Product review (3 stars): NOT Very Accurate, Even the "Second Time Around"
I'd actually give this 2 1/2 stars, just above "I don't like it", but not quite "It's OK", because it's NOT.

My unit is actually labeled a D'Addario, which is marketed by Planet Waves.  I've always been happy with products from both companies; I absolutely SWEAR by D'Addario Bass Stings.  But this hygrometer is just plain not accurate.  The first one I ordered (and returned) was even worse, but I accredited that to the fact that it was a used unit.  However, this unit is supposedly "factory fresh" and doesn't seem to have been calibrated properly.  Right now, my AC system thermostat shows the humidity in the house to be 51%, while this unit shows only 47%.  This could create a situation where I might OVER-humidify my $2000 acoustic guitar.  Knowing I need to make the allowance, I'm gonna keep this unit, but I doubt I'll buy another one.