Despite the growing popularity of other flowmeter types, the variable area flowmeter continues to command a major portion of the industrial market. Although it has not enjoyed the same degree of publicity granted more exotic meter forms in technical literature, continuing innovations and developments in variable-area metering have greatly enhanced the capabilities of this popular flow monitor. Their basic advantages--relatively low cost, accurate and reliable performance, simplicity, and inherent versatility--remain valid today. Available in a wide variety of metals and plastics, and with current alarm and control options, variable area meters can be used with a wide variety of liquid, gas, and steam applications.

Perhaps the best known of the variable area meter family is the "rotameter," first patented in the United States in 1868. In its basic form, the rotameter consists of a vertically oriented tapered glass tube with the large end at the top, and a metering float which is free to move within the tube. Fluid flow causes the float to begin to rise in the tube as the upward pressure differential and buoyancy of the fluid overcome the effect of gravity.

The float will rise until the annular area between the float and tube increases sufficiently to allow a state of dynamic equilibrium between the (upward) pressure differential and buoyancy factors, and (downward) gravity factors. The height of the float is an indication of flow rate, and the tube can be graduated in appropriate flow units.

These meters typically can have up to a 12 to 1 turndown (ratio of maximum to minimum measures flow), and industrial accuracies of +/- 2% or even 1% of the full scale rating.

To meet the demands of a variety of industrial requirements, a variety of constructions has become common. Glass is often replaced with various plastic or metal components, with some form of magnetic position sensing being used in the latter case. The use of magnetic float sensing is also used for alarm and signal transmission functions, in many cases.

SLOTTED TUBE FLOWMETERS

Another variation of industrial variable area meters uses a slotted cylinder and piston rather than a tapered tube (see figure). This piston portion of the meter float travels within a precision-honed cylinder, with the piston acting as the fourth side of the slot. As with tapered tube meters, fluid flow raises the piston until sufficient slot area is exposed to bring all forces into dynamic equilibrium. To assure constant buoyancy, a "snorkel" device allows
the top of the unit to fill with fluid in liquid applications.

The metering cylinder and piston are contained within a T-shaped body, and flow is read by means of a knife-edged disk supported by an extension from the piston. This disk moves up and down in response to flow within a transparent sight tube containing an externally mounted scale. As the tube is cylindrical and the disk always close to its walls, flow can be read in dirty fluids. If a transparent sight tube cannot be used, a magnet is added to the float and an external indicator used. Use of a magnet on or encapsulated in the float also provides alarm and control functions with appropriate accessories.

Advantages of this meter style include more compact and less costly construction, high rangeabilities of 25 to 1, and easy access to flow internals for cleaning or changing meter capacity without removing the unit from the pipeline. Standard accuracy is +/-2% full scale,
with +/-1% calibrations as an option, and repeatability +/-0.3% of rate. Pressure drops are reasonably low, and similar to tapered tube models.

VANE AND PISTON FLOWMETERS

Yet another style of variable area meter incorporates a spring-loaded orifice and tapered plug or a vane within a "bowl." Fluid flow moves the vane or orifice, exposing a greater area around the tapered plug or within the bowl until equilibrium is achieved. Flow is read by
means of a pointer mechanically positioned by the vane, or by a magnetic indicator. Although use of a spring does have drawbacks in terms of pressure drop and repeatability, the principal flow element does not rely on gravity and, therefore, these meters can be mounted in any position.

Accuracies vary from +/-2% to +/-5%, depending on the model, and average repeatability is +/-1%. Rangeability is usually 6 or 10 to 1. Because of the incorporation of the spring, pressure drops are high (often expressed in pounds rather than inches W. C. as with other
variable area meters), and generally go up from 2 psig.