(a) Area Meters. In an area flowmeter the fluid flows upward, displacing an obstructing float or piston. The float or piston is arranged so that the unobstructed area increases with upward displacement, the float or piston then moves until the area is open enough to permit the flow to pass. The basic theory of area flowmeters is the same as that of a differential-pressure meter, but, since differential pressure is held constant, or reasonably so, square-root extraction is not required A measurement of float or piston position is a
measurement of unobstructed area, and thus of flow. Area meters are of two general types rotameters and piston-type meters.

A rotameter (Fig 4 38) consists of a float inside a tapered tube, the small end of the tube being at the bottom The force exerted in the tube by the flow moves the float upward until the area of the annular space between float and tube is sufficient to permit a flow — created differential pressure to balance the weight (less the buoyancy force) of the float. Differential pressure is determined by the weight of the float and its cross-sectional area. A scale is marked on the outside of the transparent, tapered tube so that float position can be read directly. Since the unobstructed area permitting flow is almost exactly proportional to float rise (for a slightly tapered tube), the scale markings indicating flow can be uniform. For high-pressure applications or for transmitters, the tube can be metal, and the float position can be sensed by a magnetic pickup.

In a piston-type area meter, upward motion of the piston or plug uncovers ports in the sleeve or cage, increasing the area of the opening in direct proportion to plug movement and to flow rate. A spring pulls downward on the plug to increase the differential pressure beyond that which might be obtained by the weight of the parts. The vertical position of the plug establishes the rotational position of a spindle, which extends through a packing gland to operate an indicating pointer and transmitting mechanism (either pneumatic or electric).

(b) Positive-Displacement Meters. The flowing fluid is divided into separate discrete volumetric portions that are counted by a mechanical register built into the meter Alternately, the rotation of the meter mechanism may be made to generate an electrical signal with frequency proportional to the rotational speed, the signal can then be transmitted to a remote register or recorder.

(c) Velocity Meters. A turbine meter is a line-mounted meter with a rotor having helical blades Rotation generates a series of electrical pulses, which are sensed by an externally mounted electrical pickup. The receiver may be arranged to display total flow by counting the pulses by digital techniques or to display rate of flow by measuring the pulse frequency. These meters are accurate through their recommended range from maximum flow rate to about 10% maximum. At lower flow rates friction tends to cause the meter to read low. Since the bearings are exposed to and lubricated by the flowing fluid, maintenance involves removal of the meter from the line for inspection or replacement of bearings. The turbine meter is sensitive to changes in fluid viscosity and is usually individually flow-calibrated. It was developed for and has gamed wide acceptance in the Aerospace industry.

A magnetic meter is an electrically insulated section of pipe with an imposed magnetic field, perpendicular to the pipe axis, through which a conductive fluid develops an electrical potential (perpendicular to magnetic field and
pipe axis) directly proportional to its average velocity through the pipe section Electrodes flush with the pipe wall are connected to a circuit for measuring the generated voltage The magnetic meter is accurate and linear through a wide range of flow rates and is available in a wide range of sizes Because it has no internal parts to trap sediment, it is widely used for slurries and dirty fluids, it can be recommended, however, for any flow of an electrically conductive fluid where minimizing the pressure drop is important The pressure drop is no greater than that of a straight pipe of the same length. This type flowmeter is used in sodium-cooled reactors (see Vol 2, Chap 17)

4- 4.3 Liquid-Metal Flowmeters

In power reactors that use liquid-sodium coolant, flow is usually measured with magnetic meters, as noted in the preceding section. Differential-pressure devices, however, have also been used to measure liquid-sodium flow rates. The principles of operation and the methods for correcting for thermal effects, wall effects, etc, in magnetic flow­meters are described in Vol. 2, Chap 17.