the wrong humidity is destructive to wooden instruments and maintained humidity can even save re-tuning costs.
Hygrometers are notoriously inaccurate and there is only one way to (sort of) fix it. Test your hygrometer by putting in a tight container (such as a Ziploc bag) together with a small measure of salt-water slurry (in a shot glass or bottle cap). After eight hours or more, open the bag to read the hygrometer which should read 75% humidity. You will find that your hygrometer reads improbably wrong…usually low.
However many percent your hygrometer is off, you just take it into consideration for each time you want to know what the real RH is. Add the number of percentage points off it is or just do a percentage correction. Whatever you like.
You will find lots of information on the Internet about how to test the accuracy of your hygrometer. And the same with adjusting your hygrometer, but the adjustable hygrometers, that allow you to tinker with them, are more expensive ones.
Whatever you do, Music Sorb is wondrously good at gently maintaining the right humidity for your wooden instrument…hygrometer or not. And Music Sorb is probably the simplest protection that you can find on the market.