Blood pressure gauge. This dial is part of a sphygmomanometer used to measure blood pressure in millimetres of mercury (mmHg). An inflatable cuff (not seen) is placed around the upper arm and inflated using a pump (lower right) until blood flow in the arm stops. The cuff is then gradually deflated until pulsing blood flow is heard through a stethoscope. The reading on the blood pressure gauge at this point is the systolic pressure, which occurs during a heartbeat. The diastolic pressure, when the heart is relaxed, is taken by deflating the cuff until blood flows smoothly. Blood pressure is an important measure of health.