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.

-- Control Engineering, 11/17/2008

(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, peter.welander@reedbusiness.com,
Control Engineering Process Instrumentation & Sensors Monthly
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