By David W. Spitzer, P.E.

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.