Discrete and Process Automation

Video: How to display flow rate from a DP transmitter

15 February 2022

A water treatment facility wanted to display the flow rate from differential pressure (DP) transmitters with an output 4 mA to 20 mA signal to the supervisory control and data acquisition (SCADA) system. Another goal was to display the flow rate to the operators working nearby; but the signal from the DP transmitter is nonlinear.

The panel meter must be able to perform a square root calculation on the incoming signal to display the flow in gallons per minute. The display also must have large, easily read characters. The signal to the SCADA system cannot be interrupted, even if the display is down for any reason.

The solution: use Acromag’s SP336 to split the 4 mA to 20 mA signal, which will fully isolate the loop to the SCADA system from the loop to the display. The VPM3000 panel meter from Acromag can extract the square root from a 4 mA to 20 mA signal and display the gallons per minute in bright red 1.2 inch characters. The user-selectable low-flow cutoff feature gives a reading of zero when the flow rate dropsThe SP336 signal splitter. Source: AcromagThe SP336 signal splitter. Source: Acromag below the user-selected value. Options for user-settable relay alarm outputs for low and high flow rate are available.

The SP336-0700 defaults to 4 mA to 20 mA in and two 4 mA to 20 mA signals out. But it can be scaled differently with either the Acromag Windows-based software or the Agility app. The VPM3000 can easily be configured from the front panel pushbuttons or with Acromag DisplayWizard software.

The square root calculation feature saves money because no additional computation units are necessary. Combining a signal splitter with a panel meter creates a convenient and fail-safe display of the flow rate. Both units are easily configured with Acromag’s free software.

To contact the author of this article, email engineering360editors@globalspec.com

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