ROTAMETER:
The
name rotameter was chosen in the past because at one time the float was
designed with slots to produce a rotational movement for the purpose of
centering and stabilizing the float.
The
float is free to move up or down within the tapered tube. The tube is tapered
to provide linear relationship between the flow rate and the float position
within the tube. The fluid enters at the bottom and passes upward around the
float and out at the top. The float rises and falls depending on the flow rate,
giving a variable area of the annular passage. The upward hydraulic forces that
act on the float are in balance with its weight less the buoyant force; this
will maintain the float position in equilibrium. The scale shows flow rate in percent.
The
rotameter sizing is also based on Bernoulli’s theorem. Rotameters may be used
to measure gas or liquids and even some light slurry. Therefore, the sizing
where very low flows are measured, capacities are usually given in cc/minute of
air or water: this rotameter equation used for the head meter also applies to
the rotameter :
The
same fundamental equation used for the head meter also applies to the rotameter :
Q =KA√( 2gh)
In
this equation the differential pressure (h) is replaced by the factor that
causes it to remain constant. This factor denotes the volume and area of the
float as well as the density of the float and the fluid.
The
equation will be:
Q =
kA m √ ((2g Vf/Af) * (Df / (D-1))
Where:
Q =
Flow Rate
K =
Taper Constant
Am
= Annular area measured at indicating edge
G =
gravity
D =
Fluid Density
The
rotameter may be subjected to error due to changes in the fluid density small
variations in fluid viscosity will not affect measurements if the float is
designed to be insensitive to them. This will make Df /D = 2. Therefore
(Df /(D-1)) = 1.
Since
√1=1, the effect of density is eliminated . The float also must be designed to
ignore small changes in the viscosity of the fluid. The rotameter available
with a tapered glass tube or metal tube. The choice of tube depends on the
application. Where toxic or flammable fluids are to be measured, a metal tube
is selected for the rotameter . However in this case the float is not visible
and a different indication technique is required. The linear movement of the float
must be converted into a rotating movement for local indication or signal
transmission. The metering float is extended with a permanent magnet on top of
it. Glass tubes are limited up to use for low pressure say 20 Kg/cm2.
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