Flow Measurement Device

in physiology, an instrument used for measuring the volume rate of blood flow in a blood vessel (flowmeter) or for measuring expired air flow (pneumotachometer). The volume rate of blood flow, that is, the amount of blood that passes through a cross section of a blood vessel per unit time (milliliters per minute, or ml/min), is usually determined by the linear velocity rate of blood as it passes through a segment of pipe of a known cross section. The linear rate of blood flow, that is, the average velocity of the flow of blood particles in a given section of a blood vessel, is measured in the following ways: (1) by the rate of the movement of an air bubble that is introduced into the blood flow, (2) by the difference in blood pressure at two points that are located before and after a constricted section of the pipe through which the blood is flowing (the pneumotachometer has a similar circuit), (3) by the degree of cooling of the heated junction of a thermocouple or thermistor caused by the flowing blood, (4) by the difference in the rates of dissemination of ultrasound in the blood as the ultrasound moves against or with the blood flow, and (5) by the value of the electrical potential of the blood that moves perpendicularly to the magnetic field of a strong electromagnet. The electrical potential of the blood is comparable to that of an electrolytic conductor.

Other types of flow measurement devices used include blood flowmeters, rotameters, turbine meters, and gas flowmeters.