One of the two units had a fault when unpacked. It seems that the lower hairspring has become entangled which creates faulty readings of up to 20%. I am very happy that the supplier has obtained a warranty replacement unit from the factory and it has been shipped already. It is very difficult operating at a distance since the post office will NOT return faulty units without charging postage higher than the unit is worth and also requires a customs declaration filled out. What is the point of shipping back a faulty unit. Lucky this happens rarely - a third unit I had ordered arrived in 100% working order