Video listen in: Making Coriolis two-wire demands small sacrifices

Giving up a larger cable to Coriolis sensors wasn’t easy and required some very clever engineering. Coriolis flowmeter sensors require more power than many other technologies, which is why they have traditionally required four-wire cables and external power. This has effectively precluded using them in situations where only two-wire loop power is installed. That is until now. Link to video.
By Control Engineering Staff November 17, 2008

(See a video demonstration of the technology from John Herczeg, product marketing director, Micro Motion.)

Micro Motion has created a two-wire transmitter for Coriolis mass flow sensors to add versatility, but it’s not without some small functional sacrifices. How will the trade-offs to reduce power consumption affect you?

Coriolis flowmeter sensors require more power than many other technologies, which is why they have traditionally required four-wire cables and external power. This has effectively precluded using them in situations where only two-wire loop power is installed. That is until now. In response to what it characterizes as strong customer demand, Micro Motion has figured out a way to make its Elite sensor line operate within the constraints of two-wire systems. ( See earlier announcement .) Making that possible required some creative engineering and the loss of a few functions. If you want to install this technology into a loop powered environment, you’ll have to decide how important those functions are to you.

Here are some things you have to think about:

  • Not having as much power available depletes some of the “robustness” of the sensor, which means it isn’t as reliable in difficult conditions such as two-phase flow and slug flow. These are more difficult for the sensor to read.

  • Output is rescaled to 12-20 mA, rather than 4-20 mA. Some I/O cards can be adjusted for that, or there are barrier devices to scale output back to 4-20.

  • There is a small loss of turn-down ratio at the very low end of measurable flow.

  • Programming the transmitter requires removal of the cover and pressing actual buttons. It cannot be done through the glass, but it is still intrinsically safe.

  • Two-wire or four-wire, the sensor is the same, and only the transmitter has to change. Units can actually be refit in the field if required.

Micro Motion’s advice is to use the four-wire transmitter if you can for best performance, but even operating in two-wire mode, Coriolis is better than many other flowmeter technologies.

Read an earlier story on Coriolis flowmeter technology .

—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Process Instrumentation & Sensors Monthly
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