Measurement accuracy: meaning less when stage being smaller than 50%,+/-2BPM. For pulse rate Measurement Performance Oxyhemoglobin and pulse value can be shown correctly when in low perfusion: pulse-filling Rate is 0.5%Interference Resistance Capacity Deviation is smaller than+/-1% between value of Oxyhemoglobin against Ambient Light: measured in natural lighting indoor condition and present lighting sources and that measured in a dark room.                                  FREE SHIPPING