Product review (2 stars): Not a good choice for professional use
I had this as a backup for my $150 hygrometer which I can calibrate myself.  I like to take 3 meters with me letting them stabilize for 20-30 min and see if RH% is within a couple of points of each other.  Accurately measuring RH is SO MUCH harder than measuring temp. Anyone who needs to measure RH for their job should have at least 2 meters.  I found the pen style to be convenient but the RH sensor fell out of calibration in less than a month.  I measured 55-57% RH with 2 other meters but this pen said it was 65%.  That's a big difference.  It now sits in my desk.