Rotameters come in a variety of sizes and materials, but they all
have a tapered tube and a float. The "float" is the thing
that rises and falls inside the tapered tube. The float is in a continuous
state of dynamic equilibrium whose vertical position in the tube
is the balance point between the upward force of the fluid, or gas,
and the downward force of gravity on the float. The point of equilibrium
is strongly affected by specific gravity and viscosity changes.
The float material is selected to provide a certain weight, which when used
with a specific tube taper will provide the proper range. In large, 10" and
5" scale units, the most common float material is 316 st. st. or Hast
C. However, aluminum, PVC and Kynar are also used. In small rotameters using
ball floats common materials are st. st., black glass, tungsten carbide, red
sapphire and tantalum.
The accuracy statement is a function of scale length and tube material. A glass
tube is always a higher accuracy tube than a molded plastic tube.
Rotameters have accuracy's stated as a % of the full scale value, which provides
increasing error as a % of flow rate as the flow rate drops. Try to size a
rotameter so as to operate in the upper 50% of the range to minimize this effect.
For a discussion on how accuracy's are defined, see our "Practical considerations
in choosing a flowmeter".
Large rotameters:
10" scale length ...... glass
tube is 2% FS, plastic is 23%
5" scale length .......
glass tube is 3% FS, plastic is 34%
Small purge rotameters:
5" scale length .......
glass & plastic are about the same at 4% FS
3" scale length .......
glass & plastic are about the same at 6% FS
1.5" scale length ....... glass & plastic
are about the same at 10% FS
Viscosity handling characteristics vary with the float design.
Ball floats ......... no viscosity immunity at all.
(Only found on small meters.)
Cylindrical floats ........ large "drag" surface,
little immunity.
Sharp edged floats ..... maximum immunity to viscosity. (1" and
larger)
Practical viscosity limits using sharp edged floats:
1/2" size ......... 10 cP
3/4" size ......... 15 cP
1" size ............ 20 cP
1.5" size ......... 40 cP
2" size ............ 60 cP
Construction  if rupturing of the tube can in any way create a hazard, either
use a protective case design or switch to a metal tube meter. If severe water
hammer occurs in the line, the glass or plastic tube is what normally ruptures.
Float instability occurs on gas applications with low pressure on the discharge
port, typically, in situations where the discharge is to atmosphere or to the
intake of a vacuum pump. The easiest way to stop float cycling is to reposition
the rotameter to a higher pressure part of the loop, or to install a valve
downstream of the indicator to provide some back pressure. Be sure to tell
us if you anticipate having this type of installation.
The Hedland meter is a special form of rotameter with a spring replacing gravity.
On gaseous application, it should have a 25 psig backpressure to avoid instability.
Sizing and Correcting Liquid Service Rotameters. Sizing is the process of selecting
a rotameter. If your fluid is water, you can select the appropriate tube and
float directly from the specification sheet. If your fluid is not water you
need to calculate the equivalent water flow rate, and then select the tube
and float with your calculated data.
Example  You want a rotameter for isocyanate, which has a specific
gravity of 1.25 @ 25 C. Your max flow rate is about 20 GPM.
sp.gr.
of isocyanate
Equivalent flow rate water = Max flow isocyanate X 
sp.gr
of water
1.25
Equivalent flow rate water = 20 GPM x  = 20
GPM x 1.118 = 22.36 GPM 1.00
You would specify a rotameter tube and float combination that would place 22.36
GPM close to the top of the scale. Never put your maximum flow at the top of
the scale, the operator will think that it is stuck there and giving a incorrect
reading. Normal practise is to size for operation from 20 to 80% of meter span
(range).
CORRECTING:
Correcting is what you do when you have a rotameter and changed the fluid going
through it. For example, in the above we have sized a rotameter for isocyanate
at 20 GPM. Let's say that you have received the rotameter and you need to use
it on fresh water service.
All you need to do is take the above correction factor, i.e. 1.118, and divide
the indicated flow rate by it.
sp.gr.
of water
Actual flow rate of water = Indicated flow X 
sp.gr
of isocyanate
When you need to change the float material for chemical compatilbilty or
to get the correct flow range.
