ME
M 4880
Instrumen
ntation: Rotaameter
Backgrou
und
A rotameeter consists of a tapered
d tube, typicaally
made of glass with a “float”, actu
ually a shapeed
w
is push
hed up by th
he drag forcee of the
weight, which
flow and pulled down
n by gravity
y. A schemattic and
example rotatmeter are
a shown in
n Figure 1. The
T
term “flo
oat” is actuallly contradicctory as the
material of the float must
m have a greater denssity
f
otherw
wise it would
d float to the top
than the fluid,
without flow.
f
When fluid is flow
wing through
h the
rotameterr the “float” will rise to a position where
w
both the drag force an
nd buoyant force
f
will bee
balanced
d by its own weight.
w
a)
b)
Fiigure 1. Variiable Area Rootameter a)
Scchematic and
d b) Device.
Completiing a force balance
b
arou
und the “float” the
mean vellocity of the fluid may be determined
d as shown iin Equation 1
v
Eqq. 1
where vm is the mean
n velocity paassing throu
ugh the annuular space beetween the flloat and the tube,
Vb is the volume of the float, ρb and
a ρf are the density off the float andd fluid respeectively, CD is the
drag coefficient, Afr is the projected area off the float, aand g is the accelerationn of gravity. The
volumetrric flow rate (Q) flow is then
t
found by
b Equation 2
Q = umAm
Eqq. 2
where Am is the area of the annullar space. A higher volum
metric flow rate throughh a given area
increasess flow speed and drag force, so the float
fl will be ppushed upwaards.
Floats aree made in many
m
differen
nt shapes, wiith spheres aand ellipsoidds being the m
most commoon.
The floatt may be diagonally groo
oved and parrtially coloreed so that it rrotates axiallly as the fluiid
passes. This
T shows iff the float is stuck since it will only rrotate if it is free. Readinngs are usuaally
taken at the
t top of thee widest parrt of the floatt; the center for an ellipssoid, or the top for a cyliinder.
Some maanufacturers use a differeent standard
d.
Rotameteers are a versatile piece of
o instrumen
ntation with wide use thrroughout inddustry.
Advantag
ges of rotam
meters includee


Requires
R
no external
e
pow
wer
Uses
U only inh
herent properrties of the fluid,
f
along w
with gravity,, to measuree flow rate


Relatively inexpensive
Since the area of the flow passage increases as the float moves up, the scale is
approximately linear
However, as with all instrumentation rotameters also have disadvantages including



Rotameter must be mounted vertically due to its use of gravity
Due to reliance upon fluid properties, graduations will only be accurate for a given
substance at a given temperature
Typically require glass as the tube material only allowing rotameters to be used with
benign fluids