Instrument and DCS display issues
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A vacuum gauge ranging from 0 to 400 mbar, with a 4-20mA signal; it is connected in a two-wire configuration to the analog input module of the S7-400, and it has been working properly over the past few years. Abnormalities have occurred recently. The actual vacuum inside the tank is 113 mbar. The displayed value is also 113 mbar, while the DCS shows only around 60 mbar. During this period, one of the signal wires marked “+” was replaced (the original cable used was a 3-core shielded wire); the display functioned normally thereafter. It became abnormal again after a few days.
Make sure nothing else has been touched – are the DCS data and the on-site readings exactly half of each other?
It’s likely that the resistance in the circuit has increased, which has resulted in a decrease in the current flowing through the DCS channel. Pay special attention to checking the resistance values of the terminals and cables. If one core was replaced earlier, the other two cores may also be damaged.
Bro, it’s simple. I’ve encountered this issue before; use the process of elimination: 1. Connect it in series to a multimeter to check whether the current signal matches what the DCS displays. If they don’t match, then the DCS channel is damaged. If the current signal, after conversion, matches what the DCS shows, it’s recommended to replace the field gauge or transmitter. 2. Check if there are any significant spikes in the instrument’s historical data; if so, there is interference either at the field site or within the cabinet. 3. Simply replace the channel and reconfigure it
Switch to a good channel; if it works properly, then the channel is faulty; if it doesn’t work, there’s still an issue on the end side.
Perform a loop test by sending a 4-20mA signal to the input terminals of the cabinet, to check whether there are any issues with the system channels. If problems are detected, replace the channel or conduct tests using an alternative channel； If there are no issues, disconnect the on-site equipment and send a 4-20mA signal to conduct another test; if there are problems, check the circuit insulation ； If both tests are successful, it means that the table’s measurement function and on-site display function are working properly; the issue lies only in the signal conversion section. Replace the corresponding module or replace the entire table.
In my opinion, such issues should be addressed by starting with the following steps: 1. Connect a multimeter in series within the circuit to measure whether the circuit current matches the displayed value. Troubleshooting instrument loops and channel failures ； 2. Move away from the site for the channel, and use a signal generator to check whether the full range is displayed correctly. Troubleshooting channel issues ； 3. Measure the resistance value of the single-core wire to ground, or use a megohmmeter to check for any short circuits in the grounding system ； 4. Since this is a PLC system, its cards and channels have poor anti-interference and isolation capabilities; therefore, it is necessary to check whether the grounding at various locations such as cabinets, cables, and racks is proper. Eliminate interference ； 5. Can adding safety isolation and deletion tests in the circuit eliminate the fault? ； 6. Replace the cable entirely to eliminate issues such as excessively high resistance or grounding short circuits caused by cable aging or damage.
Thank you all for your guidance. It has been confirmed that there are no issues with the channels, and the readings on the on-site meters are correct as well. I plan to shut down the system and replace the cable to see if that resolves the problem.
A loop test should be done to rule it out