Question:  I’m showing a difference in 4-20mA readings between a local indicator and the DCS control card.  The local indicator reads lower and closer to the actual mass flow than the DCS card.  Do I have a ground loop?  Will adding your 4-20mA signal isolator in front of the DCS Analog Input card solve the problem?

Answer:  Possibly.  You may be getting a difference in readings due to a ground loop or other reasons on the DCS card such as calibration, channel to channel interaction, wiring, or scaling.

An easy method to check if an isolator will help follows below:

If possible, disconnect the loop wired to the DCS and try injecting an isolated 4-20mA current signal from an isolated source (ie; a fluke calibrator).
Next, with the loop disconnected at the DCS, try reading the 4-20mA signal coming from the mass flow meter with an isolated current meter (ie; a fluke meter). 

If the DCS reads okay with an isolated 4-20mA source and the loop reading looked okay with an isolated meter, then you probably have a ground loop issue.  An isolator should help you out.  For suggested hardware solutions, check out Acromag’s 651T, 652T loop powered isolators or the 63xT AC/DC powered 4-wire isolators. Acromag’s isolator application notes #4, #5, and #6 will also be good reference diagrams for you to help select the right products.  If you have any questions or need more assistance, call us at 248-295-0880 or email us at sales@acromag.com .