Flow Measurement & Control Rotameters
Flow Measurement & Control - 30801 Basic Flowmeter Principles
351
FlowMeasurement&Control
How They Work
Flowmeters are used in fluid systems (liquid and gas) to indicatethe rate of flow of the fluid. They can also control the rate of flow if
they are equipped with a flow control valve.Rotameters are a particular kind of flowmeter, based on thevariable area principle. They provide a simple, precise and
economical means of indicating flow rates in fluid systems. This variable area principle consists of three basic elements: Auniformly tapered flow tube, a float, and a measurement scale. Acontrol valve may be added if flow control is also desired.In operation, the rotameter is positioned vertically in the fluidsystem with the smallest diameter end of the tapered flow tube at
the bottom. This is the fluid inlet. The float, typically spherical, is
located inside the flow tube, and is engineered so that its diameterisnearly identical to the flow tube’s inlet diameter.When fluid — gas or liquid — is introduced into the tube, the floatislifted from its initial position at the inlet, allowing the fluid topass between it and the tube wall. As the float rises, more and
more fluid flows by the float because the tapered tube’s diameter
is increasing. Ultimately, a point is reached where the flow area is
large enough to allow the entirevolume of the fluid to flow pastthe float. This flow area is called the annular passage. The float isnow stationary at that level within the tube, as its weight is beingsupported by the fluid forces which caused it to rise. This positioncorresponds to a point on the tube’s measurement scale and
provides an indication of the fluid’s flow rate.One way to change the capacity, or flow range, of a rotameter is tochange the float material, and thus its density, while keeping the
flow tube and float size constant. Floats which aremade from less
dense materials will rise higher in the tube and thereforewill yieldlower flow capacities for the same diameter flow tube. Floats made from moredensematerials will rise less thereby
yielding higher flow capacities.
Relative flow capacities for
some common float materials
are shown in Figure 1. Another way to change thecapacity is to change the
diameter of the flow tube and
the size of the float.
FigureI
Selecting The Right Flowmeter Size
Thereare certain factors which affect the measurement of a fluid’s
flow rate with a rotameter. The fluid’s temperature, pressure and
specific gravity all impact gas flow measurements.Flow capacities (ranges) for the
flowmeters described in thiscatalog are given for air at standard conditions —- 14.7 psia (101.3
KPa Abs) and 70°F(21°C). Sizing a meter for a gas other than air,or for your specific application pressure and/or temperature,requires that you first determine the equivalent flow capacity in air
at standardconditions. Once determined, the flow capacity tablesin this catalog can be applied directly. Reference Scale tables can berequested for each flowmeter ordered which will provide
conversion to your desired fluid or conditions. Note that for
flowmeters calibrated at standard conditions with a valveon the inlet, readings on the tube arecorrect provided that the outlet
pressure is close to atmospheric. When the valve is on the outlet, readings
are correct if the inlet gas pressure is equal to the pressure for which the
tube was calibrated. For your convenience, Table 1 provides correction factors for gasesother than air at standardconditions. Call a customer service
representative if you require further assistance in sizing a
rotameter for your particular application.
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