Product review (1 stars): I used two of the Planet Waves rectangular hygrometers for ...
I used two of the Planet Waves rectangular hygrometers for over a year in two of my Martin guitars.  I had to add extra D'Addario Humidification System packs to get the RH up where I needed it (40-50%).  It didn't occur to me that I'd bought an inferior product until I tried two different hygrometers which read significantly higher than the two Planet Wave units.  I then purchased a Boveda One-Step calibration kit and discovered both of the Planet Wave units were reading significantly low.  One was at 65% instead of the 75% the calibration kit called for.  The other was at 69% instead of the 75% it should have been. In other words, I had too high a RH level for over a year in my guitars.  I accept some of the blame here as I should have tested the units before using them.  Planet Waves needs to accept the majority of the blame for putting sub-standard units on the market.  Guitars are way too expensive to risk damaging by relying on this product.  I won't make that mistake again.