Variable head flow meters
The pressure drop in a flowing fluid can be created by installing an obstacle in the flow path. Flowmeters in this class detect a pressure difference across a flow element specially designed to create that pressure difference. The most common flow element is the orifice plate, but other elements also in use are flow nozzles, venturi tubes, pitot tubes, averaging pitot tubes, target plates, and pipe elbows.
Read Also: Level measuring devices
Variable area flow meters
This type includes the familiar rotameter. The differential pressure across the device is held constant, and the area through which the fluid passes changes due to the movement of the float up and down the tapered tube. These are usually limited to use with relatively small flows where visual indication is sufficient.
These use a small permanent magnet mounted on the meter tube to create a magnetic field. A small turbine is mounted inside the tube and turns at a speed proportional to the flow rate. As each vane of the turbine
passes through the magnetic field the magnetic flux is disturbed which induces a pulse in a pickup coil mounted on the outside of the meter. The pulse rate is proportional to the flow rate. Pulses are then counted and converted to standard flow units.
Positive displacement meters
Positive displacement meters and metering pumps measure discrete quantities of the flowing fluid. The rotating element is mechanically coupled to a transmitter or counter which integrates or totals the counts to provide an indication in units of gallons, liters, cubic feet, etc. Some common types are rotating vane, bi-rotor, rotating paddle, oscillating piston, and oval gear meters. They are used for custody transfer devices such as gas meters or gasoline pumps.
If an electrical conductor is moved in a magnetic field, an electrical voltage is introduced in the conductor which is perpendicular to both the direction of motion and the magnet field and whose magnitude is proportional to the magnetic field strength and the velocity of the movement. The characterization of the laws of induction also applies to the movement of a conductive fluid in a pipe through a magnetic field and is the basis for the electrostatic flowmeter.
Ultrasonic Flow Meters
Ultrasonic measurement is based on the sending and receiving of acoustic signals through the flow of the fluid. The difference in transit time between transducers, built-in at opposite sides of the pipe gives signals that can be transferred to flow. A sound wave travels faster with the flow than one propagated against the flow. The difference in transit times is proportional to the medium’s mean flow velocity. By installing more than one pair of transducers, a larger range of the flow profiles across the metering section can be covered and thereby increase the accuracy of the meter.