Will Do Something?

I bought the Wrist Blood Pressure Monitor visomat phone on the go. at home I measure the visomat Comfort 2040. The deviations are up to 20 units worth at top. the manufacturer you thought it was a normal tolleranz because of the hand, the values ​​are not as accurate. - Refrigeration, hand gesture, not so good circulation, further from the heart, are the reasons why. - So it's a very incorrect measurement with the cell phone, and the question arises - why at all?