Many Measurements, One Instrument

Paint a More Complete Process Picture by Simultaneously Measuring Variables

By Hank Hogan

Like peanut butter and jelly, certain process variables go together. There's temperature and humidity or mass flow and density. Measuring such variables simultaneously can paint a more complete process picture and produce finer control.

One illustration of multivariable process instrumentation comes from ABB Measurement Products with new multivariable transmitters in its 266 instrumentation series. The devices measure differential pressure, absolute pressure and temperature. From those three variables, the instrument directly calculates the process parameters of mass flow or standard volume flow for gases, vapors and liquids.

SEE ALSO: Time to Hit the Multivariable Switch?

The traditional method of doing this uses multiple instruments and an associated flow computer or DCS. The all-in-one approach offers savings.

"This multivariable transmitter truly helps customers with its ability to eliminate hardware, wiring and the flow computer," says Allen Hood, ABB's product manager for field instruments and devices.

ABB's new device replaces a standard differential pressure transmitter, an absolute pressure sensing gauge transmitter, a temperature-measuring RTD, wiring and a computer. Thus, the greater cost of the multivariable instrument is offset by savings elsewhere.

An added payoff can be greater measurement accuracy. The use of three separate instruments can mean a total error span of 2%, whereas a multivariable instrument might have an error of only 0.3%, Hood says.

When supplied with specifications regarding tank shape, ABB's multivariable transmitters also offer the ability to measure liquid volume. The latter is critical for safe boiler operation.

ABB has been doing multivariable instruments for years. Recent improvements have made these easier to use, as well as offering the ability to run diagnostics when trying to pinpoint the source of process problems. "Modern electronics allows us to do much more than before," Hood says of such advances.

Micro Motion, an Emerson Process Management company, expanded its line of Elite multivariable flowmeters to smaller pipe sizes. Micro Motion products are based on the Coriolis Effect, which causes vibrating tubes to move in a specific way relative to each other when fluid flows through them. From that relative motion, the instruments provide a wealth of multivariable information.

"You get mass flow," says John Herczeg, Micro Motion's director of industry marketing. "You also get volumetric flow, because we also directly measure density and temperature. So, you get three primary variables from which you can derive volume and concentration."

Importantly, the Coriolis approach works in the presence of two-phase flow. Often, liquids will contain entrained gas, which can mess up some measurement technologies. Indeed, Micro Motion often finds trapped air or other gases in the fluid of a customer's process, even though the customer was sure — before the measurement was made — that this wasn't the case, Herczeg says.

Another benefit is that little to no maintenance is required. The instruments can run a self-check on demand or according to a schedule. This capability is particularly useful when material is being transferred from one business to another, e.g., custodial transfers, as mass flow can be the basis for payments and revenue.

A final example of multivariable instrumentation comes from Honeywell's line of temperature and humidity sensors. Relative humidity and temperature are critical process parameters, with the combination needing to be within a specified band for curing and other chemical reactions.

The company says its HumidIcon instruments offer an industry-leading total error band of ±5% relative humidity over a temperature range of 5–50 °C and for relative humidity from 10–90%. The instruments can operate from -25 to 85 °C.

They come in a SIP 4 pin package, says Jacqueline Leff, Honeywell's senior global marketing manager, because, "We're trying to help them with the very smallest package that we possibly can." This configuration is useful in applications that require remote sensor mounting, such as in probes. The SIP 4 pin package also mounts above the circuit board, which minimizes the effect of PCB heat.

In addition, the new multivariable sensors illustrate the drive toward low power. They have a sleep mode that cuts power consumption to 1 µA, much less than the 650 µA demanded during full operation.

This capability is important, Leff says. "A lot of these applications want to go wireless, so they need low power consumption," she adds.

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