Product review (5 stars): (Edited) Save your money...
I am editing this review from 1 to 5 stars.  The original unit was defective so I contacted the seller and the customer service they provided was perfect.  They provided a second unit within a week.  It is rare to have the ability to personally contact a vendor and to have no-nonsense service...very professional and they stand by their product.  For this reason, they get a 5-star review!!

Original review: Extremely disappointed.  I purchased the Extech Model 445815 standard configuration with the attached probe.  A feature that lured me in was that this unit was able to be calibrated.  I looked at the NIST traceable options as I would like a somewhat accurate reading of RH in any meter I am choosing.  However at either $130 or $173, the price is certainly prohibitive for this level of hygrometer.

Using the Boveda One-Step calibration standard of 75% RH, I followed those instructions to the letter.  The Extech never registered above 56%.  No problem I thought, I will adjust the calibration bucket and line the display up with the calibration value.  To my surprise, and as noted in other negative reviews, the calibration bucket did nothing at all...no change in value with 1/2 turn...after several minutes, another half turn and still nothing.  Rinse and repeat.  Bottom line is the RH calibration does nothing.  I was able to calibrate the Temperature and that adjustment screw worked.

My experience indicates to me that this is very poor quality toy with ornamental features at best.  Save your money on this one folks as you will be disappointed.  If you need accuracy, you will need to step up to a scientific-level meter at much higher cost point...This unit is NOT the answer.