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Variable Area
By Jesse Yoder, PhD, Flow Research, Inc.
While
variable area (VA) flowmeters are limited in their functionality, they cost far
less than most other types of flowmeters. When users are looking for a simple,
low-cost solution, they continue to consider VA meters. While they
can measure flowrate, they are sometimes used when a flow/no-flow determination
is desired. They are very effective at measuring low flowrates, and
can also serve as flow/no-flow indicators. VA meters do not require
electric power, and can safely be used in flammable environments.
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History
 The history of variable area meters
goes back to 1908 when they were invented by German engineer Karl Kueppers in Aachen , Germany . At
that time they were called “rotameters,” named after the rotating float that
was originally a component of these meters. Felix Meyer recognized
the commercial potential of Kueppers’ invention, and in 1909 founded
“Deutsche Rotawerke” in Aachen . The product invented
by Karl Kueppers was the first variable area flowmeter with a rotating float.
The German
company Deutsche Rotawerke was the forerunner of the company that later became
known as the Rota Company. Originally, Meyer called his products “rotamesser.” In
1995, Yokogawa purchased the Rota Company and named the resulting company Rota
Yokogawa. Rota Yokogawa still manufactures its variable area meters,
which it also calls rotameters, in Wehr , Germany .
In the
meantime, the GEC Crawley Company in Crawley , United Kingdom ,
began manufacturing the first glass variable area meters, and registered the
name Rotameter as a trademark in the UK . This name still
exists as a trademark in the UK , but has passed down through a number
of companies, including KDG Instruments and Solartron Mobrey. In
March 2009, Emerson Process Management acquired Solartron Mobrey, presumably for
its level, density, and flow computer products. As part of the
acquisition, Emerson Process acquired the trademark to the name “rotameter”
in the United Kingdom .
The terms
“rotameter” and “variable area meter” have become synonyms over the
years. It appears, though, that Yokogawa has the rights to this name
in Germany and that Emerson Process has the rights to the name in the United
Kingdom . On the other hand, the name ‘rotameter’ has become a generic term for variable area
flowmeters, so its status as a true trademark is open to question. This
is what happened to ‘xerox,’ which is now used generically to describe any
photocopy.
Click to learn
more about
Variable Area
Principle of Operation
Most variable
area (VA) flowmeters consist of an upright, tapered measuring tube that contains
a float. Fluid flows through the tube from bottom to top. The
upward force of the fluid raises the float in the tube. The float is
counterbalanced by the force of gravity. The point at which the float
stays constant indicates the volumetric flowrate, which can be often read on a
scale on the meter tube. Variable area meters vary according to their float
shape, and the material the float is made from.
 Construction
Variable area
meters come in plastic, glass, and metal. Their cost varies with
their material of construction. Metal-tube meters are generally
highest in cost, and are designed for high pressure and high temperature
applications. Plastic meters are typically lowest in cost, followed
by glass flowmeters. Some metal tube meters range can exceed $1,000 in price. A
group of VA meters called purgemeters have been designed for low flow
applications.
For most
plastic and glass VA meters, the flowrate can be read from the scale on the
tapered meter tube. For metal VA meters, the position of the flow is
magnetically transmitted to an analog indicator. These meters also
have a scale indicating flowrate, and the position of the analog indicator
displays the flowrate. These VA meters still rely on a float to
indicate flowrate, and they also need to be read manually, like most of the tube
design meters. Metal VA meters can be used for liquid, gas, and
steam applications.
Applications
VA flowmeters
are used in research and laboratory environments and in the process industries
to measure the flow of gases and air at low flowrates. They are also
used when a visual indication is sufficient, and to check on the performance of
other meters. VA meters, including plastic meters, are also used for OEM
applications.
Plastic
meters are applicable for water, gas, and air applications. The
scales are separately marked for water and air applications, and they are
available in both English and metric units. They are most widely used
when cost is the main consideration, and high accuracy is not required.
Suppliers
KROHNE began
making variable area meters in Duisburg , Germany in 1921. Today
KROHNE makes a wide range of plastic, glass, and metal VA meters that can be
used for a variety of applications. Some typical applications for
KROHNE’s meters include hygienic and chemical processes, water and wastewater,
and offshore applications. KROHNE quotes accuracies ranging from 1 percent of
measured value to 4 – 6 percent of measured value, depending on the model.
Today
Yokogawa makes a range of metal, glass, and plastic VA meters it calls the RA
Series. Its metal RAMC VA meter has a transmitter that
supports both HART and Profibus PA. Having a transmitter output is
still the exception for most VA meters, but is becoming more common as companies
want to combine the advantages of low cost with automation. In
addition to indicating flow at a point in time, the RAMC has a totalizing
option.
Brooks
Instrument is another major supplier of variable area meters. The
company offers both low-flow and high-flow glass tube meters. The
company quotes 2 percent, 3 percent, and 10 percent accuracy for its high-flow
glass tube meters, depending on the model. It quotes accuracies of 5
percent and 10 percent on the low-flow glass tube meters. Brooks has
a line of armored metal tube VA meters called the MT Series. These
meters have corrosion resistant options for metering aggressive fluids. Transmitters
with 4-20mA and HART are offered as options for remote monitoring.
Blue-White
Industries offers primarily acrylic and polysulfone VA meters. Polysulfone
is a type of plastic that is tolerant of high temperatures. The
company offers both tube and block models. The block type meters are
rectangular instead of round, and are designed for panel mounting. Blue-White
quotes flowrates of 0.25 to 100 gallons per minute (GPM) on some of its machined
acrylic meters. The largest line size of these meters is two inches.
Where They
Stand
Variable area
flowmeters are considered to be traditional technology meters. They
are the least expensive and least sophisticated of the traditional meters. They
are relatively simple in construction when compared to a turbine or positive
displacement meter. Yet variable area meters have maintained their
market size over the years, even though this size is less than that of turbine
or positive displacement meters.
One reason
why VA meters have maintained their presence in the market is that suppliers
have offered improvements such as transmitter outputs, and have built VA meters
designed for specific applications. This often means choosing
particular materials of construction, along with a different shaped tube or
float. This increases the versatility of VA meters and makes them
more appealing to end-users with specific needs.
 Another reason for their growth is
that they fill a genuine need in the market. In some cases end-users
simply need a flow/no-flow indication, or they are satisfied with 2 percent or 5
percent accuracy. Variable area meters may also do well in regions
where cost is a major consideration and high accuracy is not always required. Depending
on application, VA meters may do well in China , India , Australia , Malaysia ,
and Indonesia . KROHNE is positioned to supply these
regions with its variable area manufacturing facility in Chengde , China . Another
major VA supplier positioned to sell into the Asian region is Tokyo Keiso of
Japan.
Variable area
meters have a fascinating past, and they could have a bright future. How
bright their future will be depends on how much research and development
suppliers decide to put into them. By incorporating more advanced
features such as transmitter outputs, more varied materials of construction,
different float designs, and more industry approvals, VA suppliers can
potentially position themselves for expansion. At the moment, much
research and development money is going into Coriolis and ultrasonic meters. That
is not likely to change, but if VA suppliers can continue to improve their
products, and develop higher accuracies, the variable area flowmeter market has
a chance to expand.
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