Product review (3 stars): Clever, but unnecessary and possibly inaccurate
I have three hygrometers, including this one. The other two hygrometers generally show readings closer to each other than to this unit. Sometimes this unit shows higher humidity than the other two, and sometimes it shows lower humidity. I can't be sure which is correct, but because the others show readings that are closer to each other and more consistent, I suspect they are more accurate.

This unit can be used at the same time one of the manufacturer's outstanding guitar humidifiers is used. Unfortunately, even though this unit showed adequate humidity, my guitar showed signs of drying out. After a few years, the sponge in the humidifier shrinks and absorbs water more slowly, even when distilled water is the only kind of water used. This seems to reduce the effectiveness of the humidifier. So plan on buying a new one after a couple of years. This is not a problem because the humidifier works so well that it's well worth the money to buy a fresh one every couple of years.

I think this gauge is unnecessary. When I use one of the company's humidifiers that is no more than a couple of years old, it keeps the guitar adequately humidified. Knowing that, I don't need this gauge.