I have ordered these twice about a year apart.  In total I purchased 6 units.  5 of them read within a % of each other.  One is way off...by 10 to 15%.  Although I would have liked all 6 to be matched this is not bad for such an inexpensive device.  I use mine to check my photographic equipment humidity box and to check the levels in the house.  Good product.  Have not bothered to contact Amazon or the vendor about the one bad unit.  Perhaps I will....perhaps too much trouble.  I am pleased with the product.  BTW two of my units (both read accurate) have been running over a year on the battery....so I think this is low battery consumption.  Very pleased.

UPDATE 5/27/16
I lowered my rating to two stars.  This review is not only the product but on Amazon.  Yes Amazon ..I have been a PRIME member for several years and make many many purchases.  I decided to return the one bad unit and the online information states I have till June 2 to initiate a return.  But when I go through the return process it states the item is not eligible for return and provides me a link to the return policy.  Of course I can not find anything that is associated with this device.  So...the product is good if you can accept a failure rate of 1 in 6 and can accept a no-return policy.  I figure this is worth 2 stars.  Sorry Amazon....I still love your service but this time you did not come through.
I have ordered these twice about a year apart. In total I purchased 6 units. 5 of them read within a % of each other. One is way off... by 10 to 15%. Although I would have liked all 6 to be matched this is not bad for such an inexpensive device. I use mine to check my photographic equipment humidity box and to check the levels in the house. Good product. Have not bothered to contact Amazon or the vendor about the one bad unit. Perhaps I will.... perhaps too much trouble. I am pleased with the product. BTW two of my units (both read accurate) have been running over a year on the battery.... so I think this is low battery consumption. Very pleased. UPDATE 5/27/16
I lowered my rating to two stars. This review is not only the product but on Amazon. Yes Amazon.. I have been a PRIME member for several years and make many many purchases. I decided to return the one bad unit and the online information states I have till June 2 to initiate a return. But when I go through the return process it states the item is not eligible for return and provides me a link to the return policy. Of course I can not find anything that is associated with this device. So... the product is good if you can accept a failure rate of 1 in 6 and can accept a no-return policy. I figure this is worth 2 stars. Sorry Amazon.... I still love your service but this time you did not come through.