The sizing tables for larger rotameters are normally based on water service,
using 316 st. st. floats. Let's say that you are going to run bleach through
the meter and the 316 st. st. will be attacked by the bleach. The solution
is to use a Hast C float.
If you are sizing a new rotameter, the factor is changed to:
sp.gr.
bleach x ( 316 SS sp.gr  sp gr water)
Water equiv flow = Max flow bleach X 
sp.gr.
water x (Hast C sp.gr.  sp.gr. bleach)
From the table at the end of this section: 316 SS = sp.gr. 8.02
Hast
C = sp.gr. 8.94
Sizing calculations, for air at STP (14.7 psia @ 70 (F)
Select directly from the standard ranges provided by the manufacturer.
Sizing calculations, for perfect gases, i.e. argon, carbon monoxide,
chlorine, helium, hydrogen, hydrogen chloride, nitrogen, oxygen.
The following discussion relates to sizing using volumetric units, i.e. CFM
The governing equation, written for conversion from a perfect gas to a air
equivalent for sizing purposes is:
(actual
sp.gr.) (air
psia) (
actual deg R )
Air equiv. = Actual flow x  x 
x 
(air
sp.gr.) (actual psia ) (
air deg R )
Example  Assuming argon @ 70 (F, 30 psig, sp.gr. = 1.380, 76 SCFM
1.380 14.7 (460degR
+ 70)
Air equiv. = Actual flow x  x 
x 
1.000 (
14.7 + 30 ) 530deg
R
Result: Air equiv. = 76 SCFM x 1.175 x
0.5735 x 1 = 51.2 cfm
Sizing calculations, for imperfect gases.
The following discussion relates to sizing using volumetric units, i.e. CFM
Step 1  From a table of the thermodynamic properties of your gas, determine
the specific volume (cu.ft./pound) at the operating temperature and pressure.
Shortcut ... Most engineering handbooks will have a table giving the weight
density of air over a wide range of temperatures and pressures. To determine
the weight density of your gas, multiply the tabulated value by your specific
gravity. Then, take the reciprocal to get the specific volume, i.e.
1
Specific Volume = 
Weight
Density
Step 2  Determine your specific gravity at the operating condition:
13.34
Operating sp.gr. = 
Specific
Volume in Cu. Ft./Pound
Step 3  Determine the equivalent air sizing flow:
Specific
gravity of your gas
Equivalent air flow = Your flow in SCFM X 
Operating
sp. gr.
Reference data for sizing rotameters
Float material Sp.
Gr. Gases
Sp. Gr. @ STP
=========== ======= ========================
316 SS 8.02 Air
....................................1.0
Hastelloy C 8.94 Ammonia
(Anh) .............. 0.593
Teflon 2.20 Argon
...............................1.38
Glass 2.53 Carbon
Dioxide ................1.52
Sapphire 3.99 Carbon
Monoxide ............ 0.97
Carboloy 15.00 Chlorine............................ 2.45
Tantalum 16.60 Helium
.............................. 0.138
Titanium 4.50 Hydrogen
......................... 0.069
Hydrogen
Chloride .......... 1.26
Methane ........................... 0.554
Natural
Gas ..................... 0.60
Nitrogen
.......................... 0.967
Oxygen
............................1.105
Propane
...........................1.523
Bypass Rotameters
The bypass rotameter is a standard rotameter, fitted with a range orifice and
designed to work in parallel with a mainline orifice plate producing 100 " w.c.
at max flow. The purchase order must include the main line flow rate which
is producing the 100" w.c. pressure differential.
If providing a 100" w.c. is a problem, perhaps the permanent pressure
loss is to great, we can work down to 25" w.c. in most cases. However,
this invariably means that the rotameter cannot provide a full 10:1 turndown.
A bypass rotameter installation requires a differential head producing device,
generally a 300# orifice flange (weld neck or slipon), orifice plate, the
piping from the orifice flange to the rotameters and 2 block valves for this
piping to allow isolation of the rotameter. The calibration of the range orifice
on the rotameter is predicated on minimal pressure drop through the piping
system to the mainline flange, keep the piping as short as possible.
