​​. There are two types of digital BP monitors: the upper-arm | Knowledge Genie

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There are two types of digital BP monitors: the upper-arm (shown above) and the wrist models (shown below). The upper-arm model has a cuff that is placed on the upper arm and is connected by a tube to the monitor that rests on a surface near the arm. The wrist model is smaller and the entire unit wraps around the wrist. Regardless of the BP monitor type, the measurement method is the same.
A digital blood pressure monitor uses an air pump to inflate a cuff surrounding an upper arm or a wrist with sufficient pressure to prevent blood flow in the local main artery. This pressure is then gradually released using a digitally-controlled solenoid valve until the moment that the blood begins to flow through the artery.
The blood pressure measured by a pressure sensor at this point determines the systolic pressure. Pulse rate is also sensed at this time. The measurement taken when the blood flow is no longer restricted determines the diastolic pressure. This complete measurement cycle is controlled automatically by the micro-controller.