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| By David W. Spitzer, P.E.
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Sometimes words get
thrown around and their meanings become muddied. In certain situations,
this lack of clarity can block our vision and limit our thinking. The
final result of such an artificial limit can restrict our ability to
develop and apply technology. Let’s consider the phrase “multivariable
flowmeter.”
What is a
multivariable flowmeter? My recollection is that widespread usage of
this term originated in the 1990s to describe differential pressure
transmitters that could measure both differential pressure and
pressure. Common usage of this term has generally been limited to
differential pressure measurement to the extent that the
instrumentation print media has used the term “multivariable
transmitter” in reference to a multivariable differential pressure flow
transmitter.
Let’s dissect the
term “multivariable flowmeter” into its individual words to determine
the meaning of the entire term. Considering the last part first, the
Instrumentation, Systems and Automation Society (ISA) Process
Instrumentation Terminology Standard ISA 51.1-1979 (R1993) defines a
flowmeter as “a device that measures the rate of flow or quantity of a
moving fluid in an open or closed conduit. It usually consists of both
a primary and secondary device.” This definition seems to be relatively
straightforward.
To my knowledge, no
technical society (such as ISA) has published a definition of the word
multivariable as it refers to field instrumentation. However, the
definition of multivariable is “having or involving a number of
independent mathematical or statistical variables” (Webster’s
Dictionary, Ninth Edition). As related to instrumentation, this
definition could be interpreted to define a measurement instrument that
has or involves a number of independent process variables. This would
be consistent with the ISA definition of “multivariable control” which
is “a control system that involves several measured and controlled
variables…” (Automation, Systems, and Instrumentation Directory,
ISA, 2003).
Per the above, a
working definition of a multivariable flowmeter can be stated as a
flowmeter that measures a number of independent process variables.
Using this definition, a differential pressure flow transmitter that
measures (and hence outputs) differential pressure and another process
variable, such as pressure and/or temperature, is a multivariable
flowmeter. But a Coriolis mass flowmeter is also a multivariable
flowmeter because it measures and outputs flow, temperature, density,
and (in some designs) viscosity. A vortex shedding flowmeter can also
be a multivariable flowmeter if it measures and outputs flow and
another variable, such as pressure and/or temperature. In other words,
there are many types of multivariable flowmeters.
Multivariable
flowmeters should also include flowmeters that use multiple process
measurements to measure only flow. For example, a differential pressure
flowmeter with a transmitter that uses differential pressure, pressure,
and temperature sensors to generate a flow signal (only) would fall under
this broader definition. It would not be a multivariable flowmeter
using the definition in the previous paragraph because it uses, but
does not provide, an external measurement of the other process variables.
Interestingly, a
differential pressure transmitter that measures differential pressure,
pressure, and/or temperature is commonly considered a multivariable
transmitter. However, many of these transmitters are installed to use
only the flow output signal. As such, they are not strictly
multivariable (they do not use other measurements even though they
could), but they would be multivariable in the broader definition
provide here.
For a number of
reasons, not the least of which is market size, differential pressure
flow transmitter suppliers have claimed sole ownership of the term
“multivariable flowmeter,” despite the existence of other multivariable
technologies. Perhaps it is time to buck this trend, defining
multivariable flowmeters as any flowmeter that measures more than one
process variable, rather than limiting the term to differential
pressure-based instruments. When this occurs, it will help us to think
more clearly and open our minds to more creative flow measurement
solutions.
About the Author
David W. Spitzer, P.E., is a regular contributor to Flow Control.
He has more than 25 years of experience in specifying, building,
installing, start-up, and troubleshooting process control
instrumentation. He has developed and taught seminars for almost 20
years and is a member of ISA and belongs to ASME, MFC, and ISO TC30
committees. Mr. Spitzer has published a number of books concerning the
application and use of fluid handling technology, including the popular
The Consumer Guide to...
series, which compares flowmeters by supplier. Mr. Spitzer is currently
a principal in Spitzer and Boyes LLC, offering engineering, product
development, marketing, and distribution consulting for manufacturing
and automation companies. He can be reached at 845 623-1830.
For More Information: www.spitzerandboyes.com
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