If your customer requests that we provide the complete package, we can do so.
We need to have the data to allow sizing the plate, i.e. what is the fluid
or gas, flow rate, pipe size and schedule, gauge pressure, flow temperature,
viscosity and specific gravity. If steam, we need the flow in pph, plus steam
condition and, if superheated the degrees of superheat.
Rotameters ... Differences between King, SK and Blue White
A variable area meter is any flow meter which has a cross sectional area between
a fixed and a moving surface, which varies as the flow rate increases or decreases.
As an example, in the classic glass tube rotameter with a tapered internal
diameter, the annular orifice between the float OD and the tube ID increases
in area as the float rises with increasing flow rate.
The float rate is read from a scale, at the widest part of the float. At this
point, the float is in equilibrium ... that is the forces acting downward are
exactly counterbalanced by the forces acting upward. If we ignore the finer
technical points which involve coefficient of discharge, fluid drag over different
float shapes, etc., we can simplistically say that the downward forces are
gravity and the weight density of the float. The upward force is the impact
pressure exerted by the fluid velocity and the weight density of the fluid.
If the fluid specific gravity changes, the reading of the rotameter has to
be corrected. To do this, take the indicated reading and divide it by the square
root of the specific gravity.
Changes in viscosity affect the fluid drag on the float. As the viscosity increases,
it will drag the float upward. The standard correction is use a float which
has a sharp quadrant edge at the widest point, to lessen this effect.
To use the same tapered tube over a variety of flow ranges, you change the
density of the float material. As the float specific gravity goes up, it's
weight density goes up. This means that greater upward forces must be applied
to reach the same point on the rotameter scale. Greater upward force means
higher flow rates. If you examine the King literature, page 12 gives the standard
flow ranges associated with a single tapered tube, available by using float
materials of teflon (sp. gr. 2.2), 316 st. st.(sp. gr. 8.02) and Hastelloy
C (sp. gr. 8.94). The heavier the float, the higher the flow range.
Hedland versus other rotameters:
Accuracy  Rotameters made by companies like Blue White and King use the force
of gravity to provide the downward force, which means that they must be installed
vertically. Hedland units compress a spring as the float moves up on the scale
and can be mounted in any position. Because the spring compression must be
limited to the linear part of the spring range, the scales are on the order
of 1.5 to 2" in length. By contrast the King meter has scale lengths up
to 10". (Some flow bench test rotameters have 24" scales). This is
the major factor affecting accuracy ... if you have a 20200 GPM range, what
kind of reading accuracy can you get with a 1 1/2 or 2" scale length?
The second factor is the repeatability of the force constant of the spring.
The spring is operated within the linear portion (below the elastic limit),
but not all springs have the exact same force constant and the scales are not
made up on a per spring basis, therefore a error can be introduced. This same
error is not present in standard rotameters using gravity.
Cleanliness of the fluid  The Hedland unit was designed specifically for use
with hydraulic fluids to meet the requirements of a sister company producing
hydraulic jack hammers. As such, fluid cleanliness was not a serious design
consideration. Hedland recommends the use of a 200 mesh sieve or 74 micron
filter if dirty fluids will be metered (exception  their meters intended for
water service). By contrast a standard rotameter
has a tendency to be self cleaning and can pass good size particles. When a
large particles enters it hits the float, causing it to rise and allowing the
particle to pass by. The self cleaning aspect comes about because the fluid
velocity increases around the float OD and this tends to scour the tube ID
as well as the float OD.
High pressure  The Hedland unit shines in this area... 3000 psi.
Some general comments 
The limitations of the Hedland meter, i.e. short scale and spring based design
are also seen in spherical rotameters of the type made by Erdco and Universal.
A variable area meter can be used for things other than measuring flows rate:
If you hold the flow rate constant and increase the density of the fluid,
a greater force is exerted and the float will rise to a higher point of equilibrium.
This means that you can use the rotameter as a specific
gravity measuring device. Not so long ago, one could find rotameters calibrated
in terms of specific gravity to determine acid strength.
Another way to use the rotameter is as a mass flow rate indicator. This never
became popular because it requires that the float density be exactly twice
the fluid density.
