A rotator flowmeter, also known as a rotameter, is a device for measuring fluid flow. The rotameter measures the fluid flow according to the throttling principle, but it changes the flow area of the fluid to keep the differential pressure above and below the rotor constant, so it is also called a variable flow area constant differential pressure flowmeter. The rotameter is based on the position of the float. A variable area flow meter for measurement. Also known as a float flowmeter.