Product review (5 stars): Tested with NIST traceable calibrated hygrometer & is accurate!
As someone who works with precision instruments and requires NIST certificate calibrated by certified companies, I was interested to see how accurate these hygrometers were for home use.  NIST standard is basically what companies go by to have it calibrated to a known precision test equipment to insure their own equipment is in fact has correct readout and is not broken or giving false readings.  This could be anything from pressure meter, torque wrench, digital calipers and so on.  I tested using a known calibrated industrial hygrometer with a fast sample rate.

There are a lot of factors people are missing when they do a review on a consumer level hygrometer and why their review may or may not be accurate.  I'm not doubting their reviews but when I see a review that compares another product without stating how their 'other' equipment was calibrated and validated for their accuracy, no one can say for sure which is accurate.

Like someone else mentioned, even the industrial hygrometers should be in its testing environment and acclimate before getting the actual reading.  With our hygrometer we use, because the unit travels from one station to another, 10-15 minutes is given to give a better reading.  If someone put this ThermoPro hygrometer with another and sees an instant reading, then chances are, it'll be all over the place.  It's more so if the unit came from an environment that was very humid and/or with a big swing in temperature than what he or she is trying to measure.

Keep in mind, these inexpensive products unfortunately is not a product I would suggest if the environment has heavy swing in temperature and humidity that would affect their use.  Why?  These hygrometers sample rate isn't fast enough for certain application and will respond very slowly to changes in temperature and humidity.  If these were industrial hygrometers, you're looking at spending few hundred dollars for a handheld model.

Another example; a $400 digital industrial hygrometer we use have a sample rate of 1 second and that can be changed as well as measure the dew point.  This product wouldn't be bad as long as you don't have high expectation and just a general readout of your home's room or the garage as an example.  For the price, in my opinion, it's an excellent 'general' tool.  Our acceptance for humidity and temperature is no more than 2% for humidity and 2 degrees for temperature.

If you're looking to measure temperature and/or humidity that require precise measurement every time with faster response, you'll want to look at industrial hygrometer that can also be calibrated.  With the products we deal with, an non-calibrated equipment could give us false reading and possibly damage customer products.

So you have to ask yourself if this hygrometer meet your needs.  Either you simply want a general reading and don't care for sudden change in temperature, or you'll have to invest in a hygrometer that can be calibrated and certified.  Actual calibration of hygrometer isn't too expensive (under $100) and suggest you calibrate once a year.  Used hygrometer is fine as well as long as they're functioning and can be calibrated.